NERC Reliability Coordinator Exam Vocabulary (STUDY GUIDE)

Accelerating Power In a GENERATOR, the difference between the MECHANICAL INPUT POWER and the electrical output POWER (assuming no LOSSES). For a GENERATOR's TORQUE ANGLE to change, the GENERATOR must have accelerating P ...

Accelerating Power In a GENERATOR, the difference between the MECHANICAL INPUT POWER and the electrical output POWER (assuming no LOSSES). For a GENERATOR's TORQUE ANGLE to change, the GENERATOR must have accelerating POWER. Accumulated Inadvertent The accumulation over time of INADVERTENT ENERGY. The accumulated inadvertent account of a BALANCING AUTHORITY represents the amount of excess ENERGY a BALANCING AUTHORITY has either supplied to the INTERCONNECTION or absorbed from the INTERCONNECTION. BALANCING AUTHORITIES monitor their accumulated inadvertent to ensure the accounts do not grow too large Active Power The component of the COMPLEX POWER that performs the work. The common unit of active power is the MW and the symbol is "P". Active power is sometimes called REAL POWER. Actual Interchange The metered INTERCHANGE over a specific INTERCONNECTION between two PHYSICALLY ADJACENT BALANCING AUTHORITIES. Actual Load The actual MW drawn by a LOAD from the POWER SYSTEM. The actual load will be different from the rated or NOMINAL LOAD if the LOAD's VOLTAGE or FREQUENCY vary from their NOMINAL values Adequacy The ability of the electric SYSTEM to supply the aggregate electrical DEMAND and ENERGY requirements of the end-use customers at all times, taking into account SCHEDULED and reasonably expected unSCHEDULEd OUTAGEs of SYSTEM ELEMENTS.ua Adequate Regulating Margin The minimum on-line CAPACITY that can be increased or decreased to allow the SYSTEM to respond to all reasonable DEMAND changes in order to be incompliance with the CONTROL PERFORMANCE STANDARDS. Adjacent Balancing Authorities A BALANCING AUTHORITY AREA that is interconnected to another BALANCING AUTHORITY AREA either directly or via a multi-party AGREEMENT or TRANSMISSION TARIFF. AGC Pulses The AGC SYSTEM sends signals to selected (regulating) GENERATORS to adjust their set-points. These signals are sent via telecommunication EQUIPMENT. The signals are often called AGC pulses. Air- Gap The air space between the ROTOR and STATOR of a motor or GENERATOR. Alternating Current (AC) In AC SYSTEM the CURRENT and VOLTAGE magnitudes constantly vary or alternate. CURRENT and VOLTAGE magnitudes do not alternate in a DC SYSTEM. Alternator A Rotating machine whose output is alternating VOLTAGE and CURRENT Aluminum Cable Steel Reinforced (ACSR) A common type of TRANSMISSION line CONDUCTOR that comes in many different sizes and designs. The aluminum is the CONDUCTOR and the steel provides strength. Amortisseur Winding A machine winding consisting of a number of conducting bars attached between the magnetic POLES of the machine's ROTOR. Amortisseur windings may be used as starter windings or to help dampen POWER OSCILLATIONS Ampere (amp) The unit of measurement for CURRENT flow, often stated as "amp". Amplitute The value or magnitude that a waveform has at a specific point in time. Analog Electronics Electronic circuitry in which the magnitudes of quantities are used in a continuous manner to perform functions. For example, input CURRENT values may be amplified and used to perform work in an analog CIRCUIT. Analog is different than digital in the sense that analog CIRCUITS use VOLTAGE and CURRENT magnitudes continuously while digital CIRCUITS use VOLTAGE and CURRENT to determine the state (on or off) of ELEMENTS. Ancillary Services (FERC Definition) Those services that are necessary to support the TRANSMISSION of CAPACITY and ENERGY from RESOURCES to LOADS while maintaining reliable operation of the TRANSMISSION SERVICE PROVIDER's TRANSMISSION SYSTEM in accordance with GOOD UTILITY PRACTICE. Angle Instability The opposite of ANGLE STABILITY. When a POWER SYSTEM loses ANGLE STABILITY it enters a PERIOD of angle instability. An angle unstable SYSTEM has lost some portion of the magnetic bound that holds sections of the POWER SYSTEM in synchronism with one another. Angle Stability An angle stable POWER SYSTEM is one in which all ELEMENTS of the POWER SYSTEM are bound together via magnetic forces. For example, an angle stable GENERATOR's internal MAGNETIC FIELD rotates in synchronism (in-step) with the MAGNETIC FIELD of the 3Φ POWER SYSTEM to which it connects. Apparent Power The product of VOLTAGE and CURRENT PHASORS. Apparent power comprises both active and REACTIVE POWER, usually expressed in KILOVOLTAMPEREs (kVA) or megavoltamperes (MVA). Another term for apparent power is COMPLEX POWER. ARC The discharge of CURRENT through the air or in a gas. Arcing Horns An enhancement to a DISCONNECT SWITCH to increase the switch's CURRENT interrupting capability. Area Control Area (ACE) The instantaneous difference between net actual and SCHEDULED INTERCHANGE, taking into account the effects of FREQUENCY bias including a correction for meter error. Armature The component of a machine in which the VOLTAGE is induced. In a SYNCHRONOUS GENERATOR the armature is usually the STATOR. The armature winding is wrapped about the armature. Asynchronous To be OUT-OF-STEP with a reference. An induction GENERATOR is an asynchronous GENERATOR as its ROTOR does not rotate in synchronism with the POWER SYSTEM. Auto-Load A feature on some GENERATORS that automatically moves the GENERATOR to a TARGET LOAD (MW) level. Automatic Generation Control (AGC) EQUIPMENT that automatically adjusts GENERATION in a BALANCING AUTHORITY AREA from a central location to maintain the BALANCING AUTHORITY's INTERCHANGE SCHEDULE plus FREQUENCY BIAS. AGC may also accommodate automatic INADVERTENT payback and TIME ERROR CORRECTION. Automatic Operations Systems SPECIAL PROTECTION SYSTEMS, REMEDIAL ACTION SCHEMEs, or other operating SYSTEMs installed on the electric SYSTEMs that require no intervention on the part of SYSTEM OPERATORS. Auto-Transformer A POWER TRANSFORMER with a single coil. The entire coil acts as the PRIMARY WINDING while a portion of the same coil acts as the SECONDARY WINDING. Auxiliary Relay A RELAY whose function is to supplement the actions of other types of RELAYS. For example, a LOCKOUT RELAY is an auxiliary relay with numerous contacts that each can perform an action. The IEEE has numbers assigned to two types of auxiliary relays; 86 (lock-out) and 94 (tripping). Availability A measure of time a generating unit, TRANSMISSION line, or other FACILITY is capable of providing service, whether or not it actually is in service. Typically, this measure is expressed as a percent available for the PERIOD under consideration. Available Flowgate Capability (AFC) The available CAPACITY in MW or MVA on a FLOWGATE, which a TRANSMISSION SERVICE PROVIDER has determined, is available for sale. The available CAPACITY determined will be dependent on the GENERATION, LOADS and TRANSMISSION configuration assumed for the time period studied and therefore is referred to as a capability. The AFC will account for all thermal, VOLTAGE and STABILITY LIMITS under both pre and post CONTINGENCY conditions, along with any TRM or CBM. There is no one set of SOURCE and SINK points associated with an AFC. The AFC can be used to determine the amount of MW that can be transferred between a specific set of SOURCE and SINK points with respect to the FLOWGATE, by dividing the AFC by its corresponding PTDF. Available Transfer Capability (ATC) A measure of the TRANSFER CAPABILITY remaining in the physical TRANSMISSION network for further commercial activity over and above already committed uses. It is defined as TOTAL TRANSFER CAPABILITY less existing TRANSMISSION commitments (including retail customer service), less a CAPACITY BENEFIT MARGIN, less a TRANSMISSION RELIABILITY MARGIN. Avoided Cost The cost the utility would incur but for the existence of an independent GENERATOR or other ENERGY service option. Avoided cost rates have been used as the POWER purchase price utilities offer independent suppliers. Back EMF (Conductors Back EMF) When AC CURRENT flows through a CONDUCTOR an alternating MAGNETIC FIELD is created. This alternating MAGNETIC FIELD induces a VOLTAGE in the CONDUCTOR. The induced VOLTAGE is called the CONDUCTOR'S back EMF. Back-To-Back HVDC An HVDC SYSTEM in which AC is converted to DC and then immediately back to AC. The HVDC TRANSMISSION path is very short, normally consisting of a short section of buswork. Backup Power POWER provided by contract to a customer when that customer's normal SOURCE of POWER is not available. Balancing Authority The responsible ENTITY that integrates resource plans ahead of time, maintains LOADINTERCHANGE-GENERATION balance within a BALANCING AUTHORITY AREA, and supports INTERCONNECTION FREQUENCY in real time. Balancing Authority Area The collection of GENERATION, TRANSMISSION, and LOADS within the metered boundaries of the BALANCING AUTHORITY. The BALANCING AUTHORITY maintains LOAD-RESOURCE balance within this area Bandwidth The acceptable range of a quantity. For example, if it is desired to hold the VOLTAGE at a 345 kV BUS between 355 and 360 kV then the bandwidth is 355 - 360 kV or 5 kV. Banking ENERGY delivered or received by a utility with the intent that it will be returned in kind (as ENERGY) in the future. Base Load The minimum amount of electric POWER delivered or required over a given period at a constant rate. Base Load Capacity CAPACITY used to serve an essentially constant level of customer DEMAND. Base load generating units typically operate whenever they are available, and they generally have a CAPACITY FACTOR that is above 60%. Base Load Generating Units GENERATORs that normally run continuously to serve a BALANCING AUTHORITY'S LOAD. Bilateral Payback INADVERTENT INTERCHANGE accumulations paid back via an INTERCHANGE SCHEDULE with another BALANCING AUTHORITY. Bipolar HVDC An HVDC SYSTEM that uses two CONDUCTORS. One CONDUCTOR is energized with a positive VOLTAGE and the other a negative VOLTAGE. Black-Start Unit (BSU) A generating unit that has the ability to go from a shutdown condition to an operating condition and start delivering POWER without assistance from the POWER SYSTEM the GENERATOR normally connects to. Blackstart Capability Plan A documented procedure for a generating unit or STATION to go from a shutdown condition to an operating condition delivering electric POWER without assistance from the ELECTRIC SYSTEM. This procedure is only a portion of an overall SYSTEM RESTORATION plan. Blade (Steam) The component of a steam TURBINE upon which the steam impacts. Blades are also called "buckets". Blocked Governor A GOVERNOR CONTROL SYSTEM that has somehow been blocked or prevented from responding to a FREQUENCY DEVIATION. There are many ways to block a governor. Boiler The component of a steam POWER plant in which the steam is created. Boiler Follow A MODE of operation of a fossil unit's TURBINE/BOILER CONTROL SYSTEM in which the unit's TURBINE is allowed to immediately respond to a LOAD (MW) change request while the BOILER response "follows" with the resulting temperature and pressure swings. Braking Resistor A device used to enhance the angle stability of a power system that uses hydro-electric generation. A Braking Resistor is a large resistive load that is inserted to absorb excess energy when an accelerating condition is detected in the power system. Broker A third party who establishes a TRANSACTION between a seller and a purchaser. A Broker does not take title to CAPACITY or ENERGY. Brush A sliding contact, usually made of carbon, located between the excitation CURRENT SOURCE and the ROTOR FIELD WINDING leads of a SYNCHRONOUS GENERATOR. Bulk Electric System (BES) As defined by the REGIONAL RELIABILITY ORGANIZATION, the electrical GENERATION RESOURCES, TRANSMISSION LINES, INTERCONNECTIONS with neighboring SYSTEMS, and associated EQUIPMENT, generally operated at VOLTAGES of 100 kV or higher. Radial TRANSMISSION facilities serving only LOAD with one TRANSMISSION SOURCE are generally not included in this definition. Bulk Transmission A functional or VOLTAGE classification relating to the higher VOLTAGE portion of the TRANSMISSION SYSTEM, specifically, lines at or above a VOLTAGE level of 115 kV. Burden Operation of the BULK ELECTRIC SYSTEM that violates or is expected to violate a SYSTEM OPERATING LIMIT or INTERCONNECTION RELIABILITY OPERATING LIMIT in the INTERCONNECTION, or that violates any other NERC, REGIONAL RELIABILITY ORGANIZATION, or local operating RELIABILITY standards or criteria. Bus (Busbar) An abbreviation of the word busbar. A bus is a location in an electrical network where one or more ELEMENTS are connected together. Capacitance (C) The property of an ELECTRICAL CIRCUIT that opposes VOLTAGE changes by storing ENERGY in its ELECTRIC FIELD. The symbol for capacitance is "C" and the unit is the FARAD. All energized EQUIPMENT has a natural capacitance. Capacitive Load A LOAD that supplies LAGGING REACTIVE POWER to the SYSTEM. Capacitive Reactance (Xc) The opposition that CAPACITANCE provides to AC CURRENT. The capacitive reactance (XC) in a 60 HZ CIRCUIT is: XC = 1 / 2π fC Capacitively Coupled Voltage Transformer (CCVT) An instrument TRANSFORMER that is similar in function to a potential TRANSFORMER (PT). A capacitive VOLTAGE divider CIRCUIT is used in a CCVT to reduce POWER SYSTEM VOLTAGE magnitudes to usable (≈120 Volt) levels. Capacitive VOLTAGE TRANSFORMERS (CVTs), Coupling CAPACITOR Potential Devices (CCPDs), and Bushing Potential Devices (BPDs) are similar in design and function to a CCVT. Capacitor A device intentionally designed to act as a capacitor and store ENERGY in its ELECTRIC FIELD. A shunt capacitor acts as a SOURCE of REACTIVE POWER to the SYSTEM. Series capacitors are seldom seen devices that are used to reduce the INDUCTIVE REACTANCE of a TRANSMISSION path. Capacitor Bank A CAPACITOR is an electrical device that provides REACTIVE POWER to the SYSTEM and is often used to compensate for reactive LOAD and help support SYSTEM VOLTAGE. A bank is a collection of one or more CAPACITORS at a single location. Capacity The rated continuous LOAD-carrying ability, expressed in megawatts (MW) or megavolt-amperes (MVA) of GENERATION, TRANSMISSION, or other electrical EQUIPMENT. Capacity Benefit Margin (CBM) That amount of firm TRANSMISSION TRANSFER CAPABILITY preserved by the TRANSMISSION PROVIDER for LOAD SERVING ENTITIES (LSEs), whose LOADs are located on that TRANSMISSION SERVICE PROVIDER's system, to enable ACCESS by the LSEs to GENERATION from INTERCONNECTED SYSTEMS to meet GENERATION RELIABILITY requirements. Preservation of CBM for an LSE allows that entity to reduce its installed generating CAPACITY below that which may otherwise have been necessary without INTERCONNECTIONS to meet its GENERATION RELIABILITY requirements. The TRANSMISSION TRANSFER CAPABILITY preserved as CBM is intended to be used by the LSE only in times of EMERGENCY GENERATION deficiencies. Capacity Emergency A capacity emergency exists when a BALANCING AUTHORITY AREA's operating CAPACITY, plus firm purchases from other systems to the extent available or limited by TRANSFER CAPABILITY, is inadequate to meet its DEMAND plus its regulating requirements. Capacity Factor The ratio of the total ENERGY generated by a generating unit for a specified period to the maximum possible ENERGY it could have generated if operated at the maximum CAPACITY RATING for the same specified period, expressed as a percent. Cascading The uncontrolled successive loss of SYSTEM ELEMENTS triggered by an incident at any location. Cascading results in widespread electric service INTERRUPTION that cannot be restrained from sequentially spreading beyond an area predetermined by studies. Cascading Outages The uncontrolled successive LOSS of BULK ELECTRIC SYSTEM FACILITIES triggered by an incident (or condition) at any location resulting in the INTERRUPTION of ELECTRIC SERVICE that cannot be restrained from spreading beyond a pre-determined area. Cavitation Noise or vibration causing damage to a hydro unit's TURBINE BLADEs as a result of bubbles that form in the water as it goes through the TURBINE which causes a loss in CAPACITY, HEAD loss, efficiency loss, and the cavity or bubble collapses when they pass into higher regions of pressure. Centrifugal Ballhead governor A mechanical governor that uses rotating FLYWEIGHTS to sense shaft speed. A very common governor due to its durability and accuracy. Charge An atomic force. An electron has a negative charge. A proton has a positive charge. Like charges repel one another while opposite charges attract one another. CURRENT is the flow of charge. Charging The Mvar production of a TRANSMISSION line. For example, a 100 mile long 345 kV line has approximately 75 Mvar of charging. Charging Current The LEADING CURRENT that flows into the natural CAPACITANCE of a TRANSMISSION line when it is first energized. Checkout Process The method by which any two ENTITIES in the utility industry routinely perform a confirmation of SCHEDULES for a period of time. Circuit A CONDUCTOR or a SYSTEM of CONDUCTORS through which electric CURRENT flows. Circuit Switcher (CS) A piece of POWER SYSTEM EQUIPMENT that is used to disconnect other EQUIPMENT from the POWER SYSTEM. Circuit breakers are grouped according to their insulating medium. For example, Air Circuit Breaker (ACB), Gas Circuit Breakers (GCB), Oil Circuit Breakers (OCB), etc. Circuit Switcher (CS) An enhanced DISCONNECT SWITCH that is similar in function to a CIRCUIT BREAKER. A circuit switcher will contain an interrupting device (typically gas based) to enhance its CURRENT interrupting ability. Clock Hour The 60-min PERIOD ending at :00. All surveys, measurements, and reports are based on clock hour PERIODS unless specifically noted. Cogeneration Production of ELECTRICITY from steam, heat, or other forms of ENERGY produced as a by-product of another process. Example: recycled steam from a (CC) COMBINED CYCLE Natural Gas unit. Uses exhaust heat from the Simple Cycle (CT) generator and run a secondary Steam Turbin (ST) Cold Load Pick-Up The increase in a LOAD's magnitude during the initial PERIOD after it is energized. The cold load pick-up consists of a short-term (possibly several seconds) in-rush component and a longer term (possibly many minutes) LOSS OF LOAD DIVERSITY component. Combined Cycle (CC) An electric generating technology in which electricity and process steam are produced from otherwise lost waste heat exiting from one or more combustion TURBINES. The exiting heat is routed to a conventional BOILER or to a heat recovery steam GENERATOR for use by a steam TURBINE in the production of electricity. This process increases the overall efficiency of the electric generating unit. Combustion Turbine Generator (CT) A type of GENERATOR in which a fuel (oil, gas, etc.) is ignited and the combustion products are used to drive a TURBINE. Commonly or Jointly owned units (COU/JOU) These terms may be used INTERCHANGEably to refer to a unit in which two or more GENERATION OWNERS share. Commutation The process of turning off one VALVE and turning on another in an HVDC CONVERTER. In a twelve-pulse converter there are twelve commutations per CYCLE of the AC supply VOLTAGE. Complex Power (S) The vector sum of the active (MW) and reactive (Mvar) POWER. The common unit for complex POWER is MVA and the symbol is "S". Complex Power is sometimes called APPARENT POWER. Compliance Monitor The entity that monitors, reviews, and ensures compliance of responsible entities with RELIABILITY standards. Complimentary Currents When a SUBSYNCHRONOUS CURRENT flows in the STATOR winding of a SYNCHRONOUS MACHINE it will induce two complimentary currents in the ROTOR of the machine. These complimentary currents will have frequencies of 60 ± the FREQUENCY of the SUBSYNCHRONOUS STATOR CURRENT. Compressor A machine that increases the pressure of a gas (typically air) or vapor. Condenser The component of a steam POWER plant in which the steam is cooled to water after exiting the TURBINE. Conductor A material with a low RESISTANCE to CURRENT flow. A conductor is the opposite of an INSULATOR. Congestion The condition that exists when MARKET PARTICIPANTS seek to dispatch GENERATION in a pattern which would result in POWER flows that cannot be physically accommodated by the TRANSMISSION SYSTEM. Although the TRANSMISSION SYSTEM will not normally be operated in an overload condition, it may be described as congested based on requested/desired SCHEDULES. Constant Frequency Control (CFC) An operating MODE of an AGC SYSTEM. While in constant FREQUENCY control an AGC SYSTEM will determine the ACE value by considering only the FREQUENCY ERROR. Constant Net Interchange (CNI) Control An operating MODE of an AGC SYSTEM. While in constant net interchange control an AGC SYSTEM will determine the ACE value by considering only the INTERCHANGE error. Constrained Facility An operating MODE of an AGC SYSTEM. While in constant net interchange control an AGC SYSTEM will determine the ACE value by considering only the INTERCHANGE error. Constraint A limitation placed on INTERCHANGE TRANSACTIONS that flow over a CONSTRAINED FACILITY. Contiguous To belong to a common area. Adjoining. A contiguous BALANCING AUTHORITY is selfcontained. A non-contiguous BALANCING AUTHORITY may have pieces spread over an entire INTERCONNECTION. Contingency The unexpected failure or OUTAGE of a system component, such as a GENERATOR, TRANSMISSION LINE, CIRCUIT BREAKER, SWITCH or other electrical ELEMENT. Contingency Analysis The process by which the effects of contingencies are identified. CA SOFTWARE is used to identify N-1 violations. Contingency Reserve The provision of CAPACITY deployed by the BALANCING AUTHORITY to meet the DISTURBANCE CONTROL STANDARD (DCS) and other NERC and REGIONAL RELIABILITY ORGANIZATION contingency requirements. Continuous Rating The RATING as defined by the EQUIPMENT owner that specifies the level of electrical loading, usually expressed in megawatts (MW) or other appropriate units that a SYSTEM, FACILITY, or ELEMENT can support or withstand indefinitely without loss of EQUIPMENT life. Contract Path An agreed upon electrical path for the continuous FLOW of ELECTRICAL POWER between the parties of an INTERCHANGE TRANSACTION. Contractually Adjacent Balancing Authorities Two BALANCING AUTHORITIES that are part of the same Regional Transmission Organization Control Center The physical location from which an operating entity operates. Control Performance Standard (CPS) The reliability standard that sets the limits of a BALANCING AUTHORITY'S AREA CONTROL ERROR over a specified time period. Control System A collection of electrical and mechanical components designed to produce a series of outputs based on a series of measured inputs. Common control systems related to POWER are excitation control and GOVERNOR CONTROL SYSTEMS. Coordinated Control A MODE of operation of a fossil unit's TURBINE/BOILER CONTROL SYSTEM in which the operation of the BOILER and the TURBINE SYSTEMS are coordinated. The coordination balances the pressure and temperature limits of the BOILER SYSTEM with the desire for a TURBINE to immediately respond to LOAD (MW) change requests. Core The material used within a TRANSFORMER. A TRANSFORMER's core is formed of a magnetic material such as iron. The purpose of the core is to confine the MAGNETIC FIELD to a TARGET area. Corona A condition that occurs on energized EQUIPMENT when the surface potential (VOLTAGE) is so large that the DIELECTRIC strength of the surrounding air breaks down (IONIZATION occurs). Symptoms of corona are a visible ring of light and a hissing sound. Corona is undesirable as ENERGY LOSSES and electrical interference are a consequence. Corona rings are used to reduce the gradient of the ELECTRIC FIELD and reduce the likelihood of corona occurring. Cosine The cosine of either of the unknown angles of a RIGHT TRIANGLE is the ratio of the side adjacent to the unknown angle to the HYPOTENUSE. Cracking Path A portion of the electric SYSTEM that can be ISOLATED and then energized to deliver ELECTRIC POWER from a GENERATION SOURCE to enable the startup of one or more other GENERATING UNITS. Current (I) The rate of flow of electrons in an electrical CONDUCTOR. Symbol is "I" and unit is the AMPERE, or amp. Current Transformer (CT) A low POWER TRANSFORMER (an instrument TRANSFORMER) used to reduce normally high POWER SYSTEM CURRENT magnitudes to lower values (≈ 5 amps). A CT has a low number of turns on the PRIMARY WINDING and a high number of turns on the SECONDARY WINDING. The low magnitude secondary CURRENTS of a CT are typically input to PROTECTIVE RELAYS, meters, etc. Curtailment A reduction in the SCHEDULED CAPACITY or ENERGY delivery. Cut-Out A switching device typically found in the DISTRIBUTION SYSTEM. Cut-outs often include a fuse so the device serves the dual purpose of providing electrical isolation and protection. Cyber Assets Programmable electronic devices and communication networks including hardware, software, and data. Cycle A complete sequence of a repeating waveform. For example, the magnitude of a SINE wave changes as the degrees progress from 0° to 360°. After 360° the SINE wave begins to repeat itself. One cycle of a SINEWAVE is therefore 360°. Damping Forces or CONTROL SYSTEM actions that reduce the magnitude of OSCILLATIONS. Damping is provided by various natural phenomena and by various electrical EQUIPMENT. Damping can either be positive (reduce OSCILLATIONS) or negative (amplify OSCILLATIONS). Deadband The distance about a TARGET value within which a CONTROL SYSTEM will not respond. For example, a GOVERNOR CONTROL SYSTEM may have a deadband of ±0.036 HZ. Unless the FREQUENCY DEVIATION exceeds 0.036 HZ, the governor will not respond. Delayed Fault Clearing FAULT clearing consistent with correct operation of a breaker failure PROTECTION SYSTEM and its associated breakers, or of a backup PROTECTION SYSTEM with an intentional time delay. Delivering Party The entity supplying the CAPACITY and ENERGY to be transmitted at the POINT OF RECEIPT. Delta Connection A 3Φ electrical connection in which the three PHASES are connected in series with one another. The three PHASES form a closed triangle. TRANSFORMER windings are often connected in a Delta configuration. Demand 1. The rate at which electric ENERGY is delivered to or by a SYSTEM or part of a SYSTEM, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time. 2. The rate at which ENERGY is being used by the customer. Demand Interval The time period during which a DEMAND is measured, usually in 15-, 30-, or 60-minute increments. Demand-Side Management (DSN) The term for all activities or programs undertaken by LOAD-SERVING ENTITY or its customers to influence the amount or timing of electricity they use. Derating (Generator) A reduction in a generating unit's NET DEPENDABLE CAPACITY. Deviation The variance from the SCHEDULED value. For example, if the SCHEDULED value of VOLTAGE is 142 kV and the actual value is 138 kV then the VOLTAGE deviation is -4 kV. Dielectric Insulating material used to separate and insulate. CAPACITORs use a dielectric between its two conducting plates. Differential Relay A PROTECTIVE RELAY that responds to the difference between the CURRENTS entering and leaving the protected zone of the RELAY. Differential relays are typically applied to TRANSFORMERS, SUBSTATION BUSes, and GENERATORS. Digital Electronics Electronic circuitry that operates on data in the form of digits. Most digital CIRCUITS use the binary SYSTEM in which digital components are either turned on (a data value of "1") or off (a data value of "0") based on CIRCUIT VOLTAGE magnitudes. Digital electronics have replaced ANALOG ELECTRONICS in most, but not all, applications. Direct Control Load Management (DCLM) DEMAND-SIDE MANAGEMENT that is under the direct control of the SYSTEM OPERATOR. DCLM may control the electric supply to individual appliances or EQUIPMENT on customer premises. DCLM as defined here does not include INTERRUPTIBLE DEMAND. Direct Current (DC) In DC SYSTEM the CURRENT and VOLTAGE magnitudes are constant. CURRENT and VOLTAGE magnitudes alternate in an AC SYSTEM. Disconnect Switch A mechanical device that is used to isolate EQUIPMENT from energized parts of the POWER SYSTEM. Disconnect switches are not rated for interrupting CURRENT unless additional ARC interrupting components are added to the disconnect. Dispatchable Generation GENERATION available physically or contractually to respond to changes in SYSTEM DEMAND or to respond to TRANSMISSION security CONSTRAINTS. Distance Relay A PROTECTIVE RELAY that activates if the ratio of the measured VOLTAGE divided by the CURRENT (the IMPEDANCE) falls below a pickup point. A Distance relay is also commonly called an IMPEDANCE RELAY. Distribution For electricity, the function of distributing electric POWER using low VOLTAGE lines to retail customers. These lines are typically 60hz Distribution Facilities The facilities owned or controlled by a TRANSMISSION Owner and used to provide Wholesale DISTRIBUTION Service. Distribution Factor (DF) The portion of an INTERCHANGE TRANSACTION, typically expressed in PER-UNIT, that flows across a TRANSMISSION FACILITY (FLOWGATE). Distribution Lines CONDUCTORS used to distribute POWER to the utility's customers. Distribution lines may be 3Φ or 1Φ. Distribution Network The portion of an electric SYSTEM that is dedicated to delivering electric ENERGY to an end user. The distribution network consists primarily of low VOLTAGE lines and TRANSFORMERS that "transport" electricity from the bulk POWER SYSTEM to retail customers. Distribution Provider Provides and operates the "wires" between the TRANSMISSION system and the end-use customer. For those end-use customers who are served at TRANSMISSION VOLTAGEs, the TRANSMISSION OWNER also serves as the Distribution Provider. Thus, the Distribution Provider is not defined by a specific VOLTAGE, but rather as performing the DISTRIBUTION function at any VOLTAGE. Disturbance 1. An unplanned event that produces an abnormal SYSTEM condition. 2. Any perturbation to the electric system. 3. The unexpected change in ACE that is caused by the sudden failure of GENERATION or INTERRUPTION of LOAD. Disturbance Control Standard (DCS) The RELIABILITY standard that sets the time limit following a DISTURBANCE within which a BA or RSG must return its AREA CONTROL ERROR to within a specified range. Disturbance Monitoring Equipment Devices capable of monitoring and recording SYSTEM data pertaining to a DISTURBANCE. Such devices include the following categories of recorders: •Sequence of event recorders which record EQUIPMENT response to the event. • FAULT recorders, which record actual waveform data replicating the SYSTEM primary VOLTAGES and CURRENTS. This may include PROTECTIVE RELAYS. • DYNAMIC DISTURBANCE RECORDERS (DDRs), which record incidents that portray POWER SYSTEM behavior during dynamic events such as low-FREQUENCY (0.1 Hz - 3 Hz) OSCILLATIONS and abnormal FREQUENCY or VOLTAGE excursions. Diversity Factor The ratio of the sum of the coincident maximum DEMANDS of two or more LOADS to their noncoincident maximum DEMAND for the same period. Droop A characteristic of a GOVERNOR CONTROL SYSTEM that requires a decrease in GENERATOR shaft speed to produce an increase in the GENERATOR's MW output. There are two types of droop; permanent and TRANSIENT: Permanent Droop Permanent droop is the droop used to enable all GENERATORs (with active governors) to share in FREQUENCY REGULATION and to ensure a MW response in proportion to unit size. Desired values of permanent droop are in the neighborhood of 5%. The 5% droop means that a 5% FREQUENCY change will result in the unit's governor moving the fuel (steam, water, gas, etc.) VALVES across their full range. Transient Droop A feature implemented in some GOVERNOR CONTROL SYSTEMS to ensure GENERATORS do not enter into POWER OSCILLATIONS following LOAD changes. POWER OSCILLATIONS may occur due to the inherent time delay between a request for a LOAD change by the governor and the ability of a GENERATOR to actually deliver the LOAD change. This natural time delay could lead to excessive control action by the governor. A condition of oscillation called "hunting" could result. If a GENERATOR is on isochronous control, TRANSIENT droop is a necessity. The TRANSIENT droop function or "compensation" dampens a governor's initial response following a speed change. The effect is temporary as opposed to a permanent droop function which is permanent. Droop Curve A graphical method of representing the performance of a governor. The horizontal axis is typically GENERATOR output while the vertical axis is SYSTEM FREQUENCY. When a governor with a % DROOP is plotted on such a curve the plot DROOPS from left to right with increasing GENERATOR output. Dynamic Rating The process that allows a SYSTEM ELEMENT RATING to vary with the changing environmental conditions in which the ELEMENT is located. Dynamic Reactive Reserve REACTIVE POWER held in reserve in fast responding SOURCEs. GENERATORS and STATIC VAR COMPENSATORS are possible SOURCEs of dynamic reactive reserve. Dynamic Schedule or Dynamic Interchange Schedule A telemetered reading or value that is updated in real time and used as a SCHEDULE in the AGC/ACE equation and the integrated value of which is treated as a SCHEDULE for INTERCHANGE accounting purposes. Commonly used for scheduling jointly owned GENERATION to or from another BALANCING AUTHORITY AREA. Dynamic Transfer The provision of the real-time monitoring, TELEMETERING, computer SOFTWARE, hardware, communications, engineering, ENERGY accounting (including INADVERTENT INTERCHANGE), and administration required to electronically move all or a portion of the real ENERGY services associated with a GENERATOR or LOAD out of one BALANCING AUTHORITY AREA into another. Earth Surface Potentials (ESP) VOLTAGES induced in the surface of the earth by the ELECTROJETS (CURRENT) that flow above the earth's surface. ESPs cause geomagnetically induced CURRENTS (GICs) to flow in the earth's surface. Economic Dispatch The allocation of DEMAND to individual generating units on line to effect the most economical production of electricity. Economy Energy ELECTRICAL ENERGY produced and supplied from a more economical SOURCE in one SYSTEM and substituted for that being produced or capable of being produced by a less economical SOURCE in another SYSTEM. Electric Field The invisible lines of force that surround an energized piece of EQUIPMENT. An electric field is created when a CONDUCTOR is energized by a VOLTAGE. ENERGY is stored in an electric field. Electric System Losses Total electric ENERGY LOSSES in the electric SYSTEM. The LOSSES consist of TRANSMISSION, transformation, and DISTRIBUTION LOSSES between supply SOURCEs and delivery points. Electric ENERGY is lost primarily due to heating of TRANSMISSION and DISTRIBUTION ELEMENTS. Electric Utility Person, agency, authority, or other legal entity that owns or operates facilities for the GENERATION, TRANSMISSION, DISTRIBUTION, or sale of electric ENERGY primarily for use by the public, and is defined as a utility under the statutes and rules by which it is regulated. An electric utility can be investor owned, cooperatively owned, or government owned (by a federal agency, crown corporation, state, provincial government, municipal government, and public POWER district). Electrical Distance An electrical circuit consists of a VOLTAGE SOURCE, a conducting path to a LOAD, a LOAD, and a return path from the LOAD to the VOLTAGE SOURCE. All four of these ingredients are necessary for CURRENT to flow in the electrical circuit. Electrical Distance The IMPEDANCE of a TRANSMISSION line is a measure of the electrical distance. For example, if a line has a 100 Ω IMPEDANCE then 50 Ω is ½ the line's electrical length. IMPEDANCE RELAYS are often called DISTANCE RELAYS in reference to the use of IMPEDANCE as a measure of electrical distance. Electrical Energy The GENERATION or use of electric POWER by a device over a period of time, expressed in KILOWATTHOURs (kWh), or MEGAWATTHOURs (MWh), or gigaWATTHOURs (GWh). Electrodes The connection to GROUND of an HVDC CONVERTER. The grounding electrode provides a VOLTAGE reference and may be part of the CURRENT return path. Electro-Hydraulic Control (EHC) A form of a CONTROL SYSTEM in which electrical devices are typically used to measure a quantity magnitude and HYDRAULICS used to perform a control action. For example, a GOVERNOR CONTROL SYSTEM may consist of a simple electrical machine that measures the shaft's speed of rotation (FREQUENCY) and a hydraulic SYSTEM that changes the positions of fuel VALVES. Electrojet A CURRENT flow path in the shape of a donut that situates itself above the north and south magnetic POLES of the earth. Electrojets are the result of solar DISTURBANCES. Electromagnet Temporary magnet created by passing electric CURRENT through a coil. The coil is typically wound about a magnetic CORE. Electromotive Induction The creation of a VOLTAGE in a CONDUCTOR due to a relative movement between the CONDUCTOR and a MAGNETIC FIELD. Electromagnetic induction is the basic principle of operation of TRANSFORMERS and GENERATORS. Electromotive Force (EMF) The VOLTAGE produced by a GENERATOR is called an electromotive force or EMF. The symbol "E" is often used to indicate an EMF. Emergency Assistance (EA) ENERGY and/or CAPACITY provided to a utility to assist that utility during a CAPACITY EMERGENCY. Emergency Energy ELECTRICAL ENERGY purchased by a member SYSTEM whenever an event on that SYSTEM causes insufficient Operating Capability to cover its own DEMAND requirement. Emergency Rating The RATING as defined by the EQUIPMENT owner that specifies the level of electrical loading or output, usually expressed in megawatts (MW) or Mvar or other appropriate units, that a SYSTEM, FACILITY, or ELEMENT can support, produce, or withstand for a finite PERIOD. The RATING assumes acceptable loss of EQUIPMENT life or other physical or safety limitations for the EQUIPMENT involved. Emergency Response Rate The rate of LOAD change that a generating unit can achieve under EMERGENCY conditions, such as loss of a unit, expressed in megawatts per minute. Emergency Voltage Limits The operating VOLTAGE range on the INTERCONNECTED SYSTEMS that is acceptable for the time, sufficient for SYSTEM adjustments to be made following a FACILITY OUTAGE or SYSTEM DISTURBANCE. Energy The POWER used over a period of time. Electrical energy is measured in watt-hours, kilowatt-hours (kWh), or Megawatt-hours (MWh) Energy Conversion The conversion of ENERGY from one form to another. For example, a coal fired steam TURBINE/GENERATOR converts the coal's chemical ENERGY to thermal ENERGY by burning the coal. The thermal ENERGY is then converted to MECHANICAL ENERGY by heating water and turning the TURBINE with steam. The MECHANICAL ENERGY of the TURBINE is then converted to ELECTRICAL ENERGY via ELECTROMAGNETIC INDUCTION. Energy Emergency A condition when a LOAD Serving Entity has exhausted all other options and can no longer provide its customers' expected ENERGY requirements. Energy "In-Kind" Payback INADVERTENT INTERCHANGE accumulated during "ON-PEAK" hours shall only be paid back during "ON-PEAK" hours. INADVERTENT INTERCHANGE accumulated during "OFF-PEAK" hours shall only be paid back during "OFF-PEAK" hours. Energy Management System (EMS) An EMS SYSTEM is a computer SYSTEM used by an ENERGY company. The EMS includes the subfunctions of SCADA and AGC. Envelope The imaginary boundary that surrounds the FUNDAMENTAL FREQUENCY OSCILLATIONS waveshape. The FREQUENCY of lower FREQUENCY OSCILLATIONS can be determined by noting the FREQUENCY of the oscillation envelope. Equal Area Criterion A method of determining the ANGLE STABILITY or instability of a simple POWER SYSTEM. The Equal Area Criterion states that the decelerating area of a POWER-ANGLE CURVE must be at least as large as the accelerating area for the POWER SYSTEM to be angle stable. Equipment Rating The maximum and minimum permissible VOLTAGE, CURRENT, FREQUENCY, or active and REACTIVE POWER flows on individual EQUIPMENT under steady state, short-circuit, and TRANSIENT conditions, as specified by the EQUIPMENT owner. ERCOT Acronym for the Electrical Reliability Council of Texas. Excitation System A GENERATOR CONTROL SYSTEM used to control the production of REACTIVE POWER. The excitation system's main components are the VOLTAGE REGULATOR and the EXCITER. Exciter The DC POWER SOURCE for an EXCITATION SYSTEM. Exciting Current The CURRENT drawn by a TRANSFORMER to magnetize its CORE and supply the CORE LOSSES. The exciting current is typically 1-2% of the TRANSFORMER'S full LOAD CURRENT. Extinction Advance Angle (γ) In an HVDC CONVERTER operating as an INVERTER, the angle by which the VALVE firing is advanced from its normal VOLTAGE based COMMUTATION point. The Greek lower case letter gamma (γ ) is the symbol for the extinction advance angle. The extinction advance angle is used to control the magnitude of the converter's (when operated as a INVERTER) output VOLTAGE. Facility A set of electrical EQUIPMENT that operate as a single bulk electrical SYSTEM ELEMENT (for example, a line, a generating unit, a shunt CAPACITOR). Facility Rating The maximum or minimum VOLTAGE, CURRENT, FREQUENCY, or active and REACTIVE POWER flows through a FACILITY that would not violate the applicable RATINGs of any EQUIPMENT comprising the FACILITY. Facility ratings are based upon EQUIPMENT RATINGS and other limits as applicable to the EQUIPMENT. FARAD (F) The unit of CAPACITANCE. Symbol is "F". Fast Valving A method of reducing the accelerating ENERGY in a steam TURBINE/GENERATOR. Fast valving involves the rapid adjustment of TURBINE VALVES when a GENERATOR starts to accelerate. Fast valving may assist in maintaining the ANGLE STABILITY of a POWER SYSTEM. Fault An event occurring on an ELECTRIC SYSTEM such as a SHORT CIRCUIT, a broken wire, or an intermittent connection. Federal Energy Regulatory Commission (FERC) Independent Federal agency that, among other responsibilities, regulates the TRANSMISSION and wholesale sales of electricity in interstate commerce. Feedback Loop A control loop in which CURRENT control actions are influenced by the responses to previous control actions. In a feedback loop (such as in a GENERATOR's EXCITATION SYSTEM) the response of the controlled variable is constantly monitored to determine what new control actions should be taken. Ferranti Rise Effect A phenomena in which a TRANSMISSION line, with one end closed and one end open, is exposed to its highest VOLTAGE magnitude at the open-end of the line. The Ferranti Rise Effect is due to the absorption of a LEADING CHARGING CURRENT when a TRANSMISSION line is energized but open ended. Ferroresonance A RESONANCE condition in which a portion of the INDUCTANCE is provided by an iron-core INDUCTANCE (Ferro is Latin for iron.). Iron-core INDUCTANCES change magnitude when the iron is saturated. Ferroresonance is possible in the TRANSMISSION or DISTRIBUTION SYSTEM but is much more common in the DISTRIBUTION SYSTEM. Field Winding The winding wrapped about the ROTOR of a SYNCHRONOUS MACHINE. DC excitation CURRENT is fed to the field winding to produce the ROTOR's MAGNETIC FIELD. Filter A combination of CAPACITORS, inductors, and resistors used to encourage or block the flow of a specific FREQUENCY or band of frequencies of ENERGY. Firm Capacity CAPACITY that is as firm as the seller's NATIVE LOAD unless modified by contract. Associated ENERGY may or may not be taken at option of purchaser. Supporting reserve is carried by the seller. Firm Demand That portion of the DEMAND that a POWER supplier is obligated to provide except when SYSTEM RELIABILITY is threatened or during EMERGENCY conditions. Firm Transmission Service The highest quality (priority) service offered to customers under a filed rate SCHEDULE that anticipates no planned INTERRUPTION. Flare A sudden eruption of ENERGY on the solar disk lasting minutes to hours, from which radiation and particles are emitted. Flashover An ELECTRICAL discharge through air around or over the surface of insulation, between objects of different potential, caused by placing a VOLTAGE across the air space that results in the ionization of the air space. Flat Frequency Control An alternative name for CONSTANT FREQUENCY CONTROL. Flat Tie-Line Control An alternative name for CONSTANT NET INTERCHANGE CONTROL. Flowgate A designated point on the TRANSMISSION SYSTEM through which the INTERCHANGE DISTRIBUTION CALCULATOR calculates the POWER flow from INTERCHANGE TRANSACTIONS. Flyweights The portion of a CENTRIFUGAL BALLHEAD GOVERNOR that rotates. Forced Outage 1. The removal from service AVAILABILITY of a GENERATING UNIT, TRANSMISSION LINE, or other FACILITY for EMERGENCY reasons. 2. The condition in which the EQUIPMENT is unavailable due to unanticipated failure. Forced Outage Rate An indication of the percentage of time a generating unit, TRANSMISSION line or other ENERGY FACILITY is forced out of service for EMERGENCY reasons. This percentage excludes the time during which the unit experiences a planned service shutdown (for example, SCHEDULED maintenance). This number can be used to describe a FACILITY's RELIABILITY. A weighted average of forced outage rates for all generating facilities in a utility's SYSTEM may be used to describe SYSTEM RELIABILITY. Forced Outage Hours + In-Service Hours x 100 = Forced Outage Rate. Forecast Predicted DEMAND for electric POWER. A forecast may be short term (for example, 15 minutes) for SYSTEM operation purposes, long-term (five to 20 years) for GENERATION PLANNING purposes, or for any range in between. A forecast may include PEAK DEMAND, ENERGY, REACTIVE POWER, or DEMAND profile. A forecast may be made for total SYSTEM DEMAND, TRANSMISSION LOADING, SUBSTATION/feeder LOADING, individual customer DEMAND, or appliance DEMAND. Forecast Uncertainty Probable DEVIATIONS from the expected values of factors considered in a FORECAST. Fourier Analysis A scientific process in which the various FREQUENCY components (HARMONICS) of a waveform are identified. For example, a waveform may have a FUNDAMENTAL FREQUENCY of 60 HZ but also contain 3rd and 5th harmonic components. Fourier Analysis is used to identify the order of the harmonic components. FRCC Acronym for the Florida Reliability Coordinating Council. FRCC is one of the eight NERC REGIONAL RELIABILITY ORGANIZATIONS. Frequency (F) The rate at which a repeating waveform repeats itself. Frequency is measured in CYCLES per second or in HERTZ (HZ). The symbol if "F". Frequency Bias A value, usually expressed in megawatts per 0.1 HERTZ (MW/0.1 HZ), associated with a BALANCING AUTHORITY AREA that approximates the BALANCING AUTHORITY AREA's response to INTERCONNECTION FREQUENCY ERROR. Frequency Bias Settings (B) A value, usually expressed in MW/0.1 HZ, set into a BALANCING AUTHORITY ACE algorithm that allows the BALANCING AUTHORITY to contribute its FREQUENCY RESPONSE to the INTERCONNECTION. Frequency Deviation A change in INTERCONNECTION FREQUENCY. Frequency Error (DeltaF) The difference between the actual and SCHEDULED FREQUENCY. (FA - FS) Frequency Regulation The ability of a BALANCING AUTHORITY to help the INTERCONNECTION maintain SCHEDULED FREQUENCY. This assistance can include both TURBINE governor response and AUTOMATIC GENERATION CONTROL. Frequency Response (Equipment) The ability of a SYSTEM or ELEMENTS of the SYSTEM to react or respond to a change in SYSTEM FREQUENCY. Frequency Response (System) The sum of the change in DEMAND, plus the change in GENERATION, divided by the change in FREQUENCY, expressed in megawatts per 0.1 HERTZ (MW/0.1 HZ). Frequency Response Characteristic (FRC or Beta) The MW response of the POWER SYSTEM (or a section of the POWER SYSTEM) to a FREQUENCY DEVIATION. The FRC is typically stated in terms of MW per 0.1 HZ. For example, a BALANCING AUTHORITY may have an FRC of 200 MW/0.1 HZ. This value of FRC indicates that for a FREQUENCY DEVIATION of 0.1 HZ this BALANCING AUTHORITY would respond with 200 MW. The FRC of a SYSTEM varies with changing SYSTEM conditions. Frequency Swings Constant changes in FREQUENCY from its NOMINAL or steady-state value. Fundamental Frequency The base FREQUENCY for a SYSTEM. For example, the fundamental frequency of North American POWER SYSTEMS is 60 HZ while a large portion of the world uses 50 HZ as the fundamental frequency. Gallery A passageway within a water dam created to allow inspection of the dam's structure. Gate/Grid Control The means of controlling a MERCURY ARC VALVE (MAV) or a THYRISTER VALVE. A pulse of CURRENT or VOLTAGE is applied to the GRID of a MAV or the gate of a THYRISTER. The pulse will turn the VALVE on if it is forward biased. Gate/grid control is typically only used to turn a VALVE on. Generation The process of producing ELECTRICAL ENERGY from other forms of ENERGY; also, the amount of electric POWER produced, usually expressed in kilowatt hours (kW) or megawatt hours (MW). Generation Control The process by which the GENERATION supply is adjusted to both maintain SYSTEM FREQUENCY and keep a close match between the actual TIE-LINE flows and the SCHEDULED TIE-LINE flows. Generator Generally, an electromechanical device used to convert mechanical POWER to electrical POWER. Generator-to-load Distribution Factor (GLDF) The algebraic sum of a GENERATOR SHIFT FACTOR and a LOAD SHIFT FACTOR to determine the total impact of an INTERCHANGE TRANSACTION on an identified TRANSMISSION FACILITY or FLOWGATE. Generator Operator The entity that operates GENERATING unit(s) and performs the functions of supplying ENERGY and INTERCONNECTED OPERATIONS SERVICES. Generator Owner Entity that owns and maintains GENERATING units. Generator Shift Factor (GSF) A factor to be applied to a GENERATOR's expected change in output to determine the amount of flow contribution that change in output will impose on an identified TRANSMISSION FACILITY or FLOWGATE. Geomagnetic Disturbance (GMD) Solar (sun) induced DISTURBANCES to the earth's MAGNETIC FIELD. GMDs may result in large low FREQUENCY CURRENTS flowing in the earth's surface. These CURRENTS (called GICs) may enter the POWER SYSTEM and damage TRANSFORMERS. GMDs are also called SOLAR MAGNETIC DISTURBANCES, or SMDs. Geomagnetic Induced Currents Low FREQUENCY CURRENTS induced in the surface of the earth by EARTH SURFACE POTENTIALS (ESPs). ESPs are created by the ELECTROJETS that form above the earth's magnetic POLES following solar DISTURBANCES. Geomagnetic Storm A worldwide disturbance of the earth's magnetic field, distinct from regular diurnal variations. Good Utility Practice Any of the practices, methods and acts engaged in or approved by a significant portion of the ELECTRIC UTILITY industry during the relevant time period, or any of the practices, methods and acts which, in the exercise of reasonable judgment in light of the facts known at the time the decision was made, could have been expected to accomplish the desired result at a reasonable cost consistent with good business practices, RELIABILITY, safety and expedition. Good Utility Practice is not intended to be limited to the optimum practice, method, or act to the exclusion of all others, but rather to be acceptable practices, methods, or acts generally accepted in the REGION. Governor Characteristic Curve A graphical method of representing the performance of a governor. The horizontal axis is typically GENERATOR output while the vertical axis is SYSTEM FREQUENCY. When a governor with a % DROOP is plotted on such a curve the plot DROOPS from left to right with increasing GENERATOR output. Governor Control System A CONTROL SYSTEM for a GENERATOR that is used to control the speed of the GENERATOR's rotating shaft. In a steam TURBINE/GENERATOR the governor control system controls the amount of steam striking the TURBINE BLADEs. In a hydroelectric TURBINE/GENERATOR the governor control system controls the amount of water striking the TURBINE BLADEs. Governor control systems are a key ingredient in maintaining a SCHEDULED INTERCONNECTION FREQUENCY. Greek Alphabet (For reference) α alpha δ delta μ mu Φ Phi β beta θ THETA π pie ω omega γ gamma λ LAMBDA ρ rho Ω Omega Grid An electrical TRANSMISSION and/or DISTRIBUTION NETWORK. Gross Generation The output POWER (in MW) at the STATOR terminals of a GENERATOR. Ground A conducting connection between an ELECTRICAL CIRCUIT or device and the earth. A ground may be intentional, as in the case of a safety ground, or accidental, which may result in high overcurrents. Half-Cycle Saturation A magnetic SATURATION of a TRANSFORMER's CORE due to the presence of a DC CURRENT in the TRANSFORMER windings. The operating point of the TRANSFORMER on its SATURATION curve is shifted such that for a portion of ½ of each CYCLE the TRANSFORMER saturates. Harmonics Integer multiples of the FUNDAMENTAL FREQUENCY. If the FUNDAMENTAL FREQUENCY is 60 HZ then the 2nd Harmonic has a FREQUENCY of 120 HZ, 3rd Harmonic 180 HZ, etc. Head Vertical change in elevation, expressed in either feet or meters, between the head water level and the tailwater level of a hydro-electric FACILITY. Heat Rate An expression for the efficiency of a thermal POWER plant. The heat rate is the amount of heat (measured in British Thermal Units or BTU) that is required to produce a kWh or electrical output. The lower the heat rate, the more efficient the POWER plant. Henry (H) The unit of INDUCTANCE. The symbol for a Henry is "H". Hertz (HZ) Hertz is the unit of FREQUENCY equal to one CYCLE per second. High Side Winding (HS) The high VOLTAGE winding of a TRANSFORMER. Host Balancing Authority 1. A BALANCING AUTHORITY that confirms and implements INTERCHANGE TRANSACTIONs for a PURCHASING-SELLING ENTITY that operates GENERATION or serves customers directly within the BALANCING AUTHORITY's metered boundaries. 2. The BALANCING AUTHORITY within whose metered boundaries a jointly owned unit is physically located. Hourly Value Data measured on a clock-hour basis. HVDC Acronym for High VOLTAGE DIRECT CURRENT. The term HVDC is commonly used when the DC VOLTAGE is above 100 kV. HVDC Modulation An arrangement of EQUIPMENT designed and operated to convert between AC and DC POWER. A converter can be operated as an INVERTER (DC to AC) or a RECTIFIER (AC to DC). Hydraulics Modulation A feature added to the controls of an HVDC SYSTEM. HVDC modulation modulates (adjusts) the POWER flow into an HVDC CONVERTER in order to dampen POWER OSCILLATIONS in the AC supply SYSTEM. HVDC modulation assists with DAMPING AC SYSTEM POWER OSCILLATIONS. Hydraulics The use of fluid forces to perform work. For example, hydraulics are often used in GOVERNOR CONTROL SYSTEMS to develop the large forces required to move steam or water VALVES. Hypotenuse The side of a RIGHT TRIANGLE which is opposite the 90° angle. IEEE Acronym for the Institute of Electrical and Electronic Engineers. The IEEE is an international standards organization that publishes guidelines for, among other areas, POWER SYSTEMS. Igneous Rock Rock that was created by volcanic activity. Ignition Delay Angle (Alpha) In an HVDC CONVERTER operating as a RECTIFIER, the angle by which the VALVE firing is delayed from its normal VOLTAGE based COMMUTATION point. The Greek lower case letter alpha (α) is the symbol for the ignition delay angle. The ignition delay angle is used to control the magnitude of the converter's (when operated as a RECTIFIER) output VOLTAGE. Imbalance A condition where the GENERATION and INTERCHANGE SCHEDULEs do not match DEMAND. Impedance (Z) The total opposition to the CURRENT flow in an ELECTRICAL CIRCUIT. The symbol for the impedance is "Z". The impedance includes the RESISTANCE (R), CAPACITANCE (C), and the INDUCTANCE (L). Impedance Relay (Distance Relay) A PROTECTIVE RELAY that activates if the ratio of the measured VOLTAGE divided by the CURRENT (the IMPEDANCE) falls below a pickup point. An impedance relay is also commonly called a DISTANCE RELAY. Impulse Turbine A water TURBINE in which high velocity water is directed through nozzles at the TURBINE buckets. A Pelton Wheel is an example of an impulse turbine. Inadvertent Energy When INADVERTENT INTERCHANGE exists for a period of time, inadvertent energy will be accumulated. Inadvertent Energy Balancing A BALANCING AUTHORITY'S accounting of its INADVERTENT INTERCHANGE, which is the accumulated difference between actual and SCHEDULED INTERCHANGE. Inadvertent Energy Balancing When the INADVERTENT ENERGY that a BALANCING AUTHORITY accumulates exceeds a specified value, the BALANCING AUTHORITY should arrange for an inadvertent payback. Inadvertent Interchange The difference between the BALANCING AUTHORITY's NET ACTUAL INTERCHANGE and NET SCHEDULED INTERCHANGE. (IA - IS) Incremental Cost The cost associated with producing an additional MWh of ENERGY from a generating unit. Incremental cost is typically stated in $/MWh or MILLs/kWh. Incremental Energy Cost The additional cost that would be incurred by producing or purchasing the next available unit of ELECTRICAL ENERGY above the CURRENT base cost. Incremental Heat Rate The amount of additional heat that must be added to a thermal generating unit at a given loading to produce an additional unit of output. It is usually expressed in British thermal units per kilowatt hour (Btu/kWh) of output. Incremental Losses The increase in LOSSES due to an increase in POWER flow. For example, assume the POWER flow on a TRANSMISSION line is initially 100 MW. If the POWER flow is increased to 101 MW there will be incremental ENERGY LOSSES associated with the 1 MW increase in POWER flow. The percentage of the Incremental Loss increases with increasing levels of POWER flow. Independent Power Producer (IPP) Any entity that owns or operates an electricity generating FACILITY that is not included in an ELECTRIC UTILITY's RATE BASE. This term includes, but is not limited to, COGENERATORS and SMALL POWER PRODUCERS and all other nonutility electricity producers, such as EXEMPT WHOLESALE GENERATORS, who sell electricity. Independent System Operator (ISO) An organization responsible for the reliable operation of the POWER GRID under its purview and for providing open TRANSMISSION ACCESS to all MARKET PARTICIPANTS on a nondiscriminatory basis. An ISO is usually not-for-profit and can advise utilities within its territory on TRANSMISSION expansion and maintenance but does not have the responsibility to carry out the functions. Indirect Demand-Side Management Programs such as conservation, improvements in efficiency of ELECTRICAL ENERGY use, rate incentives, rebates, and other similar activities to influence the customer's DEMAND. Inductance (L) The property of an ELECTRICAL CIRCUIT that opposes a change in CURRENT flow. The symbol for inductance is the letter "L" and the unit is the HENRY (symbol "H"). Induction Machine An AC machine that can be operated as a GENERATOR or as a motor. When operated as a GENERATOR the induction machine's ROTOR is driven at a speed greater than SYNCHRONOUS SPEED. When operated as a motor the induction machine's ROTOR is driven at a speed less than SYNCHRONOUS SPEED. Induction GENERATORs are rarely used by large scale POWER GENERATORs. Induction motors are the most common type of AC motor. Induction machines absorb REACTIVE POWER (always a LAGGING LOAD) and cannot be used to produce REACTIVE POWER as a SYNCHRONOUS MACHINE can. Inductive Load A LOAD that absorbs LAGGING REACTIVE POWER from the SYSTEM. Inductive Reactance (XL) The opposition that INDUCTANCE provides to AC CURRENT. The inductive reactance (XL) in a 60 HZ CIRCUIT is: XL = 2π fL Inertia The property of an object that resists changes to the motion of the object. For example, the inertia of a rotating object resists changes to the object's speed of rotation. The inertia of a rotating object is a function of its mass, diameter, and speed of rotation. Inertial Energy ENERGY stored within a rotating mass. For example, a spinning GENERATOR contains inertial energy. The terms inertial energy, STORED ENERGY, and ROTATIONAL ENERGY are often used interchangeably to refer to the ENERGY stored in the rotating ELEMENTS (motors and GENERATORS) of the POWER SYSTEM. In-Rush Current The sudden rush of CURRENT when a TRANSFORMER or a motor is first energized. The peak magnitudes of the in-rush current last only a few CYCLES but can reach levels more than 10 times the device's full LOAD CURRENT. Installed Reserve The difference between a utility's expected annual peak MW GENERATION capability and their annual peak MW LOAD. Installed reserves are a rough approximation of a utility's spare or reserve GENERATION. Instaneous Demand The rate of ENERGY delivered at a given instant. Instrument Transformers (IT) A low POWER TRANSFORMER classification. Instrument TRANSFORMERs (IT) include CURRENT TRANSFORMERS (CTs), POTENTIAL TRANSFORMERS (PTs) and capacitive devices. Insulator A material with a high IMPEDANCE to CURRENT flow. An insulator is the opposite of a CONDUCTOR. Integrated Demand The average of the INSTANTANEOUS DEMANDS (MW) over the DEMAND INTERVAL (usually hours). Inter-Area Mode A POWER oscillation MODE (FREQUENCY) in which a large section of an interconnected POWER SYSTEM oscillates with respect to another large section of the same INTERCONNECTION. The Inter-area mode ranges from 0.05 to 0.5 HZ. Interchange ENERGY transfers that cross BALANCING AUTHORITY boundaries. Interchange Authority The responsible entity that authorizes implementation of valid and balanced INTERCHANGE SCHEDULES between BALANCING AUTHORITY AREAS, and ensures communication of INTERCHANGE information for reliability assessment purposes. Interchange Block Accounting ENERGY accounting that assumes a beginning and ending ramp time of zero minutes. For accounting purposes, this moves the ENERGY associated with the starting and ending ramps into the adjacent starting and ending clock time of the INTERCHANGE. Interchange Distribution Calculator (IDC) The mechanism used by RELIABILITY COORDINATORs in the Eastern INTERCONNECTION to calculate the DISTRIBUTION of INTERCHANGE TRANSACTIONs over specific TRANSMISSION INTERFACES, which are known as "FLOWGATES." The IDC includes a database of all INTERCHANGE TRANSACTIONS and a matrix of the DISTRIBUTION FACTORS for the Eastern INTERCONNECTION. Interchange Schedule An agreed-upon INTERCHANGE TRANSACTION size (megawatts), start and end time, beginning and ending ramp times and rate, and type required for delivery and receipt of POWER and energy between the SOURCE and SINK BALANCING AUTHORITIES involved in the TRANSACTION. Interchange Transaction An AGREEMENT to transfer ENERGY from a seller to a buyer that crosses one or more BALANCING AUTHORITY AREA boundaries. Interchange Transaction Curtailment The complete or partial INTERRUPTION of an INTERCHANGE TRANSACTION that has started or "holding" of a new INTERCHANGE TRANSACTION that has not yet started by a TRANSMISSION PROVIDER, Reliability Coordinator, or BALANCING AUTHORITY to maintain OPERATING SECURITY. Interchange Transaction Tag (TAG) The details of an INTERCHANGE TRANSACTION required for its physical implementation Interconnect When two POWER SYSTEMs tie together with AC TRANSMISSION, it is referred to as an interconnect. Interconnected Operations Service (IOS) A service (exclusive of basic ENERGY and TRANSMISSION SERVICEs) that is required to support the reliable operation of Interconnected BULK ELECTRIC SYSTEMS. Interconnected System A SYSTEM consisting of two or more individual electric SYSTEMS that normally operate in synchronism and have connecting tie lines. Interconnection When capitalized, any one of the three BULK ELECTRIC SYSTEM networks in North America: Eastern, Western, and ERCOT. When not capitalized, the facilities that connect two SYSTEMS or BALANCING AUTHORITIES. Interconnection Reliability Operating Limit (IROL) A SYSTEM OPERATING LIMIT that, if violated, could lead to instability, uncontrolled separation, or CASCADING OUTAGES that adversely impact the reliability of the BULK ELECTRIC SYSTEM. Interconnection Reliability Operating Limit Tv The maximum time that an INTERCONNECTION RELIABILITY OPERATING LIMIT can be violated before the risk to the INTERCONNECTION or other RELIABILITY COORDINATOR AREA(s) becomes greater than acceptable. Each Interconnection Reliability Operating Limit's TV shall be less than or equal to 30 minutes. Interconnection Time Monitor An ENTITY that monitors TIME ERROR and initiates and terminates TIME ERROR CORRECTIONs.v Interface The specific set of TRANSMISSION ELEMENTS between two areas or between two areas comprising one or more electrical SYSTEMS. Intermediate Balancing Authority A BALANCING AUTHORITY AREA that has connecting facilities in the SCHEDULING PATH between the Sending BALANCING AUTHORITY AREA and RECEIVING BALANCING AUTHORITY AREA and operating AGREEMENTS that establish the conditions for the use of such facilities. Interruptible Load/Interruptible Demand DEMAND that the end-use customer makes available to its LOAD-SERVING ENTITY via contract or AGREEMENT for CURTAILMENT. Interruptible Responsive Reserve INTERRUPTIBLE LOAD controlled by high-set under frequency relaying. Interruption A reduction in NON-FIRM TRANSMISSION SERVICE due to economic reasons. Intra-Area Mode A POWER oscillation MODE (FREQUENCY) in which a pocket of GENERATION in a POWER SYSTEM oscillates with respect to another pocket of GENERATION in the same POWER SYSTEM. The intra-area mode ranges from 0.4 to 1 HZ. Intra-plant Mode A POWER oscillation MODE (FREQUENCY) in which several GENERATORs in a multi-unit POWER STATION oscillate with respect to one another. The intra-plant mode ranges from 1.5 to 3 HZ. Inverter An HVDC CONVERTER operated to convert DC POWER to AC POWER. Ionization Ionization occurs when an atom (or group of atoms) receives enough ENERGY to split into one or more free electrons and a positive ion. Ionization is a special case of CHARGING. The visible CORONA effect that often surrounds energized EQUIPMENT is due to ionization of the air by the strong ELECTRIC FIELD surrounding the energized CONDUCTOR. Island An electrically ISOLATED portion of an INTERCONNECTION. An islanded SYSTEM maintains its own FREQUENCY. Islands are frequently formed after major DISTURBANCES or during RESTORATION following a major DISTURBANCE. Isochronous Governor Control A governor operated with a 0% DROOP. When in isochronous control, a governor will try to maintain 60 HZ. Isochronous control may be used during a SYSTEM RESTORATION. Isolated To be electrically separated from the remainder of the INTERCONNECTION. An isolated system does not have TRANSMISSION ties to the rest of the INTERCONNECTION. An isolated system is an electrical ISLAND. Joint Control AUTOMATIC GENERATION CONTROL of JOINTLY OWNED UNITS by two or more BALANCING AUTHORITIES. Jointly Owned Unit (JOU) This term refers to a unit in which two or more ENTITIES share ownership. Journal Bearing The bearing that supports the actual weight of a GENERATOR. K-Index A 3-hourly quasi-logarithmic local index of GEOMAGNETIC activity relative to an assumed quiet-day curve for the recording site. Range is from 0 to 9. The K-index measures the deviation of the most disturbed horizontal component. K-Index Warning K-index Warnings are issued and/or extended for any period with expected values of K equal to or greater than 4. Higher K-index warnings supersede lower ones. Kilovar (kVAr) Unit of REACTIVE POWER equal to 1,000 Var. Kilovolt (kV) Unit of electrical potential equal to 1,000 Volts. Kilovolt-amperes (kVa) Unit of COMPLEX POWER equal to 1,000 volt AMPEREs. Here, COMPLEX POWER is in contrast to ACTIVE POWER. On AC SYSTEMS the VOLTAGE and CURRENT will not be in PHASE if REACTIVE POWER is being transmitted. Kilowatthour (kWh) Unit of ENERGY equaling one thousand WATTHOURs, or one kilowatt used over one hour. This is the normal quantity used for METERING and billing electricity customers. The retail price for a kWh varies from approximately 4 cents to 20 cents. At a 100% conversion efficiency, one kWh is equivalent to about 4 fluid ounces of gasoline, 3/16 pound of liquid petroleum, 3 cubic feet of natural gas, or ¼ pound of coal. Kirchhoff's Voltage Law A basic electrical law that states that the sum of all the VOLTAGEs around any closed ELECTRICAL CIRCUIT equal zero volts. L Sub-Ten (L10) The BANDWIDTH that ACE is bounded in order to comply with CPS2. An ACE value (±) whose width is proportional to a BALANCING AUTHORITY's size. Lagging Term used when comparing VOLTAGE and CURRENT waves. The wave that is heading positive and crosses zero last is the lagging wave. In a lagging LOAD the CURRENT wave lags the VOLTAGE wave. Lambda The INCREMENTAL COST of GENERATION. Lambda is commonly expressed in $/MWh or Mils/kWh. The symbol for lambda is "λ" (the Greek letter lambda). Leading Term used when comparing VOLTAGE and CURRENT waves. The wave that is heading positive and crosses zero first is the leading wave. In a leading LOAD the CURRENT wave leads the VOLTAGE wave. Lighting Arrester A piece of EQUIPMENT that is designed to protect the POWER SYSTEM from high VOLTAGES. Lightning arresters activate when TRANSIENT over VOLTAGEs (TOVs) occur and harmlessly shunt the VOLTAGE SURGE to GROUND. Limit Compliance Violation An INTERCONNECTED RELIABILITY LIMIT VIOLATION that has occurred for more than 30 consecutive minutes. Limit Violation The operating state during which one or more facilities are outside SYSTEM OPERATING LIMITS or INTERCONNECTED RELIABILITY LIMITS. A violation occurs at the instant the established limit is exceeded. This could be a result of a change in one or more operating parameters. Line Trip Refers to the automatic opening of a TRANSMISSION line by its CIRCUIT BREAKERS. These openings or "TRIPS" are controlled by PROTECTIVE RELAYS and are designed to protect the POWER SYSTEM during faulted conditions. Load An end-use device or customer that receives POWER from the ELECTRIC SYSTEM. Load Duration Curve A nonchronological, graphical summary of DEMAND levels with corresponding time durations using a curve, which plots DEMAND magnitude (POWER) on one axis and percent of time that the magnitude occurs on the other axis. Load Factor A measure of the degree of uniformity of DEMAND over a period of time, usually one year, equivalent to the ratio of AVERAGE DEMAND to PEAK DEMAND expressed as a percentage. Load factor is calculated by dividing the total ENERGY provided by a SYSTEM during the PERIOD by the product of the PEAK DEMAND during the PERIOD and the number of hours in the PERIOD. Load Following An ENERGY based ancillary service that is provided via a linear change in a SCHEDULE through a PERIOD (typically one hour). Load Overshoot A short term increase in LOAD magnitude due to an increase in the customer's VOLTAGE. Load overshoot results from downstream tap changers boosting the customer's VOLTAGE prior to the upstream tap changer responding. Load Reference Set-Point In GOVERNOR CONTROL SYSTEMS this setting determines the position of the controlled VALVE when the FREQUENCY is at the SCHEDULED value. From a SYSTEM operations perspective, the load reference set-point is the MW a GENERATOR will produce when the FREQUENCY is 60 HZ. Load Rejection The rejection of LOAD by a GENERATOR. If a GENERATOR suddenly loses its TRANSMISSION path, it has undergone a load rejection. The GENERATOR will speed up until its mechanical POWER input can be removed or the unit tripped. Load/Frequency Relationship The relationship between FREQUENCY DEVIATIONS and the LOAD magnitude. In general, the LOAD magnitude varies with the FREQUENCY. If the FREQUENCY rises the LOAD magnitude rises and vice versa. Load Serving Entity (LSE) Secures ENERGY and TRANSMISSION SERVICE (and related INTERCONNECTED OPERATIONS SERVICES) to serve the electrical DEMAND and ENERGY requirements of its end-use customers. Load Shedding The process of deliberately removing (either manually or automatically) pre-selected customer DEMAND from a POWER SYSTEM in response to an abnormal condition, to maintain the integrity of the SYSTEM and minimize overall customer OUTAGES. Local Mode A POWER oscillation MODE (FREQUENCY) in which a GENERATOR oscillates with respect to the remainder of the POWER SYSTEM. The local mode ranges from 0.8 to 2 HZ. Lockout A state of a TRANSMISSION line following CIRCUIT BREAKERS operations where the condition detected by the PROTECTIVE RELAYING was not eliminated by temporarily opening and reclosing the line. In this state, the CIRCUIT BREAKERS cannot generally be reclosed without resetting a lockout device. Loop Flow See UNSCHEDULED POWER FLOW. Loss of Load Diversity An increase in the TOTAL LOAD that occurs due to a majority of the customer's LOAD drawing POWER from the SYSTEM at the same time. During normal SYSTEM operations only a percentage of the customer' TOTAL LOAD is drawing POWER at any one time. When LOAD diversity is lost a larger percentage of the customer LOAD draws POWER simultaneously. Loss of Synchronism The loss of the magnetic bond between ELEMENTS of a POWER SYSTEM. Loss of synchronism and OUT-OF-STEP refer to the same concept. Losses The ENERGY losses in the POWER SYSTEM. The total SYSTEM losses consist of the TRANSMISSION, transformation, and DISTRIBUTION SYSTEM losses. Low Side Winding (LS) The low VOLTAGE winding of the TRANSFORMER. Magnetic Field The invisible lines of force between the north and south POLES of a magnet. A magnetic field is created when CURRENT flows through a CONDUCTOR. ENERGY is stored in a magnetic field. Magnetism A property of matter associated with moving CHARGEs. A material may be a permanent magnet or it may acquire magnetic characteristics due to CURRENT flow through the material. Market Clearing Price The price at a location at which supply equals DEMAND—all DEMAND at or above this price has been satisfied, and all supply at or below this price has been purchased. Market Operator An ENTITY that administers a market that integrates CAPACITY, ENERGY, BALANCING RESOURCES, and TRANSMISSION RESOURCES to achieve an economic, reliability-constrained dispatch of RESOURCES. Market Participant An entity participating in the ENERGY marketplace by buying/selling TRANSMISSION rights, ENERGY, or ANCILLARY SERVICES into, out of, or through an ISO-controlled GRID. Marketer An entity that has the authority to take title to electrical POWER generated by itself or another entity and remarket that POWER at market-based rates. Mechanical Energy ENERGY of a mechanical nature. For example, a rotating mass possesses mechanical energy. Mechanical Input Power POWER input of a mechanical nature. For example, a steam or water TURBINE input mechanical POWER to the ROTOR of a GENERATOR. Megawatthour (MWh) One million WATTHOURS. Mercury ARC Valve (MAV) A high POWER switch that utilizes older "tube" based technologies. MAVs were commonly used in HVDC CONVERTERs and other POWER CONVERTER applications. MAVs have largely been replaced by THYRISTERS Metered Value A measured electrical quantity that may be collected by TELEMETERING, SCADA, or other means. Mill A unit of currency equal to 1/10 of a cent. Missoperation Any failure of a PROTECTION SYSTEM ELEMENT to operate within the specified time when a FAULT or abnormal condition occurs within a ZONE OF PROTECTION. • Any operation for a FAULT not within a ZONE OF PROTECTION (other than operation as backup protection for a FAULT in an adjacent zone that is not cleared within a specified time for the protection for that zone). • Any unintentional PROTECTION SYSTEM operation when no FAULT or other abnormal condition has occurred unrelated to on-site maintenance and testing activity. Mode A specific oscillation FREQUENCY. For example, a steam/TURBINE GENERATOR's shaft has specific frequencies at which it is susceptible to SSR. These frequencies are called modes. Monopolar HVDC An HVDC SYSTEM that uses one CONDUCTOR energized with either a positive or negative VOLTAGE and a CURRENT return path. Motor Load A simplified grouping of all spinning type LOAD. Motor load includes air conditioner COMPRESSORS, motor drives, etc. TOTAL LOAD is composed of motor load plus NON-MOTOR LOAD. MRO Acronym for the Midwest Reliability Organization. MRO is one of the eight NERC REGIONAL RELIABILITY ORGANIZATIONs. Must-Run Generation GENERATION designated to operate at a specific level and not available for dispatch. NAESB The acronym for the North American Energy Standards Board. NAESB is responsible for business practice standards in the wholesale electric ENERGY industry. Native Load (NL) The end-use customers that the LOAD-SERVING ENTITY is obligated to serve. Natural Frequency Every mechanical device has a natural frequency of oscillation. For example, when a force is applied to a bridge the bridge will oscillate at its natural frequency. ELECTRICAL CIRCUITS also have a natural frequency of oscillation. An ELECTRICAL CIRCUIT's natural frequency is determined by its RESISTANCE, INDUCTANCE, and CAPACITANCE. NERC An acronym for the North American Electric Reliability Corporation. The purpose of NERC is to enhance the RELIABILITY of the interconnected POWER SYSTEMS of North America. NERC publishes RELIABILITY Standards that provide requirements as to how to reliably operate the POWER SYSTEM. Net Actual Interchange (NIa) The algebraic sum of all metered INTERCHANGE over all INTERCONNECTIONs between two physically Adjacent BALANCING AUTHORITY AREAS. Net Generation The net POWER available from a GENERATOR to be fed to the POWER SYSTEM. Net generation is equal to GROSS GENERATION minus the GENERATOR's internal POWER usage (STATION SERVICE). Net Interchange Schedule (NIs) The algebraic sum of all INTERCHANGE SCHEDULES with each adjacent BALANCING AUTHORITY. Net Scheduled Interchange The algebraic sum of all INTERCHANGE SCHEDULES across a given path or between BALANCING AUTHORITIES for a given period or instant in time. Network Customer An entity receiving TRANSMISSION SERVICE pursuant to the terms of the TRANSMISSION SERVICE PROVIDER's NETWORK INTEGRATION TRANSMISSION SERVICE. Network Integration (NI) Transmission Service Service that allows an electric TRANSMISSION customer to integrate, plan, economically dispatch and regulate its network reserves in a manner comparable to that in which the TRANSMISSION OWNER serves NATIVE LOAD customers. Network Resource Any designated generating resource owned, or purchased or leased by a NETWORK CUSTOMER under the NETWORK INTEGRATION TRANSMISSION SERVICE Tariff. Network RESOURCES do not include any resource, or any portion thereof, that is committed for sale to third PARTIES or otherwise cannot be called upon to meet the NETWORK CUSTOMER's Network LOAD on a non-interruptible basis. No Load Tap Changer (NLTC) A tap changer that is designed to change the TURNS RATIO only when the TRANSFORMER has no CURRENT flow across its windings. The term offload tap changer (OLTC) is also used to refer to this type tap changer. NOAA The acronym for the National Oceanic and Atmospheric Administration of the U.S. Government. Nominal The design or rated value. Not necessarily the value that is intended or that occurs. For example, the nominal VOLTAGE for a piece of EQUIPMENT would be the design or rated VOLTAGE but in operation the EQUIPMENT may be operated at a different value of VOLTAGE. Nominal Load The rated or nameplate LOAD. For example, 100 MW of customer LOAD may be fed from a utility BUS. This LOAD will draw 100 MW if the VOLTAGE and FREQUENCY at the BUS are at NOMINAL values. If VOLTAGE or FREQUENCY should vary then the ACTUAL LOAD will be different than the nominal load. Non-Firm Energy ELECTRICAL ENERGY that may be interrupted by either the provider or the receiver of the ENERGY by giving advance notice to the other party to the TRANSACTION. This advance notice PERIOD is equal to or greater than the minimum PERIOD agreed to in the contract. Non-firm energy may also be interrupted to maintain SYSTEM RELIABILITY of third-party TRANSMISSION SERVICE PROVIDERS. Non-firm energy must be backed up by reserves Non-Firm Point-To-Point Transmission Service POINT-TO-POINT TRANSMISSION SERVICE under the Tariff that is reserved and SCHEDULED on an as-available basis and is subject to CURTAILMENT or INTERRUPTION. Non-Frim Transmission Service TRANSMISSION service that is reserved on an as-available basis and is subject to CURTAILMENT or INTERRUPTION. Non-Motor Load A simplified grouping of all non-spinning type LOAD. Non-motor load includes resistive heaters, lighting, etc. TOTAL LOAD is composed of MOTOR LOAD plus non-motor type LOAD. Non-Spinning Reserve 1. That generating reserve not connected to the system but capable of serving DEMAND within a specified time. 2. INTERRUPTIBLE LOAD that can be removed from the system in a specified time. Normal Clearing A PROTECTION SYSTEM operates as designed and the FAULT is cleared in the time normally expected with proper functioning of the installed PROTECTION SYSTEMS. Normal Excitation A condition in which the GENERATOR's EXCITATION SYSTEM is supplying exactly the excitation CURRENT needed to maintain the MAGNETIC FIELD of the GENERATOR. A normally excited GENERATOR is neither supplying or absorbing REACTIVE POWER from the SYSTEM. Normal Rating The RATING as defined by the EQUIPMENT owner that specifies the level of electrical loading, usually expressed in megawatts (MW) or other appropriate units that a SYSTEM, FACILITY, or ELEMENT can support or withstand through the daily DEMAND CYCLES without loss of EQUIPMENT life. Normal Voltage Limits The operating VOLTAGE range on the INTERCONNECTED SYSTEMS that is acceptable on a sustained basis. NPCC Acronym for the Northeast Power Coordinating Council. NPCC is one of the eight NERC REGIONAL RELIABILITY ORGANIZATIONS. Off-Peak Those hours or other PERIODs defined by NAESB business practices, contract, AGREEMENTs, or guides as PERIODs of lower electrical DEMAND. On-Peak Those hours or other PERIODs defined by NAESB business practices, contract, AGREEMENTs, or guides as PERIODs of higher electrical DEMAND. OHM's Law A basic electrical law that relates the VOLTAGE (V), CURRENT (I), and IMPEDANCE (Z). Ohm's law is commonly stated as: V = I × Z Open Access Same Time Information Service (OASIS) An electronic posting system that the TRANSMISSION SERVICE PROVIDER maintains for TRANSMISSION ACCESS data and that allows all TRANSMISSION CUSTOMERs to view the data simultaneously. Open Access Transmission Tariff (OATI) Electronic TRANSMISSION tariff accepted by the U.S. FEDERAL ENERGY REGULATORY COMMISSION requiring the TRANSMISSION SERVICE PROVIDER to furnish to all shippers with non-discriminating service comparable to that provided by TRANSMISSION OWNERS to themselves. Operating Authority An entity that: 1. Has ultimate accountability for a defined portion of the BULK ELECTRIC SYSTEM to meet one or more of three RELIABILITY objectives - GENERATION/DEMAND balance, TRANSMISSION security, and/or EMERGENCY preparedness; and 2. Is accountable to NERC and its REGIONAL RELIABILITY ORGANIZATIONS for complying with NERC and Regional Policies; and 3. Has the authority to control or direct the operation of generating RESOURCES, TRANSMISSION facilities, or LOADS, to meet these Policies. Operating Procedure A document that identifies specific steps or tasks that should be taken by one or more specific operating positions to achieve specific operating goal(s). The steps in an Operating Procedure should be followed in the order in which they are presented, and should be performed by the position(s) identified. A document that lists the specific steps for a SYSTEM OPERATOR to take in removing a specific TRANSMISSION LINE from service is an example of an Operating Procedure. Operating Reserve That capability above firm SYSTEM DEMAND required to provide for REGULATION, LOAD forecasting error, EQUIPMENT forced and SCHEDULED OUTAGES and local area protection. It consists of SPINNING and NON-SPINNING RESERVE. Operating Reserve - Spinning The portion of OPERATING RESERVE consisting of: • GENERATION SYNCHRONIZEd to the SYSTEM and fully available to serve LOAD within the DISTURBANCE RECOVERY PERIOD following the CONTINGENCY event; or • LOAD fully removable from the system within the DISTURBANCE RECOVERY PERIOD following the CONTINGENCY event. Operating Reserve - Supplemental The portion of OPERATING RESERVE consisting of: • GENERATION (SYNCHRONIZEd or capable of being SYNCHRONIZEd to the system) that is fully available to serve LOAD within the DISTURBANCE RECOVERY PERIOD following the CONTINGENCY event; or • LOAD fully removable from the SYSTEM within the DISTURBANCE RECOVERY PERIOD following the CONTINGENCY event. Operating Security Limit The value of a SYSTEM operating parameter (for example, total POWER transfer across an INTERFACE) that satisfies the most limiting of prescribed pre- and post- CONTINGENCY OPERATING CRITERIA as determined by EQUIPMENT loading capability and acceptable STABILITY and VOLTAGE conditions. Oscillations Cyclic variations in VOLTAGE, CURRENT, FREQUENCY, or POWER flows. The VOLTAGE and CURRENT of the POWER SYSTEM naturally oscillates at 60 HZ. The term oscillations is typically used to refer to low FREQUENCY (a few HZ) oscillations that may occur. Oscillatory Stability/Instability An ANGLE STABILITY classification. The oscillatory STABILITY LIMIT of a POWER SYSTEM is the maximum amount of ACTIVE POWER that can be transmitted across the SYSTEM without excessive POWER OSCILLATIONS leading to a LOSS OF SYNCHRONISM. No large DISTURBANCE need occur. The response of GENERATOR CONTROL SYSTEMS (governor and EXCITER) are very important to oscillatory stability / instability. Out-of-Step (OOS) To lose synchronism. Out-of-step is best viewed in terms of rotating MAGNETIC FIELDs. When a GENERATOR is out-of-step with the POWER SYSTEM it connects to, the GENERATOR's rotating MAGNETIC FIELD is no longer in-step or in synchronism with the rotating MAGNETIC FIELD of the SYSTEM. Out-of-Step Protective Relay A PROTECTIVE RELAY that is designed to detect OUT-OF-STEP conditions and initiate a user determined response. Out-of-step protective relays are often based on the same principles as IMPEDANCE RELAYs. An OUT-OF-STEP RELAY differentiates between an OUT-OF-STEP condition and a FAULT condition by measuring the speed at which the measured IMPEDANCE changes. Outage The PERIOD during which a generating unit, TRANSMISSION line, or other FACILITY is out of service. Outage Distribution Factor (ODF) Expressed in per unit as the change in POWER flow on a given TRANSMISSION path (or FLOWGATE) due to the OUTAGE of a specified TRANSMISSION ELEMENT(s) or GENERATOR(s). The affect of LOSSES is generally ignored: ODF = (Change in path flow) / (Pre-CONTINGENCY flow on OUTAGED ELEMENTS) Outage Transfer Distribution Factor (OTDF) Expressed in per unit as the portion of POWER flow on a path (or FLOWGATE) in response to a transfer, taking into consideration the PTDF of the flow gate and the effects of an OUTAGEd TRANSMISSION ELEMENT(s). The affect of LOSSES is generally ignored: OTDF (FLOWGATE) = PTDF (FLOWGATE) + (ODF) * PTDF (OUTAGED ELEMENT) Overcurrent Relay A PROTECTIVE RELAY that activates in response to a high CURRENT magnitude. Overcurrent relays can be either timed or instantaneous and directional or non-directional. Overexcite A GENERATOR is overexcited when the applied excitation is greater than that needed to support the GENERATOR's MAGNETIC FIELD. The excess REACTIVE POWER produced by the overexcitation condition flows to the SYSTEM. An overexcited GENERATOR supplies REACTIVE POWER to the SYSTEM. The terms "LAGGING" and "boosting" are also used to refer to an overexcited GENERATOR. A TRANSFORMER may also overexcite. Overexcitation of a TRANSFORMER results from either applying to high a VOLTAGE magnitude or operating at to low of a FREQUENCY. Overlap Angle (Mu) When commutating between two VALVEs in an HVDC CONVERTER, a natural overlap PERIOD occurs in which both VALVES are simultaneously conducting. The length of the overlap PERIOD is measured in degrees and is called the overlap angle. The symbol for the overlap angle is the Greek lower case letter mu (μ). Overlap Regulation Service A method of providing REGULATION SERVICE in which the BALANCING AUTHORITY providing the REGULATION SERVICE incorporates another BALANCING AUTHORITY's ACTUAL INTERCHANGE, FREQUENCY RESPONSE, and SCHEDULES into providing BALANCING AUTHORITY's AGC/ACE equation. Parallel Circuit An ELECTRICAL CIRCUIT in which all the positive terminals are connected to a common point. All the negative terminals are connected to a second point. The VOLTAGE drop is the same across each full ELEMENT of the parallel circuit. Parallel Path Flows The difference between the SCHEDULED and actual POWER flow, assuming zero INADVERTENT INTERCHANGE, on a given TRANSMISSION path. Parallel Resonance A RESONANCE condition in a CIRCUIT with a parallel combination of INDUCTANCE and a CAPACITANCE. At RESONANCE a PARALLEL CIRCUIT REACHes its maximum IMPEDANCE equal to a multiple of the CIRCUIT's RESISTANCE value. Parking Parking is a commercial term that defines ENERGY or TRANSMISSION that is not complete. The TRANSACTION SINK, SOURCE, and path are identified when tagged - prior to TRANSACTION implementation. The priority of the TRANSACTION is also identified when tagged. The merchant buys a "link" of the final TRANSMISSION path and "parks" the GENERATION without identifying SOURCE or SINK (but must show POR and POD). The merchant can also buy a "link" of ENERGY and park it. The ATC for a parked TRANSACTION should be decremented - there is an impact on a FLOWGATE, but it is not calculated until the SOURCE and SINK are known. Parking applies to both firm and NON-FIRM ENERGY and TRANSMISSION. Participation Factor Typically used to refer to generator participation in a facilities loading • For example, assume a transmission line is overloaded • Further assume Generator "A" has a 0.3 participation factor • This means that Generator "A" is responsible for 0.3 or 30% of the overloaded line's flow Parties The TRANSMISSION SERVICE PROVIDER and TRANSMISSION CUSTOMER receiving service under the Tariff. Peak Demand 1. The highest hourly integrated NET ENERGY FOR LOAD within a BALANCING AUTHORITY occurring within a given PERIOD (e.g., day, month, season, or year). 2. The highest INSTANTANEOUS DEMAND within the BALANCING AUTHORITY. Peaking Capacity CAPACITY used to serve PEAK DEMAND. PEAKING GENERATING UNITS operate a limited number of hours per year, and their CAPACITY FACTOR is normally less than 20%. Peaking Generating Units GENERATORs that are normally only run during the peak LOAD PERIODS of the LOAD CYCLE. Penstock A water pipe or conduit that carries water from the upper water reservoir to the TURBINE at a hydroelectric unit. Period The time for one complete CYCLE of a repeating wave. For example, a 60 HZ CURRENT steps through 60 CYCLES per second. The period of one complete CYCLE is therefore 1/60th of a second. Permanent Magnet Generator (PMG) A simple GENERATOR that uses a permanent magnet (a material that retains its MAGNETISM) to provide the required MAGNETIC FIELD. PMGs are often part of GOVERNOR CONTROL SYSTEMS. The PMG is used to produce an output VOLTAGE whose magnitude is representative of a GENERATOR's shaft speed. Per-Unit (P.U) A SYSTEM for reporting quantity magnitudes. A per-unit number is stated in terms of a base quantity. For example if the base VOLTAGE is 345 kV then a VOLTAGE of 359 kV is 1.04 per-unit (359/345=1.04). Phase AC POWER SYSTEMs use three CONDUCTORS to efficiently generate and transmit large amounts of POWER. Each of the CONDUCTORS is called a phase. The phases are each assigned a letter designation; "A", "B", and "C". Customer LOAD can be connected as single-phase (1Φ), two-phase (2Φ), or three-phase (3Φ). Phase Angle The angle by which one waveform leads or lags another waveform. A phase angle can exist between two VOLTAGEs, two CURRENTS, or between a CURRENT and a VOLTAGE. Phase Shifting Transformer (PST) A TRANSFORMER designed to shift the PHASE of the incoming VOLTAGEs. PSTs use a tap changing winding in the similar manner to a conventional tap changer. The VOLTAGE of a PSTs tap changer is added in QUADRATURE to the incoming VOLTAGE to create a PHASE shift. PSTs are also referred to as PHASE ANGLE regulators or PARs. Phasors Similar to a vector but also includes information about the FREQUENCY of the quantity. A phasor diagram is a collection of lines that, like VECTORS, illustrate a quantities magnitude and direction. However, phasor diagrams must also specify the FREQUENCY of the quantity. AC VOLTAGES and CURRENTS can be represented by phasors. Physically Adjacent Balancing Authorities Two BALANCING AUTHORITIES that are directly interconnected with each other. Pick-up Point An operating setting for a PROTECTIVE RELAY that determines at what point the RELAY will activate. Pilot Relay A PROTECTIVE RELAY SYSTEM typically used to protect high VOLTAGE TRANSMISSION LINES. Pilot relays use telecommunication SYSTEMS to communicate between the terminals of the TRANSMISSION line. Planned Outage An OUTAGE that is planned well in advance. Planning The process by which the performance of the electric SYSTEM is evaluated and future changes and additions to the BULK ELECTRIC SYSTEMS are determined. Planning Authority The responsible entity that coordinates and integrates TRANSMISSION FACILITY and service plans, resource plans, and protection systems. Planning Reserve The difference between an entity's expected annual peak capability and its expected annual PEAK DEMAND expressed as a percentage of the annual PEAK DEMAND. Point of Delivery (POD) A location that the TRANSMISSION SERVICE PROVIDER specifies on its TRANSMISSION SYSTEM where an INTERCHANGE TRANSACTION leaves or a LOAD-SERVING ENTITY receives its ENERGY. Point of Receipt (POR) A location that the TRANSMISSION SERVICE PROVIDER specifies on its TRANSMISSION SYSTEM where an INTERCHANGE TRANSACTION enters or a GENERATOR delivers its output. Point-to-point (PTP) Transmission Service As specified in the TRANSMISSION SERVICE PROVIDERS TARIFF, TRANSMISSION SERVICE reserved and/or SCHEDULED between specified POINTS OF RECEIPT and DELIVERY. Poles The ELECTRICAL CIRCUITS formed by the FIELD WINDING on the ROTOR of a SYNCHRONOUS GENERATOR. When DC CURRENT is passed through the FIELD WINDING the poles become magnetic north or south poles. Potential Difference A difference in VOLTAGE magnitudes between two locations. CURRENT can flow along a closed path if a potential difference exists across the path. Potential Transformers (PT) A low POWER TRANSFORMER (an instrument TRANSFORMER) used to reduce normally high POWER SYSTEM VOLTAGES to low values (≈ 110 Volt). A PT has a high number of turns on the PRIMARY WINDING and a low number of turns on the SECONDARY WINDING. The low magnitude secondary VOLTAGEs of a PT are typically input to meters, RELAYS, etc. Power (P) The rate at which ENERGY is expended to do work. Power is measured in watts (W), kilowatts (kW), Megawatts (MW), or Gigawatts (GW). Power Angle (delta) The PHASE ANGLE between two VOLTAGE waveforms. A power angle is the same as a VOLTAGE ANGLE difference. The power angle is a major factor in determining the amount of MW flow between two locations. The Greek letter delta (δ) is the symbol for power angle. Power Converter A mechanical or solid state device for converting AC POWER to DC POWER or vice versa. Modern power converters are THYRISTER based devices that are typically strong sources of HARMONICS. Power Factor (PF) The ratio of the ACTIVE POWER (MW) to the COMPLEX POWER (MVA). The COSINE of the PHASE ANGLE between a LOAD's VOLTAGE and CURRENT is the power factor of the LOAD. A unity power factor LOAD draws no REACTIVE POWER, just ACTIVE POWER. Power Flow Program A computerized algorithm that simulates the behavior of the electric SYSTEM under a given set of conditions. Power Pool Two or more interconnected electric SYSTEMs planned and operated to supply POWER for their combined DEMAND requirements. Power System The collective name given to the ELEMENTS of the electrical SYSTEM. The power system includes the GENERATION, TRANSMISSION, DISTRIBUTION, SUBSTATIONS, etc. The term power system may refer to one section of a large INTERCONNECTED SYSTEM or to the entire INTERCONNECTED SYSTEM. Power System Stabilizer (PSS) A feature added to an EXCITATION SYSTEM that is designed to assist with the DAMPING of low FREQUENCY (≈1 HZ) POWER SYSTEM OSCILLATIONS. A typical PSS provides positive DAMPING to POWER OSCILLATIONS by ensuring that VOLTAGE corrections made by the EXCITATION SYSTEM are in-PHASE with detected FREQUENCY OSCILLATIONS. Power Transfer Distribution Factor (PTDF) The portion of an INTERCHANGE TRANSACTION, expressed in per unit, that flows across a TRANSMISSION FACILITY (FLOWGATE). The values of the PTDF's can range from minus one to plus one. NERC also uses the terms DISTRIBUTION FACTOR and Transfer DISTRIBUTION FACTOR. Power-Angle Curve A graphical representation of the ACTIVE POWER transfer equation. The power-angle curve is a plot of the ACTIVE POWER transfer as the POWER ANGLE is varied between 0° and 180°. The power-angle curve is a good tool for analyzing the ANGLE STABILITY of a simple (two BUS) POWER SYSTEM. Power-Circle Diagram A graphical method of illustrating how MW and Mvar flows change as the POWER ANGLE changes. Power-circle diagrams are composed of circular characteristics of the POWER flow out of the sending end and into the receiving end of a two BUS SYSTEM. Primary Winding The winding of a TRANSFORMER that is connected to the POWER input or SOURCE end of the TRANSFORMER. Prime Mover A mechanism that converts thermal or hydraulic ENERGY into mechanical POWER. For example, a coal fired BOILER with a steam TURBINE is a prime mover as it converts the thermal ENERGY of coal into the mechanical POWER to turn the TURBINE. Pro Forma Tariff Usually refers to the standard OATT and/or associated TRANSMISSION rights mandated by the U.S. FEDERAL ENERGY REGULATORY COMMISSION Order No. 888. Production Cost The costs associated with starting, operating, and stopping generating units. Protection System PROTECTIVE RELAYS, associated communication systems, VOLTAGE and CURRENT sending devices, station batteries and DC control circuitry. Protective Relay A mechanical or electronic device used to sense POWER SYSTEM DISTURBANCES and respond to limit the possible damage. Pseudo-Tie A telemetered reading or value that is updated in real time and used as a "virtual" tie line flow in the AGC/ACE equation but for which no physical tie or energy METERING actually exists. The integrated value is used as a metered MWh value for INTERCHANGE accounting purposes. Purchasing-Selling Entity (PSE) The entity that purchases or sells, and takes title to, ENERGY, CAPACITY, and INTERCONNECTED OPERATIONS SERVICES. Purchasing-Selling Entities may be affiliated or unaffiliated merchants and may or may not own generating facilities. P-V Curve A POWER versus VOLTAGE curve. A plot of the POWER transferred to a BUS versus the VOLTAGE at that BUS. P-V curveS are a graphical tool used to analyze a POWER SYSTEM's VOLTAGE STABILITY. Pythagorean Theorem A mathematical relationship which states that in a RIGHT TRIANGLE the square of the HYPOTENUSE length is equal to the sum of the squares of the lengths of the remaining two sides Quadrature (Q) At an angle of 90°. When two VECTORS are in quadrature they are perpendicular to one another. The symbol "Q" for REACTIVE POWER is derived from the word Quadrature. Quality (Q) A factor for measuring the FREQUENCY RESPONSE of an ELECTRICAL CIRCUIT. A CIRCUIT's quality is dependent upon the relative magnitudes of its reactive and resistive ELEMENTS. Ramp Period The time between GENERATION ramp start and end times usually expressed in minutes. Ramp Rate (SCHEDULE) The rate, expressed in megawatts per minute, at which the INTERCHANGE SCHEDULE is attained during the RAMP PERIOD. (GENERATOR) The rate, expressed in megawatts per minute, that a GENERATOR changes its output. Ramp Window The time PERIOD that occurs each hour for adjusting a BALANCING AUTHORITY's GENERATION. A typical ramp window is from 10 minutes to the hour till 10 minutes after the hour. If all BALANCING AUTHORITIES use the same ramp windows, FREQUENCY DEVIATIONS will be reduced. Rate Base The value of property upon which a utility is permitted to earn a specified rate of return as established by a regulatory authority. The rate base generally represents the value of property used by the utility in providing service and may be calculated by any one or a combination of the following accounting methods: fair value, prudent investment, reproduction cost, or original cost. Depending on which method is used, the rate base includes cash, working capital, materials and supplies, and deductions for accumulated provisions for depreciation, contributions in aid ofconstruction, customer advances for construction, accumulated deferred income taxes, and accumulated deferred investment tax credits. Rate-of-Change (Protective Relay) A type of PROTECTIVE RELAY that monitors the rate at which a quantity changes. For example, a rate-of-change relay may monitor the rate at which the MW flow along a TRANSMISSION line varies. The RELAY could be set to TRIP the line if the rate of MW flow change exceeds a specified value. Rating The operational limits of a TRANSMISSION SYSTEM ELEMENT under a set of specified conditions. Reach The extent of protection that an IMPEDANCE RELAY provides to a TRANSMISSION line. The reach is typically defined in terms of the IMPEDANCE of the line. For example, a zone #1 IMPEDANCE RELAY may reach 85% into the protected line. Reaction Turbine A water TURBINE in which the pressure difference across the TURBINE BLADEs causes the BLADEs to turn. A Francis TURBINE is an example of a reaction turbine. Reactive Capability Curve A graphical method of illustrating the COMPLEX POWER output limits of a SYNCHRONOUS GENERATORs. The reactive capability curve is sometimes called a "D-Curve" as it is typically shaped like the letter "D". Reactive Power (Q) The portion of electricity that establishes and sustains the electric and MAGNETIC FIELDS of ALTERNATING-CURRENT EQUIPMENT. Reactive power must be supplied to most types of magnetic EQUIPMENT, such as MOTORS and TRANSFORMERS. It also must supply the reactive LOSSES on TRANSMISSION FACILITIES. Reactive power is provided by GENERATORS, SYNCHRONOUS CONDENSERS, or electrostatic EQUIPMENT such as CAPACITORS and directly influences electric SYSTEM VOLTAGE. It is usually expressed in KILOVARs (kvar) or megavars (Mvar). Reactor A device intentionally designed to act as an inductor and store ENERGY in its MAGNETIC FIELD. A shunt reactor acts as a SINK (absorber) of REACTIVE POWER from the SYSTEM. Series reactors are devices that are used to increase the INDUCTIVE REACTANCE (XL) of a TRANSMISSION path. Real Power The portion of electricity that supplies ENERGY to the LOAD. Real Power is sometimes called ACTIVE POWER. Real-Time Present time as opposed to future time. (From INTERCONNECTION RELIABILITY OPERATING LIMITS standard.) Real-Time Operators The instantaneous operations of a POWER SYSTEM as opposed to those operations that are simulated. Receiving Balancing Authority The BALANCING AUTHORITY importing the INTERCHANGE. Reclosing Relay A RELAY that automatically (after a few CYCLES or a few seconds) recloses a TRANSMISSION line following a FAULT. Rectifier An HVDC CONVERTER operated to convert AC POWER to DC POWER. Region One of the NERC REGIONAL RELIABILITY ORGANIZATIONS. Regional Reliability Organization (RRO) 1. An entity that ensures that a defined area of the BULK ELECTRIC SYSTEM is reliable, adequate and secure. 2. A member of the North American Electric Reliability Council. The REGIONAL RELIABILITY ORGANIZATION can serve as the COMPLIANCE MONITOR. NERC is divided into eight REGIONAL RELIABILITY ORGANIZATIONS: FRCC Florida Reliability Coordinating Council MRO Midwest Reliability Organization NPCC Northeast Power Coordinating Council RFC Reliability First Corporation SERC Southeastern Electric Reliability Council SPP Southwest Power pool TRE Texas Reliability Entity WECC Western Electricity Coordinating Council Regional Transmission Group (RTG) Voluntary organization of TRANSMISSION OWNERs, TRANSMISSION users, and other entities interested in coordinating TRANSMISSION PLANNING and expansion and use on a REGIONAL and interregional basis. Regional Transmission Organization (RTO) An organization that is independent from all GENERATION and POWER marketing interests and has exclusive responsibility for electric TRANSMISSION GRID operations, short-term electric RELIABILITY, and TRANSMISSION SERVICES within a multi-State REGION. To achieve those objectives, the RTO manages TRANSMISSION facilities owned by different companies and encompassing one, large, CONTIGUOUS geographic area. Regulating Reserve An amount of reserve responsive to AUTOMATIC GENERATION CONTROL, which is sufficient to provide normal regulating MARGIN. Regulating Transformers A TRANSFORMER used to regulate VOLTAGE or PHASE ANGLE. Conventional tap changing (ULTC) TRANSFORMERS and PHASE SHIFTING TRANSFORMERS (PSTs) are regulating transformers. Regulating Unit A GENERATOR used for the REGULATION of SYSTEM FREQUENCY. To serve as a regulating unit the GENERATOR must have available SPINNING RESERVE. Regulation The ability to maintain a quantity within acceptable limits. For example, FREQUENCY REGULATION is the control or regulation of the SYSTEM FREQUENCY to within a tight BANDWIDTH of 60 HZ. VOLTAGE regulation is the control of a VOLTAGE level within a set BANDWIDTH. Regulation Service The process whereby one BALANCING AUTHORITY contracts to provide corrective response to all or a portion of the ACE of another BALANCING AUTHORITY. The BALANCING AUTHORITY providing the response assumes the obligation of meeting all applicable control criteria as specified by NERC for itself and the BALANCING AUTHORITY for which it is providing the Regulation Service. Relative Acceleration For TORQUE and POWER ANGLEs to change, a relative acceleration must exist for a PERIOD of time. One part of the SYSTEM must accelerate with respect to another part. Once relative acceleration occurs, any speed difference that has developed will continue the increase or decrease in TORQUE or POWER ANGLES. TORQUE and POWER ANGLES will not stop changing until all sections of the SYSTEM are running at the same FREQUENCY. Relay A electrical or mechanical device that responds to a measured input with a user determined output. Types of RELAYS include AUXILIARY RELAYS, monitoring RELAYS, regulating RELAYS, and PROTECTIVE RELAYS. Reliability The degree of performance of the ELEMENTS of the BULK ELECTRIC SYSTEM that results in electricity being delivered to customers within accepted standards and in the amount desired. Reliability may be measured by the FREQUENCY, duration, and magnitude of adverse effects on the electric supply. Reliability Coordinator (RC) The entity that is the highest level of authority who is responsible for the reliable operation of the BULK ELECTRIC SYSTEM, has the WIDE AREA view of the BULK ELECTRIC SYSTEM, and has the operating tools, processes and procedures, including the authority to prevent or mitigate EMERGENCY operating situations in both next-day analysis and REAL-TIME OPERATIONS. The Reliability Coordinator has the purview that is broad enough to enable the calculation of INTERCONNECTION RELIABILITY OPERATING LIMITS, which may be based on the operating parameters of TRANSMISSION SYSTEMS beyond any TRANSMISSION OPERATOR'S vision. Reliability Coordinator Area The collection of GENERATION, TRANSMISSION, and LOADS within the boundaries of the RELIABILITY COORDINATOR. Its boundary coincides with one or more BALANCING AUTHORITIES. Reliability Coordinator Information System (RCIS) The system that RELIABILITY COORDINATORS use to post messages and share operating information in real time. Remedial Action Scheme (RAS) See "SPECIAL PROTECTION SYSTEM". Remote Terminal Unit (RTU) An electronic device, installed in a SUBSTATION or GENERATOR, which acts as an INTERFACE between a company's EMS SYSTEM and the GENERATOR or SUBSTATION. Reportable Disturbance Any event that causes an ACE change greater than or equal to 80% of a BALANCING AUTHORITY's or RESERVE SHARING GROUP's most severe CONTINGENCY. The definition of a reportable disturbance is specified by each REGIONAL RELIABILITY ORGANIZATION. This definition may not be retroactively adjusted in response to observed performance. Rerating A change in the capability of a GENERATOR due to a change in conditions such as age, upgrades, auxiliary EQUIPMENT, cooling, etc. Reserve Sharing Group (RSG) A group whose members consist of two or more BALANCING AUTHORITIES that collectively maintain, allocate, and supply OPERATING RESERVEs required for each BALANCING AUTHORITY's use in recovering from contingencies within the group. Scheduling energy from an Adjacent BALANCING AUTHORITY to aid recovery need not constitute reserve sharing provided the transaction is ramped in over a PERIOD the supplying party could reasonably be expected to load GENERATION in (e.g., ten minutes). If the transaction is ramped in quicker (e.g., between zero and ten minutes) then, for the purposes of DISTURBANCE Control Performance, the Areas become a Reserve Sharing Group. Resistance (R) The property of a material that opposes or resists CURRENT flow by converting electric ENERGY to heat. The symbol for resistance is the letter "R". Resistivity A measure of the ability of a material to conduct electricity. The greater the resistivity, the greater the opposition to CURRENT flow. An INSULATOR has a high resistivity. Resonance In an ELECTRICAL CIRCUIT, resonance is achieved when the magnitudes of the inductive and capacitive ELEMENTS match. Resonance may be achieved by inputting ENERGY at the CIRCUIT's RESONANT FREQUENCY or by varying the size of the inductive or capacitive ELEMENTS. A radio receiver is tuned to resonance at the channel the operator wants to receive. Resonant To achieve RESONANCE. Resonant Frequency (FR) The FREQUENCY at which RESONANCE is achieved in an ELECTRICAL CIRCUIT. The formula for the resonant frequency is: FR =1/ 2π square root of LC Resource Planner The entity that develops a long-term (generally one year and beyond) plan for the resource ADEQUACY of specific LOADs (customer DEMAND and energy requirements) within a PLANNING AUTHORITY Area. Resources Resources is typically used to refer to available GENERATION. For example, LOAD must match resources to maintain FREQUENCY. Response Rate The RAMP RATE that a generating unit can achieve under normal operating conditions expressed in megawatts per minute (MW/Min.). Responsive Reserves Reserve CAPACITY that is available to respond to SYSTEM FREQUENCY DISTURBANCES. Responsive Spinning Reserve That portion of SPINNING RESERVE available to respond to FREQUENCY DISTURBANCES as a result of the GENERATOR's normal governor action. Restoration The process of returning GENERATORs and TRANSMISSION SYSTEM ELEMENTS and restoring LOAD FOLLOWING an OUTAGE on the electric SYSTEM. Restrike The re-ignition of an electric ARC. RFC Acronym for the Reliability First Corporation. RFC is one of the eight NERC RROs. Right-of-Way (ROW) A corridor of land on which electric lines may be located. The TRANSMISSION OWNER may own the land in fee, own an easement, or have certain franchise, prescription, or license rights to construct and maintain lines. Right Triangle A triangle in which one of the three internal angles is equal to 90°. Root Means Squared (RMS) The effective value of an AC VOLTAGE or CURRENT. The RMS value of an AC quantity would produce the same amount of heat in a DC resistive CIRCUIT. For example if an AC CIRCUIT has an RMS CURRENT of 10 amps, then 10 amps of DC CURRENT would have the same heating effect. Most AC meters read RMS values. Rotational Energy ENERGY stored within a rotating mass. For example, a spinning GENERATOR contains rotational energy. The terms INERTIAL ENERGY, STORED ENERGY, and rotational energy are often used interchangeably to refer to the ENERGY stored in the rotating ELEMENTS (motors and GENERATORS) of the POWER SYSTEM. Rotor The rotating component of a motor or GENERATOR. Runner The rotating ELEMENT of a hydro TURBINE. Saturation After a MAGNETIC FIELD REACHes a certain strength, any further increase in the CURRENT that caused the MAGNETIC FIELD will not result in any increase in the strength of the MAGNETIC FIELD. The magnetic material is saturated at this point. When a TRANSFORMER becomes saturated, the CORE's MAGNETIC FIELD will expand and link portions of the TRANSFORMER not designed for exposure to an alternating MAGNETIC FIELD. TRANSFORMER thermal problems may result. Scheduled Interchange Electric POWER SCHEDULED to flow between entities, usually the net of all sales, purchases, and WHEELING TRANSACTIONs between those areas at a given time. Scheduled Net Interchange The sum of the intended (SCHEDULED) MW flows on all of a BALANCING AUTHORITY's TIE-LINES. Scheduled Total Interchange The net of all INTERCHANGE SCHEDULES with all ADJACENT BALANCING AUTHORITIES. It is, in essence, the SCHEDULED INTERCHANGE with the INTERCONNECTION. Scheduling Path The TRANSMISSION SERVICE arrangements reserved by the PURCHASING-SELLING ENTITY for a TRANSACTION. Scroll Case A spiral-shaped steel intake guiding the flow of water into the WICKET GATES located just prior to the TURBINE of a hydro-electric FACILITY. Scrubber A pollution CONTROL SYSTEM used in fossil fuel units for removing sulfur from the exhaust gases. Seams The boundaries between adjacent electricity-related organizations. Differences in regulatory requirements or operating practices may create "seams problems". Secondary Winding The winding of a TRANSFORMER that is connected to the POWER output or LOAD end of the TRANSFORMER. Sectionalizing The process of reducing the LOAD on a DISTRIBUTION feeder using switching actions. Self-Excitation A possible operating condition for a GENERATOR in which the CHARGING CURRENT from a high VOLTAGE TRANSMISSION SYSTEM takes over as the excitation CURRENT SOURCE of the GENERATOR. Self-excitation is a dangerous condition as high VOLTAGES can develop in the GENERATOR and in the GENERATOR auxiliaries. Sending Balancing Authority The BALANCING AUTHORITY exporting the INTERCHANGE. SERC Acronym for the Southeastern Electric Regional Reliability Council. SERC is one of the eight NERC REGIONAL RELIABILITY ORGANIZATIONS. Series Circuit ELECTRICAL CIRCUIT in which ELEMENTS are connected end to end. The same CURRENT flows through each ELEMENT of a series circuit. Series Resonance A RESONANCE condition in a CIRCUIT with a series combination of INDUCTANCE and CAPACITANCE. At RESONANCE a SERIES CIRCUIT REACHes its minimum IMPEDANCE equal to the CIRCUIT's RESISTANCE value. Servomotor A device used to create a force based on a control signal input. For example, in a GOVERNOR CONTROL SYSTEM a control signal is first developed using a control VALVE. The control signal is input to a servomotor. The servomotor then drives steam VALVES, WICKET GATES, etc. The servomotor may be an electric motor, an hydraulic piston, or any other means of developing a force. Shield Wires CONDUCTORS strung across the top of TRANSMISSION LINES that are designed to protect the TRANSMISSION line from lightning strikes. Short Circuit The introduction of a low IMPEDANCE path between CONDUCTORS energized at different VOLTAGES. A short circuit is the same as a FAULT. Short Circuit Ratio (SCR) The ratio between the SHORT CIRCUIT MVA of the local SYSTEM and the MVA RATING of a piece of EQUIPMENT. For example, the SCR of a GENERATOR is equal to the MVA that would flow to a 3Φ FAULT applied at the GENERATOR's high side BUS divided by the MVA RATING of the GENERATOR. SCRs can also be calculated for HVDC SYSTEMs and other electrical devices. Shunt Capacitor Bank Shunt capacitors are CAPACITORs connected from the POWER SYSTEM to an electrical GROUND. They are used to supply KILOVARs (REACTIVE POWER) to the SYSTEM at the point where they are connected. A shunt capacitor bank is a group of shunt capacitors. Sine The sine of either of the unknown angles of a RIGHT TRIANGLE is the ratio of the side directly opposite the unknown angle to the HYPOTENUSE. Sinewave A quantity that continually repeats itself. For example, AC VOLTAGE and CURRENT are sinewaves. The wave shape for the COSINE function is the same as the SINE function but with a 90° PHASE ANGLE difference. Single Contingency The sudden, unexpected failure or OUTAGE of a SYSTEM FACILITY(ies) or ELEMENTS(s) (generating unit, TRANSMISSION line, TRANSFORMER, etc.). ELEMENTS removed from service as part of the operation of a REMEDIAL ACTION SCHEME are considered part of a single CONTINGENCY. Sink Final POINT OF DELIVERY for the TRANSACTION: the ACTUAL LOAD. Sink Balancing Authority The BALANCING AUTHORITY in which the LOAD (SINK) is located for an INTERCHANGE TRANSACTION. (This will also be a RECEIVING BALANCING AUTHORITY for the resulting INTERCHANGE SCHEDULE.) Six-Pulse Converter An HVDC CONVERTER that has six pulses to the DC output VOLTAGE for each CYCLE of the AC supply VOLTAGE. Sliding Pressure A method of operation available in some thermal units. Normally a unit's steam VALVES are adjusted to control the amount of steam sent to the TURBINE stages. In a sliding pressure MODE of operation, the steam VALVES are operated wide open to minimize VALVE throttling LOSSES. The BOILER pressure is then varied to adjust the MW output of the unit. Slip The difference between the SYNCHRONOUS SPEED of an induction motor and the speed at which its ROTOR actually rotates. slip-rings Metal rings attached to the shaft of a SYNCHRONOUS MACHINE. The ROTOR's FIELD WINDING terminates on the inner surface of the slip-rings while the BRUSHes ride on the smooth outer surface of the slip-rings. Smoothing Reactor A series REACTOR attached to an HVDC TRANSMISSION line that smoothes the ripple of an HVDC CONVERTER's DC output VOLTAGE and assists with the POWER conversion process. Solar Magnetic Disturbance (SMD) Solar (sun) induced DISTURBANCES to the earth's MAGNETIC FIELD. SMDs may result in large low FREQUENCY CURRENTS flowing in the earth's surface. These CURRENTS (called GICs) may enter the POWER SYSTEM and damage TRANSFORMERS. SMDs are also called GEOMAGNETIC DISTURBANCES, or GMDs. Solenoid An electro-magnet that, when energized, is used to perform a mechanical function such as opening a switch. Solid-State Relay A RELAY that uses semiconductor components. Source The INITIAL POINT OF RECEIPT for the TRANSACTION: the actual GENERATION FACILITY. Source Balancing Authority The BALANCING AUTHORITY in which the GENERATION (SOURCE) is located for an INTERCHANGE TRANSACTION. (This will also be a SENDING BALANCING AUTHORITY for the resulting INTERCHANGE SCHEDULE.) Special Protection System (SPS) An automatic protection system designed to detect abnormal or predetermined system conditions, and take corrective actions other than and/or in addition to the isolation of faulted components to maintain system RELIABILITY. Such action may include changes in DEMAND, GENERATION (MW and Mvar), or SYSTEM configuration to maintain SYSTEM STABILITY, acceptable VOLTAGE, or POWER flows. An SPS does not include (a) underfrequency or underVOLTAGE LOAD SHEDDING or (b) FAULT conditions that must be ISOLATED or (c) OUT-OF-STEP relaying (not designed as an integral part of an SPS). Also called REMEDIAL ACTION SCHEME (RAS). Spinning Reserve Unloaded GENERATION that is SYNCHRONIZEd and ready to serve additional DEMAND. SPP Acronym for the Southwest Power pool. SPP is one of the eight NERC REGIONAL RELIABILITY ORGANIZATIONS. Stability The ability of an electric SYSTEM to maintain a state of equilibrium during normal and abnormal SYSTEM conditions or DISTURBANCES. Stability Limit The maximum POWER flow possible through some particular point in the SYSTEM while maintaining STABILITY in the entire SYSTEM or the part of the SYSTEM to which the stability limit refers. State Estimator (SE) Computer SOFTWARE that takes measurements of quantities related to a SYSTEM state as input and provides an estimate of the complete SYSTEM state (as its output). The SE is used to confirm that the monitored POWER SYSTEM is operating in a secure state by simulating the SYSTEM both at the present time and one step ahead, for a particular network topology and loading condition. With the use of an SE and its associated CONTINGENCY ANALYSIS SOFTWARE, SYSTEM OPERATORS can review each critical CONTINGENCY to determine whether each possible future state is within RELIABILITY limits. Static Var Compensator (SVC) A combination of shunt REACTORs and shunt CAPACITORs that use THYRISTER based switches and THYRISTER control to regulate the var output. Static Var System (SVS) A combination of an SVC and other REACTIVE POWER EQUIPMENT. A common CONTROL SYSTEM controls both the SVC and the other reactive EQUIPMENT. Station A location in an electrical network where one or more ELEMENTS are connected. Examples include generating stations and SUBSTATIONS. Station Service The electric supply for the ancillary EQUIPMENT used to operate a generating STATION or SUBSTATION. Station Service Generator A GENERATOR (usually found in hydro plants) used to normally supply electric ENERGY for STATION SERVICE EQUIPMENT. Stator The stationary component of a motor or GENERATOR. Steady State Stability Instability An ANGLE STABILITY classification. The steady state STABILITY LIMIT of a POWER SYSTEM is the maximum amount of ACTIVE POWER that can be transmitted across the SYSTEM without a LOSS OF SYNCHRONISM occurring. No large DISTURBANCE need occur. Storage ENERGY transferred from one entity to another entity that has the ability to conserve the ENERGY (that is, stored as water in a reservoir, coal in a pile, etc.) with the intent that the ENERGY will be returned at a time when such ENERGY is more useable to the original supplying entity. Stored Energy ENERGY stored within a rotating mass. For example, a spinning GENERATOR contains stored energy. The terms INERTIAL ENERGY, stored energy, and ROTATIONAL ENERGY are often used interchangeably to refer to the ENERGY stored in the rotating ELEMENTS (motors and GENERATORs) of the POWER SYSTEM. Subregion A portion of a REGIONAL RELIABILITY ORGANIZATION. Substation An ELEMENT of the POWER SYSTEM that contains CIRCUIT BREAKERS, DISCONNECT SWITCHES, TRANSFORMERS, REACTORS, CAPACITORS, and other EQUIPMENT. A central control house is often provided to house control and protective EQUIPMENT. Subsynchronous A FREQUENCY below SYNCHRONOUS SPEED. Subsynchronous System (SSR) An electric POWER SYSTEM condition where the electric network exchanges ENERGY with a TURBINE/GENERATOR at one or more of the natural frequencies of the combined SYSTEM. The FREQUENCY of the ENERGY EXCHANGE is below the SYNCHRONOUS FREQUENCY (SUBSYNCHRONOUS) of the SYSTEM. Subtransmission A functional or VOLTAGE classification relating to lines at VOLTAGE levels between the DISTRIBUTION and TRANSMISSION levels. Sunspots Large dark colored patches on the surface of the sun. Sunspots are a consequence of ENERGY DISTURBANCES on the surface of the sun. Supercritical Boiler A BOILER SYSTEM that is operated at a much higher temperature and pressure than a conventional drum type BOILER. Supercritical BOILERs are also called "once through" type BOILERs. A supercritical BOILER does not have any significant steam STORAGE, which affects its response to governor commands. Supersynchronous A FREQUENCY above SYNCHRONOUS SPEED. Supervisory Control A form of remote control comprising an arrangement for the selective control of remotely located facilities by an electrical means over one or more communication media. Supervisory Control and Data Acquisition (SCADA) A system of remote control and TELEMETRY used to monitor and control the TRANSMISSION SYSTEM. Supplemental Regulation Service A method of providing REGULATION SERVICE in which the BALANCING AUTHORITY providing the REGULATION SERVICE receives a signal representing all or a portion of the other BALANCING AUTHORITY's ACE. Surge A TRANSIENT variation of CURRENT, VOLTAGE, or POWER flow in an electric CIRCUIT or across an electric SYSTEM. Surge Impedance Loading (SIL) The MW loading on a TRANSMISSION line at which the line's natural REACTIVE POWER production equals its REACTIVE POWER usage. Switching Order A sequence of steps to accomplish a desired switching action. Switching Process The process by which the status (open, closed, etc.) of POWER SYSTEM EQUIPMENT (switches, CIRCUIT BREAKERS, etc.) is adjusted to perform maintenance or enhance operations. Switching Station POWER SYSTEM EQUIPMENT used to tie together two or more electric CIRCUITS through switches. The switches are selectively arranged to permit a CIRCUIT to be disconnected, or to change the electric connection between the CIRCUITS. Switching Surge The sudden changes to VOLTAGE and CURRENT waveforms that accompany TRANSMISSION SYSTEM switching events. Synch-Check Relay A PROTECTIVE RELAY that will not allow a CIRCUIT BREAKER to be closed unless the FREQUENCY difference, VOLTAGE magnitude difference, and VOLTAGE ANGLE across the open CIRCUIT BREAKER are within acceptable limits. Synchronize The process to bring two electrical SYSTEMS together by closing a CIRCUIT BREAKER. Synchonous To be in-step with a reference. A synchronous GENERATOR rotates in synchronism with the POWER SYSTEM. Synchronous Condenser A SYNCHRONOUS MACHINE that operates as a SYNCHRONOUS motor. The MW to turn the machine's shaft is drawn from the POWER SYSTEM. The full capabilities of the machines EXCITATION SYSTEM (to absorb and supply MVAR) are then available for VOLTAGE CONTROL purposes. Hydroelectric GENERATORS can often be operated in synchronous condenser MODE. The unit's water TURBINE is typically de-watered and the unit's ROTOR turned as if it were a motor. Synchronous Machine An AC machine whose ROTOR rotates in synchronism with the POWER SYSTEM to which it is attached. Synchronous machines can be either GENERATORS or motors. A synchronous machine also includes a SOURCE of DC excitation CURRENT (the EXCITATION SYSTEM). Synchronous Speed The speed at which a SYNCHRONOUS GENERATOR must rotate in order to stay in synchronism with the rotating MAGNETIC FIELD of the SYSTEM. The synchronous speed is determined by the FREQUENCY of the POWER SYSTEM and the number of ROTOR magnetic POLES. Synchroscope A device for comparing the FREQUENCY difference and VOLTAGE ANGLE across an open CIRCUIT BREAKER. System A combination of GENERATION, TRANSMISSION, and DISTRIBUTION components comprising an ELECTRIC UTILITY, or group of utilities. System Operating Limit (SOL) The value (such a MW, Mvar, AMPERES, FREQUENCY, or Volts) that satisfies the most limiting of the prescribed OPERATING CRITERIA for a specified SYSTEM configuration to ensure operation within acceptable RELIABILITY CRITERIA. System operating limits are based upon certain OPERATING CRITERIA. These include, but are not limited to: • FACILITY RATINGS (Applicable pre- and post-CONTINGENCY EQUIPMENT or FACILITY RATINGS) • TRANSIENT STABILITY LIMITs (Applicable pre- and post-CONTINGENCY STABILITY LIMITs) • VOLTAGE STABILITY LIMITs (Applicable pre- and post-CONTINGENCY VOLTAGE STABILITY) • SYSTEM VOLTAGE LIMITS (Applicable pre- and post-CONTINGENCY VOLTAGE LIMITS) System Operator An individual at a CONTROL CENTER (BALANCING AUTHORITY, TRANSMISSION OPERATOR, GENERATOR OPERATOR, RELIABILITY COORDINATOR) whose responsibility it is to monitor and control that electric system in real time. System Personnel Those people who have the capability to affect SYSTEM operations and who must abide by the authority vested in the SYSTEM OPERATOR. May include POWER plant operators, SYSTEM maintenance personnel, POWER schedulers, POWER MARKETERS, etc. TAG Refers to the collection of information in the electronic request for an ENERGY SCHEDULE and subsequent responses utilized in the electronic Transaction Information System (TIS) implemented by NERC. TAPS Fixed electrical contacts at different positions on a TRANSFORMER's winding. Taps are adjusted to change the VOLTAGE RATIO of a TRANSFORMER. Target An indicator on a RELAY that is displayed when the RELAY operates. The term "flag" is often used to refer to a target. Telemetering The process by which measurable electrical quantities from SUBSTATIONS and generating STATIONs are instantaneously transmitted to the CONTROL CENTER, and by which operating commands from the CONTROL CENTER are transmitted to the SUBSTATIONS and generating STATIONs. Telemetry EQUIPMENT for measuring a quantity (amps, volts, MW, etc.) and transmitting the result via a telecommunication SYSTEM (radio, microwave, etc.) to a remote location for indication or recording. Tertiary Winding An additional winding added to a POWER TRANSFORMER. The tertiary winding may be used to connect a REACTOR, CAPACITOR, or to provide STATION SERVICE. Thermal Limit A POWER flow limit based on the possibility of damage by heat. Heating is caused by the electrical LOSSES which are equal to I2 R. Thermal Rating The maximum amount of electrical CURRENT that a TRANSMISSION line or electrical FACILITY can conduct over a specified time PERIOD before it sustains permanent damage by overheating or before it sags to the point that it violates public safety requirements. Theta A Greek symbol used to indicate an angular measurement. Thrust Bearing The bearing that opposes axial forces in the GENERATOR shaft Thyrister A solid state electronic component whose ability to conduct CURRENT is controlled via its VOLTAGE polarity or a gate signal. Thyristers are also called silicon controlled RECTIFIERS (SCR). Thyristers are combined in series/parallel arrangements to perform rapid switching actions. Thyristers are used in modern STATIC VAR COMPENSATORS (SVC), adjustable speed drives (ASD), and high VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS. Tie Facility The transmission facility interconnecting BALANCING AUTHORITY AREAS. Tie-Line A TRANSMISSION LINE connecting two BALANCING AUTHORITY AREAs. Tie-line Bias (TLB) A MODE of AUTOMATIC GENERATION CONTROL that allows the BALANCING AUTHORITY to 1) maintain its INTERCHANGE SCHEDULE; and 2) respond to INTERCONNECTION FREQUENCY ERROR. Tie-line Telemetry TELEMETRY EQUIPMENT used to measure POWER flow data on a TIE-LINE connecting BALANCING AUTHORITIES. The POWER flow data is then transmitted to both BALANCING AUTHORITIES. Time Error Correction The difference between the INTERCONNECTION time measured at the BALANCING AUTHORITY(ies) and the time specified by the National Institute of Standards and Technology. Time error is caused by the accumulation of FREQUENCY ERROR over a given PERIOD. Torque The torque is a force that produces a rotating or twisting action. Torque Angle (greek lower case lamba) The angle by which the rotating MAGNETIC FIELD of SYNCHRONOUS MACHINE leads or lags the rotating MAGNETIC FIELD of the SYSTEM to which it connects. A GENERATOR has a positive torque angle while a motor has a negative torque angle. The symbol for the torque angle is the letter "δ" (Greek lower case letter delta). Torque-Speed Curves A graphical means of illustrating the relationship between the TORQUE developed by a motor and the speed of rotation of the motor shaft. Torsional A twisting force. Total Actual Interchange The algebraic sum of all INTERCHANGE metered with all PHYSICALLY ADJACENT BALANCING AUTHORITIES. It is, in essence, the ACTUAL INTERCHANGE with the INTERCONNECTION. Total Harmonic Distortion (THD) A computed value used to quantify the harmonic content of a waveform. The THD is a measure of the percent of the harmonic components content as compared to the magnitude of the fundamental component. Total Load The summation of motor and NON-MOTOR LOAD Total Transfer Capability (TTC) The amount of electric POWER that can be moved or transferred reliably from one area to another area of the interconnected TRANSMISSION systems by way of all TRANSMISSION LINES (or paths) between those areas under specified system conditions. Transaction Sale of bulk POWER via the TRANSMISSION GRID. Transaction Participation Factor (TPF) Expressed in per unit, the change in POWER flow through an area of the POWER SYSTEM due to a change in the POWER transfer between a specified SOURCE and a specified SINK, used for loss calculations. TPF = (Change in through flow) / (Change in POWER transfer) The change in POWER flow through an area is defined as one-half of the sum of the changes in flow (absolute values) of each TIE-LINE connected to the area. Transfer Capability The measure of the ability of interconnected electric SYSTEMS to move or transfer POWER in a reliable manner from one area to another over all TRANSMISSION LINES (or paths) between those areas under specified SYSTEM conditions. The units of transfer capability are in terms of electric POWER, generally expressed in megawatts (MW). In this context, "area" may be an individual electric SYSTEM, POWER POOL, BALANCING AUTHORITY, SUBREGION, or NERC REGION, or a portion of any of these. Transfer capability is directional in nature. That is, the transfer capability from "Area A" to "Area B" is not generally equal to the transfer capability from "Area B" to "Area A". Transfer Distribution Factor (TDF) The portion of an INTERCHANGE TRANSACTION, expressed in percent that FLOWS across a TRANSMISSION FACILITY (FLOWGATE). Transfer Limit The maximum amount of POWER that can be transferred in a reliable manner from one area to another over all TRANSMISSION LINES (or paths) between those areas under specified SYSTEM conditions. Transformer A device that operates on magnetic principals to increase (step up) or decrease (step down) VOLTAGE Transient A short term phenomena. Transient Stability The ability of an electric SYSTEM to maintain synchronism between its parts when subjected to a DISTURBANCE and to regain a state of equilibrium following that DISTURBANCE. Transmission An interconnected group of lines and associated EQUIPMENT for the movement or transfer of electric ENERGY between points of supply and points at which it is transformed for delivery to customers or is delivered to other electric SYSTEMS. Transmission Constraint A limitation on one or more TRANSMISSION ELEMENTS that may be reached during normal or CONTINGENCY SYSTEM operations. Transmission Line A system of structures, wires, insulators and associated hardware that carry electric energy from one point to another in an electric power system. Lines are operated at relatively high voltages varying from 69 kV up to 765 kV, and are capable of transmitting large quantities of electricity over long distances. Transmission Loading Relief (TLR) A procedure used to manage CONGESTION on the electric TRANSMISSION SYSTEM. Transmission Operator The entity responsible for the RELIABILITY of its "local" TRANSMISSION SYSTEM, and that operates or directs the operations of the TRANSMISSION FACILITIES. Transmission Owner The entity that owns and maintains TRANSMISSION FACILITIES. Transmission Overload A state where a TRANSMISSION line has exceeded either a normal or EMERGENCY RATING of the electric CONDUCTOR. Transmission Planner The entity that develops a long-term (generally one year and beyond) plan for the RELIABILITY (ADEQUACY) of the interconnected BULK ELECTRIC TRANSMISSION SYSTEMS within its portion of the PLANNING AUTHORITY AREA. Transmission Reliability Margin (TRM) The amount of TRANSMISSION TRANSFER CAPABILITY necessary to provide reasonable assurance that the interconnected TRANSMISSION network will be secure. TRM accounts for the inherent uncertainty in SYSTEM conditions and the need for operating flexibility to ensure reliable SYSTEM operation as SYSTEM conditions change. Transmission Service Services provided to the TRANSMISSION CUSTOMER by the TRANSMISSION SERVICE PROVIDER to move ENERGY from a POINT OF RECEIPT to a POINT OF DELIVERY. Transmission Service Provider (TSP) The entity that administers the TRANSMISSION tariff and provides TRANSMISSION SERVICE to TRANSMISSION CUSTOMERS under applicable TRANSMISSION SERVICE AGREEMENTs. Traveling Wave ENERGY can be viewed as a wave traveling through space. When a DISTURBANCE occurs in the POWER SYSTEM the DISTURBANCE can be viewed as a wavefront of CURRENT and VOLTAGE propagating out from the DISTURBANCE point. A FREQUENCY DISTURBANCE can also be viewed in terms of a FREQUENCY DEVIATION propagating out from the DISTURBANCE point in the shape of a wave. TRE Acronym for the Texas Reliability Entity. TRE is one of the eight Regional Reliability Organizations. Trip The opening of a CIRCUIT BREAKER or breakers on an electric SYSTEM, normally to electrically isolate a particular ELEMENT of the SYSTEM to prevent it from being damaged by FAULT CURRENT or other potentially damaging conditions. Triplen Harmonics The odd harmonic orders which are evenly divisible by the number three. For example, the 3rd, 9th, 15th, etc. are Triplen Harmonics. Turnine A rotating mechanical device. A turbine is rotated by the force of a working fluid. The working fluid is typically steam, water, or combustion gas. Turbine Follow A MODE of operation of a fossil unit's TURBINE/BOILER CONTROL SYSTEM in which the unit's response to a LOAD (MW) change request is delayed if the BOILER's temperature and/or pressure moves outside set boundaries. The MW response of the unit's TURBINE "follows" the response of the BOILER. Turning Gear A method of rotating the shaft of a horizontally mounted TURBINE/GENERATOR. The turning gear may rotate the shaft at 2 RPM to prevent shaft warpage. Turns Ratio The ratio of the number of turns in the PRIMARY WINDING of a TRANSFORMER to the number of turns in the SECONDARY WINDING. Twelve Pulse Converter An HVDC CONVERTER that has twelve pulses to the DC output VOLTAGE for each CYCLE of the AC supply VOLTAGE. Unbundling Separating ELECTRIC UTILITY service into its basic components and offering each component separately for sale with separate rates for each component. Under Load Tap Changer (ULTC) A tap changer that is designed to change the TURNS RATIO when the TRANSFORMER has CURRENT flow across its windings. The term load tap changer (LTC) is also used to refer to this type tap changer. Underexcite A GENERATOR is underexcited when the applied excitation is less than that needed to support the GENERATOR's MAGNETIC FIELD. The deficiency in the REACTIVE POWER needs of the GENERATOR is supplied by the SYSTEM. An underexcited GENERATOR absorbs REACTIVE POWER from the SYSTEM. The terms "LEADING" and "bucking" are also used to refer to an underexcited GENERATOR. Under Frequency Load Shedding (UFLS) The tripping of customer LOAD based on magnitudes of SYSTEM FREQUENCY. For example, a utility may dump 5% of their connected LOAD if FREQUENCY falls below 59.3 HZ, dump an additional 10% if FREQUENCY falls below 58.9 HZ, and dump a final 10% if FREQUENCY falls below 58.5 HZ. These three steps of LOAD SHEDDING would form this utilities UFLS plan. The purpose of UFLS is a final effort to arrest a FREQUENCY decline. Under Voltage Load Shedding (UVLS) The tripping of customer LOAD based on magnitudes of SYSTEM VOLTAGE. For example, a utility may dump 5% of their connected LOAD if VOLTAGE falls below 92% of NOMINAL and an additional 10% of their LOAD if VOLTAGE falls below 90% of NOMINAL. These two steps of LOAD SHEDDING would form this utilities UVLS plan. The purpose of UVLS is typically to avoid a VOLTAGE COLLAPSE. Unilateral Payback INADVERTENT INTERCHANGE accumulations paid back unilaterally controlling to a target of nonzero ACE. Controlling to a non-zero ACE ensures that the unilateral payback is accounted for in the CPS calculations. The unilateral payback control offset is limited to BALANCING AUTHORITY'S L10 limit and shall not burden the INTERCONNECTION. Unit Commitment The process of selecting which generating units will be placed on line to serve the LOAD and reserve requirements. Universal Coordained Time (UTC) Time standard used by the Electronic Tagging System for communication purposes; also referred to as Greenwich Mean Time (GMT). Unscheduled Power Flow POWER does not flow where it is SCHEDULED but rather according to the relative IMPEDANCE of the available paths. When POWER is SCHEDULED to flow from SYSTEM "A" to SYSTEM "B" some of the POWER may flow through an adjoining SYSTEM "C". The POWER that flows through SYSTEM "C" is called unscheduled POWER flow. Also called PARALLEL PATH FLOW or LOOP FLOW. Valve General name given to a mercury-ARC or THYRISTER based device that is used to conduct CURRENT when a control signal is applied. A valve is turned on by a gate or GRID pulse. When turned on a valve conducts CURRENT. When turned off a valve blocks the flow of CURRENT. A valve is turned off by removing the forward biased VOLTAGE and stopping the CURRENT flow. POWER CONVERTERS are composed of several valves connected in different arrangements depending on the converter design. Vectors Line segments that are used to represent the magnitude and direction of physical quantities. DC VOLTAGES and CURRENTS can be represented by vectors. Viscosity A property of a fluid that tends to prevent the fluid from flowing when subjected to an applied force. High-viscosity fluids resist flow; low-viscosity fluids flow easily. Voltage (V) The electrical force (a separation of CHARGE) that causes CURRENT to flow. Symbol is "V" and units are volts. Voltage Angle The PHASE ANGLE between two VOLTAGES. The VOLTAGE angle is the same as the POWER ANGLE. (In a GENERATOR the VOLTAGE angle between the internal GENERATOR VOLTAGE and the STATOR terminal VOLTAGE is the equivalent of the TORQUE ANGLE.) Voltage Collapse A process in which a VOLTAGE unstable SYSTEM experiences an uncontrollable reduction in SYSTEM VOLTAGE. Voltage Control The control of TRANSMISSION VOLTAGE through adjustments in GENERATOR reactive output and TRANSFORMER TAPS, and by switching CAPACITORs and inductors on the TRANSMISSION and DISTRIBUTION SYSTEMS. Voltage Limits A hard limit above or below which is an undesirable operating condition. Normal limits are between 95 and 105 percent of the NOMINAL VOLTAGE at the BUS under discussion. Voltage Ratio The ratio between the primary and secondary VOLTAGES of a TRANSFORMER. There is a strong (but not identical) relationship between a TRANSFORMER's voltage ratio and its TURNS RATIO. Voltage Reduction A procedure designed to deliberately lower the VOLTAGE at a BUS. It is often used as a means to reduce DEMAND by lowering the customer's VOLTAGE. Voltage Regulator A component of an EXCITATION SYSTEM. The voltage regulator monitors the GENERATOR's output VOLTAGE and causes an adjustment in excitation CURRENT when required. Voltage regulators can be operated in either a manual or automatic MODE. Voltage Relay A PROTECTIVE RELAY that activates in response to either a high (59) or low (27) VOLTAGE. Voltage Source A device capable of producing a VOLTAGE. GENERATORS and batteries are voltage sources. Voltage Stability The ability of a POWER SYSTEM to maintain VOLTAGE so that when the SYSTEM NOMINAL LOAD is increased the actual POWER transferred to that LOAD will increase. In a VOLTAGE stable POWER SYSTEM the POWER transfer and the SYSTEM VOLTAGES are controllable by the SYSTEM OPERATORS. V-Q Curve A VOLTAGE versus REACTIVE POWER curve. A plot of the VOLTAGE at a BUS versus the REACTIVE POWER injected into that BUS. V-Q curves are a graphical tool used to analyze a POWER SYSTEM's VOLTAGE STABILITY. Watthour (Wh) A VOLTAGE versus REACTIVE POWER curve. A plot of the VOLTAGE at a BUS versus the REACTIVE POWER injected into that BUS. V-Q curves are a graphical tool used to analyze a POWER SYSTEM's VOLTAGE STABILITY. WECC Acronym for the Western Electricity Coordinating Council. WECC is one of the eight NERC REGIONAL RELIABILITY ORGANIZATIONS. Wheeling TRANSMISSION line usage which a TRANSMISSION line owner agrees to provide to permit the transfer of CAPACITY and ENERGY by another party. Wheeling Charges Payments required for providing WHEELING services. Wicket Gate Adjustable ELEMENTs that control the flow of water to the TURBINE of a hydro-electric FACILITY. Wide Area The entire RELIABILITY COORDINATOR AREA as well as the critical flow and status information from adjacent RELIABILITY COORDINATOR AREAS as determined by detailed SYSTEM studies to allow the calculation of Interconnected RELIABILITY OPERATING LIMITS. Wye Connection A 3Φ electrical connection in which one end of each of the three PHASES is connected to a common point. The common point is often GROUNDED. TRANSFORMER windings are often connected in a Wye configuration. Zone of Protection The zone or area within which a PROTECTIVE RELAY can sense abnormal conditions.