IEEE Recommended Practice for Excitation System Models for Power System Stability Studies 1.Overview 1.1 Scope When the behavior of synchronous machines is to be simulated accurately in power system stability studies, it is essential that the excitation systems of the synchronous machines be modeled in sufficient detail(see Byerly and Kimbark [B7]).The desired models must be suitable for representing the actual excitation equipment performance for large,severe disturbances as well as for small perturbations. A 1968 IEEE Committee Report (see [B18])provided initial excitation system reference models.It established a common nomenclature,presented mathematical models for excitation systems then in common use,and defined parameters for those models.A 1981 report(see IEEE Committee Report [B20])extended that work.It provided models for newer types of excitation equipment not covered previously as well as improved models for older equipment. This document,based heavily on the 1981 report,is intended to again update the models,provide models for additional control features,and formalize those models in a recommended practice.To some extent,the model structures presented in this document are intended to facilitate the use of field test data as a means of obtaining model parameters.The models are,however,reduced order models,and they do not represent all of the control loops on any particular system.In some cases,the model used may represent a substantial reduction,resulting in large differences between the structure of the model and the physical system. The excitation system models themselves do not allow for regulator modulation as a function of system frequency,an inherent characteristic of some older excitation systems.The models are valid for frequency deviations of+5%from rated frequency and oscillation frequencies up to about 3 Hz.These models would not normally be adequate for use in studies of subsynchronous resonance or other shaft torsional interaction problems.Delayed protective and control functions that may come into play in long-term dynamic performance studies are not represented.See additional information in Annex F. Where possible,the supplied models are referenced to commercial equipment and vendor names shown in Annex I.This information is given for the convenience of users of this recommended practice and does not IThe numbers in brackets correspond to those of the bibliography in Annex J. Copyright 2006 IEEE.All rights reserved. 1
Copyright © 2006 IEEE. All rights reserved. 1 IEEE Recommended Practice for Excitation System Models for Power System Stability Studies 1. Overview 1.1 Scope When the behavior of synchronous machines is to be simulated accurately in power system stability studies, it is essential that the excitation systems of the synchronous machines be modeled in sufficient detail (see Byerly and Kimbark [B7]1). The desired models must be suitable for representing the actual excitation equipment performance for large, severe disturbances as well as for small perturbations. A 1968 IEEE Committee Report (see [B18]) provided initial excitation system reference models. It established a common nomenclature, presented mathematical models for excitation systems then in common use, and defined parameters for those models. A 1981 report (see IEEE Committee Report [B20]) extended that work. It provided models for newer types of excitation equipment not covered previously as well as improved models for older equipment. This document, based heavily on the 1981 report, is intended to again update the models, provide models for additional control features, and formalize those models in a recommended practice. To some extent, the model structures presented in this document are intended to facilitate the use of field test data as a means of obtaining model parameters. The models are, however, reduced order models, and they do not represent all of the control loops on any particular system. In some cases, the model used may represent a substantial reduction, resulting in large differences between the structure of the model and the physical system. The excitation system models themselves do not allow for regulator modulation as a function of system frequency, an inherent characteristic of some older excitation systems. The models are valid for frequency deviations of ±5% from rated frequency and oscillation frequencies up to about 3 Hz. These models would not normally be adequate for use in studies of subsynchronous resonance or other shaft torsional interaction problems. Delayed protective and control functions that may come into play in long-term dynamic performance studies are not represented. See additional information in Annex F. Where possible, the supplied models are referenced to commercial equipment and vendor names shown in Annex I. This information is given for the convenience of users of this recommended practice and does not 1 The numbers in brackets correspond to those of the bibliography in Annex J
IEEE Std421.5-2005 IEEE STANDARD constitute an endorsement by the IEEE of these products.The models thus referenced may be appropriate for equivalent excitation systems supplied by other manufacturers. A sample set of data(not necessarily typical)for each of the models,for at least one particular application,is provided in Annex H.A suffix"A"is used for the designation of models introduced or modified in IEEE Std 421.5-1992,and a suffix"B"is used for models introduced or modified in this latest recommended practice, IEEE Std 421.5-2005. Modeling work outside of the IEEE is documented in IEC 60034-16:1991 [B17].Additional background is found in IEEE Committee Report [B19]. 2.Normative references The following referenced documents are indispensable for the application of this document.For dated references,only the edition cited applies.For undated references,the latest edition of the referenced document (including any amendments or corrigenda)applies. ANSI C50.10 American National Standard for Rotating Electrical Machinery-Synchronous Machines.2 IEEE Std 115TM,IEEE Guide:Test Procedures for Synchronous Machines-Part I:Acceptance and Performance Testing;Part II:Test Procedures and Parameter Determination for Dynamic Analysis.3,4 IEEE Std 421.1TM,IEEE Definitions for Excitation Systems for Synchronous Machines. IEEE Std 421.2TM,IEEE Guide for Identification,Testing,and Evaluation of the Dynamic Performance of Excitation Control Systems IEEE Std 421.3TM,IEEE Standard for High Potential-Test Requirements for Excitation Systems for Synchronous Machines. IEEE Std 421.4TM,IEEE Guide for the Preparation of Excitation System Specifications. IEEE Std C50.13TM,IEEE Standard for Cylindrical-Rotor 50 Hz and 60 Hz,Synchronous Generators Rated 10 MVA and above. 3.Representation of synchronous machine excitation systems in power system studies The general functional block diagram shown in Figure 3-1 indicates various synchronous machine excitation subsystems.These subsystems may include a terminal voltage transducer and load compensator,excitation control elements,an exciter,and in many instances,a power system stabilizer (PSS).Supplementary discontinuous excitation control may also be employed.Models for all of these functions are presented in this recommended practice. 2ANSI publications are available from the Sales Department,American National Standards Institute,25 West 43rd Street,4th Floor, New York,NY 10036,USA (http://www.ansi.org/). IEEE publications are available from the Institute of Electrical and Electronics Engineers,Inc.,445 Hoes Lane,Piscataway,NJ 08854. USA (http://standards.ieee.org/). The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers,Inc. 2 Copyright 2006 IEEE.All rights reserved
IEEE Std 421.5-2005 IEEE STANDARD 2 Copyright © 2006 IEEE. All rights reserved. constitute an endorsement by the IEEE of these products. The models thus referenced may be appropriate for equivalent excitation systems supplied by other manufacturers. A sample set of data (not necessarily typical) for each of the models, for at least one particular application, is provided in Annex H. A suffix “A” is used for the designation of models introduced or modified in IEEE Std 421.5-1992, and a suffix “B” is used for models introduced or modified in this latest recommended practice, IEEE Std 421.5-2005. Modeling work outside of the IEEE is documented in IEC 60034-16:1991 [B17]. Additional background is found in IEEE Committee Report [B19]. 2. Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. ANSI C50.10 American National Standard for Rotating Electrical Machinery—Synchronous Machines.2 IEEE Std 115™, IEEE Guide: Test Procedures for Synchronous Machines—Part I: Acceptance and Performance Testing; Part II: Test Procedures and Parameter Determination for Dynamic Analysis.3, 4 IEEE Std 421.1™, IEEE Definitions for Excitation Systems for Synchronous Machines. IEEE Std 421.2™, IEEE Guide for Identification, Testing, and Evaluation of the Dynamic Performance of Excitation Control Systems. IEEE Std 421.3™, IEEE Standard for High Potential-Test Requirements for Excitation Systems for Synchronous Machines. IEEE Std 421.4™, IEEE Guide for the Preparation of Excitation System Specifications. IEEE Std C50.13™, IEEE Standard for Cylindrical-Rotor 50 Hz and 60 Hz, Synchronous Generators Rated 10 MVA and above. 3. Representation of synchronous machine excitation systems in power system studies The general functional block diagram shown in Figure 3-1 indicates various synchronous machine excitation subsystems. These subsystems may include a terminal voltage transducer and load compensator, excitation control elements, an exciter, and in many instances, a power system stabilizer (PSS). Supplementary discontinuous excitation control may also be employed. Models for all of these functions are presented in this recommended practice. 2 ANSI publications are available from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org/). 3 IEEE publications are available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 4 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc
IEEE FOR EXCITATION SYSTEM MODELS FOR POWER SYSTEM STABILITY STUDIES td421.5-2005 Vc TERMINAL VOLTAGE TRANSDUCER AND LOAD COMPENSATOR V VR FD EXCITATION EXCITER SYNCHRONOUS CONTROL MACHINE AND VRE ELEMENTS POWER SYSTEM POWER SYSTEM Vs STABILIZER AND Vs SUPPLEMENTARY DISCONTINUOUS EXCITATION CONTROLS Figure 3-1-General functional block diagram for synchronous machine excitation control system Excitation control elements include both excitation regulating and stabilizing functions.The terms excitation system stabilizer and transient gain reduction are used to describe circuits in several of the models encompassed by the excitation control elements shown in Figure 3-1 that affect the stability and response of those systems. Recently,modeling of field current limiters has become increasingly important,resulting in the addition to this recommended practice of Clause 9 and Clause 10 describing overexcitation and underexcitation limiters (OELs and UELs,respectively).The individual excitation system models in this document show how the output signals from such limiters(VoEL and VUEL)would normally be connected. The output of the UEL may be received as an input to the excitation system (VUEL)at various locations, either as a summing input or as a gated input,but for any one application of the model,only one of these inputs would be used. For the OEL some models provide a gate through which the output of the overexcitation limiter or terminal voltage limiter(VoEL)could enter the regulator loop. In the implementation of all of the models,provision should be made for handling zero values of parameters. For some zero values,it may be appropriate to bypass entire blocks of a model. The per unit(pu)system used for modeling the excitation system is described in Annex B. Three distinctive types of excitation systems are identified on the basis of excitation power source,as follows: a)Type DC excitation systems,which utilize a direct current generator with a commutator as the source of excitation system power(see Clause 5) b) Type AC excitation systems.which use an alternator and either stationary or rotating rectifiers to produce the direct current needed for the synchronous machine field(see Clause 6) c) Type ST excitation systems,in which excitation power is supplied through transformers or auxiliary generator windings and rectifiers(see Clause 7) The following key accessory functions common to most excitation systems are identified and described as follows: 1)Voltage sensing and load compensation(see Clause 4) 2) Power system stabilizer(see Clause 8) Copyright 2006 IEEE.All rights reserved. 3
IEEE FOR EXCITATION SYSTEM MODELS FOR POWER SYSTEM STABILITY STUDIES Std 421.5-2005 Copyright © 2006 IEEE. All rights reserved. 3 Excitation control elements include both excitation regulating and stabilizing functions. The terms excitation system stabilizer and transient gain reduction are used to describe circuits in several of the models encompassed by the excitation control elements shown in Figure 3-1 that affect the stability and response of those systems. Recently, modeling of field current limiters has become increasingly important, resulting in the addition to this recommended practice of Clause 9 and Clause 10 describing overexcitation and underexcitation limiters (OELs and UELs, respectively). The individual excitation system models in this document show how the output signals from such limiters (VOEL and VUEL) would normally be connected. The output of the UEL may be received as an input to the excitation system (VUEL) at various locations, either as a summing input or as a gated input, but for any one application of the model, only one of these inputs would be used. For the OEL some models provide a gate through which the output of the overexcitation limiter or terminal voltage limiter (VOEL) could enter the regulator loop. In the implementation of all of the models, provision should be made for handling zero values of parameters. For some zero values, it may be appropriate to bypass entire blocks of a model. The per unit (pu) system used for modeling the excitation system is described in Annex B. Three distinctive types of excitation systems are identified on the basis of excitation power source, as follows: a) Type DC excitation systems, which utilize a direct current generator with a commutator as the source of excitation system power (see Clause 5) b) Type AC excitation systems, which use an alternator and either stationary or rotating rectifiers to produce the direct current needed for the synchronous machine field (see Clause 6) c) Type ST excitation systems, in which excitation power is supplied through transformers or auxiliary generator windings and rectifiers (see Clause 7) The following key accessory functions common to most excitation systems are identified and described as follows: 1) Voltage sensing and load compensation (see Clause 4) 2) Power system stabilizer (see Clause 8) Figure 3-1—General functional block diagram for synchronous machine excitation control system
IEEE Std421.5-2005 IEEE STANDARD 3)Overexcitation limiter(see Clause 9) 4)Underexcitation limiter(see Clause 10) 5) Power factor and var control(see Clause 11) 6 Discontinuous excitation controls(see Clause 12) In addition,models for some supplementary discontinuous excitation controls are provided. Most excitation systems represented by the Type AC and ST models allow only positive current flow to the field of the machine,although some systems allow negative voltage forcing until the current decays to zero. Special provisions are made to allow the flow of negative field current when it is induced by the synchronous machine.Methods of accommodating this in the machine/excitation system interface for special studies are described in Annex G. 4.Synchronous machine terminal voltage transducer and current compensator models Several types of compensation are available on most excitation systems.Synchronous machine active and reactive current compensation are the most common.Either reactive droop compensation and/or line-drop compensation may be used,simulating an impedance drop and effectively regulating at some point other than the terminals of the machine.The impedance or range of adjustment and type of compensation should be specified. Droop compensation takes its name from the drooping (declining)voltage profile with increasing reactive power output on the unit.Line-drop compensation,also referred to as transformer-drop compensation, refers to the act of regulating voltage at a point partway within a generator's step-up transformer or,less frequently,somewhere along the transmission system.This form of compensation produces a rising voltage profile at the generator terminals for increases in reactive output power. A block diagram of the terminal voltage transducer and the load compensator is shown in Figure 4-1.These model elements are common to all excitation system models described in this document.It is realized that, for some systems,there may be separate and different time constants associated with the functions of voltage sensing and load compensation.The distinction is not recognized in this model,in which only one time constant,TR,is used for the combined voltage sensing and compensation signal.Single-phase voltage and current sensing will,in general,require a longer time constant in the sensing circuitry to eliminate ripple. When load compensation is not employed(Rc=Xc=0),the block diagram reduces to a simple sensing circuit.The terminal voltage of the synchronous machine is sensed and is usually reduced to a dc quantity. While the filtering associated with the voltage transducer may be complex,it can usually be reduced,for modeling purposes,to the single time constant TR shown.For many systems,this time constant is very small and provision should be made to set it to zero. C1 Vc1=|VT+(Rc+jXc)T斤 1 Vc 1+STR Figure 4-1-Terminal voltage transducer and optional load compensation elements 4 Copyright 2006 IEEE.All rights reserved
IEEE Std 421.5-2005 IEEE STANDARD 4 Copyright © 2006 IEEE. All rights reserved. 3) Overexcitation limiter (see Clause 9) 4) Underexcitation limiter (see Clause 10) 5) Power factor and var control (see Clause 11) 6) Discontinuous excitation controls (see Clause 12) In addition, models for some supplementary discontinuous excitation controls are provided. Most excitation systems represented by the Type AC and ST models allow only positive current flow to the field of the machine, although some systems allow negative voltage forcing until the current decays to zero. Special provisions are made to allow the flow of negative field current when it is induced by the synchronous machine. Methods of accommodating this in the machine/excitation system interface for special studies are described in Annex G. 4. Synchronous machine terminal voltage transducer and current compensator models Several types of compensation are available on most excitation systems. Synchronous machine active and reactive current compensation are the most common. Either reactive droop compensation and/or line-drop compensation may be used, simulating an impedance drop and effectively regulating at some point other than the terminals of the machine. The impedance or range of adjustment and type of compensation should be specified. Droop compensation takes its name from the drooping (declining) voltage profile with increasing reactive power output on the unit. Line-drop compensation, also referred to as transformer-drop compensation, refers to the act of regulating voltage at a point partway within a generator’s step-up transformer or, less frequently, somewhere along the transmission system. This form of compensation produces a rising voltage profile at the generator terminals for increases in reactive output power. A block diagram of the terminal voltage transducer and the load compensator is shown in Figure 4-1. These model elements are common to all excitation system models described in this document. It is realized that, for some systems, there may be separate and different time constants associated with the functions of voltage sensing and load compensation. The distinction is not recognized in this model, in which only one time constant, TR, is used for the combined voltage sensing and compensation signal. Single-phase voltage and current sensing will, in general, require a longer time constant in the sensing circuitry to eliminate ripple. When load compensation is not employed (RC = XC = 0), the block diagram reduces to a simple sensing circuit. The terminal voltage of the synchronous machine is sensed and is usually reduced to a dc quantity. While the filtering associated with the voltage transducer may be complex, it can usually be reduced, for modeling purposes, to the single time constant TR shown. For many systems, this time constant is very small and provision should be made to set it to zero. Figure 4-1—Terminal voltage transducer and optional load compensation elements
IEEE FOR EXCITATION SYSTEM MODELS FOR POWER SYSTEM STABILITY STUDIES td421.5-2005 The terminal voltage transducer output,VC,is compared with a reference that represents the desired terminal voltage setting,as shown on each of the excitation system models.The equivalent voltage regulator reference signal,VREF,is calculated to satisfy the initial operating conditions.It will,therefore,take on a value unique to the synchronous machine load condition being studied.The resulting error is amplified as described in the appropriate excitation system model to provide the field voltage and subsequent terminal voltage to satisfy the steady-state loop equations.Without load compensation,the excitation system,within its regulation characteristics,attempts to maintain a terminal voltage determined by the reference signal. When compensation is desired,the appropriate values of Rc and Xcare entered.In most cases,the value of Rc is negligible.The input variables of synchronous machine voltage and current must be in phasor form for the compensator calculation.Care must be taken to ensure that a consistent pu system is utilized for the compensator parameters and the synchronous machine current base. This type of compensation is normally used in one of the following two ways: a) When synchronous machines are bused together with no impedance between them,the compensator is used to create artificial coupling impedance so that the machines will share reactive power appro- priately.This corresponds to the choice of a regulating point within the synchronous machine.For this case,Rc and Xc would have positive values. b) When a single synchronous machine is connected through significant impedance to the system,or when two or more machines are connected through individual transformers,it may be desirable to regulate voltage at a point beyond the machine terminals.For example,it may be desirable to com- pensate for a portion of the transformer impedance and effectively regulate voltage at a point part way through the step-up transformer.For these cases,Rc and Xc would take on the appropriate neg- ative values. Some compensator circuits act to modify terminal voltage as a function of reactive and real power,instead of reactive and real components of current.Although the model provided will be equivalent to these circuits only near rated terminal voltage,more precise representation has not been deemed worthwhile.These and other forms of compensation are described in Rubenstein and Wakley [B39]. The automatic voltage regulator (AVR)feedback signal can include inputs from other synchronous machines where the machines are connected together on a low-voltage bus and share a common main output transformer.A general form of the AVR feedback signal for unit 1,VCl,is written as shown in Equation(1): Vc1=VT+(Rcn+jxcu)Ti+(Rcn2+jXc12)/T2 (1) VT ac voltage phasor common to both of the generators I ac current flow out of generator i RCij resistive component of compensation of generator i for current flow out of generator j Xcij reactive component of compensation of generator i for current flow out of generator j The subscripts identify the signals associated with each of the two generators.The first subscript indicates the unit to which the load compensation is connected,while the second subscript indicates the source of the current signal to the compensation.This is the general form of the single machine compensation found on all utility generators(i.e.,with RC12,XC12 to zero).A similar equation applies to the AVR input for the second unit with appropriate substitution of inputs and subscripts.This can be readily extended to more generators by including additional compensation terms. In practice,the resistive component of compensation is rarely required on generators synchronized to large grids over high-voltage interconnections.This component of compensation is not even available on some manufacturer's designs.To simplify analysis,the resistive component of compensation is assumed to be zero,and the current signals are resolved into two components as shown in Equation(2): Copyright 2006 IEEE.All rights reserved. 5
IEEE FOR EXCITATION SYSTEM MODELS FOR POWER SYSTEM STABILITY STUDIES Std 421.5-2005 Copyright © 2006 IEEE. All rights reserved. 5 The terminal voltage transducer output, VC, is compared with a reference that represents the desired terminal voltage setting, as shown on each of the excitation system models. The equivalent voltage regulator reference signal, VREF, is calculated to satisfy the initial operating conditions. It will, therefore, take on a value unique to the synchronous machine load condition being studied. The resulting error is amplified as described in the appropriate excitation system model to provide the field voltage and subsequent terminal voltage to satisfy the steady-state loop equations. Without load compensation, the excitation system, within its regulation characteristics, attempts to maintain a terminal voltage determined by the reference signal. When compensation is desired, the appropriate values of RC and XC are entered. In most cases, the value of RC is negligible. The input variables of synchronous machine voltage and current must be in phasor form for the compensator calculation. Care must be taken to ensure that a consistent pu system is utilized for the compensator parameters and the synchronous machine current base. This type of compensation is normally used in one of the following two ways: a) When synchronous machines are bused together with no impedance between them, the compensator is used to create artificial coupling impedance so that the machines will share reactive power appropriately. This corresponds to the choice of a regulating point within the synchronous machine. For this case, RC and XC would have positive values. b) When a single synchronous machine is connected through significant impedance to the system, or when two or more machines are connected through individual transformers, it may be desirable to regulate voltage at a point beyond the machine terminals. For example, it may be desirable to compensate for a portion of the transformer impedance and effectively regulate voltage at a point part way through the step-up transformer. For these cases, RC and XC would take on the appropriate negative values. Some compensator circuits act to modify terminal voltage as a function of reactive and real power, instead of reactive and real components of current. Although the model provided will be equivalent to these circuits only near rated terminal voltage, more precise representation has not been deemed worthwhile. These and other forms of compensation are described in Rubenstein and Wakley [B39]. The automatic voltage regulator (AVR) feedback signal can include inputs from other synchronous machines where the machines are connected together on a low-voltage bus and share a common main output transformer. A general form of the AVR feedback signal for unit 1, VC1, is written as shown in Equation (1): (1) VT = ac voltage phasor common to both of the generators ITi = ac current flow out of generator i RCij = resistive component of compensation of generator i for current flow out of generator j XCij = reactive component of compensation of generator i for current flow out of generator j The subscripts identify the signals associated with each of the two generators. The first subscript indicates the unit to which the load compensation is connected, while the second subscript indicates the source of the current signal to the compensation. This is the general form of the single machine compensation found on all utility generators (i.e., with RC12, XC12 to zero). A similar equation applies to the AVR input for the second unit with appropriate substitution of inputs and subscripts. This can be readily extended to more generators by including additional compensation terms. In practice, the resistive component of compensation is rarely required on generators synchronized to large grids over high-voltage interconnections. This component of compensation is not even available on some manufacturer’s designs. To simplify analysis, the resistive component of compensation is assumed to be zero, and the current signals are resolved into two components as shown in Equation (2): VC1 VT RC11 + jXC11 + ( )IT 1 RC12 + jXC12 ( )I = + T 2