GB/T 7409 "Excitation Systems for Synchronous Machines" comprises three parts:
——Part 1: GB/T 7409.1 "Excitation Systems for Synchronous Machines-Definitions";
——Part 2: GB/T 7409.2 "Excitation Systems for Synchronous Machines-Models for Power System Studies";
——Part 3: GB/T 7409.3 "Excitation Systems for Synchronous Machines-Technical Requirements of Excitation System for Large and Medium Synchronous Generators"
This part is the second part of GB/T "Excitation Systems for Synchronous Machines", which was firstly developed in 1987 and firstly revised in 1997, and this edition is the section revision.
GB/T 7409.2-1997 is identical to IEC 60034-16-2: 1991 "Rotating Electrical Machines--Part 16: Excitation Systems for Synchronous Machines--Chapter 2: Models for Power System Studies".
This part was revised by amending IEC 60034-16-2: 1991. This part has made reference to the domestic existing realistic model of generator excited system, the domestic current generator excited system computational model used in the stability analysis of power system and the standard IEEE Std.421.5, proposes the general and practical generator excited system computational model that can meet the requirements of the stability analysis of power system.
There have been some significant changes in this part over GB/T 7409.2-1997 as follows:
——The model of excitation system is described specifically, the established model for generator excited system is able to meet the requirement for the main domestic generator excited system to have power system stability analysis;
——The regulation link and the model for power system stabilizer are supplemented;
——The mode of the limiter and power system stabilizer to act on voltage regulator is described.
Appendix E provides the summary list of a comparison between this part and the clauses of IEC 60034-16-2: 1991-02, Appendix F provides the summary list of the technical differences of this part and IEC 60034-16-2: 1991-02 and the relevant reasons for your guidance.
Appendixes A, B, C and D in this part are normative and Appendixes and F are informative.
This part was proposed by China Electrical Equipment Industry Association.
This standard is under the jurisdiction of National Technical Committee 26 on Electric Rotating Machinery of Standardization Administration of China.
Zhejiang Electric Power Test & Research Institute is in charge of the drafting of this parts, and China Electric Power Research Institute, Harbin electric machinery Co., Ltd.(HEC), North China Electric Power Research Institute Co. Ltd., Shanghai Turbine Generator Co., Ltd., Dongfang Electric Machinery Co., Ltd., Nanjing Automation Research Institute, Guangzhou Electric Apparatus Research Institute, Shandong Jinan Power Equipment Factory, Beijing BEIZHONG Steam Turbine Generator Co., Ltd., Hydropower and Water Resources Planning and Design General Institute and other organizations participate in drafting this part.
Chief drafting staffs of this part: Zhu Shizhang, Liu Zenghuang, Li Guoliang, SuWeimin, Xu Fuan, Wu Tao, Liu Mingxing, Wang Dawei, Lv Hongshui, Xu Jingtao, Yin Guoji, Zhang Yuhua, Liu Guoyang, Pu Jun and Chen Xinqi.
The previous editions of the standards replaced by this part are as:
——GB 7409—1987;
——GB/T 7409.2—1997.
Introduction
When the behaviour of synchronous machines is to be accurately simulated in power system stability studies, the excitation system of these machines shall be modeled adequately. Since expenditure of data acquisition, programming and computation has to be limited in so far as is permissible, it is necessary to use simplified models that provide reasonable accuracy. The models shall adequately represent the actual excitation system performance:
——During the steady-state conditions prior to occurrence of the fault studied;
——During the time interval from application to clearing of fault;
——During the oscillations following fault clearing.
The excitation modeling does not account for the frequency deviations. It is assumed that in stability studies, the frequency deviations of up to ±5% from the rated frequency can be neglected as far as the excitation system is concerned.
The excitation system models shall be valid for the steady-state conditions and for the natural oscillation frequency of the synchronous machine. The oscillation frequency range to be covered will typically be from 0Hz to 3Hz.
The operation of protective functions and field discharge or overvoltage suppression equipment is beyond the scope of these models.
The excitation system modeling methods and standard models may also be used for studies of other dynamical problems regarding synchronous machines, for example, studies of out-of-step operation, sub-synchronous resonance or shaft torsional effects. However, the models shall then be checked to determine their suitability for that purpose.
The general functional block diagram in Figure 1 indicates the various excitation system components which have to be considered in the power system stability studies. These components include:
——Voltage control elements;
——Limiters;
——Power system stabilizer (if used);
——Exciter.
The main distinctive feature of an exciting power unit is the manner in which the exciting power is supplied and converted.
Figure 1 General Functional Block Diagram of Excitation Systems (within the dotted block) for Synchronous Machines
Excitation Systems for Synchronous Machines –
Models for Power System Studies
同步電機勵磁系統 電力系統研究用模型
1 Scope
The excitation system simulation block diagram and the corresponding mathematical models, as well as the terms and definitions of parameters and variables included thereinto specified in this part of GB/T 7409 apply to the power system stability studies.
2 Normative References
The following standards contain provisions which, through reference in this part of GB/T 7409, constitute provisions o this part. For dated reference, subsequent amendments to (excluding correction to), or revisions of, any of these publications do not apply. However, the parties whose enter into agreement according to these specifications are encouraged to research whether the latest editions of these labels are applied or not. For undated references, the latest editions of the normative documents are applicable to this part.
GB/T 7409.1-2008 ""Excitation Systems for Synchronous Machines-Definitions" [IEC 60034-16-1: 1991; MOD]
3 Exciter Categories: Graphical Representation and Mathematical Models for Stability Studies
3.1 DC Exciter
Although not frequently used on new machines in recent years, DC exciters are considered because many synchronous machines presently in service are equipped with this type of exciter. Figure 2 shows a graphical representation of the type with one separately excited field winding and Figure 3 shows the corresponding model of this exciter. The term KE has been introduced in model to account for the characteristics of exciters having self-excitation. Note: KE=1 in case of separately-excited exciters.
Figure 2 Exciter of DC Exciter with One Separately-excited Winding
Figure 3 Model Corresponding to Figure 2
Excitation control adopts with mechanical, electromagnetic and electronic control devices.
Considering the winding percentage and importance of units equipped with DC exciters, the simple model of Figure 3 shall prove adequate for these cases.
3.2 AC Exciter
AC exciters employ an AC exciter with static or rotating rectifier to produce the field current for the synchronous machines. The rectifiers may be controlled or uncontrolled. In case of uncontrolled rectifiers, control is effected via one or one field windings of the AC exciter.
It is essential to know the source of supply for the AC exciter field current in order to simulate this exciter. This power supply may be an auxiliary exciter or a potential or compound static power supply.
Figure 4 shows the graphical representation of an AC exciter with an uncontrolled stationary rectifier. The stationary rectifier is fed from the AC exciter and delivers DC current to the field winding of the synchronous generator via electrical brushes and slip-rings. The connection of the rotating field winding of the exciter to the excitation control equipment is also made by slip-rings and electrical brushes.
Figure 5 shows the graphical representation of an AC exciter (brushless exciter) with an uncontrolled rotating rectifier and permanent magnet auxiliary exciter for supply of the excitation control equipment. The rectifier rotates on a shaft common to the synchronous machine and the rotating armature of AC exciter. The output of rotating rectifier id connected without slip-rings or electrical brushes directly to the field winding of the synchronous machine.
Figure 4 AC Exciter with Uncontrolled Stationary Rectifier
Figure 5 Rotating Exciter with Uncontrolled Rotating Rectifier (Brushless Exciter)
The AC exciter can be modeled as shown in Figure 6. This model is used to account for both the steady-state and transient exciter loading effects (In certain cases, an even detailed model may be used to take in account the effects of transient loads).Depending on the data completeness of exciter, a model for exciter not representing the phase inversion function may be made, where XE is zero.
A simplified model for AC exciter is shown in Figure 7. Although it accounts only for the steady-state load effects by use of load saturation curve, it may be adequate for most studies. The use of the simplified model may also be indicated where complete data are not complete data are not available.
Figure 6 Detailed Model of an AC Exciter
Figure 7 Simplified Model of an AC Exciter
3.3 Potential Source Static Exciter
Potential source static exciters use rectifier transformers which can be supplied from an auxiliary generator mounted on the same shaft as the synchronous machine, from an auxiliary bus bar not independent on the main generator voltage or from the synchronous machine terminal voltage. The latter is called a self-shunt static exciter and the voltage variations of this self-shunt static excitation system shall be taken into account for the performance and modeling. Figure 8 shows the potential source static exciter. The mathematical models of potential source static exciter may be represented as Figure 9 or Figure 10.
Figure 8 Potential Source Static Exciter
The controlled rectifying device adopts fully controlled bridge or may also adopts the semi-controlled bridge with a half thyristor and a half diode. The output voltage is limited frequently by controlling the triggering corner, which is expressed with UP + and UP- . The route of semi-controlled bridge can not be inversed, and the value of UP- is equal to zero.
The most commonly-used controlled rectification bridge only allows the passage of field current in positive direction. When the disturbance at ends of synchronous machine causes negative field current, the mathematical model in Figure 9 thereby will be invalid, in this case, the voltage of the synchronous machine field winding will be no longer controlled by the regulator but depends on other factors, this is beyond this part.
Figure 9 Model of Potential Source Exciter (I)
Figure 10 Model of Potential Source Exciter (II)
Only in particular cases where the equipment is required to allow the passage of positive and negative field current, the mathematical model shown in Figure 9 will be valid.
3.4 Compound Source Static Exciter
Compound source static exciters use rectifier transformers supplied from both current and voltage sources (from synchronous machines). There are a number of designs possibilities, including: current source and voltage source are of parallel connection at the DC side, in series at the DC side, of parallel connection at the AC side and in series at the AC side, and others. The compound source static exciters are rarely used, and those in series at the AC side is illustrated only.
Figure 11 illustrates the concept of addition of voltage from two sources in series on the DC side of the rectifier. The reactors with air space will convert the current source into voltage source, or the current source transformers with air space may be used to directly convert the current source into voltage source. The corresponding model of which is given in Figure 12.
Figure 11 Compound Source Static Exciter with Addition of Voltages in Series on the AC Side
Figure 12 Model Corresponding to Figure 11
3.5 Mathematical Models for the Control Function
A considerable part of the effort required to prepare large scale system stability studies is in collecting and determining date for the mathematical model related to this system study. The use of simplified mathematical models to reduce this effort may be restricted sometimes. For example, the use of simplified model will bring many difficulties when studies extend beyond the first rotor angle swing as the determination of the power system stability of modern power grid generally requires the simulation to be continued for many seconds and many swing oscillations. Thus simplification will be ruled out in some instances.
3.5.1 Models for voltage measurement and load current compensation units
Generally, modeling of generator terminal voltage sending is common to all voltage regulators. Figure 13 shows voltage sensing combined with load current compensation on the AC side. In this case the input variables (generator voltage and current) are entered in phasor form and the resulting signal is then rectified. Load current compensation is normally used in one of the following forms:
Figure 13 Terminal Voltage Signal and Load Current Compensation
——When units are paralleled with no impedance between them, the current compensation is used to create an artificial coupling impedance so that the units will share reactive power reasonably. For this case, Xc shall have a positive value.
——When a single unit is connected through a significant impedance to the system, or when two or more units 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. For these cases, Rc and Xc take on negative values.
In most cases of load current compensation, the Rc component is negligible and only a value of Xc is required. In this case, it is sufficient to reduce the load current influence to the reactive component, the element with this reducing function is designated as reactive current compensator.
When compensator is not employed, only the filter for the rectified terminal voltage remains in Figure 13. While the filtering link may be complex, for modeling purposes, it can usually be reduced to the single time constant. For many systems, this time constant is rather small and provision shall be made set it to zero
The terminal voltage after adding load compensator influence and filtering is compared with a reference which represents the desired terminal voltage setting value. The equivalent voltage regulator reference signal, UREF, is chosen to satisfy the initial operating conditions.
When compensator is sued, it has to be noted that it may add positive or negative damping in case of power oscillations.
3.5.2 Models for regulation link
The regulation link of excitation control realizes the field adjustment and stable control functions. Generally, the regulation link includes the following several forms: serial PID regulation link, parallel PID regulation link, transient feedback regulation link and exciter time constant compensation link. Moreover, several types of regulation links may be combined.
a) Serial PID regulation link
Model for the serial PID regulation link is given in Figure 14. This regulation link is composed of two stages of lead-lag links when Kv is set to be 1.The regulation link has pure integrating element and realizes isochronous control when Kv is set to be zero.
Figure 14 Serial PID Regulation Link
b) Parallel PID regulation link
Model for the parallel PID regulation link is given in Figure 15.
Figure 15 Parallel PID Regulation Link
c) Transient feedback regulation link
Model for the transient feedback regulation link is given in Figure 16. The input signal of transient feedback link is the output Ur of regulator in the static excitation system and may be the current signal Uie of exciter field in the exciter excitation system, or the voltage Uf of generator field. The output signal of the transient feedback link is added to the addition point of voltage or the output of PID regulation link.
Figure 16 Transient Feedback Regulation Link
d) Exciter time constant compensation link
The exciter time constant compensation link is used to reduce the equivalent time constant of exciter. The input signal of this link is the generator field voltage signal or exciter field current signal, and is fed back to the output of PID regulation link, as shown in Figure 17.
Figure 17 Exciter Time Constant Compensation Link
3.5.3 Limitation
Attentions shall be paid to separate the “wind-up” limitation and “non-wind-up” limitation. Representations of “wind-up” limitation and “non-wind-up” limitation are given in Appendix D.
3.5.4 Models for power system stabilizers
Generally, the input signals of power system stabilizer include the generator active power, frequency of machine terminal voltage, generator speed or their combination. The power system stabilizer may be used to operating conditions of generator and motor, however, the parameters shall be calibrated respectively.
Generally, the output signals of power system stabilizer are superposed to the voltage addition point of voltage regulator. The output quantity of power system stabilizer superposed to the voltage addition point has the same reference value as the generator voltage. The reference value of the output quantity of power system stabilizer superposed to other points is the reference value of that superposed to the voltage addition point multiplied by the dynamic gain from the voltage addition point to the delivery phase addition point of power system stabilizer under the oscillation frequency requiring key suppression. The input quantity of power system stabilizer has the same reference value as generator.
a) Model for single input signal power system stabilizer——PSS1 Type
PSSl type model for single input signal power system stabilizer comprises signal measuring link, two-stage separate straight link, shaft torsional oscillation filter, three-stage lead-lag link, gain adjustment link and output limitation link, as shown in Figure 18.The Input signal may be generator active power machine terminal voltage frequency or generator speed.
Contents
Foreword I
Introduction III
1 Scope
2 Normative References
3 Exciter Categories: Graphical Representation and Mathematical Models for Stability Studies
3.1 DC Exciter
3.2 AC Exciter
3.3 Potential Source Static Exciter
3.4 Compound Source Static Exciter
3.5 Mathematical Models for the Control Function
3.6 Model of Excitation System
4 Nomenclature
4.1 Parameters
4.2 Variables
Appendix A (Normative) Per Unit System
Appendix B (Normative) Rectifier Regulation Characteristics
Appendix C (Normative) Saturation Function
Appendix D (Normative) Representation of Limits
Appendix E (Informative) Comparison Between Clauses of This Part and IEC 60034-16-2: 1991-02 By Numbers
Appendix F (Informative) Technical Differences of This Part over IEC 60034-16-2: 1991-02 and the Reasons
