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1、English Material:Performance of Generator Protection During Major System DisturbanceAbstract：Disturbance isan inherent part of easy power system during the transition from one steady-state operating condition to the next. Protective relays may experience abnormal operating conditions during this tra
2、nsient period. This paper reviews control actions drat play a part during the transition and provides technical guidance to the industry on the application and setting of generator protective relays that can operate during major system disturbances. Index Terms：AC generator excitation、ac generator p
3、rotection、 governors、power system control turbines.INTRODUCTION：Every power system is subject from time to time to transient disturbances primarily due to faults and/or switching of major load. Normally, the system adapts to a new steady-state condition with the help of generator excitation and turb
4、ine governor control systems. A variety of additional power system control schemes may also be used to help restore an acceptable new steady-state condition.One of the lessons learned from the past major disturbances is that some of the functions associated with generator protection may operate duri
5、ng these transient conditions. It is important for the relays to provide protection while optimizing their coordination to avoid undesirable operation during the system disturbances and thereby help preserve the integrity of the power grid.Brief descriptions of past power system disturbances as well
6、 as generator excitation, turbine governor, and power system controls are included in this paper. The balance of the paper discusses the generator protection functions that may operate during system disturbances. POWER SYSTEM DISTURBANCES：Power system disturbances are events that produce abnormal sy
7、stem conditions and the state of the system may change from normal to emergency. These disturbances can be classified into two groups-small and large. Large disturbances are a challenging problem for the utilities because of the size and complexity of the power system. Adequate protection and contro
8、l actions are required after a system disturbance to prevent further degradation of the system and restoration to the normal state.Many system disturbances can be attributed to tight operating margins and less redundancy in generation, transmission, and distribution capability. These are best addres
9、sed at the planning stage. A properly designed system is less vulnerable to large-scale disturbances. However, small disturbances cannot be eliminated due to the physical nature of the system. Overhead lines constitute a significant component of any power system and experience frequent faults that a
10、re caused by variety of reasons.Stable operation of a power system requires a continuous matching between energy supply to the prime movers and the electrical load on the system and an adequate reactive power support mechanism to maintain voltage within limits at different buses. These conditions ar
11、e not satisfied during faults and other disturbances. During a fault, the terminal voltage dips and power transfer through the faulted system are altered depending on, the type of fault. After successful clearing of the fault, the system adapts to a new steady-state condition. If the fault is not cl
12、eared before the critical fault clearing time, system instability will occur.Even the successful clearing of faults may sometimes lead to undesired relay operations because of line overloads, inadequate reactive power support, and an improper relay setting. These may, in turn, develop into large sys
13、tem disturbances due to cascading. Disoperation of relays may result in undesired and/or sympathy tripping leading to large system disturbances. Inadequate protection arrangements, such as the absence of bus-bar protection for critical system buses, may also result in system disturbances due to song
14、 fault clearing time by remote back-up protection. Loss of a large generator or a large block of load changes the system frequency and may alter the reactive power flow in the network. This requires control action, like under frequency load shedding, to restore the system frequency and voltage at di
15、fferent buses within limits.Performance of the generator excitation system and the turbine control system are important during a system disturbance. Coordination between these systems, system protection, and other control strategies are necessary to avoid system collapse. Transient and dynamic stabi
16、lity studies should be periodically conducted in order to develop adequate control and protection strategies.A. Excitation Control：The excitation system of a generator provides the energy for the magnetic field that keeps the generator in synchronism with the power system. In addition to maintaining
17、 the synchronism of the generator, the excitation system also affects the amount of reactive power that the generator may absorb or produce. If the terminal voltage is fixed, increasing the excitation power will increase the synchronizing torque of the machine and increase the reactive power output.
18、 Decreasing the excitation power will have the opposite effect and, in extreme cases, may result in loss of synchronism of the generator with the power system. If the generator is operating isolated from a power system, and there are no other reactive power sources controlling terminal voltage, incr
19、easing the level of excitation power will increase the generator terminal voltage and vice versa.There are a variety of control functions that can be applied to the excitation system, including automatic voltage regulation (AVR), constant power factor regulation, and constant reactive power regulati
20、on. The excitation system may also operate in manual control with no automatic regulation. All of the automatic control modes may have supplementary controls. These supplementary controls may ensure that even under automatic regulation of a primary parameter, the generator is always operated within
21、its capability limits. Supplementary controls may also enhance the stable operation of the generator in parallel with a power system. Supplementary controls may include the following:1、maximum and/or minimum excitation level limits (OEL/MEL respectively, these limits may be tithe dependent);2、stator
22、 current limit to prevent stator thermal overload;3、volts per hertz limit to prevent equipment damage due to excessive flux levels;4、terminal voltage limit to prevent equipment damage due to excessive dielectric stress;5、line drop compensation to increase generator response to system voltage depress
23、ions;6、reactive power sharing controls for generators trying to regulate the same parameter;7、power system stabilizer to damp low-frequency oscillations;8、under excited limit (UEL) to protect against generator stator end-winding treating while operating in the under excited mode 15.The most commonly
24、 used control mode for generators of significant size that are connected to a power system is the AVR mode. In this mode, the excitation system helps to maintain power system voltage within acceptable limits by supplying or absorbing reactive power as required and also helps maintain synchronism of
25、the generator with the power system by increasing synchronizing torque when required.In stable steady state operation, a power system has an exact match of mechanical power delivered to generators and electrical power consumed by loads. Further; the voltage is regulated within narrow limits. Small d
26、isturbances resulting in. power or voltage oscillations are quickly damped. Frequency is maintained within acceptable limits by turbine governor controls and sometimes by system load control as noted in other sections of this paper. During Large disturbances, excitation controls act to maintain syst
27、em stability, For major disturbances. the terminal voltage change is sufficient that the output of the excitation is either full on, at ceiling, or full off. The AVR is the main control function in most cases, but the supplementary controls provide important features.Large system disturbances are ty
28、pically caused by short circuits of different types. The opening of appropriate high-speed breakers isolates the fault. During the fault, the terminal voltage dips and, in response, the exciter increases its output voltage to ceiling which causes the excitation current into the field to increase at
29、a rate determined by the voltage divided by the inductance of the field.System disturbances are also caused when a generation unit is suddenly dropped or when a breaker is suddenly opened and a load is dropped. When a unit is dropped, other nearby units pick up the load of the dropped unit and in re
30、sponse to the drop in mercurial voltage, the excitation of each of these units will increase When a breaker is suddenly opened and a load is dropped, the excitation will be reduced.The electrical powertransferred from the generator, an electric machine, to the load is given by the equation(A1.1)wher
31、e internal voltage and is proportional to the excitation current; load voltage; reactance between the generator and the load; delta, the angle that the internal voltage leads the load voltage.(A1.2)where mechanical turbine power of the generating unit; electromagnetic power out of the generating uni
32、t; accelerating power.The mechanical power is provided by the turbine and the average mechanical power must be equal to the average electrical power. When a system disturbance occurs, there is a change in one of the parameters of the electrical power equation. For faults, typically, the reactance be
33、tween the generator and the load()，the load voltage()，or some combination of these two parameters causes the electrical power to change. For example, for a short circuit, the load voltage is reduced; for a breaker opening, the reactance increases. When a generation unit trips offline, the required e
34、lectrical power from the remaining generators increases.In both cases, the instantaneous mechanical power provided by the turbine is no longer equal to the instantaneous electrical power delivered or required by the load. This difference must be accounted for.For a short time after a disturbance, th
35、e turbine control will not have much affect on the turbine power and the rotor will either absorb or provide the required transient energy. In the case of a fault, the energy absorbed by the rotor increases its angular velocity. When the Load on a unit is suddenly increased, the energy furnished by
36、the rotor results in a decrease in the rotor angular velocity. The exciter will respond to these disturbances based on terminal voltage measured.To understand what is happening, let us consider the example of a three-phase solid fault at the load. The load voltage shorted (), and the reactance betwe
37、en the generate and the load () is unchanged. From (1), the electrical power during the fault is zero. Since the turbine control cannot it extraneously reduce its power output, the power that was previously input to the load now accelerates the combine rotating mass of both the generator and turbine
38、 rotors see (2) This causes angle delta to increase. The excitation, in response to the reduced terminal voltage, increases its voltage output to ceiling causing the internal voltage (E) of the generator to increase at a rate determined by the operating time constant of the field and the ceiling vol
39、tage. Assuming that a generator trip has not occurred, when the fault clears and the load voltage is restored. the new internal voltage and the new delta now determine the electrical power delivered to the load, still defined by (1). This new electrical power must be larger than the mechanical power
40、 input by the turbine in order that the kinetic energy gained by the rotor during the fault is removes If the new electrical power is less than the mechanical power the rotor will continue to accelerate and the generator will lose synchronism. Exciters with high ceiling voltages and fast response ti
41、mes help the internal voltage of the machine to increase rapidly, therefore increasing the new electric power and, thus, increasing the probability that the kinetic emerge gained during the fault will be removed from the rotor. If this energy is not removed, the generator will lose synchronism an a
42、subsequent trip will resultIn disturbances where short circuits depress the system voltage, prevail electrical power cannot fully be delivered through the transmission system. Transient stability become a threat to the power system within a time frame of less than 1 s. During the short circuit, the
43、generator rotor accelerate due to mismatch of the reduced electrical power output with the constant mechanical power input (in the transient time frame before the turbine governor control can react). Fast response of the AVR and excitation system is important to increase the synchronizing torque to
44、allow the generator to remain in synchronism with the system. After the short circuit has been cleared, the resulting oscillations of the generator rotor speed with respect to the system frequency will cause the terminal voltage to fluctuate above and below the AVR set point. Supplementary excitatio
45、n controls may be called upon to prevent the AVR from imposing unacceptable condition upon tire generator. The supplementary controls in this case are usually maximum and minimum excitation limners. The over excitation limiter (OEL) prevents the AVR from trying to supply more excitation current than
46、 the excitation system Can supply or the generator field can withstand. The OEL must limit excitation current before the exciter system short circuit or overload protection operates and before the generator field overload protection operates. The minimum excitation limiter (MEL) prevents the AVR fro
47、m reducing excitation to such low level that the generator is in danger of losing synchronism The MEL must prevent reduction of current to a level when the generator loss of field protection may operate. UEL protect against generator stator end winding heating during under excited operation.In exten
48、ded disturbances beyond the transient stability time frame, the AVR again Lies to regulate voltage, but in this case it will attempt to steadily increase or reduce excitation to regulate voltage. Periodic oscillations are not evident as in the case of challenges to transient stability and the system
49、 stress may persist for periods of up to tens of seconds or even longer. Prolonged low voltages may result from loss of important transmission capability or loss of important sources of reactive power support. These transmission and generation low voltages may be exacerbated over a long time frame as system controls, such as distribution voltage regulators, attempt to maintain distribution voltage levels. Prolonged high voltages may result in the case of sudden
