1、English Material: Performance of Generator Protection During Major System Disturbance Abstract： Disturbance is an 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
2、 transient 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 gener
3、ator protection、 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 excitatio
4、n and turbine 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 o
5、perate during 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 disturba
6、nces as well 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 prod
7、uce abnormal system 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 protec
8、tion and control 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
9、 are best addressed 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 freque
10、nt faults that are 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. Th
11、ese conditions are 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 th
12、e fault is not cleared 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, deve
13、lop into large system 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 distur
14、bances due to song 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
15、 and voltage at different 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. Transien
16、t and dynamic stability 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 ad
17、dition to maintaining 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 r
18、eactive power output. 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 t
19、erminal voltage, increasing 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 r
20、eactive power regulation. 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 a
21、lways operated within 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 t
22、ithe dependent)； 2、 stator 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 re
23、sponse to system voltage depressions； 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 u
24、nder excited mode 15. The most commonly 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
25、and also helps maintain synchronism of 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
26、 regulated within narrow limits. Small disturbances 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,
27、excitation controls act to maintain system 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 f
28、eatures. Large system disturbances are typically 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 a rate determined by the voltage divided by the
