外文翻译--大型发电机干扰下发电机保护的性能(编辑修改稿)

外文翻译--大型发电机干扰下发电机保护的性能(编辑修改稿)内容摘要：

now determine the electrical power delivered to the load, still defined by (1). This new electrical power must be larger than the mechanical power input by the turbine in order that the kiic 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 times help the internal voltage of the machine to increase rapidly, therefore increasing the new electric power and, thus, increasing the probability that the kiic emerge gained during the fault will be removed from the rotor. If this energy is not removed, the generator will lose synchronism an a subsequent trip will result In disturbances where short circuits depress the system voltage, prevail electrical power cannot fully be delivered through the transmission system. Transient stability bee a threat to the power system within a time frame of less than 1 s. During the short circuit, the 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 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 excitation 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 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 from 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 extended 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 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 loss of load together with the inability of available connected reactive power sinks to absorb reactive power generated by unregulated sources such as capacitor banks. Again, the supplementary controls of QEL and MEL may operate, and additional controls such as the volts per hertz or terminal voltage or stator current limners may also operate. The voltage deviations from normal levels may persist for extended durations so coordination of the supplementary controls with other protection systems over a long time frame, and possibly even in steady state, is important. Supplementary excitation controls such as line drop pensation and reactive power sharing are sometimes applied to maintain system voltage within tolerable limits. These controls must coordinate with system controls (such as reactive power equipment switching) that also regulate system voltage. Power system stabilizers (PSSs) are normally required to damp small system oscillations. PSSs must be in service and properly tuned, but are not normally coordinated with any generator protection systems. It is evident from the above review of excitation control systems that the AVR is a vital control system that should be in service at all times. In North America. regional reliability criteria require that system transmission operators have positive assurance that generator excitation controls are in service and that specified generator real and reactive power capability is available. Assurance of generator capability may require periodic testing of the controls to ensure their steadystate and dynamic coordination with protection systems. Assurance of control system status may require supplementary SCADA control points to provide real time information to operators. B. Turbine Governor Control： The major role of the turbine governor control is to maintain proper speed regulation and Load division for the generating units on the power system. Two types of control are used droop and frequency or isochronous (constant speed), depending on the units operation and control requirements. Droop (speed/load) control behaves with a characteristic that as load increases speed drops. With synchronous machines, their operation is locked at system frequency. Therefore, the droop governor bees a load controller. As load increases, the governor signals the governor valves to open to maintain the established speed setting and acmodate the additional system load. Governor droop control prevents one generator from trying to pick up the entire additional load. Important benefits are the。