【正文】 quired electrical 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, the 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 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 threephase solid fault at the load. The load voltage shorted ( t 0E? ), and the reactance between the generate and the load ( tX ) 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 bine rotating mass of both the generator and turbine 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 voltage. 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 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 stabil