专业英语 翻译

There are a number of abnormal conditions which do not occur in the stator or rotor winding, but are undesirable since they can damage the generator. Each of these conditions is discussed in the following.

(1)Loss of synchronism. This condition can occur either due to loss of field excitation or governor becomes defective. During out of step condition, as the swing angle between the generated voltage of the machine and that of other units in the system changes, the current in any such unit varies in magnitude. The current surges that result are cyclical in nature, their frequency being a function of reactive rate of slip of the poles in the machine. The resulted high peak currents and off-frequency operation can cause winding stresses, and pulsating torques which can excite mechanical resonances that can be potentially damaging to the generator and to the shifts. Thus generator should be tripped without any delay within the first slip cycle to avoid any major damage.

(2)Overspeed. The cause of overspeed is sudden loss of a very large load; sometimes this happens due to tripping of circuit breaker near the generator end. In the case of steam turbine, the steam can be shut off immediately but in case of hydro turbine, the water flow cannot be stopped quickly, due to the mechanical and hydraulic inertia. The governor controls the over speeding so as to avoid any high voltage, high frequency and mecheanical damage to the generators. The setting of an overspeed rating may be 115% for steam turbines and 140% for hydro-turbiners.

(3)Motoring. In a multi-generator system, when prime mover fails to provide required speed, the generator may act as a motor, drawing power from the system, instead of supplying power. Generally motoring is prevented by sensitive reverse power relay which operates on about 0.5% reverse power.

(4)Underspeed. Due to failure of steam or water supply to the prime mover, the speed of the generator will reduce and if the reverse power relay fails, then underspeed and/or underfrequency relay comes into picture and trips the circuit breaker.

(5)Loss of excitation. Excitation failure may be caused by a faulty field circuit breaker or failure of the exciter. It can be detected by an undercurrent dc relay. Due to failure of excitation, the synchronous generator may act as an induction generator thereby absorbing reactive power (i.e. sink of reactive power). Turbine generator tends to overheat the rotor and the slot wedges under these conditions because of heavy currents in these parts and sometimes arcing occurs at metal wedges in the slots. (6)Overvoltage. This may be caused due to overspeed or overexcitation when speed governor or voltage regulator fails to act as desired.