Double Earth Faults in Power Stations

Single-phase-to-ground faults in power stations sometimes cause a second phase-to-ground fault in another phase. If the phase-to-ground faults are located on different sides of the generator circuit breaker (CB), one has a double earth fault of a special and so far not analyzed kind with respect to the CB. It may lead to high stresses for the last CB pole to clear. The general expressions for the current, the rate of rise of recovery voltage, and the power frequency recovery voltage across this CB pole have been deduced as function of the sequence values of the short-circuit impedances and surge impedances, respectively. The corresponding results are discussed for typical cases. A detailed case analysis by means of the Electromagnetic Transients Program shows that at rated load, such a double earth fault, produces CB stresses comparable to out-of-phase conditions. A combination of double earth fault and out-of-phase conditions, however, causes a peak value of the transient recovery voltage, which will significantly exceed the values laid down in the standards.