FIELD OVERVOLTAGES IN RECTIFIER EXCITED SYNCHRONOUS MACHINE DURING SEQUENTIAL 3-PHASE FAULT

Abstract

In a synchronous machine with rectifier excitation, large reverse voltages may be induced during a disturbance when the field current tends to reverse its direction and is blocked by rectifier. Earlier investigations showed that a line to line fault was perhaps the severest from the field overvoltage point of view. In this paper closed form solutions for estimating the field overvoltages for a line to line fault and also a sequential 3-ph. fault have been developed with some simplifying assumptions. Estimated currents and overvoltages tally closely with the numerical solutions obtained for specific cases. These results show that although a simple 3-ph. fault may not cause field overvoltages, a line-to-line fault developing into a sequential 3-ph. fault may cause, under worst conditions, higher overvoltages than due to a simple line-to-line fault.