Effect of Prefault Load on Field Overvoltages in Rectifier-Excited Synchrondus Machine During Sequential 3-Phase Fault

ABSTRACT

In a Synchronous machine with rectifier excitation, large reverse voltages may be induced across the field during a transient condition when the field current tends to flow in the reverse direction and is blocked by rectifiers. Earlier investigation on an initially unloaded machine showed that a sequential 3-phase fault, in which a line to line fault develops into a 3-phase fault after a short time, produces field overvoltages whose magnitudes may be comparable to those due to line to line fault. In this paper, analysis has been carried out to estimate the magnitude of the field overvoltage during a sequential 3-phase fault when the machine is assumed to operate on a prefault load. Numerical results show that a sequential 3-phase fault on a machine having prefault loads may be much more severe compared to a line to line fault from the field overvoltage point of view. The results also indicate that very large field overvoltages may be produced during a sequential 3-phase fault on a salient pole machine operating initially on leading power factor loads.