Use of discharge resistor to improve transient de-excitation in brushless synchronous machines

Abstract

The discharge resistor is only used in case of electrical trip to reduce the field current as fast as possible and to minimize the damages produced by the short-circuit current supplied by the synchronous machine. The connection of the discharge resistor is done by opening the field breaker and it implies a large negative voltage in the field winding. This negative voltage is limited to 80% of the winding insulation voltage. On the other hand, in case of a transient de-excitation, at the first moment, the automatic voltage regulator (AVR) reduces the field voltage to the minimum. In case of one-quadrant rectifier type AVR, the minimum voltage is zero and in case of two-quadrant rectifier AVR, the minimum voltage is close to the ceiling voltage with negative polarity. In both cases, the minimum voltages are much smaller than the negative voltage produced by the connection of the discharge resistor. This paper presents a new system that improves the transient de-excitation of synchronous machines using the discharge resistor by an additional static field breaker (SFB). The control of the static field breaker and consequently the connection and disconnection of the discharge resistor is done based on the output field voltage supplied by the AVR. This allows the exciter field current to be reduced in a faster way and continue with the normal operation of the machine after the transient. In this study, the correct operation of the additional static field breaker (SFB) has been validated by computer simulations and experimental test in a 15 MVA generator comprising a commercial one-quadrant rectifier AVR type obtaining excellent results.