MV generator low-resistance grounding and stator ground fault damage

Most of the in-plant medium-voltage generators are grounded through resistors ranging from 200 A to 400 A. In some unusual situations, the resistors may even be as high as 1200 A. low-resistance grounding is a preferred choice for a medium-voltage power distribution system. However extensive generator stator ground fault damages had been reported due to the prolonging generator neutral ground current, which would continue to flow even after the main and field breaker opened. This ground fault current could continue to flow for as long as 5 seconds depending on the generator open-circuit time constant T'/sub do/. As a result, the lower the grounding resistor i.e. higher the generator neutral current, would lead to the higher stator ground fault damages. An IEEE Working Group has completed its study and made recommendations for medium-voltage generator grounding in a multiple source industrial environment. Based on the considerations of transient over-voltage, ground fault damages and ground fault protection, the working group suggests a few variations of grounding systems, which basically are low-resistance grounding systems during normal operating conditions, and the generator would either be on high-resistance grounded or be switched to a high-resistance grounded system from a normal low-resistance grounded system when a ground fault occurs in the generator stator. The purposes of this paper are (1) to examine the generator transient over-voltage and currents under the low-resistance ground fault conditions, and also to evaluate their corresponding stator ground fault damages, and (2) to establish an acceptable maximum system ground fault level. For comparison purpose, three versions of low-resistance grounding systems have been considered and they are: a low-resistance grounding system with a neutral breaker at the generator (neutral breaker system); a low-resistance grounding system with the generator neutral low resistor being switched to a high-resistor after a stator ground fault (hybrid system); and a low-resistance grounding system similar to the current practice (tradition system). The simulation study is conducted with the aid of the Electromagnetic Transient Program (EMTP). An experimental analog generator model is also used to verify the simulation results.