Power transformer neutral grounding reactor application and distribution feeder protection issues at CenterPoint Energy

Abstract

This paper discusses the ramifications of grounding a substation power transformer through a reactor on the CenterPoint Energy distribution system. The purpose of the reactor is to reduce the magnitude of ground fault currents. This is expected to increase transformer life expectancy by significantly reducing transformer short circuit forces. The higher ground impedance provided by the neutral grounding reactor (NGR) also causes an increased voltage drop in the ground return path, raising the temporary overvoltage on the unfaulted phases and deepening the voltage sag on the faulted phase during a ground fault. Under normal operating conditions with large load imbalances, voltage regulation can be also impacted. The reduction in ground fault current requires the modification of ground protective device coordination and settings. The paper discusses how these issues are addressed for a successful implementation of an NGR in an effectively grounded, 4-wire distribution system.