Design and Specification for NGR installing in MEA Substation

As an electricity distribution utility in Thailand, Metropolitan Electricity Authority (MEA) realizes the importance of quality and reliability of the distribution system and aims to improve its services. MEA has a plan to change the neutral grounding scheme of power transformers from solid ground to resistance ground via Neutral Grounding Resistance (NGR). This implementation can reduce the magnitude of ground fault short circuit current and the severity of voltage dip. This article provides the detailed information on the NGR design concepts and methodology behind NGR specification determination. The related equipment in power system which has to be replaced due to the change in magnitude of temporary overvoltage and short circuit current are also proposed. Finally, the suitable resistance value of NGR will be installed at substation to reduce short circuit current and maintain line to line voltage about 90% of the nominal voltage during a single line to ground fault. However, this method can only be applicable in case of a single line to ground fault.