Adaptive Ground Fault Relay Protection for Transformers with Different Neutral Grounding Resistors

Neutral Grounding Resistor systems can be inserted between the neutral and ground in a power system to provide a ground fault protection through resistance. The fundamental purpose of a Neutral Grounding Resistor (NGR) is to limit the ground fault currents to safe levels so that all the electrical equipment in a power system is protected. Neutral Grounding Resistors are also commonly referred to as Neutral Earthing Resistors and Earth Fault Protection Resistors. This paper will discuss the protection of two parallel HV /MV transformers during normal operating condition and under contigency. Another case discussed is a generator with a different NGR value connected to the MV system. When depicted on a symmetrical components circuit, the short circuit current will vary according to the system operating conditions, either in normal mode or contingency modes. To overcome this, a protection scheme is proposed, which adjusts the settings based on the system's operating conditions.