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Zero-sequence components in unit-connected generator with ungrounded neutral during ground-faults

An analysis of voltage zero-sequence components in a generator during normal operation (pre-fault conditions) and during ground-faults in stator winding of generator with ungrounded neutral is presented. The changes in voltage zero-sequence components of fundamental frequency and 3rd harmonic during generator pre-fault conditions, ground-faults in generator stator winding and ground-faults in a power system are shown. The analysis was done for a generator with an ungrounded neutral for different values of the generator and main transformer parameters and additional capacitance to the ground of the generator breakers. Different resistances of a breakdown channel and different locations of the ground-faults in the generator stator winding were considered in analysis. It has been found that in the generator with ungrounded neutral, the parameters of generator and transformer, additional capacitance to ground of the generator breakers, phase of interrupted arcing ground-fault (level of the fault resistance) and the ground-fault location have a substantial influence on the zero-sequence voltages feeding different generator protection systems. The voltage zero-sequence components in generator neutral and the distribution of 3rd harmonic voltages in generator stator winding are determined mainly by all these parameters.