The Influence of Cable Capacitance, Generator Neutral Grounding and Zig-zag Transformer to Third Harmonic Produced by Synchronous Generator

Salient pole synchronous generator is one of harmonic sources that causes various power system harmonic problem especially related to neutral. This paper aims to study the characteristics of third harmonic voltage and current from generator when connected to balanced load via cable, generator load connection with different generator neutral grounding resistor and generator load connection shunted by zig-zag transformer. Lab scale experiments have been conducted to vary all parameters related to cable capacitor and generator neutral grounding resistor under various combined resistive and inductive load. Cable capacitance which depends on cable length, size and number of parallel cable, combined with load impedance determines the third harmonic current magnitude. The presence of cable capacitance that interact with generator inductance resulted to series harmonic resonance at 9th and 15th harmonic order that causes high 9th and 15th harmonic current flowing between generator and cable capacitor. Generator neutral grounding resistor can effectively reduce the third harmonic in the phase and neutral. However care should be taken not to use very high generator neutral grounding resistor that can also restrict earth fault current that may pose problem to the sensitivity of earth fault protection system. Zig-zag transformer efficiently divert third harmonic current from generator to enter the load but high circulating third harmonic current between generator and zig-zag transformer may pose heating problem to the equipments.