Impact of 3rd Harmonic Circulating Currents on Medium Voltage Low Resistance Grounding and Ground Fault Protection : Copyright Material IEEE Paper No. IAS-(do not insert number)

Industrial facility with large islanded 13.8kV electrical system consists of existing four gas powered turbine generators connected to main switchgear line-up. Outgoing 13.8kV feeders to various facility loads are 13.8kV cables of various sizes ranging from #2 AWG to 750 MCM with total cable network length of approximately 28 miles. Since all existing generators and the new planned generator are 5/6 winding pole pitch, zero-sequence triplen (3rd, 9th, 15th, etc) harmonic currents are expected to circulate between all connected generators through the closed loop of the main switchgear bus and also between all generators and the equivalent system cable capacitance to ground.This paper investigate the impact of 3^rd harmonic circulating currents between all generators and the system cable’s capacitance to ground on system performance and ground fault protection schemes. This study is limited to the 13.8kV system as most facility loads are fed from area transformers with delta/wye windings that isolate its zero-sequence loop from the main 13.8kV system.This investigation discovered significant 3^rd harmonic circulating currents in the existing system and the new planned generator is expected to make this problem worse. Therefore, system grounding resistors is upgraded to non-standard units to withstand increased steady state 3^rd harmonic circulating currents. Ground fault protection adjustments was found to be necessary to prevent nuisance and sympathetic trips.