Unbalance in Parallel Interconnector Circuits

This paper investigates current unbalance in parallel cable interconnector circuits. Several instances have been observed where an MV switchboard loses supply during a motor starting transient. In each instance the affected switchboard was supplied via two interconnector cables operating in parallel. During motor starting, the transient current was not shared equally between the interconnector red, blue and yellow phases. The protection on each interconnector interpreted the unbalance in the phases as an earth fault and initiated a trip. To establish that the unbalance is not due to load or current transformer spill current, steady-state and transient measurements have been made. Steady-state and transient PSCAD simulations have been performed to investigate possible causes of the unequal current sharing between parallel interconnectors.