EHV transformer bank unbalance: practical issues and solutions

An unconventional tap configuration has been implemented in AEP's South Canton 765/345/34.5 kV 3*500 MVA transformer bank to counter the combined effects of impedance mismatch within the bank and the natural unbalance of the EHV transmission system. These effects manifest themselves as unequal phase voltages and loadings and as elevated neutral and tertiary circulating currents in the bank, capable of reaching high magnitudes during heavy loading conditions. This paper describes the South Canton bank unbalance, the concerns it raised, and the actions taken by AEP to control its possible adverse effects on the integrity and operation of the South Canton Station (USA) facilities.