EHV TRANSFORMER BANK UNBALANCE - PRACTICAL ISSUES AND SOLUTIONS

An unconventional tap configuration has been implemented in AEP's Sout h Canton 765/345/34.5 kV, 3-500 MVA transformer bank to counter the co mbined effects of impedance mismatch within the bank and natural unbal ance of the extra-high-voltage (EHV) transmission system. These effect s manifest themselves as unequal phase voltages and loadings and as el evated neutral and tertiary circulating currents in the bank, capable of reaching high magnitudes during heavy loading conditions. This pape r describes the South Canton bank unbalance, the concerns it raised, a nd the actions taken by AEP to control its possible adverse effects on the integrity and operation of the South Canton Station facilities.