Tertiary Voltage Unbalance Compensation for 500kV Single Phase Autotransformer Banks

In an ungrounded transformer tertiary system, most often the phase-to-ground voltages are slightly unbalanced. Therefore all equipment supplied by the tertiary must have extra insulation to withstand the expected phase-to-ground overvoltages that arise from such unbalance. However, in a recent 3 × 250MV A single phase transformer bank replacement project, it was found that the tertiary phase-to-ground overvoltages were abnormally high therefore the transformer bank could not be put back into service. After extensive investigation, it was found that the transformer inter-winding capacitances and tertiary phase-to-ground capacitances were unbalanced, resulting in excessive unbalance in the tertiary phase-to-ground voltages, which was directly responsible for the overvoltage. A mitigation plan was developed. The plan was to modify the tertiary phase-to-ground capacitances in such a way that the voltage unbalance due to the asymmetry in the inter-winding capacitances cancels the voltage unbalance due to the asymmetry in the phase-to-ground capacitances. Based on theoretical derivation and EMTP simulation, two CVTs (Capacitive Voltage Transformer) were selected from the warehouse and installed on the transformer tertiary. The voltage unbalance was successfully mitigated and the transformer bank was put back into service. The mitigation methodology developed in the project achieved significant cost saving and will have great value in future project development.