An adaptive algorithm for the differential protection of the UHV voltage-regulating transformer

1000 kV ultra-high voltage (UHV) power transformer plays an important role for HVAC transmission in China. The voltage-regulating transformer (VRT) is an independent part of the UHV power transformer and the exclusive differential protection is needed. The voltage regulation of VRT can range from 0 to 100% which takes great challenges for the differential protection. In engineering, a fixed tap ratio, which is probably different from the online tap ratio, is used to calculate the VRT's differential current. The unbalanced current unavoidably increases and even may cause the mal-operation of VRT's differential protection. This paper proposes an adaptive algorithm that the online ratio can be obtained during the voltage-regulating process according to the electrical connections of the UHV power transformer. The result shows application of the adaptive online ratio overcomes the limitation that upraises the minimum operation current at the expense of the sensitivity. Several groups of comparison results are given to verify the effectiveness of the proposed algorithm.