A Novel Algorithm to Avoid the Maloperation of UHV Voltage-Regulating Transformers

A typical maloperation case of the differential protection utilized in the ultra-high voltage (UHV) voltage-regulating transformer (VRT) under inrush conditions is investigated in this paper. It indicates that the second harmonic restraint principle cannot identify the inrush current reliably under such conditions. This paper proposes a normalized equivalent instantaneous inductance (NEII)-based inrush blocking method, which improves the equivalent instantaneous inductance (EII)-based method in the UHV VRT protection. Although the EII-based method is capable of discriminating inrush currents and internal faults, some factors constrain its application, such as the field limitation of measuring the terminal voltage of the transformer and the difficulty of determining a reasonable threshold due to the diversity of transformer magnetizing characteristics. The proposed NEII-based method overcomes limitations of the EII-based method and can be adopted in UHVVRT protection. A number of simulation tests have verified the effectiveness of the proposed method.