Research on a mal-operation case of differential protection in UHV transformer

According to a field case on the mal-operation of transformer differential protection in the ultra-high voltage (UHV) experiment project in China, a new scheme of second harmonic blocking criterion is proposed in this paper. Based on the simulation model of UHV transformer established in MATLAB/SIMULINK, the causes of mal-operation is analyzed. The factors such as initial angle and remnant flux which could affect the attribute of inrush are studied. However, besides these possible factors, the special structure of UHV transformer may also result in mal-operation. The maloperation case indicates that the reliability of differential protection for voltage-regulating transformer is low. To avoid needless trip by inrush current, a comprehensive restraint scheme is proposed, which is based on a 2-out-of-3 method and takes the tendency of second harmonic ratio (2^ndHR) as an auxiliary criteria to identify inrush. In addition, the reliability and selectivity of the proposed method is verified by the field recorded data and simulation tests respectively.