Research On Transformer Differential Protection With New Anti-CT Saturation Principle

Power transformer, one of the most important elements, is widely used in electric power transmission and distribution system. To prevent transformer from damage, current differential protection principle is applied to transformer main protection.At present, the problem is that disturbance, such as switching operations and external faults, cause transformer differential protection to mal-operate and the complicated electromagnetic transient coming with both transformer and current transformer (CT) is difficult to be described accurately by the dynamic simulation experiments. In addition, the increase of power system short-circuit fault capacity and time constant results in CT saturation more serious.This thesis is focused on the two critical problems of transformer inrush and CT saturation. On one hand, the digital simulation models of transformer during external faults and two transformers in parallel or in serial switching-on without load is built up, so the characteristics and influences on differential protection of recovery inrush after the clearance of external faults and sympathetic inrush are analyzed in detail; On the other hand, CT transient is investigated when faulted current and exciting inrush current pass through CT. In the following, the viewpoint is presented that CT saturation probably results in the mal-operation of transformer differential protection nowadays.Transformer differential protection with new anti-CT saturation principle is based on the fact that linear data always exist per cycle during CT transient saturation. The specific strategy is to make full use of linear data to prevent ratio restraint differential protection from mal-operating during external faults. Exactly operating current phasor is calculated by half-cycle Fourier algorithm while restraining current phasor is calculated by full-cycle Fourier algorithm. Digital simulation results and dynamic simulation data indicate that the strategy will eliminate the mal-operation caused by CT saturation. Finally the prospective application of transformer differential protection with new anti-CT saturation principle is depicted.