Application of a Recursive Phasor Estimation Method for Adaptive Fault Component Based Differential Protection of Power Transformers

An adaptive differential protection method is proposed for power transformers. First, the phasors of captured current signals are computed using an online recursive S-transform-based method of phasor estimation with low computational burden. Then, the fault components of currents are calculated as a measure of disturbance severity. The relay employs the fault components of the restraint and differential currents to adjust slopes and pickup settings of its dual-slope characteristic curve according to the disturbance, and discriminates external and internal faults. To avoid malfunctions caused by inrush currents, an average power based index is also proposed, which has high values during inrush currents while it is near zero for faulty cases. The proposed method is implemented using MATLAB and electromagnetic transients program (EMTP) software packages and its performance is compared with the multiregional and self-adaptive approaches. In addition, real-data records of two differential relays located in the South West Interconnected System in Western Australia have also been used to determine the accuracy of the proposed power transformer protection algorithm. Both the simulation and the real data test results show that the proposed method is more reliable and secure for faulty and nonfaulty cases in analogy to the other two protection approaches.