Abstract Differential protection based on percentage check and second harmonic restraint is extensively adopted in transformer protection. In this paper, the merits and shortcomings of this protection are studied, the characteristics of various digital filter methods are analyzed .By comparing the calculation methods of second harmonic restraint, the synthesize method is adopted and the digital filter is designed by the pole and zero estimation method. The simulation suggests that the basic current and second harmonic can be obtained exactly. The percentage check can be more exactly used. It is helpful to avoid the postpone mistake act when the transformer is energized without load for the potential fault.
2013, Vol. 14 
