The research of sampling frequency conversion algorithm for differential protection

As prescribed in IEC60044-8 Standard, the rated values of electronic current transformer output data rate can be 1000Hz, 2400Hz, or 4000Hz (industrial frequency is 50Hz). In intelligent substations, it is necessary to convert different sampling rates to the same one when intelligent electronic devices of transformer differential protection are connected to ECTs with different sampling rates. This paper studies the sampling rate conversion mechanism with cascade connection of decimation and interpolation to realize sampling rate conversion in rational factor, and the impact on differential protection current is also analyzed. To reduce the group delay of high-order finite impulse response (FIR) in sampling rate conversion, a zero-phase filtering algorithm is designed. Combined that with the decimation and interpolation algorithm, the branch current in any different sampling rate can be converted to the same one, and the seamless connection of differential protection and ECT is realized. Simulation results show that the proposed algorithm can improve the operation speed of differential protection.