In digital substations， it is necessary to convert different sampling rates to the same rate when the differential protection relay is connected to electronic current transducers（ECTs） with different sampling rates. This paper studies the impact of the multi-rate signal processing technique and rate conversion of sampling data on the differential current of the differential protection and proposes an effective sampling rate conversion mechanism with cascade connection of decimation and interpolation， by which sampling rate conversion in a rational factor can be realize. An optimum equiripple low-pass filter based on the Chebyshev approximation principle for multi-rate signal processing is designed and combined with multi-rate signal processing technique to reduce the frequency aliasing error. Based on the above design， the branch current in differential relays with different sampling rates can be converted to the same one， so an accurate differential protection can be realized under the ECT circumstances. Simulation results show the feasibility and effectiveness of the method proposed.