Abstract: When the frequency response analysis (FRA) method is applied to detect winding deformation of a transformer during its operation, the high frequency FRA currents at the high voltage terminal of the windings are measured by coil-type of current sensors. The power frequency load currents of the transformer interfere greatly with the measurement of the FRA current, which has an amplitude of only microamperes. Based on the approximate balance of three-phase power frequency currents of transformers in the transmission and transformation grid, a parallel sensing system composed of three-phase current sensors is proposed in this paper. This sensing system has excellent performance to suppress power frequency interference, and can measure the FRA currents of microampere when the amplitude of power frequency currents is several hundred amps. The principle of the system is completely different from the existing filter concept. Therefore, its effect is excellent, which is difficult to achieve by ordinary filters. Based on circuit theory, the mechanism and characteristics of the sensing system against power frequency interference are analyzed. Tests are carried out in the laboratory and at a power station to verify the mechanism and characteristics of the sensing system. The frequency response curve of the 110 kV winding of the transformer is obtained when it is in operation. Therefore, it is feasible to obtain the frequency response curve online by injecting the excitation signal and measuring the current response signal at the high-voltage terminal of the windings.