Research on the methods of measuring high frequency small current in large power frequency current by current transformer

Current transformer (CT) with the structure of iron-core coil can detect the partial discharge to reflect the insulation state of the equipment, or measure the sweep current in the frequency response method to judge the mechanical state of the transformer winding. In these cases, the CT only need to measure the small current with high frequency in the conductor, but sometimes there will be large power frequency current in the measured conductor. The signal induced by this power frequency current will be superimposed on the output of the CT, so that the high frequency signal is easily submerged in the interference of the power frequency signal. To improve the signal-to-noise ratio of measurement, this paper proposes the double coils compensation method and the double integral resistors method from the perspectives of coil and integral circuit respectively, and conducts experimental verification. The experimental results show that due to the nonlinearity of iron-core, the double coils compensation method cannot obtain satisfactory measurement results. While the double integral resistors method can effectively suppress the interference caused by the power frequency current, and has little effect on the measurement of high frequency current, which significantly improves the signal-to-noise ratio of measurement.