Two methods AC and DC, are recommended to determine the excitation characteristics of the core in the national standard. At present, the AC method is often used to measure the relationship between the secondary turn flux and the excitation current of TA in transformer manufacturers. However, the main disadvantage of the AC method is that 50 Hz power frequency will cause the winding and secondary terminal to bear over-voltage. Therefore, the test is limited by the insulation level of the winding and the frequency of the power supply. During the measurement, the low-frequency generator or other high-voltage frequency conversion equipment that can generate the low-frequency sine wave voltage is required, and the whole test system is cumbersome. The traditional analog RMS instrument is generally effective for the standard sine wave. When the signal waveform is distorted, the reading is not the real RMS value, so the RMS error is relatively large. In addition, the accuracy is low due to the small number of points in the visual surveying and mapping curve.