The transformer error test generally uses the measured transformer to compare with the standard transformer, and the secondary current difference between the two transformers is the measured transformer error. This test method is called the comparative method. The standard transformer requires two levels higher than the measured transformer, and the error of the standard transformer is negligible at this time. If the standard transformer is only one grade higher than the measured transformer, the error of the test result should be considered to add the standard transformer error. The secondary current difference between the measured transformer and the standard transformer is generally measured with a transformer calibrator. Read the ratio difference fx(%) and the phase difference δx(’) directly from the transformer calibrator. Since the transformer calibrator measures the ratio of the current difference between the measured transformer and the standard transformer to the secondary current, the requirements for the transformer calibrator are not high. What level of transformer must be calibrated is basically determined by the standard transformer. The standard transformer is the key core of the transformer calibration system. To calibrate the measured transformer, in addition to the standard transformer and transformer calibrator, there must be a current booster that provides primary current to the transformer, and a voltage regulator and load that can adjust the current of the booster.