In the transformer error test, the measured transformer is generally compared 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. 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 standard transformer is the key core of the transformer calibration system. To calibrate the measured transformer, in addition to the standard transformer and the transformer calibrator, there must also 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.