The preventive test of high-voltage test transformers can be carried out through the following aspects: measuring insulation resistance, measuring leakage current, measuring dielectric loss angle, AC withstand voltage test, etc., among them:

1. Measuring insulation resistance: It is a commonly used and simplest test method, usually measured with a megohmmeter. According to the measured insulation resistance of the sample at 1 minute, it can be detected whether the insulation has penetrating concentrated defects, overall dampness or penetrating dampness.

2. Measuring leakage current: The principle is basically the same as that of measuring insulation resistance, and the nature of the detected defects is also roughly the same. However, the power supply used in leakage current measurement is generally provided by high-voltage rectifier equipment, and the leakage current is directly read with a microammeter. It has the test voltage that can be adjusted at will; the sensitivity is high, the measurement repeatability is good; the insulation resistance value is converted; the measurement absorption ratio can be used to judge insulation defects and other characteristics.

3. Measuring the medium loss angle: it is a highly sensitive test item, it can find the overall dampness, deterioration, deterioration of the electrical equipment insulation and the partial defects of the small-volume test equipment through or not through. But when the volume of the test product is large, and the volume occupied by the defect is small, it is difficult to find with this method. It is widely used in electrical manufacturing and electrical equipment handover and preventive tests.

4. AC withstand voltage test: one of the destructive tests, it can further diagnose the insulation defects of electrical equipment. The AC withstand voltage test is the most rigorous, effective and direct test method to identify the insulation strength of electrical equipment. It can detect the smallest weaknesses of the insulation during normal operation and has a decisive role in judging whether the electrical equipment can continue to be put into operation.