When selecting a test transformer, the following points should be considered:

(1) Voltage. According to the requirements of the test sample, first select a test transformer with appropriate voltage, so that the rated voltage Un on the high voltage side of the test transformer is higher than the test voltage Us of the test sample, i.e. Un>Us. Secondly, the low-voltage side voltage required for the test transformer should be checked to ensure that it matches the on-site power supply voltage and voltage regulator.  

(2) Current. The rated output current In of the test transformer should be greater than the current Is required by the test sample, that is, In>Is. The required current for the test sample can be estimated based on its capacitance, Is=Us w Cx， Cx includes the sample capacitor and additional capacitor.  

(3) Capacity. Based on the rated current and rated voltage output of the test transformer, the capacity of the test transformer can be determined, that is, P=UnIn. According to the standards issued by the Ministry, the voltage levels of test transformers in China include: 5, 10, 25, 35, 50, 100, 150, 300kV, etc; The capacity levels include: 3, 5, 10, 25, 50, 100, 150, 200kVA, etc.

Based on the calculation results, the required test transformer can be selected by checking the departmental standards. 

For example, if the voltage level and capacity of a distribution transformer are 10kV and 1000kVA, and the test sample encountered is basically 10kV, a 50kV and 5kVA test transformer can be selected, because the factory test voltage of a 10kV and 1000kVA distribution transformer is 35kV, and the AC test voltage is 30kV; at the same time, it can meet the requirements of 10kV insulator and high-voltage switchgear testing (test voltage of 42kV) and 10kV cable DC testing (DC voltage of 60kV, corresponding to AC voltage of 42.83kV). In terms of test capacity, a 10kV, 1000kVA tested transformer has a capacitance current of approximately 80-110mA during the 30-35kV test. Since 35kV × 110mA<5kVA, 5kVA can meet the requirements. For example, for a 35kV, 2000-4000kVA transformer, the capacitance current is approximately 150-260mA when the test voltage is between 72-85kV, 300-420mA for 6000-8000kVA, and 800-1000mA for 10000kVA. At this time, the capacity of the selected test transformer must be greater than the capacity corresponding to the capacitance current of the test sample. It is generally believed that the capacity of the test transformer is 5 ‰ of the capacity of the test object (power transformer).  

The test transformers provided by Wuhan UHV include dry-type test transformers, portable withstand voltage testers, oil immersed test transformers, etc