The first is the voltage, according to the requirements of the test object, first choose the test transformer with the right voltage, so that the rated voltage of the high voltage side of the test transformer is higher than the test voltage of the test object, and secondly, check whether the low voltage side voltage required by the test transformer is matched with the field supply voltage and regulator.

The second is the current, the rated output current of the test transformer should be greater than the current required by the tested product, and the current required by the test product can be estimated by its capacitance.

There is also the capacity, according to the rated current and rated voltage output of the test transformer, the capacity of the test transformer can be determined.According to the regulations issued by the Ministry of Industry and Information Technology, the voltage levels of China's test transformers are: 5, 10, 25, 35, 50, 100, 150, 300KV, etc. Capacity classes: 3, 5, 10, 25, 50, 100, 150, 200KVA and so on. If you have special requirements, you can generally order special specifications of the test transformer.

For example, the voltage level and capacity of the distribution transformer is 10KV, 1000KV, the test products encountered are basically 10KV, you can choose 50KV, 5KV test transformers, because 10KV, 1000KV distribution transformers factory test voltage of 35KV, AC test voltage of 30KV; At the same time meet the requirements of 10kV insulator and high voltage switchgear test (test voltage 42kV) and 10kV cable DC test (DC voltage 60kV, corresponding to AC voltage 42.83KV).

In terms of test capacity, a 10KV, 1000KVA transformer under test, in the 30-35kV test, its capacitive current when charging is about 80~110mA, because 35KV×110mA<5KVA, 5KVA can meet the requirements. For example, 35KV, 2000-4000KVA transformer, when the test voltage is 72~85KV, the capacitance current is about 150~260mA, 6000~8000KVA is about 300~420mA, 10000KVA is about 800~1000mA capacity. The selected test transformer must be larger than the capacity corresponding to the current of the test capacitor. It is generally believed that the capacity of the test transformer is 5‰ of the capacity of the test product (power transformer). Transformer capacity is a measure of the rated power of the transformer, if the transformer capacity is too large, it will make the contract power larger, the user needs to pay a lot of basic electricity every month. In addition, the efficiency will be reduced due to the reduction of the transformer load rate. If the transformer capacity is selected too small, it will cause the transformer overload and increase the loss, but also limit the development of future production.