1) Voltage. According to the requirements of the test product, first select a test transformer with a suitable voltage so that the rated voltage Un of the high-voltage side of the test transformer is higher than the test voltage Us of the test product, that is, Un>Us. Secondly, check whether the low-voltage side voltage required by the test transformer can match 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 tested product, that is, In>Is. The current required by the test product can be estimated by its capacitance, Is=Us w Cx, where Cx includes the test product capacitance and additional capacitance.
3) Capacity. According to the rated current and rated voltage output by the test transformer, the capacity of the test transformer can be determined, namely P=UnIn.
According to the regulations issued by the ministry, the voltage levels of test transformers in my country are: 5, 10, 25, 35, 50, 100, 150, 300kV, etc.; the capacity levels are: 3, 5, 10, 25, 50, 100, 150, 200kVA Wait.
From the calculation results, the required test transformer can be selected by checking the standards issued by the ministry. If you have special requirements, you can generally order test transformers with special specifications.
For example, the voltage level and capacity of the distribution transformer are 10kV, 1000kVA, and the test product encountered is basically 10kV, you can choose the test transformer of 50kV, 5kVA, because the factory test voltage of the 10kV, 1000kVA distribution transformer is 35kV , AC test voltage is 30kV; at the same time, it can meet the requirements of 10kV insulator and high-voltage switchgear test (test voltage is 42kV) and 10kV cable DC test (DC voltage is 60kV, corresponding AC voltage is 42.83kV).
In terms of test capacity, a 10kV, 1000kVA transformer under test, its capacitive current when charging is about 80~110mA during a 30~35kV test, because 35kV×110mA<5kVA, 5kVA can meet the requirements. Another example is a 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, etc. The capacity of the selected test transformer must be greater than the capacity corresponding to the capacitor current of the test sample. It is generally believed that the capacity of the test transformer is 5‰ of the capacity of the test product (power transformer).