When selecting a test transformer, the following aspects are mainly considered:
1) Test 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) Test 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 according to its capacitance, Is=UswCx, where Cx includes the test product capacitance and additional capacitance.
3) Test capacity. According to the rated current and rated voltage output by the test transformer, the capacity of the test transformer can be determined, that is, P=UnIn.
According to the relevant test transformer standards, the voltage levels of my country's test transformers are: 5, 10, 25, 35, 50, 100, 150, 300kV, etc.; the capacity levels are: 3, 5, 10, 25, 50, 100, 150 , 200kVA, etc.
The required test transformer can be selected from the calculation results and relevant standards. If there are special requirements, test transformers with special specifications can be ordered from the manufacturer Wuhan Hezhong Electric for testing.
Example of Test Transformer Selection
For example, the voltage level and capacity of the distribution transformer are 10kV and 1000kVA, and the test product is basically 10kV, so the test transformer of 50kV and 5kVA can be selected. The test voltage is 30kV; at the same time, it can meet the requirements of the test of 10kV insulators and high-voltage switchgear (the test voltage is 42kV) and the DC test of 10kV cables (the DC voltage is 60kV, and the corresponding AC voltage is 42.83kV).
In terms of test capacity, a 10kV, 1000kVA transformer under test, the capacitive current during charging is about 80~110mA during the 30~35kV test, because 35kVx110mA<5kVA, so 5kVA can meet the requirements. Another example is a 35kV, 2000~4000kVA transformer, when the test voltage is 72~85kV, the capacitive 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 above-mentioned test capacitor current. For routine test requirements, the capacity of the test transformer is 5% of the capacity of the tested product (power transformer).