How is the External Withstand Voltage Test conducted?

The External Withstand Voltage Test is a one-minute AC high-voltage test conducted on the tested transformer, also known as the AC Withstand Voltage Test. This test is employed to assess the insulation performance between different winding sides and between windings and ground, specifically evaluating the main insulation of the transformer. Therefore, it is only applicable to fully insulated transformers. During the test, the different winding sides of the tested transformer are connected together, with one side subjected to voltage and the other side grounded, as illustrated in Figure 98-1.

During the External Withstand Voltage Test, the circuit should be closed when the power supply voltage is relatively low. The voltage ramp-up speed can be arbitrary when the test power supply voltage is below 40% of the test voltage. When it exceeds 40%, it should ramp up uniformly at a rate of 3% per second. Once the specified voltage and duration are reached, the voltage should be quickly and uniformly reduced to below 25% of the test voltage within 5 seconds before disconnecting the power supply.

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