The purpose of the induced overvoltage test is to test whether there are defects such as short circuits in the inter-turn, inter-layer, and inter-segment insulation of the voltage transformer. Since the voltage value of the longitudinal insulation withstand test is much greater than the rated primary voltage value of the transformer, during the test, the magnetic flux density in the iron core far exceeds the value at the saturation point on the magnetization curve, causing the excitation current to increase sharply. As a result, the iron core overheats, which can easily lead to thermal breakdown and damage the voltage transformer.


Test Method


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The specific method is chosen by the manufacturer based on the test equipment available. If the test equipment permits, the specified test voltage can be applied directly to the primary winding for excitation. If limited by the frequency multiplier transformer, the PT's own transformation ratio can be utilized to apply sufficient excitation voltage from the secondary winding, causing the primary winding to induce the specified test voltage value.



Capacitance Rise Effect


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In practice, the induced overvoltage test of a voltage transformer involves connecting a capacitive voltage divider to the high-voltage terminal of the primary winding to measure the induced voltage, and then exciting the secondary winding with a medium-frequency power supply voltage several times the 50Hz frequency. When the indication on the low-voltage meter of the divider reaches the specified requirement, it should be maintained for a certain duration. At this time, the primary winding of the voltage transformer carries a capacitive load.