The purpose of the induced overvoltage test is to check whether there are defects such as short circuits in the insulation between turns, layers, and sections of the voltage transformer. Since the voltage value of the longitudinal insulation withstand test is much greater than the rated primary voltage of the transformer, the magnetic flux density in the core far exceeds the saturation point on the magnetization curve during the test, causing a sharp increase in the excitation current. As a result, the 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 allows, 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 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, then exciting the secondary winding with a medium-frequency power supply voltage several times the 50Hz frequency. When the reading on the low-voltage indicator 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.