Transformer leakage current testing can be divided into four core stages: equipment preparation, wiring operation, voltage application testing, and result analysis. It is a non-destructive test for accurately detecting moisture in the transformer insulation, surface contamination, and through-going defects.
1. Core Testing Process
① Instrument Preparation: Requires a DC high-voltage generator (voltage ripple factor <3%, output current >0.5mA), a 0.5-grade microammeter, a 1.5-grade voltmeter, a discharge rod, and a thermometer/hygrometer.
② Standardized Wiring: Disconnect all winding leads of the transformer. Connect the test high voltage to the winding under test, and short-circuit and ground all other windings. The microammeter should be connected to the high-voltage side and connected via a shielded cable to reduce external interference.
③ Step-by-Step Voltage Application Testing: Gradually increase the voltage in 4-6 steps. After stabilizing the voltage for 1 minute at each step, record the microammeter reading. Finally, maintain the rated test voltage for 1 minute and read the final leakage current.
④ Result Judgment: Compare the test results with historical data of the same type of equipment. If the leakage current exceeds the specified threshold or the voltage-current curve shows a steep rise or non-linear increase, it is determined that there is a defect in the insulation.
2. Key Precautions
① Before testing, the surface of the transformer bushings must be wiped clean to prevent surface contamination from affecting test accuracy.
② For graded insulation transformers, the test voltage must not exceed the insulation withstand voltage level of the neutral point.
③ After the test, the equipment must be fully discharged for 1-3 minutes using a high-resistance discharge rod. Direct short-circuit discharge is prohibited to avoid burning out the microammeter.
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