The preventive test of distribution transformer has the following items:
1) Insulation resistance measurement: The standard generally does not stipulate. Compared with the previously measured insulation resistance value converted to the same temperature, generally shall not be lower than 70% of the previous measured results.
2) AC withstand voltage test: the standard is 6kV plus 21kV; 10kV plus 30kV; low-voltage 400V winding plus 4kV.
3) Determination of leakage current: generally not specified, but there should be no significant change compared with the value over the years.
4) Measuring the DC resistance of windings: the standard is that the difference between the DC resistances of the windings of each phase of the transformer of 630kVA and above should not be greater than 2% of the average value of the three-phase, and the relative change compared with the previous measurement results (converted to the same temperature) does not change. It should be greater than 2%; the phase-to-phase difference of transformers below 630kVA should not be greater than 4% of the three-phase average value, and the line-to-line difference should not be greater than 2% of the three-phase average value.
5) Electrical strength test of insulating oil: The oil test standard in operation is 20kV.