(2) Before measurement, the transformer casing clean, so as not to cause leakage current, affecting the accuracy of the measurement results. (3)Measurement of non-measured winding should be grounded. First ground the transformer casing and high-voltage winding, measure the low-voltage winding; then ground the transformer casing and low-voltage winding, measure the high-voltage winding.
(4)Measurement, such as hand-operated megohmmeter, should be 120r/min speed to turn the handle, so that the pointer gradually rise until the pointer indicates the stability of the record readings.
(5) insulation resistance should be transformer windings immersed in oil when the value shall prevail (refers to the oil-immersed transformer), the transformer should be static 5?6h after oil injection and then measured. Measurement results and put into operation before the value for comparison, if the decline is significant (less than 70%), it should be tested to make a comprehensive analysis.
(6) Precautions.
①Measurement should be carried out in good weather conditions, and the measured transformer and the ambient temperature should not be less than 5. According to the Ministry of Hydropower Electric Power Research Institute statistics, the ambient temperature is less than 5 when the measurement results are often inaccurate.
② Measurement, the ambient humidity should not be greater than 75%, because humidity has a greater impact on the surface leakage current, so that the insulation resistance is reduced.
③After each measurement of insulation resistance, the charge should be discharged (with a wire shorting the terminals); otherwise, the second measurement, there will be an illusion of increased insulation resistance