The insulation resistance and dielectric loss from transformer preventive test and the conventionalparameters for evaluating overall insulation state of the transformer. However

because the insulation resistance testis affected by the field environment and interference

the test data scatter

so it is difficult to give accurate thresholdvalue. And the obtained threshold value of dielectric loss is a bit larger. Therefore

this paper uses mathematicalstatistics method to calculate the test data of insulation resistance and dielectric loss of the main transformers in allpower plants in Yunnan Province

adopts SPSS to analyze these data

and discusses the distribution law of the testdata. Consequently

the thresholds of the insulation resistance and dielectric loss based on the significant differenceare put forward to provide reference for accurate judgment of the insulation state of a transformer.