Return voltage measurement（RVM） is a non-destructive method to diagnose insulation condition of power transformers. In order to make full use of the advantages of the recovery voltage method in the oil-paper insulation diagnosis

we put forward a method of using the gray relational model to diagnose insulation condition of power transformers. First

RVM is used to extract the characteristics which can reflect insulation condition of power transformers. Then

a large amount of test data are used to establish a standard state vector. Finally

the gray relational diagnostic model is used to diagnose insulation condition of power transformers. According to the grey correlation degree between diagnosing transformers and standard state

the insulation condition of power transformers can be diagnosed. Evaluation results show that the diagnosis results are in accordance with the actual insulation state of transformer.