The possibility of diagnosing the pre-breakdown state of polymeric insulators by the thermovision method is represented in this paper. The diagnostic method is based on the monitoring of time of the appearance of the local temperature difference and the value of the local temperature difference on the insulator surface due to the defect presence inside the insulator volume. Time corresponding to the critical or pre-breakdown state of dielectric may be estimated by means of above mentioned parameters. Good agreement between the experimental results and the model predicted data was found. The deviation between the experimental results and the model predicted time corresponding to the critical (or pre-breakdown) state of dielectric does not exceed 15%.
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