Evaluation of unconventional partial discharge and tan delta assessment techniques on medium voltage cable terminations with artificial defects

Abstract

Reliable cable systems are of utmost importance for electricity distribution- and grid networks. Cable terminations are key components of cable systems but also contribute significantly towards unwanted cable failures and power outages. Although well-developed standards exist in the electricity industry, it is not always possible to execute the most effective tests at power frequency due to financial or logistical reasons. Hence, withstand voltage- and Partial Discharge tests at power frequency are only conducted at a low percentage of electricity utilities. This paper presents the evaluation of unconventional partial discharge fault detection techniques and Very Low Frequency Tan Delta measurements on different cable termination defects. Five artificial cable termination defects were created on single core 6.35/11kV cross linked polyethylene cables, with one “defect-free” termination and five defective ones. Power frequency voltage was applied at increased voltage steps to initiate aging acceleration due to partial discharge. This resulted in insulation deterioration and eventual termination failure. Different data sets were obtained from the Very Low Frequency Tan Delta measurements as well as for the unconventional Partial Discharge measurements. The sequence of failures is compared between the two data sets and evaluated against the root cause failures of the terminations as evaluated by an industry expert. High Frequency Current Transformer and capacitive coupling sensors were used together with an advanced partial discharge data acquisition system. Partial discharge analysis was done to relate the specific on-line trending data with termination failure. Good correlation was found between the two data sets which supports the utilization of unconventional partial discharge measurement for cable acceptance testing. It would be beneficial for the electricity industry to consider unconventional Partial Discharge technology as a suitable alternative for on-site acceptance tests for cable terminations.