Published Papers Volume 30 - Issue 4 (Award Issue) Awarded Paper Use of insulation resistance imbalance degree for the condition assessment of power distribution cables

Abstract

Medium-voltage (MV) underground power distribution cables are critical assets of The Hong Kong Electric Co., Ltd. (HK Electric) for ensuring reliable customer power delivery. The inspection and condition assessment of these power distribution cables are conducted regularly to determine their health. During diagnostic tests, all MV cables have to pass a threshold of insulation resistance (IR) before they can be energised. Low IR readings may indicate a deterioration in insulation related to a high leakage current. However, defining a threshold value to assess the intrinsic condition of the power distribution cables for every test is challenging. This is because the reference value is determined by several factors such as cable length, the composition of the cable circuit, e.g., cable type, number and types of cable joints, service years and surface leakage from cable splices and terminations. In order to tackle this problem, a concept of imbalance degree in IR testing is developed to supplement the existing tests on dielectric losses for the purpose of enhancing the condition assessment of three-phase power MV distribution cables when performing IR measurement. The effectiveness of this method has been validated with historical testing records of 11kV distribution power cables at HK Electric. Based on our experience and study results, this improved method could further enhance the diagnosis and maintenance efficacy of HK Electric’s power distribution cables.