Condition Assessment of Underground Cable System Using Health Index and Conditional Multiplying Factor

Abstract

This paper presents condition assessment of underground cable system for an independent power producer in Thailand. The underground cable is very important in both underground transmission and distribution system. Nowadays, the underground cable system is widely utilized to improve system reliability and aesthetic view. When cables have been used for a period of time, they become deteriorated and finally damaged because of electrical and mechanical stresses, and etc. If the failures were not recognized and repaired in time, the catastrophic failure could probably occur. Furthermore, this affects to customer’s reliability and trustworthy. Therefore, the condition assessment and health index calculation are needed to effectively maintain the system. The underground cable system consists of power cable, termination, joint, manhole, and duct bank. Normally, visual inspection and special testing with those components are required for assessment. Without power interruption, on-line testing consists of visual inspection, infrared thermography inspection and partial discharge measurement. The off-line testing, which is performed during maintenance, consists of polarization index measurement, dissipation factor measurement, leakage current measurement and very low frequency withstand test as required. These testing methods are considered together to evaluate the actual condition of all components by using weighting and scoring method. A conditional factor, considering service year, load current, failure frequency, laying, and environment, is proposed to provide more accurate result in term of practical usage condition. The web-application software is applied to setup systematic database for technical data and test results. It has been developed for processing condition assessment and facilitating maintenance activities.