What are the maintenance procedures for underground distribution?

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Although a distribution system with underground cables is less maintenance intensive than one with overhead lines, it is necessary to have clear procedures for preventive, corrective and condition-based maintenance in order to ensure a long trouble free operational life for the cables.

Preventive maintenance to be carried out annually comprises:

• Visual inspection of structures, fixtures, transformers, switchgear and cables for signs of deterioration or damage, cleanliness and physical condition with corrective action as necessary.
• Visual inspection of cable joints, splices and ground connections for integrity and condition and cleaning of termination joint outer surfaces of dust and contaminant deposits.
• Testing contact resistance of terminations for signs of deterioration.
• Insulation test between conductors, and resistance to ground using a Megger.
• Infra-red thermal scanning of all joints. If a poor electrical contact is detected through an increase in temperature, it should be cleaned, re-torqued and re-scanned.
• Repairs / replacement of faulty cable sections and re-jointing where necessary.

Corrective maintenance is required in response to a fault that could occur due to an internal failure (e.g. due to improper handling and installation, manufacturing defects, overloading etc.) or externally caused damage (e.g. due to digging). The objective is to identify the nature and location of fault and take remedial action in the quickest possible time. The recommended steps are:

• Visual and physical inspection of the cable route for signs of possible damage e.g. due to roadworks or digging.
• Insulation resistance tests between conductor and shield ground, shield ground and cable ground to identify the presence of a fault.
• Use of cable fault locator and sheath tester to determine the location of the fault.
• Repairs followed by testing and recommissioning.
• Root cause analysis and remedial measures to prevent / minimize future occurrence.

Condition based maintenance is based on assessing the condition of cables and equipment that have already suffered partial damage and thereby predicting the need and timeline for future maintenance or replacement. Equipment such an Oscillating Wave Testing System ( OWTS) can be used to identify, evaluate and detect partial discharge conditions in cables, joints and terminations. Depending on the magnitude of partial discharge conditions, a decision can be made to either replace the faulty cable section immediately or to test it again after an interval that can stretch from a week to 12 months.

Reference: Power Cables Handbook, Volumes 1, 2 & 3, Conversion of Overhead Lines to Underground Cables, Copyright@2014, International Copper Association South East Asia Ltd. And Provincial Electricity Authority, Thailand