Continued maintenance of power cables is vital to ensure the reliability of power distribution systems because fixing faults after they occur can be challenging, expensive and time consuming. Routine maintenance minimises the occurrence of cable faults. Underground cables require less maintenance than overhead lines. Maintenance can be classified as preventive, corrective or condition based.
Preventive maintenance should be carried out annually as it helps check faults arising from negligence of the distribution infrastructure. The integrity and condition of ground connections, joints and splices should be checked by visual inspection and any issue found must be corrected. It is also important to check the cleanliness of switchgear, transformers, other above-ground fixtures and outer surfaces of joints as the accumulation of dust and debris leads to stress on cables and their eventual breakdown. Damage due to wear and tear at the
termination points can be assessed by conducting contact resistance tests. Meggers can be used to conduct insulation resistance tests between ground connections and conductors. Infrared thermal scanning reveals defective electrical contacts and joints. Joints that register high levels of heat should be cleaned, retorqued and then retested.
Corrective maintenance needs to be undertaken at points where faults are detected due to dig-ins, defects, overloading or mistakes during installation, and suitable remedial action determined. Underground cables, especially in congested urban and metropolitan areas where space is limited, are susceptible to nicks and damages due to spiking during excavations. Hence, electric cable routes must be inspected for possible damage after such work has been carried out. Visual inspection is an indispensable practice for maintenance.
Recurrence of faults can be reduced by performing root-cause analysis and putting in place remedial solutions. It is also helpful to establish clear signages along the routes of underground cables, especially where channels are shared with other utilities.
Condition-based maintenance is undertaken along portions of underground transmission cables and above-ground fixtures that have already experienced partial damage. This is necessary because it helps predict future repairs, or signals the need for immediate replacement. Oscillating Wave Testing System equipment can be used to identify and evaluate conditions of partial discharge in cables, joints and terminations. Depending on the gravity of the condition detected, the faulty cable sections may be replaced immediately, or scheduled to be retested after an adequate interval.
Locating underground cable faults
Because of their location, identifying underground cable faults can be challenging. However, modern technologies have made this process more efficient. Two basic techniques for detecting faults in underground cables are the time domain reflectometry (TDR) and high voltage radar (HVR) methods. TDR tests underground cables by using low-energy signals, making it possible to locate a faulty section without causing insulation damage. The results are displayed as graphs. Properly functioning cables send return signals within a known time frame, based on set impedance settings.
HVR methods are of three types: arc reflection, surge pulse reflection and voltage pulse reflection. In arc reflection, a TDR device, filter and thumper work collectively to provide an approximate distance to any possible electric cable fault. Surge pulse reflection uses a storage oscilloscope, thumper and current coupler to ionise distant or hard-to-locate faults. The voltage pulse reflection method uses a voltage coupler and analyser, equipped with a proof tester or dielectric test, to locate faults at voltages higher than 25 kV.
With regular and precise maintenance, the durability and performance of cables can be optimised. By minimising the downtime due to faults and related repair work, power distributors can provide more reliable service while reducing operational costs.