Cables and conductors are the most vital component of a power transmission network. The faults in electrical systems are unavoidable, but it is essential that they undergo routine maintenance in order to ensure the reliability of the power distribution systems. While fixing faults after they occur can be challenging, expensive and time-consuming, regular maintenance will optimise the durability and performance of the cables.
Predictive and preventive maintenance
The aim of predictive maintenance is to anticipate when an equipment failure might occur, and monitor for such future failures. In general, predictive maintenance approaches inspect, monitor, assess and analyse electrical equipment. Following this, 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.
Online partial discharge (OLPD) monitoring is another predictive maintenance technique that allows for trends in partial discharge activity to be observed over time. Partial discharge testing thus gives an indication of deterioration of the insulation and is an indicator of incipient faults. OLPD testing and monitoring gives an accurate picture of a high voltage cable’s health and performance under normal service conditions, including the effect of load, temperature and humidity. As partial discharge activity is often present well in advance of insulation failure, it is possible to make strategic decisions about refurbishing and renewal programmes.
It is also crucial 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. Infrared (thermographic) inspections can be done where specialised cameras are used to detect anomalies which are not visible to the naked eye. In situations where this technique cannot be deployed, continuous surveillance by way of online monitoring can be performed to evaluate the current condition and detect abnormalities at an early stage. Joints that register high levels of heat should be cleaned, retorqued and then retested.
Routine inspections can also be conducted via aerial patrolling and surveillance of the transmission lines to check for their safety and security in the eventuality of any natural disaster or sabotage. Such surveillance uses features of LIDAR, thermovision, corona scanning and high definition video and image sensors, which provide high resolution visual images, video images and thermo clips. Such inspections can also be carried out via drones, which will completely eliminate the need for tower climbing for a detailed inspection.
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. Hotline maintenance is another upcoming method, that can be used for overhead line maintenance. This can be deployed for energised lines working at high voltages and for changing the damaged parts of conductors.
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.
Maintaining underground cables
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.
Considering the indispensible nature of cables, it is important to ensure timely maintenance. Such routine check-ups give the actual information regarding fault location and situation, and quantify the severity of faults in cables and conductors. Fault recurrence can be reduced by performing root cause analysis and putting in place remedial solutions.