Although power cables are manufactured in controlled environments with sophisticated monitoring, cable failures do occur. Different types of cables are prone to specific types of failures. Since the 1990s, cross-linked polyethylene (XLPE) cables have become the norm for 132 kV power lines. Failure in cables or deviation from their expected performance occurs due to manufacturing defects, adverse installation conditions or unintentional damage during the laying of cables. The points where cables have been joined to extend the network are also prone to certain predictable types of failure, especially in underground networks. Cable joints are inherently weak owing to three types of stresses – thermal, electrical and mechanical. Before XLPE cables became the norm, paper-insulated lead-covered (PILC) cables were used and certain portions of the network continue to use that technology. PILC cables are prone to failures that are distinct from those of XLPE cables. Transition joints, where the two types of cables are joined, tend to be more prone to failure.
Manufacturing defects in cables occur in the form of voids in insulation, eccentricity in the cable core and defects in taping or protrusions. Voids in the insulation of a power cable due to faulty manufacturing decrease the performance of the cable over time, ultimately leading to failure. This happens because ionisation and partial discharge within the voids gradually cause the insulation to erode. Cable core eccentricity leads to a reduced distance between the sheath and core, causing an increased electric field concentration on one side of the cable. The concentration of the electric field initiates partial discharge, eventually leading to the formation of a conducting channel through the insulation, leading to dielectric breakdown. In the case of PILC cables, inadequate impregnation of the paper insulation during manufacturing leads to brittle cables that are less durable.
Errors during installation
There are certain ideal conditions for the installation of power cables, deviations from which result in decreased performance and viability. Cables must be laid at an adequate depth, failing which, the chances of damage due to spiking increase. Cables laid at an inadequate depth also experience fatigue of conductor strands due to vibration from vehicular movements. In the case of PILC cables, which have been in service for more than 50 years, adverse conditions during installation and cyclic overloading cause accelerated ageing of cables. The dielectric strength of paper insulation can degrade due to irregular load patterns, frequent overloading due to high thermal resistivity and bending of the cable beyond the safe radius.
Failure due to thermal stress
Thermal breakdown is a very common form of breakdown in cable insulation. It occurs when the rate of energy and heat transferred to the insulation material exceed the rate of heat dissipation and absorption due to the electric field. This is not very common in XLPE cables unless the insulating material is made of sub-par materials.
Failure due to electrical breakdown
Electrical breakdowns in polymeric cables occur due to treeing. Treeing takes place in two forms: electrical and water. Electrical treeing occurs because the voltage stress appearing across the cable insulation is very high in high tension cables. Voids and impurities in the insulation cause localised discharge and the resulting heat causes carbonised paths in the insulation. The formation of a series of such carbonised pockets creates a carbonised conducting tracking path, which ultimately results in a dielectric breakdown.
Water treeing occurs because polymeric insulations are hygroscopic. The presence of moisture degrades the insulation in extra high voltage cables. Furthermore, moisture seeps through the cable sheath and percolates through the insulation. These water molecules get charged as the conductor acts as a cathode and the screen as an anode. The charged water molecules travel through the insulation from the earthed screen towards the live conductor, forming a conducting path and causing dielectric failure in the long run.
Failure due to mechanical stress
The conductor jointing region is prone to stress during the cyclic loading of the cable. The development of transient over-current during fault conditions results in the expansion and contraction of the conductor, causing mechanical stress, which leads to fatigue and, ultimately, breakdown. In the case of underground cables in congested metropolitan areas, the route of cables is often shared with other utilities like telecom, municipal sewage, water supply works, etc. Excavation related to any of these utilities may result in direct spiking of the underground power cable. While direct spiking can damage the cable, it does not cause immediate feeder tripping, especially if the penetration is not significant. However, a nick in the outer sheath of the cable and armour of PILC cables can cause corrosion of the lead sheath, providing a path for moisture to seep into the paper insulation and cause dielectric breakdown.