Estimating and extending electric cable life

Use of Electric cables is of paramount importance to any Country’s building exercise. Africa has experienced its share of growth in the electric cable industry since it is still developing its electricity sector and other related infrastructure.

According to Madhurendu Bajpai of CMI Limited a manufacturer of electric cables in India, the growth in renewable power generation is also one of the primary factors that have contributed to the growth of the electric power cable and wire in Africa.

However, the insulating properties of power cables deteriorate with time and, at some stage of life, the cable will be unable to meet performance requirements, and suffer repeated failures.

End of life (EOL) vary with the cable type, the installation and conditions under which the cable is operated, and vary from installation to installation. Cable life can be extended in most cases, depending on the condition of the cable.

The lifetime of a power cable depends on its ability to carry the rated current safely at the rated voltage without excessive loss or failures. Cable failure is a breakdown of the insulation surrounding the conductor and subsequent leakage of current to earth or between conductors. Insulation breakdown may be total, resulting in a high fault current, resulting in leakage current.

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Cable failure

According to Ms. Tortigue of Cablerie Daumesnil an independent company specializing in the sales of electrical cables and wires to wholesalers in France, sub-standard quality of the components such as copper and aluminium, and vandalism especially because of copper can cause cable failure. However these are phenomenons that can be avoided by being vigilant and ensuring proper legislation are put in place to avoid the same.

In the technical side, medium voltage cables fail due to a phenomenon called water trees. Water trees can grow from the inside of the cable out, from the outside of the cable in, or from defects within the insulation. As they grow, they look like trees or bushes. Water trees grow until they can no longer hold the voltage stress and an electric tree is formed. Once an electrical tree is formed, the cable usually fails within two weeks.

Failure results as partial discharges erode the wall of the void in which they occur. The erosion of each discharge in a void increases the size of the void and so decreases the partial discharge inception and extinction voltages for that void. This self-acceleration means that any PD occurring at operating voltages is likely to lead to rapid failure.

Cable monitoring and testing

Monitoring and testing are essential to determine the life expectancy of power cables. The two most common methods of testing the condition of cable insulation are partial discharge (PD) and tan-delta (TD) testing. Partial discharge can occur at voids, gaps and similar defects in medium and high voltage cable systems. If allowed to continue, partial discharge will erode the insulation, usually forming a tree-shaped pattern of deterioration (electrical tree) and eventually result in complete breakdown and failure of the cable or accessory. TD testing can reveal the presence of PD paths in the cable.

Data obtained through PD and TD testing and monitoring can provide critical information regarding the quality of insulation and its impact on cable system health. By detecting and trending PD as well as TD, it is possible to observe its development over time to assist with strategic decisions regarding the repair or replacement of the cable.

In a pure capacitor, the current leads the voltage by 90°. The insulation, in a pure condition, will behave similarly. However, if the insulation has deteriorated, the current which flows through the insulation will also have a resistive component. This will cause the angle of the current to be less than 90°. This difference in the angle is known as the loss angle. The tangent of the angle gives us an indication of the condition of the insulation. A higher value for the loss angle indicates a high degree of contamination of the insulation.

The cable whose insulation is to be tested is first disconnected and isolated. The test voltage is applied from the very low frequency power source and the TD controller takes the measurements. The test voltage is increased in steps up to the rated voltage of the cable.

The readings are plotted in a graph against the applied voltage and the trend is studied. Healthy insulation would produce a straight line. A rising trend indicates weak insulation which may fail if the test voltage is increased beyond the rated voltage of the cable.

Cable life extension

Injection technology, otherwise known as cable insulation rejuvenation, is a well-established option to cable replacement. Cable injection technology involves the injection of a diffusive, water-reactive material into the conductor core of a buried power cable insulated with solid dielectric materials.

Once inside the cable, the fluid diffuses into the cable’s insulation and chemically combines with the water content inevitably contained within. This process retards the growth of water trees, the primary cause of cable failure in aged solid dielectric cable, simultaneously increasing overall insulation breakdown strength.

The silicon-based fluid is injected under pressure through the interstitial spaces of the conductor strands. The properties of the injection fluid causes oligomerization with water molecules in the water tree. The resulting larger molecules fill the void, repairing the dielectric properties.

Treated cables demonstrate long term survival rates on par with new cables. With application costs in the order of one third to half of the cost of cable replacement it is not surprising that injection technology has experienced remarkable market success. Most of this market growth, however, has been limited to medium voltage distribution cables.

Solid dielectric cables which transmit power at voltages above 46 kV have construction details similar to distribution cables, however questions remain regarding the potential effectiveness of injection technology in light of the increased insulation thickness required by the higher voltages.