SebaKMT August 2013 1st Edition www.sebakmt.com

In the first of a series of articles on cable fault location, Peter Herpetz looks at the construction of modern power cables and examines the most common types of fault that affect them.

The importance of cable testing

Fault location on power cables is a very special area of electrical technology, and the results obtained depend very much on good logistics and knowledge. Accurate prelocation is the foundation for fast and reliable fault location, because it means that pinpoint-ing procedures only need to be carried out on a short section of cable.

The importance of cable testing, cable fault diagnosis and par-tial discharge analysis are certain to become increasingly important in the future, as condition-based maintenance of cable networks more and more displaces event-oriented maintenance.

A good, detailed knowledge about the cable network, cable types and cable accessories greatly simplifies the evaluation of test results and, in many cases, such knowledge is an essential prereq-uisite for making correct decisions. Among the most important things that technicians need to know are the types of cable faults and the steps needed to carry out cable fault location and diagno-sis.

Construction of power cables The function of power cables is the distribution of electrical energy, and they must carry out this function reliably and safely for very long periods. Depending on the application, the external environment and local conditions, such as the presence of ground water and the type of ground voltages, different types of cable are used. Cables with impregnated insulation, such as PILC (paper insulated lead covered) types were widely used until the late 1960s and are still in service in some areas. These cables have, however, mostly been replaced by cables with PVC (polyvinylchloride), EPR (ethylene-propylene rubber), PR, or XLPE (cross-linked polyeth-ylene) insulation. As a result of these changes in the type of in-sulation used, cable faults and cable testing techniques have also changed considerably.

The following sections cannot cover all of the possible types of cables, insulating materials and cable construction, so they fo-cus on the most important variants. In many cases, details are ex-plained primarily as an aid to understanding the terminology used in the later sections of this guide to cable fault location.

Conductor

The conductor is the part of the cable that transmits current, and is usually soft electrolytic copper or pure aluminium. The con-ductor can be round or sector-shaped, and made of single wire or multi-stranded.

Insulation The purpose of the insulation is to prevent the flow of current between the conductors in the cable, and from the conductors to the cable’s metallic outer covering, which may be armour or a lead sheath. Typical insulating materials are:

n 1 to 10 kV: mass impregnated paper (PILC), polyvinylchloride (PVC)n 1 to 30 kV: mass impregnated paper (PILC), cross-linked polyethylene (XLPE), ethylene propylene rubber (EPR)n above 60 kV: paper with oil or gas, cross- linked polyethylene (XLPE)

As well as these typical materials, there are many other types of insulation.

Semiconducting layers (at nominal voltages above 6 kV)

The purpose of semiconducting layers is to reduce the strength of electric fields within the cable, and to eliminate partial discharge. Semi-conducting layers reduce the electric field that develops around the conductors, and thereby eliminate the potentially damaging discharges associated with high electric field strengths.

On modern cables, another type of semi- conducting layer is some-times integrated with the outer insulating sheath/jacket. The purpose of this type of layer is to aid the location of sheath faults on cables that are installed in ducts, where there is no return path through the earth for fault currents.

Metallic sheath The metallic sheath performs multiple functions. It seals the cable against the entry of humidity, it provides a conductive path for leakage and earth-fault currents, it provides potential equalisation and it can be used as either an earth conductor or a concentric neutral conductor. For cables used in critical or subsea applications, the metallic sheath can be designed to provide robust mechanical protection.

Shield (for MV and HV cables) The shield provides electric field control, and also offers a conductive path for leakage and earth fault currents.

Armour The armour provides mechanical protection. It may consist of steel bands, flat steel wire, round steel wires, etc. In some cases, the armour may be made up of several different layers.

Plastic sheath The plastic sheath provides outer protection for the cable, and usually consists of either PVC or polyethylene.

Cable faults When diagnosing and locating cable faults, the procedure depends on the type of cable fault. Cable faults are generally divided into the types listed here.

Conductor-to-conductor fault (parallel fault)

Unwanted connection between two or more conductors. The resist-ance of the fault may be anywhere between zero ohms (low resistance) and several megohms (high resistance).

Conductor-to-shield fault (parallel fault)

Connection between a conductor and the shield or between multiple conductors and the shield. The resistance of the fault may be anywhere between zero ohms (low resistance) and several megohms (high resist-ance).

Experience shows that the majority of faults fall into this category.

Flashing fault (parallel fault)

This is a very high resistance fault, and can be present when the cable is charged. Typically, the flashover occurs at several kV, and is very often located in cable joints. The cable behaves in the same way as an arc gap, where the distance between the electrodes determines the breakdown voltage. The resistance of this type of fault is typically infinity up to the breakdown voltage.

Open-circuit fault (series fault)

Faults of this type can be very high resistance, up to infinity if the conductor is completely severed. Very often, this type of fault is a com-bination of series and parallel resistances. The reason for this is that if the conductor is completely cut or pulled out of a joint, this not only produces a complete open circuit, but also allows all possible variations of flashover. If the conductor is partially burned, this type of fault is called a longitudinal fault.

Earth faults and sheath faults

These are faults between the metallic shield and the surrounding soil for plastic-insulated cables, or between the conductor and the surrounding soil for LV and plastic-insulated cables. Great care must be taken when using high voltages to test for or locate this type of fault, as the voltage discharges directly into the earth, creating shock hazards for people and animals.

Humid/wet faults

On multicore cables, all conductors are often affected by this type of fault, but the flashovers do not always occur at the point where the water entered the cable. Impedance changes occur at the fault position. De-pending on the cable construction (for example, the type of longitudinal water sealing), these faults can be confined to a single point or widespread throughout the cable. Humidity/wet faults are the most difficult faults to locate. They have a tendency to change during the fault location pro-cedure, often very considerably. Particularly in joints, this means that the fault becomes highly resistive after one or two discharges, as the water is blown out of the joint and dries up. When this happens, the fault can no longer be localised. Underwater faults are another form of wet fault. With these, the water pressure prevents effective ignition of the fault when high voltage is applied. These faults can be very difficult to localise.

CONCLUSION

A clear distinction must be made between short-circuit, resistive and high-resistance faults, because this distinction has a significant influence on the procedures that should be used for fault location. These proce-dures will be described in future articles in this series.

Vaina/cubierta

Protección/pantallaSemiconductor

Aislamiento/dialéctico

semiconductor interior

Núcleo/conductor

Cable faultPeter HerpertzProduct manager, power