Cable Fault Location

Before attempting to locate underground cable faults on direct buried primary cable, it is necessary to know where the cable is located and what route it takes. If the fault is on secondary cable, knowing the exact route is even more critical. Since it is extremely difficult to find a cable fault without knowing where the cable is, it makes sense to master cable locating and tracing and to do a cable trace before beginning the fault locating process. Success in locating or tracing the route of electrical cable and metal pipe depends upon knowledge, skill, and perhaps, most of all, experience. Although locating can be a complex job, it will very likely become even more complex as more and more underground plant is installed. It is just as important to understand how the equipment works as it is to be thoroughly familiar with the exact equipment being used.

Testing of Underground Power Cables

Probably 80% of all testing performed in electrical power systems is related to the verification of insulation quality. This chapter briefly describes the fundamental concepts of insulation testing including – insulation behavior, types of tests, and some test procedures. Most electrical equipment in utility, industrial, and commercial power systems uses either 50 or 60 Hz alternating current. Because of this, the use of an alternating current source to test insulation would appear to be the logical choice. However, as will be described a little later, insulation systems are extremely capacitive. For this and other reasons, DC has found a large niche in the technology. Before we can really evaluate the value of one system as opposed to the other (e.g. AC vs DC), let us examine how each type of voltage affects insulation. Testing of underground power cables are reported by NS161. (2014). IEC 6038. (1979). IEC Standard 60228. (1979). IEC60229. (2007). IEC60230. (1974). IEC60233. (1981). IEC 60332 (1974). IEC 6071 (2008). IEC 60270. (2000), IEC 60287. (2002).

Effect of Dry-Zone Formation around Underground Power Cables on Their Ratings

Current ratings of buried cables are determined by the characteristics of surrounding soils and cable properties as given in IEC 60287-1-3 (1982). In this standard the soil thermal resistivity of the surrounding soil is supposed to be varies from 0.5 oC m/w to 1.2 oC m/w but under loading the heat dissipated from underground power cables increases the soil thermal resistivity and this may leads to cable thermal failure and thermal instability of the soil around the underground cables. For this reason de-rating factors for cable loading taking the dry zone formation into consideration has to be considered during distribution cable network design. Several approaches have been adopted to establish current ratings of buried cables based on constant values of soil thermal conductivities. Mathematical models are suggested by many researches to study the drying out phenomenon around underground power cables. In this chapter de-rating factor for underground power cables taking dry zone formation into account is calculated depending on IEC 60287-1-3 (1982). This chapter also contains an experimental work carried out on different types of soils to investigate the formation of dry zone phenomena under loading by heat source simulates the underground cables.

Dry Band Formation around Underground Cable

The current ratings of underground power distribution cables are affected by ambient temperature, cable laying depth, number of cables in parallel circuits, sheath bonding and thermal resistivity of soil. One important factor usually ignored is the formation of dry zones around the underground power cables. Dry zones are usually formed around underground cables when they are loaded due to the migration of soil moisture content. This in turn may cause an abrupt rise in temperature of the cable sheath, leading to thermal damage of cable insulation or reduces the insulation life time.

Underground Cables Layout

This chapter discusses the different ways of cables installation such as open wire, aerial cable, above-ground conduits, underground ducts and underwater (Submarine) cables. The chapter explains the precautions to protect cables from moisture, general drum handling, power cable Installation guide and after installation field tests

Partial Discharges Measurements for Power Cable Insulation System

On-site PD measurements on high voltage cables have to concentrate on the cable accessories because there is a remaining risk for assembling faults on site. PD sensors with an appropriate coupling behavior to accessory-internal PD give sensitivities of a few pC or even better. Unfortunately, two main reasons prevent the general use of PD sensors in cable accessories. First of all, the costs for PD sensors have to be balanced with the costs of the accessories, importance of the cable link, consequential costs for outage etc. This is the reason why PD sensors were mainly used EHV cable systems. The second reason is limited accessibility: the PD sensor cable at the accessory has to be connected to a PD detection unit. Accessibility is much more difficult for direct buried cable systems than for cable terminations and tunnel-laid cable systems: the senor cable must pass the ground and the end up in a box on the surface to provide access. This solution causes additional costs and new problems like sealing the sensor cable against humidity, capability to withstand sheath testing etc. By looking for alternative access to PD signals from cable joints of long cable systems, a very simple solution proved suitable: detecting PD at cross-bonding links. To investigate the high frequency propagation of PD pulses in cross-bonding links, computer simulations and laboratory measurements were done.

Electrical and Water Treeing of Cable Insulation

Partial discharges (PD) have been recognized as a harmful ageing process for electrical insulation at the last century when the high voltage technology was introduced for the generation and transmission of electrical power. Since that time numerous papers and books appeared, dealing with the physics and recognition of partial discharges. First industrial PD tests of HV apparatus were introduced at the beginning of 1940. The method applied was based on NEMA 107, which specifies the measurement of ratio influence voltages (RIV) expressed in terms of µV. One disadvantage of this method is, however, that the RIV level is weighted according to the acoustical noise impression of the human ear, which is not correlated to the PD activity. Therefore, the IEC Technical Committee No.42 decided the issue of a separate standard on electrical PD measurement associated with the PD quantity apparent charge, which is expressed in terms of pC.

Sheath Bonding and Grounding

With the rapid increase in demand for electric energy and the trend for large infra-structures and vast expansion of highly-populated metropolitan areas, the use of underground power cables has grown significantly over the years. Three separate single-core cables are usually used instead of three-core cables. The principal reasons are:

Sheath Overvoltage Due to External Faults in Specially Bonded Cable System

In chapter 10, It is shown that the types of the bonding are one of the important factors which affect the sheath losses in single-core cables, and it is concluded that both single-point bonding and cross bonding, which are known as special bonding, introduce the lowest losses in the metallic sheath of the cable. To take the advantages of the specially bonded cable systems it is necessary to insulate the cable sheath from earth to avoid corrosion. This is achieved by having an extruded serving of PVC or PE on the cables and housing the joints in compound filled fiberglass boxes to insulate them from the surrounding soil. The use of special bonding gives rise to sheath over-voltages at sheath sectionalizing insulators in cross bonded cable system and insulators in a single-point bonded cable system due to lightning, switching surges or faults. One of the factors affecting the sheath losses in single-core underground power cables in case of special bonding types is the sheath overvoltage. Those over-voltages may cause the sheath multi-points break-down which result in a large sheath currents and losses and hence may cause overheating of the cables and finally leading to operation faults. As mentioned before, faults are one of reasons which cause sheath over-voltages. System faults may be divided into internal faults occurring within the cables themselves and external faults for which the cables carry some or the entire fault current. The sheath voltages resulting from internal faults may greatly exceed those caused by external faults. A fault in the cables themselves inevitably involves repair work and hence it is not so important if the sheath insulation adjacent to the fault is also damaged. The sheath bonding design should preclude the damage cascading to other parts of the cable system i.e. the cable installation must clearly be capable of safely withstanding the effects of any fault in the system external to the cables . So it is important to consider the performance of special sheath bonding methods in relation to power frequency external fault currents.

Experimental Analysis of Backfill Soils

In this chapter different types of backfill soils are investigated to select the most suitable type of soil that can be used to increase the underground ampacity by selecting the soils that limit the dry zones formation around the underground power cables. The tests carried out on the soils under study are: