While it is difficult to predict all trends and developments in regard to future application of underground power cables, one can reasonably forecast that voltage levels will continue to increase to extra high voltages. At present, the most important factor in the decision between overhead line and cable is still cost and the former remain the preferred means of power transmission. But this cost differential is changing in favour of cable due to new materials and improved production conditions. At the same time, other decision criteria are becoming more relevant, including aesthetics, network reliability, government regulation and public opposition to overhead lines. Especially in urban areas, the aspect of aesthetics will become more important. Increasingly, MV overhead lines will disappear in urban networks while the share of overhead lines compared to cables will likely decrease in rural areas as well. The reason for this is superior reliability. The outage rate of new XLPE cables is outstanding, far superior to older PE or paper-insulated cable and also better than for a MV overhead line (see Table 1)
Fig. 1 compares the mean unavailability value with cable rate in the Netherlands, Germany, Italy and Spain and makes it apparent that a higher MV network cable rate has great inuence on system reliability. Indeed, Sweden has begun increasing its MV cable rate with the goal of reducing impact of severe weather events. For these reasons the MV cable share will continue to increase.
The situation at the high and extra high voltage levels, however, is entirely dierent. Because of higher cost, overhead lines will remain the dominant transmission system for electrical power. Still, in certain megacities, there will continue to be cable applications up to 400 kV for parts of the system and numbers of such projects will increase. Another option is utilization of gas insulated lines (GIL). But while GIL combines the advantages of EHV cable with the capacity of an EHV overhead line, it comes with much higher investment cost than cable.
Design of medium voltage cables can reasonably be considered as mature. Nevertheless, the high number of dierent cable constructions used worldwide will require the industry to converge to some standardized, maintenance-free and environmentally- friendly designs. Among the nearly 100 dierent MV cable constructions presently available, the conductor can be stranded copper or aluminum or solid aluminum. The insulating material can be XLPE or EPR having a fully-bonded or peelable outer semi-conducting layer. The cable screen can be a copper wire or tape screen or a laminated aluminum screen covered by an outer sheath made of PVC or PE. A MV system can consist of three single core cables or one three core cable. In the case of the latter, steel wire armoring is common. Selection among these many possible alternative cable constructions depends on tradition as well as on local laying and economic variables.
HV and EHV AC cable designs can also be considered as basically mature. Here, the conductor is stranded aluminum or copper and the insulation material is XLPE, always with a fully-bonded outer semi-conducting layer. The screen can be a lead or welded aluminum sheath, a copper wire screen or a combination of copper wire screen and laminated aluminum sheath.
For subsea interconnections and to connect more and more o-shore wind parks to the grid, AC and DC submarine cables are increasing in application and this applies to DC land cables as well. Three core submarine cables are used for AC applications and single core cables for DC.
The price and weight advantages of aluminum at the medium voltage level is the main reason why use of solid aluminum conductors is expected to increase. In some cases, the weight factor also favors aluminum conductors for HV and also for some submarine cable applications. At EHV, however, an aluminum conductor needs much more insulating material to reach the same transmission capacity for the cable system. That is why copper remains the preferred conductor material, particularly for the upper range of cross-sections. In order to reduce the skin effect in AC cables, individually insulated strands as a so-called ‘Milliken conductor’ or enameled wire conductors are possible and require special connectors.
The insulating materials for MV cables will be predominantly XLPE and EPR with possible growing use of polypropylene as a kind of ‘green’ cable insulation. The limit for EPR insulation is the HV level because of its loss factor, which is much higher than for XLPE. HV DC and EHV AC polymeric cables will only be XLPE type while some EHV DC submarine cables will continue to see use of special paper-impregnated insulation.
For reasons of cost reduction, screen design of MV cable is expected to go in the direction of laminated aluminum screens. In the case of HV cables, a combination of copper wire screen and laminated aluminum sheath is a cost optimized and installation friendly solution. To reduce screen losses in single core cables, cross bonding of the screen is important and a wire screen offers an easy way to put the screen out of the joint. The six screen terminations external to the joint can be connected in a screen connection box. To apply cross bonding for other screen solutions, an additional contact device is required and makes installation more complex.
Application of high temperature superconductor cables for medium and high voltage has moved from the prototype stage to initial field installations. For example, a 10 kV threecore coaxial such cable of one kilometer length and nominal current 2.31 kA has been installed in Essen, Germany.
Connection of cable conductors can be accomplished by soldering or welding or by use of compression or shear-bolt type connectors. Since heat is a problem for XLPE insulation, use of compression or mechanical connectors has become the standard at all voltage levels. For LV and MV applications, the classic compression connector is increasingly being replaced by the shear bolt connector. Given the range of different sizes, materials and shapes of cable conductors, the advantage of a mechanical connector is evident. More and more shear bolt connectors, even at HV and EHV and for cross-sections up to 2500 mm², are now available and connector technology will continue to move in this direction.
The connector is one of the most important components in a joint and can become the reason for thermal destruction of its insulation. That is why the connector should aim to minimize risk of mistakes during installation. A mechanical connector fulfils this requirement better than does a compression connector. Each type of connector must be able to carry the nominal current of the cable as well as the maximum short-circuit current of the network for a certain period without overheating. This must be confirmed by long-term as well as short circuit tests.
A straight joint is a connection of two identical single or three core cable ends. It consists of connector, insulating body including stress control device, screen connection and outer protection against moisture and mechanical damage. A transition joint is a connection of two different cables ends, most often used to link a paper and a polymeric insulated cable. At the MV level, heat shrink joints are widely used today for straight and transition joint applications. This will likely be the case in the future since it is a cost-efficient and robust solution, suitable for all cable types and sizes. The main disadvantage is the number of components and different installation steps. If application of heat is not possible or if there are other disadvantages to a heat shrink joint, use of a slip-on (or cold shrink) joint has also become common for MV applications. The material used for the joint insulation body, which can be a single piece or a three piece solution, is typically silicone or EPDM.
For HV and EHV cables, there is no application of heat shrink joints. Rather, it is necessary to use a pre-fabricated, factory tested silicone or EPDM insulating body and the market trend is from slip-on to cold shrink technology. The main reason is easier installation of cold shrink accessories.
A one piece solution is more complex, both in production and installation, and needs a parking position on the cable insulation. However, it can be completely pre-tested at the factory. Cold shrink technology combines the advantage of a single piece, pre-tested insulating body with an easy installation procedure.
For moisture and mechanical protection of joints, it is now common to use heat shrink components or cast resin lled shells in combination with metallic radial water barriers. It is indispensable to apply a stress control element at the end of the outer semi-conducting layer in all joints and terminations. In the case of polymeric insulated cables, two different design and material solutions have been accepted by users, each with dierent advantages and application limits – the classical geometric stress control system that is suitable for all voltages and refractive stress control applicable mainly for medium voltages.
Terminations & Plug-In Connectors
Terminations and plug in connectors are necessary to connect the cable end to an overhead line or to switchgear or transformers. All types of installation technologies are used at the medium voltage level, from heat shrink to slip-on to cold shrink. For HV XLPE cables, the classical outdoor termination is uid lled and equipped with a porcelain insulator. This type of termination has had long service experience and oers excellent resistance against tracking, radiation and attack by birds. Still, in recent years composite type housings have captured an ever growing share of this application due to advantages such as hydrophobicity, low weight, resistance to earthquake and safer failure mode. A new design of termination especially suitable for polymeric cables consists of a stress control element and a composite insulator lled with silicone gel or insulating gas.
A new generation of terminations, coming from experience at medium voltages, are called ‘dry type’ and oer advantages such as easier installation, earthquake and explosion resistance and no need for oil lling.
Especially at medium voltages, more and more cables are connected to SF6 insulated switchgear. In this case, plug-in connectors are used. There are two standardized plug-in systems on the market: the outer cone connector system used widely in medium voltage ring main units; and the inner cone system used in MV primary substations and also for high voltage applications. The latter system, for HV applications up to 400 kV, is also used for outdoor terminations and joints. It is expected to become one of the main elements of cable accessories of the future, at all voltage levels.