Cable accessories perform the key function of ensuring required insulation either at the connection points of cables (straight joints) or at their ends (terminations). These accessories have to be easy and safe to install over a broad range of different cable cross-sections and ideally should consist of as few components as possible. Since the start of the shift away from medium voltage (MV) paper-insulated cables and toward polymeric cables more than 45 years ago, there have been a steadily increasing number of different types in use throughout the world. For all these different cables, successive generations of accessories have been introduced. Heat shrink technology, with polyolefin material used for terminations as well as joints, was gradually replaced by slip-on and cold shrink technology, using either silicone or EPDM. For example, the substitution of air-insulated switchgear by gas-insulated switchgear (GIS) led to the replacement of terminations by separable connectors made from these polymers. Due to the increasingly large variety of different types of cable accessories, questions naturally arose as to which offered the best material, the best installation technology and the optimal design for each application. Since the answer in each case depends on application requirements and service conditions, this has naturally been a topic of debate within the industry. This past INMR article outlined the advantages and disadvantages of different material combinations and design features as they apply to MV polymeric cables, with focus on outdoor terminations and straight joints.
To better appreciate the diversity of MV cables available these days, it is useful to examine their different components. As evident in Fig. 1, which shows but a small sample of the many different cables now available, the conductor can be copper or aluminium, with a solid or a stranded conductor and with rounded or sector shape. Standard cross-sections vary from 10 mm² up to as much as 800 or 1000 mm², although the large majority of MV cables in use today fall between 95 and 300 mm².
The insulation material and inner and outer semi-conductive layers of cables are generally made of cross-linked polyethylene (XLPE) or ethylene propylene rubber (EPR). The outer semi-conductive layer can be fully bonded with the insulation (removable only with a special tool) or strippable by means of knife, which is the case for most sector shaped and EPR cables. Typically, the screen for single and three core cables consists of copper wire, but it can also be made of copper tape, aluminium tape, aluminium wire, copper or steel mesh or even a lead sheath. Some three-core cables have armouring using steel wire or tape while single core cables usually have armouring made of a non-magnetic material such as aluminium wire. The outer sheath material can be PE or PVC, available in different colours. Accessories for MV cables should ideally be applicable over this wide range of different cable types and constructions. Fig. 2 outlines some of the most important applications, including:
1. Indoor termination, i.e. connecting a cable with air-insulated switchgear
2. Outdoor termination, i.e. connecting a cable with an overhead line
3. Straight joint connecting two cables
4. Transition joint, i.e. connecting polymeric with paper-insulated cable
5. Separable connector, i.e. connecting a cable with a gas insulated switchgear
In certain cases, the number of accessory variants can effectively be reduced by using only one type for different voltage classes, different cross-sections and even different applications (e.g. outdoor termination for 24 kV or alternatively indoor termination for 36 kV). While paper-insulated cables are still in use in some parts of the globe, even for new applications, their importance is declining rapidly. As such, the following review does not cover accessories for this technology.
With increasing voltage beyond 10 kV, the problem of electrical field control (or stress control) has become more and more important and a major requirement in the design of cable accessories. Fig. 3 shows the main components of a MV cable.
Because of components 2, 3 and 4 in a MV cable, the internal field is slightly non-homogeneous (see sectional drawing at right of Fig. 3).
In order to install a joint or termination, it is necessary to remove the outer semi-conductive layer along a certain length. At this end of the outer semi-conductive layer (i.e. the cut edge), electric field intensity increases considerably (see function E along the cable insulation surface). To reduce this field intensity, two different solutions are considered suitable. One sees geometric field control (Fig. 4a), using a stress cone made of semi-conductive material. The other involves refractive field control (Fig. 4b) using a tube of an insulating material with a dielectric constant between 10 and 20. The advantage of the former is that it represents a loss-free solution that is also suitable for high and extra high voltage. On the other hand, it requires more material consumption and is not that good a solution for multi range type accessories. Refractive field control (b), by contrast, is a good solution for a wide range of different MV cable diameters because of its slim tube construction. This type of field control can compensate for any mistakes made during cable preparation (such as shown in Fig. 5) but will also result in low thermal losses at this critical point in the termination.