High-voltage cable

A high voltage cable - also called HV cable - is a device for electric power transmission at high voltage. In practice there are three types of HV cable:
1. X-ray cable, used in small lengths in X-ray equipment, electron microscopes and other scientific equipment,
2. AC power cable, used in lengths of many kilometers for underground transmission of large blocks of electric power at AC voltage,
3. DC power cable, or HVDC cables, used in lengths up to several hundreds of kilometers at DC voltage, often executed as submarine cable.

X-ray cable

X-ray cables ^[1] are used in lengths of some meters to connect the HV source with an X-ray tube or any other HV device in scientific equipment. They transmit small currents, in the order of milliamperes at DC voltages of 30 to 200 kV, or sometimes higher.
The cable shall be flexible, the insulation is therefore made of a rubber-like material or elastomer and the outer sheath of braided copper-wire; in principle the construction is not different from that of HV power cables as shown in figure 1 in the following section.

AC power cable

Although high voltage is defined as any voltage over 1000 volts and cables for 3000 and 6000 volts exist, the majority of these cables are used from 10 kV onwards ^[2]. Those of 10 to 33 kV are usually called medium voltage cables, those over 50 kV high voltage cables.

The construction of a modern HV cable is simple, but is refined at the same time. Simple, because the design includes a few elements only; refined, because the physics behind it are rather sophisticated.
A conductor of copper or aluminum wires transports the current, see (1) in figure 1. Conductor sections up to 2000 mm² may transport currents up to 2000 amperes.
The insulation (3) consists of cross-linked polyethylene, also called XLPE. It is reasonably flexible and tolerates operating temperatures up to 120 °C.
At the inner (2) and outer (4) sides of this insulation, semi-conducting layers are fused to the insulation ^[3]. The function of these layers is to prevent air-filled cavities between the metal conductors and the dielectric so that no electric discharges can arise and endanger the insulation material ^[4].
The outer conductor or sheath (5) serves as an earthed layer and may conduct return currents if needed.

Quality

During the development of the HV insulation, which has taken about half a century, two characteristics proved to be paramount.
First, the introduction of the semi-conducting layers. These layers shall be absolutely smooth, without any protrusions even as small as some microns. Further the fusion between the insulation and these layers shall be absolute^[5]; any fission, air-pocket or other defect - of the same micro-dimensions as above - is detrimental for the breakdown characteristics of the cable.

Secondly, the insulation shall be free of inclusions, cavities or other defects of the same sort of size. Any defect of these types shortens the voltage life of the cable which is supposed to be in the order of 30 years or more ^[6].
Cooperation between cable-makers and manufacturers of materials has resulted in grades of XLPE of absolute cleanliness with tight specifications about the number and size of foreign particles per pound or per kilogram. ^[7]. Also the way of packing the raw material and unloading it within a cleanroom environment in the cable-making machines has been developed into a highly sophisticated procedure.

Furthermore, the development of extruders for plastics extrusion and cross-linking has resulted in high-tech cable-making installations^[8] for making defect-free and pure insulations.

HVDC cable

A high voltage cable for HVDC transmission has the same construction as the AC cable shown in figure 1. The physics and the test-requirements are, however, much different ^[9]. In this case the smoothness of the semi-conducting layers (2) and (4) is of utmost importance, whereas the cleanliness of the insulation remains imperative.

Many HVDC cables are used for DC submarine connections, because at distances over 30 km AC cannot longer be used. The longest submarine cable today is the NorNed cable between Norway and Holland that is almost 600 km long and transports 700 megawatts, a capacity equal to two large power stations.
Most of these long deep-sea cables are made in an older construction, using oil-impregnated paper as an insulator.

Cable terminals

Any high voltage cable—whether flexible, AC power or DC power—shall be terminated with a construction that takes care of the electric fields at the ends ^[10]. Without such a construction the electric field will concentrate at the end of the earth-conductor as shown in figure 2.
Equipotential lines are shown here which can be compared with the contour lines on a map of a mountainous region: the nearer these lines are to each other, the steeper the slope and the greater the danger, in this case the danger of an electric breakdown. The equipotential lines can also be compared with the isobars on a weather map: the denser the lines, the more wind and the greater the danger of damage.

In order to control the equipotential lines (that is to control the electric field) a device is used that is called a stress-cone, see figure 3 ^[11]. It consists of a rubber or elastomer body that is stretched over the cable end ^[12]. On this rubber-like body R an earthelectrode is applied that spreads the equipotential lines. These lines pass the surface of the body after they have sufficiently been spread out to guarantee a low electric field.

The crux of this device, invented by NKF in Delft in 1964 ^[13], is the fact that the bore of the elastic body R is narrower than the diameter of the cable. In this way the (blue) interface between cable and stress-cone is brought under mechanical pressure so that no cavities or air-pockets can be formed between cable and cone. Electric breakdown in this region is prevented in this way.

This construction can further be surrounded by a porcelain insulator for outdoor use ^[14], or by contraptions to enter the cable into a power transformer under oil, or switchgear under gas-pressure ^[15].

Cable joints

Connecting two high-voltage cables with one another poses two main problems. First, the outer conducting layers in both cables shall be terminated without causing a field concentration ^[16], similar as with the making of a cable terminal. Secondly, a field free space shall be created where the cut-down cable insulation and the connector of the two conductors safely can be accommodated ^[17]. These problems have been solved by NKF in Delft in 1965 ^[18] by introducing a device called bi-manchet.

Figure 4 shows a photograph of the cross-section of such a device. At one side of this photograph the contours of a high voltage cable are drawn. Here red represents the conductor of that cable and blue the insulation of the cable. The black parts in this picture are semi-conducting rubber parts. The outer one is at earth potential and spreads the electric field in a similar way as in a cable terminal. The inner one is at high-voltage and shields the connector of the conductors from the electric field.
The field itself is diverted in a sophisticated way as shown in figure 5, where the equipotential lines are smoothly directed from the inside of the cable to the outer part of the bi-manchet (and vice versa at the other side of the device).

The crux of the matter is here, like in the cable terminal, that the inner bore of this bi-manchet is chosen smaller than the diameter over the cable-insulation ^[19]. In this way a permanent pressure is created between the bi-manchet and the cable surface and cavities or electrical weak points are avoided. This simple, but sophisticated construction has proven its reliability during more than 40 years.

Although the construction is simple, the job of installing a terminal or bi-manchet is highly skilled work. Removing the outer semi-conducting layer at the end of the cables, placing the field-controlling bodies, connecting the conductors, etc., ask for high skill and extreme precision.

Sources and notes

This article is based on:

[1] F.H. Kreuger, Industrial High Voltage, Delft University Press, 1991, ISBN 90-6275-561-5. Parts 1, 2 and 3 in one Volume.
[2] ibid, Industrial High Voltage, Delft University Press, 1992, ISBN 90-6275-562-3. Parts 4, 5 and 6 in one Volume.
[3] E. Kuffel, W.S. Zaengl, High Voltage Engineering, Pergamon Press, Oxford; later edition 2004, Butterworth-Heinemann, ISBN 0-7506-3634-3.

Notes

^ see [1] pages 65, 133
^ see [1] pages 133-137
^ see [3] and [2] page 118
^ see [3] section Discharges
^ see [2] picture 8.1e
^ see [2] pages 87-91
^ Most producers of XLPE-compound, like AKZO-ICI, DOW, Borealis, DuPont, specify an “extra clean” grade where the number and size of foreign particles are guaranteed
^ see Wikimedia File:Extruder section.jpg
^ see [2]pages 15-19
^ see [1] pages 53,147,153
^ see [1] pages 147-153
^ see [1] fig. 10.7
^ Dutch Patent 123795, Netherlands Cable Works NKF, submitted 21-4-1964, granted 27-3-1968
^ see a similar case in [1] page 160
^ see a similar case in [1] page 157
^ see [1] page 156
^ see [1] page 154
^ Dutch Patent 149955 of Netherlands Cable Works NKF, submitted 4-11-1965, granted 17-11-1976
^ see [1] page 155

[1] see [1] pages 65, 133

[2] see [1] pages 133-137

[3] see [3] and [2] page 118

[4] see [3] section Discharges

[5] see [2] picture 8.1e

[6] see [2] pages 87-91

[7] Most producers of XLPE-compound, like AKZO-ICI, DOW, Borealis, DuPont, specify an “extra clean” grade where the number and size of foreign particles are guaranteed

[8] see Wikimedia File:Extruder section.jpg

[9] see [2]pages 15-19

[10] see [1] pages 53,147,153

[11] see [1] pages 147-153

[12] see [1] fig. 10.7

[13] Dutch Patent 123795, Netherlands Cable Works NKF, submitted 21-4-1964, granted 27-3-1968

[14] see a similar case in [1] page 160

[15] see a similar case in [1] page 157

[16] see [1] page 156

[17] see [1] page 154

[18] Dutch Patent 149955 of Netherlands Cable Works NKF, submitted 4-11-1965, granted 17-11-1976

[19] see [1] page 155

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