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Title05 High Voltage Cables
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High Voltage Cables

5.0 High Voltage Cables

High Voltage Cables are used when underground transmission is required. These cables are laid in ducts or
may be buried in the ground. Unlike in overhead lines, air does not form part of the insulation, and the
conductor must be completely insulated. Thus cables are much more costly than overhead lines. Also, unlike
for overhead lines where tappings can easily given, cables must be connected through cable boxes which
provide the necessary insulation for the joint.

Cables have a much lower inductance than overhead lines due to the lower spacing between conductor and
earth, but have a correspondingly higher capacitance, and hence a much higher charging current. High voltage
cables are generally single cored, and hence have their separate insulation and mechanical protection by
sheaths. In the older paper insulated cables, the sheath was of extruded lead. Figure 5.1 shows three such
cables, as usually laid out.

The presence of the sheath introduces certain difficulties as currents are induced in the sheath as well. This is
due to fact that the sheaths of the conductors cross the magnetic fields set up by the conductor currents. At all
points along the cable, the magnetic field is not the same, Hence different voltages are induced at different
points on the sheath. This causes eddy currents to flow in the sheaths. These eddy currents depend mainly on
(a) the frequency of operation, (b) the distance between cables, (c) the mean radius of the sheath, and (d) the
resistivity of the sheath material.

5.1 Power loss in the Cable

Power loss in the cable can occur due to a variety of reasons (Figure 5.2). They may be caused by the
conductor current passing through the resistance of the conductor - conductor loss (also sometimes called the
copper loss on account of the fact that conductors were mainly made out of copper), dielectric losses caused
by the voltage across the insulation, sheath losses caused by the induced currents in the sheath, and
intersheath losses caused by circulating currents in loops formed between sheaths of different phases. The
dielectric loss is voltage dependant, while the rest is current dependant.

Figure 5.1 - Layout of three, single-core cables

������������������������������������� Heat generated
↑ �������������������
conductor ↑ ↑ ↑ ↑ ↑ ↑
loss dielectric sheath & intersheath
loss loss loss

Page 27

High Voltage Engineering - J R Lucas, 2001 ��


A condenser bushing for an r.m.s. voltage of 30 kV to earth is designed to have a uniform radial voltage
gradient (Figure 5.38). The insulating material used has a maximum permissible working voltage stress of 10
kV/cm (peak). Assuming a uniform and very small thickness of insulation between each successive foil,
determine the radial thickness t' of the bushing.

If the length of the bushing at the outermost radius is 10 cm, determine the length at the surface of the
conductor (radius 2 cm).

Estimate also the thickness t for the bushing without foils, if it is to have the same maximum radial stress.

Since stress is uniform,

The profile of the bushing has the equation y = a/x,

at x = t' + 2, y = 10 cm, so that a = 10(t' + 2) = 10(4.24 + 2)

therefore, x.y = a = 62.4

at x = r = 2, y = l

therefore, l = 62.4/x = 62.4/2 = 31.2 cm

In the absence of foils,

Thus in the absence of grading, it is seen that a much greater thickness of insulation is required (14.68 cm as
compared to 4.24 cm).

In addition to the simple cylinder bushing, and the condenser type bushing, there are other types of bushings,
which may consist of more than one material.

10 cm


������������������������ l �����������������������

Figure 5.38 - Length of Condenser Bushing


4.24 =


= t

cm 14.68 = 7.342 x 2 = t 8.342 = t +1 .e.i





2.121 = 2 1.5 = t) + (1

2 + t


2 30
= 10

t+ r







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