Page 66 - Energize October 2022

TECHNICAL

Effects of cable capacitance

on MV and LV distribution cable

operational limits

Underground cables are now commonly used for the MV and “last mile” LV portion of distribution

systems in urban areas, to avoid the visual and other effects in modern networks. The main
argument against constructing underground systems is usually financial, but costs are not the only
limitation. The basic laws of electricity limit how long, physically, an underground cable can be.

by Mike Rycroft, Energize

hese factors are relatively unimportant on overhead the charging current could equal the total amount of current the
lines but will limit the length of underground MV and LV line can carry. This will severely limit its ability to deliver power.
Tcable systems. The limiting factor is the capacitance of Charging current can also have an impact on the operation
the cable, which is much higher than that of overhead lines and of protective devices, and must be taken into account in the
has a much greater effect on performance. Generally, because settings of such devices.
of the lower voltage used, and the relatively short length of The Ferranti effect results in the voltage at the end of a cable
distribution feeders, the effects of capacitance on distribution exceeding the voltage at the connection point under unloaded or
cables is not a major factor, but the growth of DER systems lightly loaded conditions. The effect can impact the operation of
on distribution lines is changing the operating conditions to surge protection devices, and can affect inverter based systems
the point where the cable’s capacitive effects can become connected to the network.
significant.
There are two main limiting effects of cable capacitance: Cable capacitance
charging current, and the Ferranti effect. The cable charging Three-core cables
effect results in capacitive current flowing in the cable under Capacitance exists between the conductors of a cable and
both loaded and unloaded conditions. With high enough value between the conductors and sheath. The capacitance existing in
of capacitance the charging current can exceed the rated a three-core cable is illustrated in Figure 1. In an armoured cable
current. At low loads the current is largely reactive, resulting in the sheath consists of conductive armouring wires which have a
a low power factor. great effect on capacitance
If a cable is long enough, and the voltage is high enough, Consider a three-cored symmetric underground cable as

(a) (b) (c)
Figure 1: Capacitance of underground cables (Circuitglobe )

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