Page 68 - Energize October 2022

TECHNICAL

Rated voltage Current rating Capacitance Cutoff length CCL = 10% CCL = 5% CCL = 1%
(kV) (A) μF/km (km) cutoff (km) cutoff (km) cutoff (km)
35 195 0,142 133 13,3 6,7 1,3
22 200 0,203 143 14,3 7,2 1,4
11 195 0,221 156 15,6 7,8 1,6
6,6 195 0,221 262 26,2 13,1 2,6
6,6 300 0,327 442 44,2 22,14,4
Table 1: Cut-off length due to charging capacitance for XLPE MV cable (M-tech cables data)

Reactive power effect due to capacitance
Capacitive charging current also affects the power factor of
the distribution network. Under low loading conditions the
charging current predominates and causes the power flow to
be mainly reactive, with an accompanying low power factor and
high network losses. Low loading conditions can result from
feed-in from DER systems, a factor which is often ignored.

Ferranti effect and voltage rise effects Figure 4: Nominal Π model of the line at no load (elprocus.com)
The Ferranti effect results in the voltage at the receiving end
of a cable rising above the voltage at the sending-end, on approximate basis can be obtained by lumping the inductance
lightly loaded or unloaded power cables. In extreme cases and capacitance parameters of the line into a single Π section as
the voltage can exceed the rated value of the cable, and can shown in Figure 4.
affect the cable protection devices on the line. The effect is
due to the capacitance and inductance of the cable. It occurs Where:
on very long transmission lines, but because the capacitance C = The capacitance per unit length (μF/km)
of cables is much higher, it occurs at much shorter lengths L = the inductance per unit length (Mh/km)
of cable than transmission line and is more prevalent. The
Ferranti effect has a pronounced effect in underground From the Π model of the cable [3]
cables, possibly even in short lengths, because of their high
capacitance
The effect occurs when current drawn by the distributed
capacitance of the transmission line itself is greater than the where Z = the series impedance = R + jωLl, Y = the shunt
current associated with the load at the receiving-end of the admittance = jωCl, l is the length of the cable (km), and Ir is the
line and tends to be a problem on lightly loaded lines. The current flowing at the receiving-end.
capacitive line charging current produces a voltage drop across
the line inductance that is in phase with the sending-end Under no load conditions Ir = 0 and can be ignored.
voltage. The Ferranti effect will be more pronounced the longer
the cable and the higher the voltage applied.
The Ferranti effect is not a problem with lines that are
loaded because line capacitive effect is constant independent
of load, while the inductive current will vary with load. Both Ignoring resistance:
the line inductance and capacitance are responsible for this
phenomenon. The relative voltage rise is proportional to
the square of the cable length. The extent of the voltage
rise may be estimated using a simplified model of the cable. This equation shows that (Vs – Vr) is negative. That is, Vr > Vs.
Underground cable is usually modelled as lumped T or Π This equation also shows that the Ferranti effect depends on
sections. (Figure 3). frequency and electrical length of the line. The Ferranti effect
A simplified explanation of the Ferranti effect on an voltage rise factor is the ratio of the receiving-end voltage to the
sending end voltage

Figure 3: Lumped Π section model of underground cable

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