CABLES, WIRES AND CABLE INSTALLATION PRACTICES 2139.4.3.2 Volt-drop in long cables
Let the series resistance beRohms, the series inductive reactance beXlohms and the total shunt
capacitive reactanceXcohms for a cable of l kilometre. Manufacturers usually quote the shunt
capacitance data in microfarads/km.
The method described in sub-section 9.4.3.1 may not always be sufficiently accurate for long
cables where the shunt capacitive reactance cannot be neglected. Two more accurate methods can be
used in which the cable is treated as an equivalent ‘Tee’ or an equivalent ‘Pye’ circuit, see Figures 9.2
and 9.3.
In these methods the complete solution must be found without the simplification made in (9.2).
These methods will be shown by an example.
9.4.3.2.1 Worked example
A 240 mm^2 3-core polymeric insulated cable 25 km in length feeds a static load of 20 MVA at a
power factor of 0.95 lagging. The nominal system voltage is 33,000 V and the sending end voltage
Figure 9.2 Equivalent Tee circuit of a long cable.Figure 9.3 Equivalent Pye circuit of a long cable.