Medium Voltage Design Guide

Prefabricated metal-enclosed switchgear

In order to design a medium-voltage cubicle, you need to know the following basic magnitudes: Voltage, Current, Frequency and Short-circuit power. The voltage, the rated current and the rated frequency are often known or can easily be defined, but how can we calculate the short-circuit power or current at a given point in an installation?

Knowing the short-circuit power of the network allows us to choose the various parts of a switchboard which must withstand significant temperature rises and electrodynamic constraints.

Disconnection, control and protection of electrical networks is achieved by using switchgear. Metal enclosed switchgear is sub-divided into three types: metal-clad, compartmented and block.

Short-circuit power

The short-circuit power depends directly on the network configuration and the impedance of its components: lines, cables, transformers, motors… through which the short-circuit current passes. It is the maximum power that the network can provide to an installation during a fault, expressed in MVA or in kA rms for a given operating voltage.

The customer generally imposes the value of short-circuit power on us because we rarely have the information required to calculate it. Determination of the short-circuit power requires analysis of the power flows feeding the short-circuit in the worst possible case.

Possible sources are:

1. Network incomer via power transformers.
2. Generator incomer.
3. Power feedback due to rotary sets (motors, etc); or via MV/LV transformaters.