MV design guide - Schneider Electric

Design rules
Short-circuit currents
Here is the solution
to the problem with the
calculation method
Solving the exercise
c Determining the various short-circuit currents
The three sources which could supply power to the short-circuit are
the two transformers and the alternator.
We are supposing that there can be no feedback of power through
D4, D5, D6 and D7.
In the case of a short-circuit upstream of a circuit breaker (D1, D2,
D3, D4, D5, D6, D7), this then has the short-circuit current flow
through it supplied by T1, T2 and G1.
c Equivalent diagram
Each component comprises a resistance and an inductance.
We have to calculate the values for each component.
The network can be shown as follows:
Zr = network impedance
Za = alternator impedance different
according to state
(transient or subtransient)
Z15 = transformer
impedance 15 MVA
Z20 = transformer
impedance
20 MVA
busbars
Experience shows that the resistance is generally low compared with,
reactance, so we can therefore deduce that the reactance is equal to
the impedance (X = Z).
c To determine the short-circuit power, we have to calculate the
various values of resistances and inductances,
then separately calculate the arithmetic sum:
Rt = R
Xt = X
c Knowing Rt and Xt, we can deduce the value of Zt by applying the
equation:
Z =
( ∑R 2 + ∑X 2 )
N.B.: Since R is negligible compared with X, we can say that Z = X.
Schneider Electric
Merlin Gerin MV design guide
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