Short-circuit current at the secondary terminals of a MV/LV distribution transformer
From Electrical Installation Guide
Contents |
The case of one transformer
- In a simplified approach, the impedance of the MV system is assumed to be negligibly small, so that:
where 
and:
P = kVA rating of the transformer
U20 = phase-to-phase secondary volts on open circuit
In = nominal current in amps
Isc = short-circuit fault current in amps
Usc = short-circuit impedance voltage of the transformer in %.
Typical values of Usc for distribution transformers are given in Figure G31
| Transformer rating (kVA) | Usc in % | |
| Oil-immersed | Cast-resin dry type | |
| 50 to 750 | 4 | 6 |
| 800 to 3,200 | 6 | 6 |
Fig. G31: Typical values of Usc for different kVA ratings of transformers with MV windings y 20 kV
- Example
400 kVA transformer, 420 V at no load
Usc = 4%
The case of several transformers in parallel feeding a busbar
The value of fault current on an outgoing circuit immediately downstream of the busbars (see Fig. G32) can be estimated as the sum of the Isc from each transformer calculated separately.
It is assumed that all transformers are supplied from the same MV network, in which case the values obtained from Figure G31 when added together will give a slightly higher fault-level value than would actually occur.
Other factors which have not been taken into account are the impedance of the busbars and of the circuit-breakers.
The conservative fault-current value obtained however, is sufficiently accurate for basic installation design purposes. The choice of circuit-breakers and incorporated protective devices against short-circuit fault currents is described in Chapter H.
Fig. G32: Case of several transformers in parallel
