Medium Voltage Application Guide

13846.A
SWITCHGEAR
Terminal voltage drop
If a load draws more current than the transformer's rated secondary current (I-load > I r ), the transformer's output
voltage will drop from its rated value, U r . The amount of voltage drop is determined by the internal impedance of
the transformer and the level of overload.
Voltage drop analysis is useful for determining the suitability of a transformer, when a motor is a large portion of the
load. It is recommended that the transformer output voltage should not drop more than 10% of its nominal value
when using a soft starter to start a motor.
U-int
Ir
I-load
E
Z-int
(Z%)
U r
1
Load
Where
I r = rated secondary current (A)
S = transformer power (kVA)
U r = rated secondary voltage (kV)
E = transformer internally generated EMF (kV)
Z% = percentage impedance (%)
Z-int = transformer internal impedance ()
U-int = internal voltage drop (kV)
I-load = load current (A)
U r' = secondary output voltage due to overload (kV)
Example
Transformer power S = 30 MVA
Secondary rated voltage U r = 6.6 kV
Impedance Z% = 10%
Assume infinite power system
Rated secondary current, I r
r
r
The rated secondary current is 2625 A.
Transformer internally generated EMF, E
r
The internally generated EMF is 7.33 kV.
710-12280-00A Medium Voltage Application Guide Page 145