Medium voltage switchgear application guide

Medium voltage switchgear application guide
Appendix 1:
transformer
A transformer is defined by its rated power, its rated primary and
secondary currents, its rated primary and secondary voltages, its
insulation level, its short-circuit voltage and its coupling.
The peak-switching current is also a parameter that should not be
forgotten in the determination of protective switchgear.
1 - RATED POWER
The
S = U
rated power (S in kVA or MVA) of a transformer is equal to:
• 3 • I r
2 - RATED CURRENT OF ONE WINDING
The rated current Inis equal to:
I r = 3 S •U
3 - RATED VOLTAGE AND INSULATION LEVEL OF ONE WINDING
This in “no-load” is the specified operating voltage between to be the applied terminals (primary) of one or transformer developed (secondary)
The rated primary voltage must be higher than or equal to that of the winding. network.
rated
voltage
U r (kV)
insulation level
rms kV (50 Hz - 1 mn) kV impulse (1.2/50 µs)
7.2 20 60
12 28 75
17.5 38 95
24 50 125
36 70 170
4 - SHORT-CIRCUIT VOLTAGE
The short-circuit short-circuit current voltage to be (expressed calculated. in %) allows the transformer’s secondary
The short-circuit current is given in the following equation:
•
I k =In
Uk: Ir: rated
U
100
short-circuit current
k
andP k =U r •I k • 3
in voltage Ampsin %
given The usual in the Ukvalues tables on depending the following on transformer page. voltage and power are
The real values must be requested from the manufacturer.
date
11/94
- B•3•2 -
revised
05/2004
page 1