Medium voltage switchgear application guide

Medium voltage switchgear application guide
Appendix 4: generator
A generator is defined by its power, its voltage and its internal
impedance.
1 - RATED POWER
The
S
rated power (S in kVA or MVA) of a generator is equal to:
In
r = U • 3 •Ig
large power stations, the power ranges between 500 and 1500 MVA
2 - RATED CURRENT
The rated current is equal toIg = 3
S
•U
r
3 - RATED VOLTAGE
Generator standardised. voltage — contrary to distribution network voltage — is not
There a variety of rated voltage values for generators.
generator power
(MVA)
S n < 200 10 and 16
200 - 400 14 and 21
400 - 600 16 and 23
600 - 1000 18 and 26
S n > 1000 20 and 27
generator rated voltage
(kV)
4 - INTERNAL IMPEDANCE
Manufacturers The resistance generally values are specify always generator negligible impedance before the reactance expressed values. in %.
subtransient reactance reactance (X''d ≈20%),
■the earth reactance (X'd ≈30%), (X'o ≈6%).
These making values capacities will allow as well us to as determine protection the setting. circuit breaker breaking and
Ik: (three-phase) short-circuit kA current
Ur: Sr: generator generator rated rated power voltage (MVA)
X'cc: transient reactance expressed (kV) in % by the manufacturer.
date
11/94
- B•3•2 -
revised
05/2004
I k = 3
S
•U
r •
r X' 1
cc = 345
•10 100 30 •
= 8.66 kA
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