MV design guide - Schneider Electric

Switchgear
definition
Medium voltage circuit breaker
Rated cable-charging breaking current
(cf. § 4 .108 IEC 60 056)
The specification of a rated breaking current for a circuit breaker located
at the head of no-load cables is not compulsory and is considered as not
being necessary for voltages less than 24 kV.
c Normal rated breaking current values for a circuit breaker located at the
head of no-load cables:
Rated voltage
(Ur in kV)
(Ic in kA)
7.2 10
12 25
17.5 31.5
24 31.5
36 50
Rated breaking current
for no-load cables
Rated line-charging breaking current
(cf. § 4.107 IEC 60 056)
The specification of a rated breaking current for a circuit breaker switch
situated at the head of no-load lines is limited to overhead, three-phased
lines and to a rated voltage ≥ 72 kV.
L
A
B
Rated single capacitor bank breaking current
(cf. § 4.109 IEC 60 056)
G
U
Ic
C
The specification of a breaking current for a circuit breaker switch located
upstream of capacitors is not compulsory. Due to the presence of
harmonics, the breaking current for capacitors is equal to 0.7 times the
device's rated current.
Rated current Breaking current for capacitors
(A)
(A)
400 280
630 440
1250 875
2500 1750
3150 2200
By definition
pu =
r
Ur
e
c The normal value of over-voltage obtained is equal to 2.5 pu, this being:
X1
2.5 • Ur r
e
G
U
Rated back-to-back capacitor bank breaking current
(cf. § 4.110 IEC 60 056)
The specification of a breaking current for multi-stage capacitor banks is
not compulsory.
c If n is equal to the number of stages, then the over-voltage is equal to:
C1 C2 C3
2n
2n + 1
• pu with pu =
r
Ur
e
Schneider Electric
Merlin Gerin MV design guide
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