Medium Voltage Application Guide

SWITCHGEAR
Step 3: Calculate the equivalent capacitance of banks which are switched in (C eq )
C eq = C 2 +C 3 +C 4
= 55+55+55
165 µF
The equivalent capacitance is 165 µF.
Step 4: Calculate the equivalent inductance of banks which are switched in (L eq )
1
L eq =
1 + 1 +
L 1
2
L 3
L 4
=
1
1 + 1 + 1
40 40 40
= 40
3
=13.3 µH
The equivalent inductance is 13.3 µH.
Step 5: Calculate the peak inrush current, I p
I p
= U×
2 C
× 1 × C eq 1
×
3 C 1 + C eq L 1 + L eq
= 6300 ×
2 55 × 165 1
× ×
3 55 + 165 40 + 13.3
= 6300 ×
2 9075 1
× ×
3 220 53.3
= 6300 ×
1
0.67 × 41.25 ×
53.3
= 6300 × 0.67 × 41.25 × 0.019
= 6300 × 0.52
= 4543 A
The peak inrush current is 4543 A.
To be suitable, the peak inrush current (I p ) must be less than 100 times the capacitor's nominal
current: I p 100 x I nom
Q
Inom
=
3×
U
689
=
3×
6.3
689
=
10.91
= 63.15 A
In this example, 4543 100 x 63.15 A
This installation is acceptable.
NOTE
When selecting line fuses for upstream protection, the fuse pre-melt figure must be greater than the
capacitor bank's peak inrush current.
If using a circuit breaker for upstream protection, the circuit breaker's making capacity at rated voltage
must be at least equal to the capacitor bank's peak inrush current.
Page 142 Medium Voltage Application Guide 710-12280-00A