Medium Voltage Application Guide

13812.A
SWITCHGEAR
Calculations: peak inrush current of a fixed capacitor bank
To calculate the peak inrush current (I p ) of a fixed capacitor bank with no extra inrush reactance:
S trans = 5000 kVA
Z = 5%
U = 6.3 kV
I nom
Ip
Q = 250 kVAr
Step 1: Calculate capacitor bank nominal current, I nom
Q = capacitor bank power (kVAr)
U = system voltage (kV)
The capacitor bank's nominal current is 23 A.
Step 2: Calculate the short circuit power of the system, S sc
S trans = transformer nominal power rating (kVA)
Z = transformer impedance (%)
S trans
S SC = × 100
Z
=
5000
× 100
5
= 100000 kVA
The system short circuit power is 100,000 kVA.
Step 3: Calculate the peak inrush current, I p
I nom = capacitor bank nominal current (A)
S sc = transformer short circuit power (kVA)
Q = capacitor bank power (kVA)
The peak inrush current is 650 A.
To be suitable, the peak inrush current (I p ) must be less than 100 times the capacitor's nominal current (I nom ).
I p 100 x I nom
In this example, 650 100 x 23 A
This installation is acceptable.
Page 140 Medium Voltage Application Guide 710-12280-00A