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Capacitor Switching
Interactive Calculator

Assuming capacitor bank 1 is already switched on or energized.
Calculate the peak inrush current and resonant frequency that will flow through CB2 when capacitor bank 2 is switched on.

Reference: IEEE Std C37.012-2005 APPLICATION GUIDE FOR CAPACITANCE CURRENT
SWITCHING FOR AC HIGH VOLTAGE CIRCUIT BREKAERS

Back to back switching of series capacitor can generate high frequency (575 Hz) transient peak current that can flow from already charged capacitor bank 1 through disconnect switch or circuit breaker going to capacitor bank 2 can be visualize using a tuned LC circuit as shown below.

The animation speed was slowdown to 3 seconds for us to see the transient speed of high frequency inrush current. The actual speed of one (1) cycle of 575 Hz resonant frequency is 1/575 second or 1.74 milliseconds.

Tuned circuit animation 3 300ms Credit to : Chetvorno from Wikimedia Commons

Step 1. Calculate the line reactance of cable, XL = ? Formula Memory Recall

Ω , line reactance of cable, XL
=
Ω * feet
feet

Step 2. Calculate the lumped inductance of cable, L = ? Formula Memory Recall

If the line reactance is given , for example you can enter the number in XL = 0.001 Ω

milliHenry (mH), inductance of cable, L
=
XL
* * Hz

Step 3. Calculate the lumped reactance of capacitor bank 1 & 2, X C 1 & X C 2 = ? Formula Memory Recall

Ω , reactance of capacitor bank 1, X C 1
=
2 kV
MVAR1


Ω , reactance of capacitor bank 2, X C 2
=
2 kV
MVAR2

Step 4. Calculate the lumped capacitance of capacitor bank 1, C 1 = ? Formula Memory Recall

microfarad (µF) , of capacitor bank 1.
=
* * Hz * Ω


microfarad (µF) , of capacitor bank 2.
=
* * Hz * Ω

Step 5. Calculate the equivalent capacitance of two series capacitor bank, C eq = ? Formula Memory Recall

microFarad (µF) , C eq
=
microFarad (µF)

Step 6. Calculate the resonant frequency of peak current, F res Formula Memory Recall

Hz, F res
=
* * √  H * F  

Step 7. Calculate the surge impedance, Z o = ?

Ω , Z o
=
√  H  
√  F  

Step 8. Calculate the peak currrent, I peak = ? Formula Memory Recall

Amp , peak line current
=
* kV
Ω


The calculated peak current did not consider the system resistance and the inductance of the capacitors so it is always higher compare to power quality meter reading. The power quality meter like BMI/Electrotek is measuring the system resistance and the inductance of the capacitors. Hence the peak current reported by power quality meter is usually lower than the calculated peak current.

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