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Series Capacitor Bank Switching Reviewer
and Interactive Calculator
From capacitor bank 1 to capacitor bank 2, the cable distance is 3,990 ft. The cable reactance is X = 0.124 Ω / 1000 ft
Capacitor bank 1 and bank 2 have the same rating of 1.8 MVAR at rated voltage of 23 kV. Assuming capacitor bank 1 is already energized and you needed to switch the capacitor bank 2. What is the estimated peak inrush current and its resonant frequency?
Can your power circuit breaker and disconnect switches able to withstand the high frequency and peak current ?
Back to back switching of series capacitor can generate high frequency (2068 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.
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 2068 Hz resonant frequency is 1/2068 second or 0.483 milliseconds.
Credit to : Chetvorno from Wikimedia Commons
Step 1. Calculate the line reactance of cable, XL = ? Formula Memory Recall
|
Ω
| = |
Ω * feet
feet
|
Step 2. Calculate the lumped inductance of cable, L = ? Formula Memory Recall
|
H
| = |
*
*
Hz
|
Step 3. Calculate the lumped reactance of capacitor bank 1, X C 1 = ? Formula Memory Recall
|
Ω
| = |
2 kV
MVAR
|
Step 4. Calculate the lumped capacitance of capacitor bank 1, C 1 = ? Formula Memory Recall
|
F
| = |
*
*
Hz *
Ω
|
Step 5. Calculate the equivalent capacitance of two series capacitor bank, C eq = ? Formula Memory Recall
When C1 = C2 it becomes
|
F
| = |
F
|
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 = ? Formula Memory Recall
|
Ω , Z o
| = |
√
H
√
F
|
Step 8. Calculate the peak currrent, I peak = ? Formula Memory Recall
|
A
| = |
* 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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