Large capacitor banks are finding application and acceptance in medium voltage distribution networks and industries. The switching of the capacitor bank is important, and the transients should be considered. Load flow and network stability studies determine the size and location of the capacitor bank. Capacitor bank switching transients are at a frequency higher than the power frequency.
The source of the switching transients is:
- Switching of a shunt capacitor bank, which may include switching to fault.
- Back-to-back switching, i.e. switching a second capacitor bank on the same bus in the presence of an already energized bank.
- Tripping or de-energizing of a bank in normal operation and under fault conditions.
- Possible secondary resonance when the capacitors are used at multiple voltage levels in a distribution network.
- Restrike or pre-strike in switching device
- Autoclosing with precharge on the capacitors
All these conditions should be simulated using EMTP software during the design phase of the shunt capacitor bank.
Inrush current of the capacitor bank
When a capacitor bank is energized, high current flows through the circuit. When the circuit breaker (CB) receives the closing command, its mechanism is activated. The moving contact begins to move. At the peak of the voltage there is a pre-strike and then an arc. The result is a higher inrush current through the circuit breaker. Figure 1 shows the inrush current of the 20MVAr, 63kV capacitor bank when energized. The amplitude of the inrush current is 2900A. This is 15.82 times its rated current. In addition, the amplitude of the overvoltage reaches up to 1.5 times of the rated voltage. The overvoltage frequency is up to 1500 Hz. Therefore, an inductance is used in series with the capacitor bank. It limits the inrush current of the capacitor bank when it is switched on. It is necessary to calculate the value of the inductor.
Fig1. a) inrush current of the capacitor bank b) voltage across the terminals of the capacitor bank during energizing at peak of the phase a
Back-to-back switching of the capacitor bank
If one bank already exists and the second bank is switched on, the higher voltage and current will occur than switching on a single bank. Figure 2 shows the waveforms of the current and voltage during the back-to-back switching operation. The current amplitude is up to 5500A with 1880Hz frequency.
Fig2. Waveform of the a) current through the CB b) voltage across the capacitor bank during back-to-back switching
These high-frequency overvoltage and overcurrent can damage power system equipment. Power transformers, instrument transformers, circuit breakers and control systems are at risk of insulation failure.
To reduce the overvoltage and inrush current amplitude, an inductor is used in series with the bank. This inductor should be carefully designed. Its presence will increase the continuous voltage across the capacitor bank.