Capacitor switching can cause disruptive transients which travel through the power grid and create problems for motor drives, process controls, data communications or any load that cannot tolerate sub-cycle transients.
This is a fairly common power quality issue. The problem occurs when power factor correction capacitors are switched on, first causing the voltage on the line to fall, followed by a sudden rise in voltage. This process repeats itself until the system settles down, typically within a few cycles. After the initial voltage drop, there is an oscillating (harmonic) current caused by the interaction between the capacitor bank and the system inductance.
In a whitepaper, Pre-insertion Resistors in High Voltage Capacitor Bank Switching, the authors state that
Installing detuning reactors in series with the power factor correction capacitors can help reduce the damage caused by the transient currents. These reactors are designed to make the correction circuit inductive to the higher frequency harmonics. Line reactors are the least expensive option, depending on the size of the bank. However, adding the inductance in series with the capacitors will reduce their effectiveness at the supply frequency. Reducing the resonant or tuned frequency will reduce the effective capacitance further.
Another possibility is to use tuned filters to eliminate specific harmonic frequencies. A tuned harmonic filter is essentially a power factor correction capacitor combined with a series iron core reactor. A filter provides power factor correction at the fundamental frequency and becomes an inductance (like a motor) at frequencies higher than its tuning point.
Both line reactors and tuned filters have a long track record, are reasonably priced and work effectively. However, increasing line impedance and filtering the capacitor switching transients may not completely eliminate the harmonics problem.
Other solutions include the addition of a zero threshold surge suppressor (ZTSS), or a surge protection device (SPD). The ZTSS shunts transient energy away from sensitive equipment. An SPD absorbs and lessens the impact of voltage transients. SPD devices come in a range of sizes, but they may be an expensive solution for this size application.
Do you have more questions about transients or other power quality issues? Use the Ask an Expert service to get the answers.
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