Switching devices for Capacitors



Capacitors have been an important part of LT power distribution system typically as APFC (Automatic Power Factor Controller) units. Using APFC for Power factor correction gives a significant value addition to the system and provides following benefits:
1)      Lower KVA Demands.
2)      Better Voltage regulation due to lower voltage drops.
3)      Lower voltage fluctuations and reduced electrical stresses due to lower currents.
4)      Reduction in Tariff Penalties with rebate in selected cases.

Due to above listed benefits; APFC units have been very popular in the Industry. However, Capacitor switching causes certain stresses on the system which need to be addressed. Arrangements need to be made to either minimize the stresses or make the system suitable to withstand them. Let us discuss effects of capacitor switching as below:

a)      Switching over-voltages:  A Capacitor unit retains its charge and prevents any change in current through it. Hence, in a capacitor, current leads the voltage by 900 . Due to this behaviour, there always exists a switching overvoltage across the capacitor terminals.
The extent of overvoltage depends upon arrangement of capacitors. In Delta connected capacitors, the Transient Recovery Voltage (TRV) can reach up to 3 times the system voltage. Thus, the system should be able to withstand this voltage and Impulse withstand Voltage of all the components used in the APFC should be above the specified limit.

b)      Harmonic effects:  Any electrical system shall have some amount of harmonics present in it. They affect the electrical system by causing additional losses as well as interference with electronic systems.
However, the effects are pronounced in Capacitors as capacitive impedance is inversely proportional to frequency. Thus, Harmonic current in Capacitors is amplified for every harmonic order. E.g. For voltage harmonics of 10%, Harmonic current rises to the tune of 30% of the fundamental current.  Care must be taken to incorporate the same while selecting switching as well as protection devices.
The Harmonic effect can also be suppressed by using a Series Reactor which can provide a resonance at a frequency much below the desired harmonic frequency. E.g. Providing a 7% reactance in series will resonate at 189 Hz thereby blocking out all harmonics beyond the 5th Harmonic. 

c)       Excessive charging currents: An uncharged capacitor acts as a short circuit with very high natural frequency thereby causing Xc   to drop. This can cause very high transient switching currents. The magnitude of this current depends on system voltage and system impedance. The case is more severe in case other charged capacitors are connected on the bus.  The international standard IEC 60947-3 specifies utilization category AC-6b exclusively for capacitor switching. The making current will vary to the tune of 12 to 20 times the full load current (Depending upon system fault MVA).
After considering the above factors, we can calculate the minimum current rating of switching device required for same safe switching of Capacitor banks should be at least 1.6 times the full load current. 

Considering a  415 V system, the minimum current rating would be
I = [KVAR *1000 / (√3 * 415)]* 1.6 =2.25 KVAR
(Note – This is the minimum rating specified. Rating may rise based on system parameters)

Thus, by using this rule of thumb, we can select switching devices for Capacitor banks. However, APFC bank selection should be done only after careful system analysis.
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