Capacitor bank :

When a single unit of capacitor is not sufficient for high voltage rating as well as kilo or mega VAR rating, a number of unit of capacitor is connected in series-parallel combination, is called capacitor bank.

There are three types of capacitor bank are available namely externally fused capacitor bank, internally fused capacitor bank and fuse less capacitor bank.
In case of externally fused capacitor bank fuse can be easily replaceable, but in fuse less system number of capacitor units are connected in series, so with one unit damage, no harm occur. But cost is higher than other two.
In three phase system, capacitor bank is connected either in star or delta form.

Capacitor bank is connected in delta in case of L.V line, whereas, in H.V transmission, it is connected by star connection.

Delta connected capacitor bank vs. Star connected capacitor bank.

→ The capacitor connected in delta form, has 3 times higher capacitance value than star connected capacitor bank. More capacitance means more reactive power.
→ In delta connection phase current is line current/√3. With smaller current less loss occurred.
→ In case of unbalancing circulating current or 3rd harmonic current, the current flows within the bank, which is not harmful for the total system.
→ In star connection, required value of C will be higher than delta connection for same value of required KVAR/MVAR.
→ In star connection, phase voltage is 1/√3 times of line voltage, so lower insulation is required.
→ There is no 3rd harmonic current flows through ungrounded star connection.
→ In high voltage transmission, star connection of capacitor bank is economical, whereas, in low voltage distribution and industrial purpose, delta connection is preferable.
So, we can conclude that, at low and medium voltage, capacitor bank is connected in delta form and at high voltage transmission, generally capacitor bank is connected in star form with neutral ungrounded. As the star point is ungrounded, the ground path for 3rd harmonic current is absent.
Normally a current limiting inductor is connected in series with shunt capacitor.

Why inductor is connected in series with capacitor bank?

Capacitor bank is connected to improve the power factor of the system. We know that, in case of D.C voltage, at starting capacitor act as short circuit, for which a high inrush current flows. If we switch on the capacitor even in A.C voltage, in dot time it acts as D.C, and a high inrush current flows which is harmful for capacitor unit as well as total system. Now, if we connect an inductor in series with capacitor bank, it opposes the sudden change of current. Thus limiting the inrush current and protect the system from hazard.

Specification of capacitor bank :

When manufacturer supplies capacitor bank, the following specifications should be mentioned.
a) System voltage and frequency.
b) Ambient temperature- maximum and minimum.
c) Rated output- KV and KVAR.
d) Number of phase- either single or three phase.
e) Types of connection.
f) Number of units per bank.
g) Power frequency withstands voltage, KVrms.
h) Impulse withstands voltage, KVpeak.
i) Capacitor unit’s output- voltage, current, dielectric type, losses.
j) Clearances from phase to phase and phase to earth.
k) Fuse type and specification.
l) Total weight of unit and bank, etc.

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