The choice of capacitor bank to install in a system is closely depended from:
- cosϕ2 value that we would obtain
- cosϕ1 starting value
- installed active power
By the following equation
QC= Required capacitors reactive output (kvar)
P = active power
QL, Q’L = Inductive reactive output before and after the installation of the capacitor bank
A, A’ =apparent power before and after the power factor correction.
can be also written
where the parameter k is easily calculable using table 1 below.
We have installed a load that absorb an active power of 300 kW with a beginning power factor 0.7 and we want to increase it until 0.92.
From the table 1 we find:
and then we find
A typical example of power factor correction, sometimes not much considered but surely important, concerns the power factor correction of transformers for the distribution of energy. It is essentially a fixed power factor correction must compensate for the reactive power absorbed by the transformer in its no load condition (this happens often during the night). The calculation of the needed reactive output is very easy and it bases itself on this equation
IO%= magnetising current of the transformer (AS%);
AN = Apparent rated power in kVA of the transformer.
If we don’t have these parameters, it is possible using the following table 2.
Another very important example of power factor correction concerns asynchronous three-phase motors that are individually corrected. The reactive power we must install is reported on table 3.
Be careful: the capacitor output must not be dimensioned too high for individual compensated machines where the capacitor is directly connected with the motor terminals. The capacitor placed in parallel may act as a generator for the motor which will cause serious overvoltages (self-excitation phenomena).
What about the wound rotor motor the reactive power of the
capacitor bank must be increased by 5%.