POWER FACTOR CORRECTION: HOW MANY CAPACITORS?

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

where
Q_C= Required capacitors reactive output (kvar)
P = active power
Q_L, 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.

The equation

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:

Table 1

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

where
I_O%= magnetising current of the transformer (AS%);
A_N = Apparent rated power in kVA of the transformer.

If we don’t have these parameters, it is possible using the following table 2.

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.

Table 3