Capacitor Selection

Here is a generic calculation:

Let's assume these input data,

The total load in KW = kw

Initial power factor = pf1

The target power factor = pf2

System Voltage in KV = V

System frequency = f

For power factor improvement, pf2 shall be greater than pf1 (pf2 value closer to 1)

Calculation:

The initial KVAR, kvar1 = sqrt{(kw/pf1)² - (kw)²}

The target KVAR, kvar2 = sqrt{(kw/pf2)² - (kw)²}

Since pf2 > pf1, kvar1 > kvar 2

To improve power factor from pf1 to pf2, the required KVAR of the capacitor, kvar3 = kvar1 – kvar2

Again assuming it is a 3 phase system and the capacitors to be connected in Y, the required capacitor/phase, kvar = kvar3/3

Phase to neutral voltage vn = V/1.732

The required capacitive reactance Xc = (vn)²/(kvar x 1000) ohm

The required capacitor is C (in μf), then

Xc = 1/(2 x PI x f x C) (PI = 3.14)

C = 1/(2 x PI x f x Xc) μf (micro-farads)

For additional help, see these links:

http://www.powerstudies.com/articles/PowerFactorBasics.pdf

http://www.nepsi.com/kvar_calculation.htm

- MS