How to calculate Size of Capacitor Bank in KVAR?

The application of capacitors generally falls in one of two categories - correction of power factor and correction of low voltage. The power factor correction aspect has been addressed above. The impact of the capacitors on voltage level is directly proportional to its kVar rating and the inductive reactance of the circuit. This is related by the following formula:

Percentage voltage rise = (kvar)(X)(l)÷((10)(kV)²)

where kvar = 3Ø kilovars
X = reactance in ohms per unit length
l = length of circuit
kV = Ø-Ø kiloVolts

In a distribution system where the length unit is in miles and the construction is based on 8' crossarms, the value of X per mile ranges from about 0.7 for smaller conductors (#4, #2) to 0.6 for larger (336.4). So a "quick" calculation for a 600kVar capacitor 2mi from the source on a 11kV system is approximately:

%VD ≈ 600 x 0.7 x 2 ÷ 1200 ≈ 0.7%

Likewise, for a 22kV system the approximate formula would be kVar x 0.7 x l ÷ 5000. Unfortunately, as the voltage doubles, the effect of the capacitor is reduced by a factor of 4 due to the kV² in the denominator. And the values in the formula can easily be converted to metric by taking 5/8ths of the impedance (or approximate 0.4 ohms/kilometer).

The voltage rise at the bank location will apply to the entire line beyond that point. The voltage rise between the substation and the bank will be proportional to the distance from the substation.

Using this approximate formula, it is easy to make a quick engineering determination of voltage rise due to their application. Also note that for rural distribution lines, bank sizes on the order of 150 to 600kVar are more practical. Banks in the 1200 to 1800kVar size located far from the substation can cause a major shock to the system when energized. It is often common in rural applications to split the bank and make it half fixed and half switched. The fixed capacitor may be de-energized during off-peak months to keep voltage levels from going too high.

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