Answer / zan

The small capacitors used in electronic circuits, or with
single-phase motors, are always rated in capacitance units,
like microfarads or picofarads. Some much bigger
capacitors, used for motor surge protection, are also rated
that way. But the large capacitors for power factor
correction are rated in kilovars.
Why the difference and how are the two rating systems
related?

Units intended to add specific capacitance to some circuits
are rated in capacitance units. Analysis of circuit
behavior involves formulas that directly include
capacitance, inductance, and resistance. This applies to
the smaller units, particularly in electronic circuits, and
to surge protection.
For power factor adjustment or voltage stabilization on a
power circuit, system analysis calculations involve amounts
of reactive volt-amperes and watts, making it more
straightforward to rate such capacitors in kvar units. The
two kinds of rating are related by this formula:
kvar = 0.377(C)(kV)^sup 2^
in which C is the capacitance in microfarads and kV is the
voltage applied to the capacitor in kilovolts. One kvar at
460 volts=12.54 microfarads; at 2300 volts, 0.502
microfarad; at 4000 volts, 0.165 microfarad.

Answer / n.sivasankar

VAR = reactive power = V*I (for a capacitor)

For a capacitor in a steady state AC circuit, V = I/(jwC) = I/(2*pi*f*C) or I = 2*pi*f*C*V

Substituting, then

VAR = C*(2*pi*f*V^2)
kVAR = VAR / 1000

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