Power factor correction capacitor selection

Capacitance(in Farads) = VAR /(2 x Pi x Frequency(cycles per second) x Voltage^2)

where VAR is the amount of reactive power desired in VA. Voltage is voltage across terminals of capacitor. If this is wrong, I know I'll hear about it.

Power factor correction capacitors usually have a rating stated in VA so that this calculation is not necessary. Building something special?

Well there may be two ways to select the size of a power factor correction capacitor.

The dificult way is to do a whole lot of calculations ( example above) and derive the value of inductive VA you need to correct .

The second way is to measure the power factor and VA demand.

If you know your intial power fector(pf) and your final pf then you will know how much capacitive VA you need to correct the pf.

Power factor correction capacitors always do carry a VA rating for a particular voltage.

This makes it really easy to apply.

Word of caution, though, in motor load circuits , you never correct to unity since if capacitor is in parallel to motor, during motor shutdown , you can end up with a resonance condition with can harm you motor.

If you are correcting for the entire plant or factory, beaware that some utilities do not penalise you for over correction ( leading final powerfactor) but some do.

hi

Iam venkat, how to calculate the VA from intial And final Power Factor, just explain through formulation if there.....my mail id is ktva361_eee@yahoo.co.in

thank u

hi frnd

our aim is to select the capacitor . ten how we know the voltage across the capacitor for calculation and wat s the formulation to calculate reactive power.My mail id is ktva361_eee@yahoo.co.in

thank u

The formula given above calculates the capacitance given the amount of reactive power desired in VARs, the frequency of your system in cycles per second, and the voltage across the capacitor. The voltage is your system voltage; if you connect the capacitors from line to line, then use line to line voltage. If the capacitors are connected from line to neutral, then use line to neutral voltage.

If you still need more information, tell us what is known, for example the amount of load with its power factor, system voltage, etc., and then tell us what you need to know from it, for example the capacitance, reactive power, etc. Perhaps a more detailed description of your setup, such as power factor correction for a motor. We might be able to point you in the right direction.

The voltage is 208 3 phase,existing PF is about 85%, energy on the system is

9000kwhr,demand is 226 kw and load factor of 47.5 %. Looking for a 3 phase

capacitor or individual single phase for each incoming line.

I'll assume 60 hertz. You stated 226 kW, but my guess is that this is total power, or 226 kVA. If the load is 226 kVA at a 85% power factor (assume lagging), then the reactive power is about 119 kVAR. Using the equation given above, that works out to 7296 microfarads of total capacitance connected across 208 volts. Three separate banks, each 2432 microfarads, connected in delta (assuming voltage given is line to line).