kVAr calculator: which capacitor bank do you need?

How the size is worked out

• Power factor from the bill. Units (kWh) ÷ kVAh. 1,20,000 ÷ 1,36,000 = 0.88.
• Load to correct. Maximum Demand × power factor, so the panel covers your busiest half hour. Without an MD, units ÷ running hours, which is the average load.
• kVAr needed. Load in kW × the multiplier in the table below, for your power factor now and the one you want.
• Steps. One small step and the rest twice its size, so the panel can switch on almost any amount in small jumps. The relay gets one spare stage.
• Own transformer. Its magnetising kVAr is on your bill but the APFC panel cannot see it, so a small fixed capacitor goes on the transformer: about 1.5% of its kVA rating is typical.

kVAr = kW × multiplier. A 500 kW load at 0.85, corrected to 0.98: 500 × 0.417 = 208 kVAr.

kVAr to microfarad and amps

For checking a capacitor on the bench, or sizing its fuse and cable.

Capacitor for a motor

A capacitor fixed at a motor and switched with it corrects that motor only. Keep it below the motor’s own no-load kVAr, or the motor can self-excite when switched off and damage itself and the capacitor.

Safety points for panel builders

• Discharge. Each capacitor needs discharge resistors that bring it to 75 V or less within 3 minutes of switch-off.
• Reconnection delay. Set the relay so a step is not switched back on while it is still charged.
• Contactors. Use capacitor-duty contactors, with resistors that limit the switch-on surge.
• Fuses and cable. Size them for about 1.5 times the step’s rated current: capacitors run above nameplate current with harmonics and high voltage.
• Heat. Heat is what ages capacitors fastest. Give the panel fans and filters, and keep it out of the sun.
• Relay CT. On the incomer, before every load and before the panel itself, in the phase the relay expects.
• On DG. Keep the panel off, or limited to a few steps. A leading power factor can push the DG’s voltage up and trip it.

Standards this follows

The calculator follows the practice in these standards. They are guidance here; the panel maker certifies the panel.

• IS 7752 (Part 1): guide for improving power factor in consumer installations.
• IS 13340-1 and -2, the same as IEC 60831-1 and -2: low-voltage shunt power capacitors.
• IEC 61921: low-voltage power factor correction banks.
• IS/IEC 61439-1 and -2: low-voltage switchgear assemblies, the panel itself.
• IEC 60076-6: reactors, for detuned panels.
• IEEE 18 and IEEE 1036: shunt capacitors and how to apply them.
• IEEE 519: harmonic limits, referred to by the CEA connectivity regulations.

Two worked examples

• A new panel. A 500 kW plant at power factor 0.85 needs about 208 kVAr to reach 0.98. Ask for a 225 kVAr APFC panel: one 25 kVAr step and four 50 kVAr steps.
• A panel already installed, bill still at 0.88. 1,20,000 units, 1,36,000 kVAh, 16 hours a day for 26 days, 200 kVAr installed. About 95 kVAr of it is not working. Fixed, the bill drops by about 13,551 kVAh a month, about ₹96,212 at ₹7.10.

Power factor in KONDUKTOR® EMS

• Power factor and lagging and leading kVArh are logged for every meter, every few seconds. A failed step, or a leading power factor at night, shows the same day, not on next month’s bill.
• It flags a low power factor under load, the usual sign of a capacitor step that is off or has failed.
• The savings page sizes the capacitor for each meter from its own readings.

Alerts when a panel’s power factor drops are set up for your plant as part of your project.