Bill guide
How to improve power factor: and keep it there.
Most plants already have capacitors. The problem is that they slowly stop doing their job: steps fail, fuses blow, settings drift, and power factor slips without anyone noticing until the bill shows it. Here is how to size correction, and how to find out why it is not working.
How much correction you need
Capacitor kVAr = kW × (tan φ1 − tan φ2), where φ1 is the angle at today’s power factor and φ2 at the target.
For 400 kW from 0.85 to 0.98: tan φ1 = 0.620, tan φ2 = 0.203, so about 167 kVAr. Work out your own figure, and what low power factor costs, with the PF penalty calculator.
Fixed or automatic correction
- Fixed capacitors suit loads that run steadily, such as a large motor that is on whenever the plant is. Fit them at the motor.
- APFC panels (automatic power factor control) switch capacitor steps in and out as the load changes. They suit plants with varying load across the day and between shifts.
- Detuned banks, with a reactor in series with each step, are needed where VFDs, UPS or rectifiers make harmonics. See power quality.
Why power factor is low with an APFC panel
| Symptom | Common cause | What to check |
|---|---|---|
| PF slowly falls over months | Capacitors losing capacitance with age and heat | Current in each step against its rating |
| Some steps never seem to help | Blown fuses or failed contactors | Fuses and contactor tips, step by step |
| Relay shows good PF, bill shows low PF | Relay CT on the wrong cable, wrong phase or reversed | Relay CT on the incomer, on the phase the relay expects |
| PF good by day, leading at night | Steps left in at light load, or fixed capacitors larger than the night load | Minimum step size and fixed capacitors |
| Steps switching in and out constantly | Steps too large for the load, or relay settings too tight | Step sizes, switching delay and target band |
| Capacitors failing early | Harmonics, or resonance with the supply | THD; use detuned steps |
Leading power factor
Too much correction pushes power factor leading. Some kVAh tariffs count leading kVArh the same as lagging, so over-correction raises kVAh units just as under-correction does. Others ignore leading kVArh, or ignore it only at night. Check your own tariff order, and keep power factor just below unity, lagging. How kVAh billing treats power factor is explained in kVAh billing.
Power factor on DG
A DG set is rated for a lagging power factor, typically 0.8. A capacitor bank left in while running on DG at light load can push the DG leading, which it handles poorly and which can trip it. Many plants switch the APFC off, or limit it, when the DG takes over.
Steps to improve power factor
- Measure power factor at the incomer, and per panel, over a full week.
- Size the correction from the lowest-PF load period, not the average.
- Check every APFC step and fix failed ones before adding new kVAr.
- Set the target at 0.98–0.99 lagging and a minimum step small enough for the night load.
- Watch PF afterwards: lagging by day, not leading at night, steady on DG.
How KONDUKTOR® EMS helps
- Power factor on every meter, live and in history, shown as lagging or leading.
- An alert when power factor falls below the limit you set.
- The rupee cost of low power factor on your tariff, with the capacitor kVAr needed to reach the target.
- Per-panel PF, so a failing step or a panel without correction stands out.
Common questions
Why is our power factor low even with an APFC panel?+
Usually because capacitor steps have lost capacitance, fuses or contactors have failed, the relay’s CT is on the wrong cable or phase, or the target is set too low. Check each step’s current and the relay CT position first.
What is the leading power factor penalty?+
Some kVAh tariffs count leading reactive energy the same as lagging, so over-correction raises your billed kVAh. Others ignore leading reactive energy, or ignore it only at night. Your state’s tariff order sets the rule.
Should capacitors be switched off when running on DG?+
Often, yes, or limited. A DG set is rated for lagging power factor, and capacitors left in at light load can push it leading, which can make it unstable or trip it.
What power factor should a factory aim for?+
Around 0.98 to 0.99 lagging at the incomer. Unity is the ideal, but aiming for it exactly risks going leading at light load.
See your power factor before the bill does.
Book a live demo and see lagging and leading PF on every panel, on meters like yours.