A factory that runs at a low power factor draws more current than its work needs. The network may bill the excess, and the main switch and the supply cable carry it. This guide sizes a capacitor bank for one factory with the power factor correction tool. The theory is in Power factor and reactive power.
The site
- A 400 V three phase factory. The maximum demand reading is 250 kW at 0.78 lagging.
- The network bills reactive power below 0.95, so the target is 0.95.
- The load has several variable speed drives. A power quality logger reads 12 % THDi.
- The supplier offers banks in 25 kvar steps.
Step 1. Find the kvar to remove
The real power stays at 250 kW. At 0.78 the load draws 200.57 kvar. At 0.95 it would draw 82.17 kvar. The bank must supply the difference, 118.40 kvar.
Open the tool with this load. The primary result is 118.40 kvar.
Step 2. Round up to whole steps
Five 25 kvar steps give 125 kvar. With 125 kvar on line, the power factor is 0.957, just above the target. The controller switches steps in and out as the load moves, so the bank never sits at full output on a light load.
A larger step can overshoot. If one step is bigger than the reactive power of the lightest load, the site turns leading at night. The tool warns when the whole steps pass unity.
Step 3. Read the current and the released capacity
| Before | After | |
|---|---|---|
| Apparent power | 320.51 kVA | 261.17 kVA |
| Current at 400 V | 462.62 A | 376.97 A |
| Power factor | 0.78 | 0.957 |
The supply carries 85.65 A less, and 59.34 kVA of capacity comes free. That is room for new machines without a larger supply. Check the new load with the maximum demand tool.
Step 4. Choose a detuned bank
At 12 % THDi, the tool warns that the site needs a detuned bank. A plain capacitor and the supply transformer form a tuned circuit. If it tunes near the 5th or 7th harmonic, the drives' harmonic currents are amplified, and capacitors fail early.
A detuned bank puts a reactor in series with each step, so the circuit tunes below the 5th harmonic. Ask the supplier for a detuned bank. Above about 20 % THDi, get a harmonic study first.
If you know the fault level at the bank, enter it. On a 10 MVA fault level, 125 kvar resonates near the 9th harmonic, clear of the 5th, 7th and 11th.
The result
| Item | Value |
|---|---|
| Correction required | 118.40 kvar |
| Bank | 5 × 25 kvar, detuned |
| Power factor with the bank | 0.957 |
| Current saved | 85.65 A |
| Released capacity | 59.34 kVA |
The bank needs its own circuit. Its rated current at 400 V is 180.42 A. AS/NZS 3000:2018 Cl 4.15.2.3 asks the conductors to carry at least 135 % of it, 243.57 A, or the breaker setting if that is higher. Size the cable for that current with the cable size tool. Switch the steps with contactors rated for capacitors (Cl 4.15.2.2), and give each step a discharge path and a warning notice (Cl 4.15.3.1).
See also: Power factor and reactive power.