What power factor is
In an industrial installation with motors, transformers and devices with windings, the current drawn from the grid is not in phase with the voltage. Part of the energy transported through the grid does not produce useful work — it circulates between the generator and the load, producing only losses along the way. This part is called reactive energy.
The ratio between active power (the useful one) and apparent power (the total transported) is the power factor, denoted cos φ. The ideal value is 1 (all energy does useful work). Values below 0.92 trigger additional billing from the distribution operator and put strain on transformers, cables and switchgear.
Why regulation requires cos φ > 0.92
The Romanian electricity distribution operator (Electrica, E.ON, Distribuție Energie Oltenia) bills inductive reactive energy above the threshold of cos φ = 0.92. Practically, if an installation consumes 100 MWh active per month and its power factor is 0.78, the distribution operator issues an additional invoice for reactive energy above the threshold — amounts that, in industries with high consumption, reach thousands or tens of thousands of RON per month.
The customer has two options:
- Pay the penalty monthly, indefinitely.
- Invest once in a capacitor bank that compensates reactive at the receiver, bringing cos φ to 0.95-0.99 and eliminating the additional invoice.
The investment typically pays back in 12-24 months — the safest ROI an industrial electrician can present to a customer.
How to calculate compensation needs
Step 1 is analysing the actual consumption of the installation: measurement with a network analyser over at least 24 hours, ideally a week, to capture diurnal and day/night variations.
The analyser data produces:
- Average and peak active power (kW)
- Average and peak reactive power (kVAr)
- Harmonic distribution (distortion factor, THD-i)
- Variation of power factor over time
With this data, the classic formula for the required capacity of the capacitor bank is:
Q_comp = P × (tan φ_current − tan φ_desired)
where Q_comp is in kVAr, P is active power in kW, and tan φ is calculated from the measured and desired cos φ values.
For a 500 kW installation with cos φ = 0.78 (tan φ = 0.802) that we want to bring to 0.95 (tan φ = 0.329), the requirement is:
Q_comp = 500 × (0.802 − 0.329) ≈ 237 kVAr
Capacitor bank types
Three architectures are in use:
Fixed bank (global compensation) — capacitor banks permanently connected to general bus bars. Simple and cheap solution, but does not adapt to load variations. At low load, the risk is overcompensation (capacitive cos φ), penalised just like undercompensation.
Automatic stepped bank — multiple capacitor steps switched automatically by a controller based on the cos φ measured in real time. The standard solution for installations with variable load: production sections, administrative sites with HVAC.
Bank with anti-harmonic reactors — in installations with many frequency converters, electronic welding, switched-mode supplies, harmonics (especially the 5th and 7th) circulate in the grid. Classic capacitors resonate with harmonics and self-destruct. The solution is a bank with reactors connected in series, sized to shift the resonance frequency below the important harmonic spectrum. More expensive, but the only correct one in modern installations.
Frequent sizing mistakes
Four pitfalls encountered in projects coming for audit:
- Compensation based on installed power, not actual consumption — oversizes the bank, generates capacitive cos φ at low load.
- Ignoring harmonics — installing a bank without reactors in an environment with THD-i above 10% destroys the capacitors in months.
- Global compensation in installations with large, intermittent motors — a 200 kW motor started/stopped cyclically generates variations a global bank cannot follow. More efficient: capacitors at the motor terminals (individual compensation).
- Lack of maintenance — capacitors age, fuses blow, contactors wear out. Without annual checks, a 2-3 year old bank may compensate only partially.
What we deliver in a compensation project
A complete project includes:
- Measurements with a network analyser (1-2 weeks)
- Technical report with consumption profile, harmonic distribution, recommendations
- Signed electrical design (schematic, calculation, equipment list)
- Equipment supply with dry capacitors (not oil-filled — for safety and environmental reasons)
- Installation, grid connection, commissioning
- Controller programming, calibration with real load
- Acceptance documentation and operating instructions
- Annual maintenance contract (capacitor check, contactors, controller adjustment)
For installations with monthly consumption above 50 MWh, the amortisation period is under 18 months in most cases — a technical-economic case we are happy to present to any interested customer.