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Power factor correction is one of the least glamorous upgrades a factory can make, and one of the more useful. Most industrial loads, especially induction motors, draw reactive power as well as the real power that does the work. That reactive power does not turn a shaft or heat an oven, but it still flows through your transformers, generators and cables, adding current and losses. Capacitor banks supply that reactive power locally, so the rest of the system does not have to carry it.
This guide explains what power factor is, what correction achieves, how to choose and place capacitor banks, and the harmonic problems that catch many plants out. It is written for factory owners, plant engineers and maintenance managers, particularly those running on a mix of grid and generator supply.
What power factor means
Every AC load draws current made up of two parts:
- Active power (kW), which does useful work.
- Reactive power (kvar), which builds the magnetic fields motors and transformers need to operate.
Together they make up the apparent power (kVA) that your supply equipment must deliver. Power factor is the ratio of active power to apparent power. A power factor of 1.0 means all the current is doing useful work. A lower figure means part of the current is reactive.
Typical sources of poor power factor in a factory are lightly loaded induction motors, transformers, welding sets and some older lighting.
Why power factor correction matters
ABB’s technical application paper on power factor correction sets out the main benefits clearly. When reactive power is supplied locally by capacitors:
- Current falls for the same load. Lines, generators and transformers can be sized for a lower apparent power, or existing equipment can carry more useful load.
- Losses fall in every part of the installation upstream of the point of correction.
- Voltage drop is reduced. ABB notes that for the same active power, the higher the power factor, the smaller the voltage drop.
- Reactive energy charges may be avoided where the supply tariff penalises low power factor. ABB gives the example of tariffs that charge for reactive energy above that corresponding to a power factor of 0.9.
The Nigerian context
Many Nigerian factories rely heavily on diesel or gas generators, with the grid as one source among several. Generators are rated in kVA, so a plant with poor power factor uses more of the generator’s capacity to deliver the same kilowatts. Improving power factor can free up headroom on an existing set and reduce the current through cables and switchgear. Whether you also face reactive energy charges from your distribution company depends on your metering and tariff arrangement, so check your bill and supply agreement before assuming savings on that side.
Because low grid voltage at the end of long feeders is a common complaint, reduced voltage drop inside the plant is a practical benefit in its own right.
Types of power factor correction
ABB describes several arrangements. Each suits different load patterns.
| Type | How it works | Best suited to |
|---|---|---|
| Individual (distributed) | Capacitor connected directly at a large load, switched with it | Large motors or equipment with steady load running long hours |
| Group | One bank serves a group of loads that operate together | Production lines or areas with similar duty |
| Centralised | One bank at the main switchboard | Plants with many small, varying loads; uses less total capacitance |
| Combined | Individual correction on large loads, centralised for the rest | Larger factories with a mix of big motors and general loads |
| Automatic | Bank divided into steps switched in and out by a controller | Any plant where load varies through the day |
The trade-off is straightforward. Correction close to the load reduces current in more of the installation, but needs more capacitors overall. Centralised correction uses less capacitance but the cables between the main board and the loads still carry the reactive current.
Most factories end up with an automatic centralised bank at the main LV switchboard, sometimes supplemented by fixed capacitors on large, constantly running motors. ABB notes that control accuracy improves with more steps.
Sizing a capacitor bank
Sizing starts with data, not guesswork. The steps are:
- Measure the load. Use a power quality analyser at the main incomer, ideally over a full production cycle, to record kW, kvar, power factor and harmonics.
- Set a target power factor, often driven by tariff requirements or generator capacity.
- Calculate the kvar needed. ABB’s method uses a factor K, taken from a table, so that the required capacitor output Qc = K × P, where P is the active power.
- Select step sizes so the bank can follow the load without over-correcting at light load.
Care with motors and transformers
ABB gives two important limits. For capacitors connected directly to a motor, the capacitor output should not exceed 90 per cent of the reactive power the motor draws at no load. Larger values risk self-excitation and overvoltage when the motor is switched off while still spinning. For fixed compensation of a transformer, the capacitor output should not exceed the reactive power the transformer absorbs at minimum load. Over-correction at light load can push voltage up and leave the power factor leading.
Harmonics: the hidden risk
Modern factories contain many variable speed drives, rectifiers, UPS units and LED lighting. These non-linear loads draw distorted current and create harmonics. Capacitors and harmonics do not mix well.
ABB explains why. A capacitor’s impedance falls as frequency rises, so capacitors attract harmonic currents and can be overloaded. Worse, a capacitor bank combined with the inductance of the supply transformer forms a resonant circuit. If that resonant frequency sits close to a harmonic present in the plant, currents and voltages can be amplified, damaging capacitors and other equipment and causing nuisance tripping.
Detuned capacitor banks
The usual remedy is a detuned bank: each capacitor step has a reactor in series, sized so the resonant frequency of the combination falls below the lowest significant harmonic in the plant. The bank still corrects power factor at the fundamental frequency but no longer resonates with the harmonics.
Harmonic filters
Where harmonic levels are high, or the supply authority sets limits, filters may be needed:
- Passive filters use a capacitor and inductor tuned to a specific harmonic to absorb it.
- Active filters inject compensating current electronically. ABB notes they can handle many harmonic orders without being designed for one specific frequency.
The rule is simple: measure harmonics before buying capacitors. Installing a standard bank into a plant full of drives without checking is one of the most common causes of capacitor failure.
Installation and protection
A capacitor bank is a piece of switchgear and should be engineered as one.
- Rating for harmonics. ABB notes capacitors must tolerate steady-state current up to 30 per cent above rated because of harmonics, and protection and cables should be selected with that in mind.
- Inrush current. Switching a capacitor in creates a high, brief inrush, especially when other steps are already connected. Use capacitor-duty contactors and, where needed, inrush-limiting inductances.
- Protection settings. Short-circuit protection must not trip on inrush.
- Discharge. Capacitors hold charge after disconnection. ABB refers to IEC 60831-1, which requires the voltage to be reduced to 75 V or less within 3 minutes. Discharge resistors are fitted for this purpose. Staff must still prove a capacitor dead before touching it.
- Ventilation. Capacitors and reactors generate heat and their life shortens at high temperature. Hot Nigerian switchrooms need adequate ventilation or cooling, and harmattan dust should be kept out of the cubicle.
- Location of the controller CT. The automatic controller needs a current transformer positioned to see the total load current. A badly placed CT is a common reason for banks that never switch properly.
Generator operation
When a plant runs on generators, check how the capacitor bank behaves. Over-correction on a lightly loaded generator can lead to a leading power factor, which some generators and their voltage regulators do not tolerate well. The control scheme should prevent this, for example by limiting the steps available on generator supply. Discuss it with both the bank supplier and the generator supplier.
Testing and maintenance
| Activity | What to check |
|---|---|
| Commissioning | Controller settings, CT polarity and location, step sequencing, target power factor reached, no hunting |
| Routine visual check | Bulging or leaking capacitors, discoloured terminals, fan operation, filter cleanliness |
| Periodic electrical check | Capacitor current per step compared with rating; loss of capacitance shows as reduced kvar |
| Thermography | Hot connections, contactors and fuses |
| Power quality review | Re-measure harmonics after adding drives or major loads |
Capacitors lose capacitance as they age. A bank that worked well when installed may gradually stop reaching its target, so trend the results.
Regulatory note
The Nigerian Electricity Management Services Agency (NEMSA) has a statutory role in the technical inspection, testing and certification of electrical installations and equipment. For larger installations, factor NEMSA requirements into the project alongside your insurer’s and corporate safety requirements.
FAQ
What power factor should a factory aim for?
It depends on your tariff and equipment. ABB’s paper uses 0.9 as an example of a tariff threshold, and many plants choose a target somewhat above whatever their tariff requires, to leave a margin. The right target comes from your supply agreement, generator capacity and an analysis of your load.
Will power factor correction reduce my energy bill?
It reduces current and losses in your own installation, and may avoid reactive energy charges if your tariff includes them. It does not reduce the real power your machines need, so savings depend on your situation.
Why do our capacitors keep failing?
Common causes are harmonics from drives, resonance, high ambient temperature and over-voltage. A power quality survey usually reveals the cause. A detuned bank is often the solution.
Can we add capacitors to individual motors driven by VSDs?
No. Capacitors should not be connected on the output side of a variable speed drive. Correction for drive-fed motors is handled elsewhere in the system.
Talk to Maven about power factor correction
Maven Technical Services installs and tests industrial electrical systems, from switchboards and motor control centres to cabling and earthing. If you want to understand your plant’s power factor, deal with failing capacitors or free up capacity on your generators, speak to Maven’s engineers about a measured, properly engineered solution.
