Why Is Power Factor Still Low After Installing a Capacitor Bank?

Installing a capacitor bank should improve power factor, reduce reactive power demand and release capacity in transformers and cables. However, some facilities find that their power factor remains below the utility target even after the capacitor bank has been commissioned.

This does not necessarily mean the capacitor bank is defective. The cause may be incorrect sizing, changing loads, harmonic distortion, controller settings, failed capacitor stages or measurements taken at the wrong point.

Finding the right solution begins with understanding what a capacitor bank can—and cannot—correct.

How Does a Capacitor Bank Improve Power Factor?

Motors, transformers, pumps and compressors require reactive power to produce magnetic fields. This reactive demand causes current to lag behind voltage and lowers the displacement power factor.

A capacitor bank supplies capacitive reactive power locally, reducing the reactive current drawn from the upstream network.

Power factor is calculated as:

Power factor = Active power (kW) ÷ Apparent power (kVA)

The required capacitor rating can be estimated using:

Qc = P × (tan φ₁ − tan φ₂)

Where Qc is the required compensation in kVAr, P is the active load in kW, φ₁ represents the existing power factor and φ₂ represents the target.

This calculation works well for stable loads with low harmonic distortion. Modern electrical systems are often more complicated.

Why Does Power Factor Remain Low?

1. The Capacitor Bank Is Undersized

The capacitor bank may have been selected using the original connected load instead of the facility’s present operating demand. Additional motors, production lines or HVAC equipment can increase reactive power after installation.

Undersizing can also occur when the design target is too low. A bank designed to improve power factor from 0.80 to 0.90 will not necessarily achieve a utility requirement of 0.95 or 0.98.

Measure the actual maximum kW and kVAr during representative operating conditions. Do not size the bank from transformer capacity alone.

2. The Load Changes Too Quickly

Automatic capacitor banks compensate in fixed steps. A controller connects or disconnects these stages after detecting a change in reactive demand.

This works well for stable loads but can be too slow for welding machines, cranes, elevators, rolling mills and other rapidly changing equipment. The required compensation may change again before the bank responds.

A Static Var Generator provides continuously variable reactive power with a much faster response. A hybrid system can also use capacitors for the stable base load and an SVG for rapid fluctuations.

3. Harmonics Are Lowering the True Power Factor

A capacitor bank corrects displacement power factor. It does not remove distorted current produced by nonlinear loads such as variable frequency drives, UPS systems, rectifiers, EV chargers and switched-mode power supplies.

True power factor includes both phase displacement and waveform distortion. A facility can therefore have a displacement power factor of 0.98 while its true power factor remains significantly lower.

Adding more capacitors may make the situation worse. Capacitors can interact with system inductance and create harmonic resonance, amplifying current or voltage distortion. Capacitor banks correct displacement power factor, while harmonic mitigation requires a different approach. Schneider Electric

Before increasing the capacitor capacity, measure:

• True and displacement power factor

• Current and voltage THD

• Individual harmonic orders

• Capacitor current

Where harmonics are responsible for the remaining low power factor, an Active Harmonic Filter may be required.

4. The Controller Is Receiving the Wrong Signal

An automatic capacitor bank depends on correct voltage and current measurements. If the current transformer is installed in the wrong location, connected with reversed polarity or assigned to the wrong phase, the controller may switch incorrectly.

The CT should normally measure both the load current and the effect of the capacitor bank. Important settings include:

• CT ratio and polarity

• Voltage-phase reference

• Target power factor

• Switching and reconnection delays

• Step sequence

• C/k or response-current value

These settings should be verified during commissioning and after major changes to the distribution system.

5. One or More Capacitor Steps Have Failed

A controller may show that a stage is connected even when that stage is no longer producing its rated output.

Possible causes include blown fuses, damaged contactors, deteriorated capacitors, failed reactors, loose connections or excessive temperature. Capacitance can also decline with age, gradually reducing the available kVAr.

Measure the current of each stage and compare it with the expected value at the actual operating voltage. Capacitor current, connections and protection fuses should be checked periodically, especially after nonlinear loads are added. ABB

6. The Bank Is Not Producing Its Nameplate kVAr

Capacitor output changes approximately with the square of voltage:

Qc actual = Qc rated × (V actual ÷ V rated)²

A capacitor rated at 50 kVAr and 440 V will produce only about 41.3 kVAr when operated at 400 V:

50 × (400 ÷ 440)² ≈ 41.3 kVAr

This difference can leave the system under-compensated even when the nameplate total appears sufficient. Voltage rating, system frequency, reactor configuration and actual bus voltage must all be considered.

7. The System Has Significant Phase Unbalance

A conventional three-phase capacitor bank supplies equal compensation across all three phases. It cannot independently follow a system in which one phase requires more reactive compensation than the others.

This is common in buildings and facilities with many single-phase loads. The bank may overcompensate one phase while another remains under-compensated.

Measure current, power factor and reactive power on each phase. An SVG with independent phase compensation may be more suitable when the imbalance changes with operating conditions.

How to Diagnose Low Power Factor Correctly

Use a power-quality analyzer at the point where the utility measures the installation. Record at least one complete operating cycle instead of relying on a single reading.

The survey should include:

1. kW, kVAr, kVA and power factor

2. True and displacement power factor

3. THDi and THDv

4. Individual phase currents

5. Capacitor-stage status

6. Load changes over time

7. Measurements with the bank connected and disconnected

These results will show whether the facility needs additional capacitance, maintenance, harmonic filtering or faster reactive power compensation.

Choosing the Correct Solution

Frequently Asked Questions

Why did the power factor improve but not reach the target?

The bank may be undersized, one or more stages may have failed, or its actual output may be lower because of the operating voltage. Harmonic distortion can also keep true power factor below the displacement power factor.

Can I solve low power factor by adding more capacitors?

Not without taking measurements. Additional capacitance can cause leading power factor, overvoltage or harmonic resonance. Confirm the actual reactive power requirement and harmonic conditions first.

Does a capacitor bank reduce harmonics?

A standard capacitor bank does not remove harmonics. Detuned reactors help protect capacitors and avoid resonance, while an Active Harmonic Filter compensates harmonic current.

What power factor should an industrial facility target?

The correct target depends on the utility tariff and grid requirements. Many facilities target at least 0.95, while others aim for 0.98. Operating unnecessarily close to unity can create a leading power factor during light-load periods.

When should an SVG be used instead of a capacitor bank?

An SVG is suitable when reactive demand changes rapidly, fine adjustment is required, phases are unbalanced or both inductive and capacitive compensation are needed. Capacitor banks remain economical for stable base reactive loads.

Conclusion

A low power factor after capacitor bank installation should not automatically be treated by adding more kVAr. The cause may be insufficient capacity, slow switching, harmonics, incorrect controller settings, failed components, low operating voltage or phase unbalance.

A complete power-quality survey provides the information required to repair the existing bank or determine whether detuning, an AHF, an SVG or a hybrid solution is more appropriate.