AKKN ElectronicsThe capacitor bank in an industrial network
An industrial power-factor-correction (PFC) bank typically consists of several three-phase capacitor steps switched by contactors or thyristor modules. Each step is sized in kVAR to compensate the reactive power of loads such as motors, transformers, and welding equipment. The bank is usually connected to the low-voltage bus, 400 V or 480 V, and is operated by a reactive-power regulator that steps units in and out based on measured cos φ.
The electrical stress on each capacitor is not limited to the fundamental 50/60 Hz voltage. The bus voltage itself rises above nominal when the bank is switched in, because compensation reduces the current drawn from the distribution transformer and therefore reduces voltage drop. In addition, the presence of non-linear loads — VFDs, rectifiers, UPS systems — injects harmonic currents into the bus. These harmonics increase the RMS current through the capacitor, even if the fundamental reactive power stays within the nameplate rating.
A capacitor rated for a certain kVAR at a certain voltage must therefore be assessed not only for its capacitance at 50 Hz but for its ability to carry harmonic current, survive overvoltage events, and operate at the prevailing ambient temperature of the enclosure. An aged bank, one that has been in service for 10–15 years, frequently shows failures caused by a combination of these factors rather than by a single abnormal event.
What the replacement capacitor must meet
When replacing capacitors in a PFC bank, the parameters below are the ones to compare between the existing unit and the candidate replacement.
Rated voltage. The rated voltage of a PFC capacitor is deliberately chosen above the nominal bus voltage. Typical ratings for a 400 V network are 440 V, 450 V, or 480 V. For a 480 V network, ratings of 525 V or 550 V are common. This margin covers tolerance on the transformer taps, the voltage rise caused by switching the bank in, and a portion of the harmonic voltage distortion. Choosing a replacement with a lower voltage class than the original, even if the kVAR appears correct at the lower voltage, is a common sourcing error.
Reactive power. The kVAR rating of a capacitor is always declared at a specific voltage, often the rated voltage of the capacitor, not the bus voltage. When the capacitor is connected to a bus that is lower than its rated voltage, the actual kVAR it produces is lower than the nameplate value, because reactive power is proportional to the square of the voltage. A spec that says "40 kVAR at 400 V" and a spec that says "40 kVAR at 480 V" are two different products.
Ripple and harmonic current. IEC 60831-1 is the relevant standard for low-voltage PFC capacitors. It sets an upper limit for the thermal current, typically 1.3–1.5 times the rated current for units with an internal overpressure disconnector. In practice, the harmonic content of the network determines whether this margin is sufficient. On a bus with substantial 5th- and 7th-order harmonics, the RMS current can exceed 1.5 times the fundamental current while the peak voltage is still within the rated limit. For such locations, a detuned filter approach — adding a reactor to shift the resonant frequency — is preferable to simply installing a capacitor with a higher current rating.
Temperature class. PFC capacitors are classified by their maximum ambient temperature in the enclosure. The common classes are B (−45 to +45 °C), C (−25 to +50 °C), and D (−25 to +55 °C). Capacitors installed in metal enclosures with multiple steps and no forced ventilation endure a higher internal temperature than open-rack installations. The lifetime of a polypropylene-film PFC capacitor is linked to the hot-spot temperature; every 8–10 K rise reduces the expected life by a factor of roughly two, following the Arrhenius model.
Screening table for a replacement candidate
The table below summarizes the parameters to compare when a replacement unit is proposed for an existing step in a PFC bank.
| Parameter | What to check | Typical value for a 400 V bus | Notes |
|---|---|---|---|
| Rated voltage | Nameplate voltage class | 440–480 V | Select above bus voltage to cover harmonics and switch-in rise |
| Rated kVAR | Declared at which voltage | 25–50 kVAR per step | Actual output falls with the square of the bus/rated voltage ratio |
| Capacitance per phase | Measured at 50 Hz, at reference temperature | 3 × 100–400 µF typical per step | Tolerance per IEC 60831 is −5 % / +10 % |
| Maximum harmonic current | Thermal limit at nominal voltage | 1.3–1.5 × fundamental current | Check if a detuning reactor is required on harmonic-rich buses |
| Temperature class | Maximum ambient in enclosure | Class B or C | Class D for unventilated enclosures |
| Dielectric | Polypropylene film | Standard for low-loss PFC | Loss angle tan δ ≤ 0.002 at 20 °C, 50 Hz |
| Discharge resistor | Internal or external | ≤ 75 V within 3 min after de-energizing | Required for safe access |
Installation and test notes
Before switching a rebuilt PFC bank back into service, the following checks are worth performing.
Measure capacitance per phase. A capacitance meter at 100 Hz or 1 kHz gives a first screening. The three phases of a three-phase capacitor should agree within about 5 %. A replacement unit with a phase imbalance larger than that can unbalance the neutral point of a star-connected bank and trigger protection in the regulator.
Verify the discharge path. IEC 60831-1 requires that the residual voltage at the terminals falls to 75 V or below within 3 minutes after disconnection. Capacitors with internal discharge resistors comply when new; after 10 years in service, however, the resistor can drift or crack. An external discharge arrangement is a safe retrofit for aged banks.
Check the switching device rating. PFC capacitors experience inrush current at switch-on that is several times their steady-state current. A contactor rated only for the fundamental current can weld its contacts after a number of switching cycles. The replacement of capacitors in a bank is a suitable moment to verify that the contactors and fuses are rated for the step size and the expected switching frequency.
Test with a PQ analyzer. After energizing the bank, a power-quality meter on the bus will show whether the voltage distortion has moved. If the total harmonic voltage distortion (THD-V) rises noticeably after some of the steps are switched in, a detuning reactor or a lower capacitance step is required. The check is not a one-time measurement: harmonic sources in the plant change when new drives are added, so a re-test after any significant load change is good practice.
The material above is intended as a practical guide for sourcing and commissioning PFC capacitor replacements. The decisive screening variables are rated voltage class, the voltage at which the kVAR is declared, harmonic current capability, and the temperature class. These four parameters, checked against the data sheet of a candidate replacement unit, are sufficient to eliminate most mismatches in a PFC bank rebuild.
