An electrician opens the APFC panel and finds a bulged capacitor can with the overpressure disconnect pushed up. The controller still operates the step, but the measured capacitance has drifted and the power factor no longer reaches the setpoint. Replacing the cell with the nearest standard kvar value is a workable repair; doing it without checking the bus voltage level and the harmonic content on the bus does not remove the root cause.
Stress Paths That Fail Step Capacitors
The equipment in question is a conventional APFC cabinet: contactor-switched steps, each step containing one three-phase capacitor or three single-phase cylindrical capacitors, a discharge resistor network and, in better-protected designs, a detuned reactor. Step ratings commonly range from 5 kvar to 50 kvar on 380–480 V networks. Failures develop through three identifiable stress paths.
Bus overvoltage. Energizing a step raises the local voltage because the capacitor current reduces the voltage drop across the distribution transformer. On a lightly loaded night shift, a 400 V network can run at 415–425 V continuously. A 400 V capacitor on that bus is operated at its rated ceiling all day. The accepted derating practice is to select a 440 V or 480 V rated can on 400 V networks, and a 525 V class on 480 V networks. Note that the delivered kvar changes with the square of the voltage ratio: a 30 kvar, 440 V rated step connected to a 400 V bus delivers roughly 24.8 kvar. Size the controller’s compensation step on that actual value, not on the nameplate rating.
Harmonic current and resonance. VFDs, rectifiers and UPS front ends inject harmonic currents. The capacitor bank and the transformer leakage inductance form a resonant circuit; when the resonant frequency is close to the 5th or 7th harmonic, harmonic currents several times the fundamental can circulate into the capacitors. This raises dielectric losses and internal temperature, accelerating film aging. A detuned reactor of 7% impedance moves the resonance below the 5th harmonic (about 189 Hz at 50 Hz); 14% reactors are chosen when low-order distortion dominates.
Switching inrush. A discharged capacitor behaves as a near-zero impedance at the instant of contactor closing. When other steps are already charged, the peak inrush current can reach several hundred times the rated peak current. Each switching event erodes the contactor contacts and stresses the internal connections of the capacitor. Rapid cycling by the controller shortens both switch and capacitor life.
Requirements for the Replacement Capacitor
The following parameters form a reasonable minimum specification when sourcing replacement units for export and stock repair programs. These values follow common practice for metallized polypropylene PFC capacitors on 50/60 Hz industrial networks.
| Parameter | Typical requirement | Verification note |
|---|---|---|
| Rated voltage class | 440 V or 480 V on 400 V buses; 525 V on 480 V buses | Record bus voltage over 24 h before ordering |
| Capacitance tolerance | –5 % / +10 % per IEC 60831-1 | Compare each phase with the nameplate using an LCR meter at 100/120 Hz |
| Dissipation factor | ≤ 0.002 at 20 °C | Higher values indicate degraded film or moisture entry |
| Continuous overcurrent | 1.3 × rated current, harmonics included | Verify with a true-RMS clamp meter on each phase |
| Overvoltage withstand | 1.10 × UR for 8 h/day; 1.15 × UR for 30 min/day; 1.30 × UR for 1 min/day | Daily operating voltage should stay below 1.10 × UR |
| Discharge time | Residual voltage ≤ 75 V within the time marked on the can (typically 60–180 s) | Measure with a high-impedance voltmeter after disconnection |
| Thermal class | –25/D (max 55 °C mean over 24 h, 45 °C mean over 1 year) | Vent the cubicle if internal temperature approaches these values |
When one can of a three-phase step fails, replacing all three cans keeps the phases balanced and avoids mixing aged and new dielectric. If the panel already has detuned reactors, the replacement capacitor must keep the same voltage and kvar rating at the reactor output; verify the reactor impedance percentage and tuned frequency before changing the step’s kvar value. The dielectric should be self-healing metallized polypropylene, and the can should be fitted with an overpressure disconnect.
Incoming Inspection and Installation Notes
- Measure capacitance per phase at 20 °C. A spread above 5% between the three cans of the same step points to one cell degrading.
- Confirm the discharge behavior: residual voltage must fall below 75 V within the marked time. An open discharge resistor leaves a dangerous voltage on the terminals.
- Use the terminal torque marked on the can. For M12 studs, typical values are 10–15 N·m. Under-torque leaves contact resistance that heats the joint; over-torque can damage the internal connection to the terminal.
- After energizing, measure the per-phase current of every step. A step drawing above 1.3 times its rated current indicates resonance or an undersized capacitor, and replacing it with the same value will not correct that condition.
- Observe the switching pattern after commissioning. If a step cycles more than once per minute, the controller’s reconnection delay is too short for the discharge resistor.
For procurement, RoHS and REACH declarations should accompany each batch of PFC capacitors. Because the common voltage classes — 440 V, 480 V and 525 V — cover most industrial APFC panels, ordering from stock reduces export lead time. Confirm the marked discharge time, thermal class and capacitance tolerance at the quotation stage; these parameters decide whether the unit can be swapped into the panel without changing controller settings.
