Why DG Sets Trip When the APFC Panel Is On — and How to Fix It

In short. An APFC panel adds capacitors to keep your power factor close to unity on the grid. On generator supply, especially at light load, those same capacitors can push the power factor leading. The generator’s voltage regulator loses control, voltage rises or swings, and the set trips. The usual cure is simple: switch the capacitors off, or limit them, whenever the generator is supplying the load.

Who this page is for: For plant electrical and maintenance teams: find out whether your capacitor bank is tripping your generator, and fix it.

What happens, step by step

1. On mains, the APFC panel switches capacitor steps in to cancel the lagging current of your motors.
2. Power fails and the load moves to the generator. Some capacitor steps are still connected.
3. The load on the generator is lighter than usual, or motors have not restarted yet. The capacitors now supply more reactive power than the load needs — the power factor becomes leading.
4. Leading current raises the alternator’s voltage. The automatic voltage regulator cuts back excitation to compensate.
5. If excitation is pushed too low, the regulator loses control: voltage swings, the set becomes unstable, and over-voltage or excitation protection trips it.

Symptoms to look for

• Set trips a few seconds or minutes after taking load, but runs fine with the APFC switched off
• Voltage on the generator panel higher than normal, or swinging
• Power-factor meter showing leading (capacitive) on DG supply
• Repeated AVR or alternator faults with no clear engine cause
• Lamps flickering or electronics failing during generator operation

Fixes that work

Harmonics make it worse

Drives, UPS systems and LED lighting create harmonic currents. Capacitors can resonate with them, drawing excess current, overheating and failing. A generator has a higher internal impedance than the grid, so harmonic distortion is usually worse on DG supply. If your site has many electronic loads, detuned capacitor banks are worth the extra cost.

A service engineer’s view

When a DG set “trips for no reason” in a factory, the APFC panel is one of the first things to check. The quickest test is to run the set on the same load with the capacitors switched off. If the problem disappears, the fix is in the wiring between the panels, not in the engine.

What you get from us

• Diagnosis of DG tripping, hunting and over-voltage problems
• DG-ON interlock wiring between AMF, changeover and APFC panels
• AVR checks and replacement
• Advice on step sizes and detuned capacitor banks

How much leading kVAr can an alternator actually absorb?

Every alternator has a capability chart: a map of the kW and kVAr it can supply. On the lagging side the limit is field-winding heat — more lagging kVAr needs more excitation current. On the leading side the limit is quite different. Leading current through the stator aids the rotor’s magnetic field (armature reaction works in the same direction), so the regulator must reduce excitation to hold voltage. There is a floor: the regulator cannot reduce field current below zero, and long before that the set becomes unstable.

For a typical stand-alone industrial set, the usable leading region is small — often in the order of 10–30% of rated kVA as leading kVAr. Light load is the danger point not because this limit shrinks, but because the load’s own lagging kVAr, which offsets the capacitors, is then smallest. The actual figure comes from the alternator’s own capability chart, which is worth asking for whenever capacitors will remain on DG supply.

Self-excitation: when the regulator has nothing left to cut

If enough capacitance is connected, the alternator can hold up its voltage with almost no field current at all. Residual magnetism produces a small voltage; that voltage drives leading current into the capacitors; the leading current strengthens the field; the voltage rises further. It settles only where the alternator’s magnetisation curve meets the capacitors’ voltage–current line — which can be well above 415 V. The regulator has already reduced excitation to its minimum and cannot intervene. This is why some sets show over-voltage even with the AVR apparently “off”.

Capacitor kVAr rises with voltage

Capacitor output rises with the square of voltage. A bank rated 200 kVAr at 415 V delivers 200 × (440 ÷ 415)² ≈ 225 kVAr at 440 V. So as leading power factor pushes voltage up, the capacitors produce even more leading kVAr — a self-reinforcing loop rather than a self-correcting one.

Reconnection and discharge time

Capacitors need time to discharge after switching off — commonly up to about a minute through their discharge resistors. If the changeover transfers load to the DG while steps are still charged, or the APFC relay re-inserts them too quickly, the set sees a sharp current transient at the moment of closing. A DG-ON interlock should therefore switch steps off before the DG breaker closes, not after.

Worked kVAr calculation

Illustrative case. A 500 kVA set (rated at 0.8 power factor: 400 kW, 300 kVAr lagging). The alternator chart allows an assumed 100 kVAr leading. During the day on mains, the APFC has switched in four 50 kVAr steps (200 kvar). An evening outage arrives during a shift change; only 80 kW of load remains, at 0.85 lagging.

The set is lightly loaded in kW, yet the 150 kVAr leading is half as much again as the 100 kVAr the alternator can tolerate. Voltage rises, the capacitors give more kVAr, and the set trips. With the DG-ON interlock, all four steps drop out and the set sees 80 kW at 0.85 lagging — comfortably inside its chart.

If the plant wants some correction on DG, the rule is simple: capacitor kVAr left connected must stay below the load’s own lagging kVAr at the lowest load the set can drop to before the relay has time to remove steps. At 300 kW and 0.8 lagging (225 kvar), four 50 kVAr steps leave 25 kVAr lagging and a power factor of 300 ÷ √(300² + 25²) ≈ 0.997 lagging — acceptable. At the 80 kW night load, not even one 50 kVAr step can be allowed, because 50 kVAr exceeds the 49.6 kVAr the load needs.

The Indian angle

• kVAh billing: where tariffs bill on kVAh, sites push APFC targets to unity and run aggressive step settings — ideal on mains, risky on DG.
• Evening and night outages: outages often come at shift change or on the night shift, exactly when kW is low and capacitor steps are still in from the daytime load.
• Panel heat: APFC panels in un-ventilated rooms at 45 °C ambient run hotter still inside. Capacitors lose capacitance as they age, so staff sometimes force extra steps in manual mode — and forget them.
• Monsoon: lower air-conditioning load in the rains reduces lagging kVAr, so a step arrangement that was safe in May can lead in August.

Light-load audit worksheet

Record these with the set on its lowest normal load — typically late evening or a holiday — to see how close it runs to the leading region.

Decision table: what the readings mean

Alternator and regulator checks that go with this audit are covered on the AVR & excitation service page; the arithmetic above can be repeated with the power-factor calculator.

Common mistakes

• Leaving the APFC panel in automatic on generator supply.
• Blaming the AVR for over-voltage trips that only happen at light load with capacitors connected.
• Fitting motor-terminal capacitors without checking how they behave on generator supply.
• Ignoring harmonics, which overload capacitors and add to nuisance trips.

What to send us

Send the set’s kVA, the APFC panel size and number of steps, how the panel is switched on generator supply, the alarm or trip shown, the load and power factor at the time, and the time of day the trips occur. Use the application engineering enquiry, or WhatsApp the details and photographs.

Related pages

• Generator Running but Low Voltage or No Output
• Generator Hunting
• AMF Panels

Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 3 November 2026.