6 min read
One of the things we hear most often on site: “We have a correction panel but the bill still shows a reactive penalty.” Having a capacitor bank does not mean it is working. Four causes usually lie behind a persisting penalty, and only measurement tells them apart.
1. Steps are faulty or not switching in
Capacitors lose capacitance as they age and some drop out entirely. Opening the panel and reading the controller display is often enough: if the relay keeps searching for steps but never reaches target, some steps are not working. Bulging, leakage and discolouration on the capacitor body are visible signs too.
2. Contactors are welded or burnt
Correction contactors behave differently from normal loads: they draw a very high inrush at switch-in. That is why purpose-built capacitor contactors with pre-charging resistors or early-make contacts must be used. In a panel fitted with standard contactors, welded contacts and early failure are inevitable.
3. The controller is set incorrectly
- The CT ratio may be entered incorrectly, so the controller sees the load as smaller or larger than it is.
- If the CT is fitted at the wrong point, the controller may be measuring part of the plant rather than all of it.
- The target power factor may be set too low.
- A wrong C/k value makes the controller hunt, switching steps in and out continuously.
4. Harmonics are stressing the capacitors
As the number of drives grows, harmonic distortion on the network rises. Because capacitor impedance falls with frequency, harmonic currents flow straight into the capacitors: they heat up, lose capacitance and eventually fail. The answer here is not more capacitors but moving to detuned correction.
| Symptom | Likely cause | First check |
|---|---|---|
| Controller never reaches target | A failed step | Measure capacitance step by step |
| Capacitors fail often | Harmonics | Measure harmonic distortion |
| Welded contacts | Wrong contactor type | Are they capacitor contactors? |
| Penalty despite an “OK” controller | Wrong CT ratio or position | Check the CT and settings |
Frequently asked questions
How often does a correction panel need servicing?
We recommend a full check at least once a year: measuring step capacitance, inspecting contactor contacts, torque checks and thermal inspection. In plants with high harmonic levels the interval should be shorter.
Is a detuned reactor needed in every plant?
No. Where electronic loads such as drives, UPS units and LED drivers make up a small share of total power, standard correction is enough. Where that share is high, a detuned solution both removes the penalty and extends capacitor life.
How is the reactive penalty calculated?
The utility measures inductive and capacitive reactive energy relative to the active energy consumed. Exceeding the defined ratios appears as a separate line on the bill, which is why over-correction can also be penalised.
Why do capacitive penalties appear at night and at weekends?
When production stops the inductive load falls, but if correction steps stay switched in the plant exports capacitive reactive power. Configuring the controller to drop steps at low load, and isolating the bank if needed, solves it.