Short answer
Capacitor bank step size sets the voltage jump each stage causes on the bus. Switching a stage changes reactive power flow through the transformer and source, so bus voltage steps by about the reactive change divided by the short-circuit power at the bus. On a weak low-voltage bus, that step can be visible. In an illustrative case, a 1,000 kVA, 6 % impedance transformer on a stiff source gives about 16.7 MVA short-circuit power at the low-voltage bus, so a 100 kvar step moves voltage about 0.6 %. With a weaker upstream network reducing short-circuit power to 12 MVA, the same step gives about 0.8 %, and a 200 kvar step about 1.7 %. Large steps can cause visible flicker and controller hunting. Finer steps reduce voltage change but add switching operations and cost.
Voltage step per stage
| Step | Stiff 16.7 MVA | Weak 12 MVA |
|---|---|---|
| 50 kvar | 0.30 % | 0.42 % |
| 100 kvar | 0.60 % | 0.83 % |
| 200 kvar | 1.2 % | 1.7 % |
ΔU / U ≈ ΔQ / Ssc
S_sc includes the transformer impedance and the upstream source. Load also matters: motor loads respond to voltage and partially offset the change.
Illustrative values
Illustrative example. 1,000 kVA transformer, 6 % impedance; LV bus. Two upstream conditions: stiff (bus S_sc ≈ 16.7 MVA) and weak (bus S_sc ≈ 12 MVA). Values chosen for demonstration.
| Step | Stiff, 16.7 MVA | Weak, 12 MVA |
|---|---|---|
| 50 kvar | 0.30 % | 0.42 % |
| 100 kvar | 0.60 % | 0.83 % |
| 200 kvar | 1.2 % | 1.7 % |
At 400 V, 1.7 % is about 7 V, enough to be seen as a lighting change if it repeats.
Hunting
A power-factor controller with a deadband narrower than one step may switch on, overshoot the target, switch off, and repeat. Large steps relative to load variation worsen this. Controller settings and step sizes must be chosen together. Measurement location of the controller’s CT also matters; see power-factor controller CT location with parallel transformer incomers.
Finer steps versus operations
| Choice | Voltage step | Operations | Cost |
|---|---|---|---|
| Few large steps | Larger | Fewer | Lower |
| Many small steps | Smaller | More | Higher |
| Thyristor-switched stages | Reduced switching transient; the steady voltage step remains | No mechanical contact wear; electrical and thermal limits remain | Higher, with losses |
Contactor life depends on operations and inrush; detuned banks reduce inrush; see detuning factor in transformer-fed capacitor banks.
Weak buses and overcompensation
At light load, a large step may push the bus leading and raise voltage; see transformer reactive demand at low load and capacitor overcompensation. Voltage steps also interact with transformer tap control.
Checks
- Bus S_sc in normal and weakest configurations, including one transformer out; see harmonic amplification after one transformer is removed.
- Step size against allowable voltage change and flicker.
- Controller deadband and delay against step size.
- Switching count per day against contactor life.
What this article does not cover
Transformer tap step size is a separate topic and is not covered here.
Where this reasoning stops. The ΔQ/Ssc estimate ignores resistance and load response.
Practical takeaway
Estimate each stage’s voltage step from bus short-circuit power in the weakest configuration, size steps and controller deadband together, and balance smaller steps against switching operations.
Reference framework
- IEC 61921, Power capacitors: low-voltage power factor correction banks.