Short answer
Capacitor bank measured kvar at site usually differs from nameplate, and most of the difference is supply conditions, not the product. Reactive output of a capacitor is proportional to voltage squared and to frequency. In an illustrative 300 kvar bank rated at 400 V and 50 Hz, measured at 380 V and 50 Hz, the expected output is about 271 kvar. If 262 kvar is measured, capacitance is about 3.3 % below nominal, inside an illustrative acceptance band of −5 % to +10 %; the actual product standard and contract must define the accepted band. Detuned banks have their output referred to bus voltage with the reactor’s 1/(1 − p) factor, so the capacitor’s own rating and the branch rating must not be mixed. Separating capacitance measurement from supply conditions shows whether a shortfall comes from the product or the supply. Low site voltage is not proof of a defective bank.
Corrections for voltage and frequency
| Quantity | Value | Note |
|---|---|---|
| Nameplate | 300 kvar | 400 V, 50 Hz |
| Expected at 380 V | ≈ 270.8 kvar | Voltage squared |
| Measured | 262 kvar | Site |
| Capacitance ratio | ≈ 0.967 | Inside the example −5 % lower limit |
Q = Q_r × (U / U_r)^2 × (f / f_r)
Illustrative example. Plain bank: 300 kvar at 400 V, 50 Hz; site measurement at 380 V, 50 Hz gives 262 kvar. Values chosen for demonstration.
Qexpected = 300 × (380 / 400)^2 ≈ 270.8 kvar
C / C_r ≈ 262 / 270.8 ≈ 0.967
Capacitance is about 3.3 % below nominal. This example assumes a −5 % to +10 % acceptance band. Confirm the permitted capacitance band, test conditions and corrections from the actual product specification and contract.
Detuned banks
For a detuned branch with detuning factor p, the branch output at bus voltage is:
Qbranch = Ubus^2 ω C / (1 − p)
A detuned bank sold as “300 kvar at 400 V bus” uses capacitors rated at a higher voltage; their own nameplate kvar differs from the branch output. Comparison must use the same reference; see detuning factor in transformer-fed capacitor banks and capacitor terminal voltage with series detuning reactors.
Separating product and supply
| Measurement | Indicates |
|---|---|
| Capacitance per phase (bridge or current/voltage) | Product condition |
| Bus voltage and frequency | Supply condition |
| Harmonic content | Extra current, not fundamental kvar |
| Stage-by-stage current | Failed units or contactors |
Measuring capacitance directly, or computing it from fundamental current and voltage, removes supply effects. Unit losses in fused banks show as stepwise capacitance reductions; see external-fuse capacitor banks and internally fused capacitor unbalance.
Acceptance record
- Voltage, frequency and temperature at measurement.
- Fundamental current and voltage per phase and stage.
- Calculated capacitance and comparison with nominal and tolerance.
- Corrected kvar at rated conditions.
- Harmonic content noted separately.
What this article does not cover
It does not treat reduced kvar at low site voltage as a product defect.
Where this reasoning stops. Tolerance figures must be taken from the applicable standard and contract.
Practical takeaway
Correct measured kvar for voltage squared and frequency, measure capacitance directly, use the right reference for detuned banks, and record each contribution before judging the product.
Reference framework
- IEC 60831-1, Shunt power capacitors of the self-healing type for a.c. systems up to 1 kV.
- IEC 61921, Power capacitors: low-voltage power factor correction banks.