You find a 29 kvar, 400 V capacitor bank on site. A meter reads capacitance between L1 and L2. Is that the actual capacitance of one internal capacitor? Not necessarily.
This guide explains what the meter sees, how to calculate kvar from capacitance or inductance, and when those shortcuts stop being reliable.
What are capacitance (C) and inductance (L)?
Capacitance
Farad (F), often µFA capacitor stores energy in an electric field. In AC, ideal capacitor current leads voltage.
Inductance
Henry (H), often mHAn inductor stores energy in a magnetic field. In AC, ideal inductor current lags voltage.
Impedance
Ohms (Ω)Impedance includes resistance and reactance. It is not another name for inductance.
Think of C and L as physical properties. Their effect on AC is described by reactance X. Total impedance Z also includes resistive losses.
Why frequency changes the result
At higher frequency, an ideal capacitor lets more current flow while an ideal inductor opposes more current. This is why a 50 Hz capacitor-bank calculation cannot automatically be used at 60 Hz without recalculating.
For a real reactor, the total impedance includes winding resistance and may depend on magnetic saturation or coupling between phases.
How delta and star differ
For a balanced three-phase bank, each delta branch sits across the full line-to-line voltage. Each star branch sees the phase voltage when its center is floating and all three branches are balanced.
Why measured L-L capacitance is different
When measuring between two terminals of a disconnected, isolated three-phase capacitor network, the third terminal is left floating. The meter sees combinations of the internal branches—not just one capacitor.
With three ideal uncoupled inductors, the corresponding terminal inductance follows the inverse network-equivalent relationships:
How to estimate kvar from terminal measurements
For a balanced ideal three-phase capacitor-only bank, the same kvar expression works for delta and isolated star when you use the measured L-L equivalent capacitance:
Here Q is in var, voltage in V and C in farads. For ideal, uncoupled inductors, the corresponding magnitude is:
A three-phase reactive-power magnitude corresponds to approximate balanced line current:
Use var rather than kvar inside these unscaled formulas.
29 kvar capacitor bank, 400 V, 50 Hz, delta
For an isolated, balanced capacitor-only bank, the expected line-to-line capacitance is:
Because the bank is delta-connected, the capacitance of each equivalent branch is smaller:
Now imagine you read 285 µF, 289 µF and 288 µF between the three line-terminal pairs. Take their average and estimate the actual kvar with the calculator. Compare the result with the manufacturer's permitted tolerance—not an arbitrary fixed pass/fail threshold.
Does the same calculation work for reactors?
For simple uncoupled ideal inductors, yes: star/delta series-parallel algebra gives the terminal equivalent and an ideal inductive kvar magnitude. That is why KiloVolt offers an Inductive (L) tab.
But a real reactor may have winding resistance, magnetic coupling between phases, iron-core saturation, varying inductance with excitation and external capacitor/inductor networks. In those cases, a single L-L inductance reading is not enough to determine operating kvar accurately.
To assess a detuned capacitor bank or actual reactor, consult the manufacturer's rated voltage/frequency, test method and instrument measurement conditions.
Work safely with capacitor banks and reactors
Do not take capacitance or inductance measurements on energized equipment. Follow the applicable isolation and lockout procedure, account for stored capacitor energy and verify safe discharge with an appropriate instrument before handling terminals.
Disconnect external reactors, discharge circuits and other components as required by the equipment manufacturer's test procedure. Measurement at the assembled bank terminals may otherwise produce misleading values.
Common mistakes to avoid
The meter sees a network equivalent. On an isolated delta bank, the L-L measurement is 1.5 times the equal branch capacitance.
For the same capacitance, kvar changes with the square of voltage. Compare at the specified rating.
Unbalanced readings indicate a need for closer investigation; the average provides only a preliminary kvar estimate.
Real impedance includes resistance and other effects; frequency also matters.
A detuned bank does not behave like an isolated three-capacitor network.
Further reading: ABB — Power Factor Correction and Harmonic Filtering and LIFASA — Capacitor Measurement Technical Note.
Calculate C, L, kvar and terminal equivalents
Switch between capacitive and inductive components, three-phase star/delta and L-L measurement modes.
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