Short answer
Capacitor bank pre-insertion devices reduce the inrush and overvoltage when a bank is energised. A resistor or inductor is connected in series for a short time before the main contacts close; both types limit initial current, but a resistor dissipates energy and damps oscillation while an inductor mainly limits current and changes the oscillation frequency. A bypass contact then shorts out the insertion device. Bypass causes a second, usually smaller, transient. A pre-insertion resistor may dissipate energy of the order of the bank’s stored energy per operation, illustratively about 1 kJ per phase for a 1 Mvar bank at 11 kV. An insertion inductor requires its own current, magnetic and loss assessment; it does not dissipate the same stored energy merely by being inductive. Both devices need a manufacturer-supported repetitive duty. Controlled switching is an alternative: for a discharged bank, each phase closes near its voltage zero so that the charging transient is reduced. Its effectiveness depends on timing accuracy; a 1 ms error at 50 Hz leaves a voltage step of about 0.31 per unit. Insertion time, energy and post-bypass behaviour must be recorded and checked against the study.
Options
| Timing error | Angle | Voltage step |
|---|---|---|
| 0.5 ms | 9° | ≈ 0.16 pu |
| 1 ms | 18° | ≈ 0.31 pu |
| 2 ms | 36° | ≈ 0.59 pu |
| Method | Principle | Main concern |
|---|---|---|
| Pre-insertion resistor | Damps inrush and overvoltage | Resistor energy, bypass transient |
| Pre-insertion inductor | Limits inrush current and frequency | Less overvoltage damping |
| Fixed current-limiting reactor | Permanent series inductance | Losses, voltage rise |
| Controlled switching | Closes at voltage zero | Timing accuracy, residual charge |
Insertion and bypass
- Insertion: the device and bank charge with damped current.
- Insertion time: long enough for the first transient to decay, set by design and the manufacturer.
- Bypass: main contacts short the device; a second step occurs equal to the voltage across the device at that moment.
Both transients must be within switch and bank ratings. Back-to-back conditions are the most severe; see back-to-back capacitor switching near a transformer bus.
Energy order of magnitude
Illustrative example. 1 Mvar bank at 11 kV, 50 Hz; 26.3 µF per phase; phase peak voltage 8.98 kV. Values chosen for demonstration.
E ≈ (1/2) C Upk^2 ≈ 0.5 × 26.3 × 10-6 × 8,981^2 ≈ 1.06 kJ per phase
The resistor absorbs energy of this order per energisation; repeated operations need thermal recovery time per the device rating.
Controlled switching
ΔU ≈ sin(2π f Δt)
Δt = 1 ms at 50 Hz: sin(18°) ≈ 0.31 pu
Closing 1 ms late still leaves a 0.31 pu step. Mechanical scatter, temperature and ageing of the switch affect timing; adaptive controllers compensate. A charged bank needs closing at the matching voltage, not zero; see residual charge and capacitor reclosing duty.
Records to keep
- Insertion time measured per phase.
- Inrush current and overvoltage during insertion and at bypass.
- Device temperature or energy counter where available.
- Steady state after bypass: current, voltage and harmonics normal.
Effects on downstream sensitive loads are discussed in capacitor switching transients at transformers.
What this article does not cover
It does not give resistor or inductor values; these must come from a transient study and manufacturer data.
Where this reasoning stops. Energy is an order-of-magnitude estimate; actual absorption depends on device value and timing.
Practical takeaway
Choose pre-insertion or controlled switching from a transient study, rate devices for energy per operation and repetition, check both insertion and bypass transients, and record timing and waveforms at commissioning.
Reference framework
- IEEE C37.012, Guide for the application of capacitance current switching for AC high-voltage circuit breakers.