Short answer
Capacitor switching transients transformer paths explain why a utility switching a capacitor bank on the medium-voltage network can trip drives in a plant. Energising an MV bank produces a voltage oscillation at a frequency set by the bank and source, approximately the fundamental times the square root of short-circuit power over bank reactive power. In an illustrative case, a 5 Mvar bank on a 200 MVA bus oscillates at about 316 Hz. That oscillation passes through distribution transformers to the low-voltage bus. If the plant has power-factor capacitors whose resonance with the transformer lies near the same frequency, illustratively about 373 Hz for a 1,000 kVA transformer with 300 kvar, the transient can be magnified at the LV bus, depending on damping and the complete connection. Variable-speed drives can trip on DC-link overvoltage. Cables and filters change the response, and simultaneous records at several points show where magnification occurs.
Two oscillating circuits
| Circuit | Basis | Frequency |
|---|---|---|
| MV switching | 200 MVA, 5 Mvar | ≈ 316 Hz |
| LV natural | 16.7 MVA, 0.3 Mvar | ≈ 373 Hz |
| LV detuned 7 % | Inductive above 189 Hz | Reduced LC risk; verify transient response |
- MV circuit: source inductance and switched capacitor bank; oscillation at f_MV.
- LV circuit: transformer leakage inductance and LV capacitors; natural frequency f_LV.
When f_MV and f_LV are close, energy transfers efficiently and the LV voltage overshoot can exceed the MV overshoot.
Illustrative frequencies
Illustrative example. MV bus 200 MVA short-circuit; 5 Mvar bank switched; LV bus with 1,000 kVA, 6 % transformer and 300 kvar PF capacitors (plain). 50 Hz. Values chosen for demonstration.
fMV ≈ 50 × sqrt(200 / 5) ≈ 316 Hz
fLV ≈ 50 × sqrt(16.7 / 0.3) ≈ 373 Hz
The frequencies are close; magnification is likely. With detuned LV capacitors, the LV branch is inductive above its tuning order, which can reduce this magnification mechanism but does not guarantee a transient-free bus; see detuning factor in transformer-fed capacitor banks.
Effects on sensitive loads
| Load | Effect |
|---|---|
| Variable-speed drive | DC-link overvoltage trip |
| UPS | Transfer to battery |
| Electronic controls | Resets |
| Surge protective devices | Energy absorption, ageing |
Drive trips often occur without visible damage and are blamed on “voltage spikes” without measurement.
Factors that change the response
- Cables between transformer and loads add capacitance and delay.
- Harmonic filters at LV alter the LV natural frequency; see single-tuned and damped filters on transformer buses.
- Load provides damping; light-load periods give higher overshoot.
- Switching instant on the voltage wave; controlled switching or pre-insertion at MV reduces the source transient; see capacitor bank energization with pre-insertion devices.
Multi-point records
- MV bus voltage at the switching instant.
- LV bus voltage, same time base.
- Drive terminals or DC link if possible.
- Sampling of at least tens of kilohertz.
Comparing overshoot at each point shows whether the LV bus magnifies. Back-to-back MV switching produces higher-frequency transients that couple differently; see back-to-back capacitor switching near a transformer bus.
What this article does not cover
Current chopping by vacuum breakers when switching transformers is a separate phenomenon and not covered here.
Where this reasoning stops. Simple formulas give frequencies, not overshoot magnitudes; magnification needs transient simulation.
Practical takeaway
Compare MV switching frequency with LV capacitor resonance, avoid plain LV capacitors that resonate near it, use controlled switching or pre-insertion at MV, and confirm with simultaneous records at MV, LV and load.
Reference framework
- IEEE 1036, Guide for the application of shunt power capacitors.
- IEEE C37.012, Guide for the application of capacitance current switching for AC high-voltage circuit breakers.