Answer first
Short Answer
A momentary interruption brings supply voltage close to zero, so the protected load cannot rely on residual grid power.
Ride-through therefore requires a source of real energy sized for the load power, interruption duration, conversion losses, and process reserve.
Flywheel-backed, battery UPS, or hybrid systems may be considered according to duration, cycling, load behavior, and required recovery sequence.
When This Applies
- Measured events include near-zero residual voltage.
- A critical drive, feeder, or control system must survive without an immediate process stop.
- Transfer to backup generation is too slow for the sensitive load.
- The required interval is defined and short-duration storage is being evaluated.
Engineering Explanation
A compensator can restore missing voltage during a sag while the grid still provides part of the load energy. Near zero volts, that contribution disappears. The ride-through system must deliver the protected load's real power until the source returns or another supply takes over.
The total design envelope includes detection time, converter response, load inrush or torque, the interruption interval, recovery ramp, and any required overlap with a generator or alternate feeder.
| Event | Residual supply | Typical design question |
|---|---|---|
| Voltage sag | Reduced but present | How much voltage must be injected, and for how long? |
| Momentary interruption | Near zero | What source can supply full load power for the required interval? |
| Sustained outage | Zero for extended time | What long-duration backup and fuel/energy strategy is required? |
Calculation Example
A 1 MW critical load with a 1.5-second zero-voltage requirement has an ideal energy demand of 1.5 MJ, or about 0.417 kWh. The energy number is small compared with long-duration storage, but the source and converter must still deliver the full 1 MW power plus design margin.
Alternative Solutions
| Option | Consider when | Check carefully |
|---|---|---|
| Flywheel-backed ride-through | High power, short duration, frequent cycling | Verified duration, converter interface, mechanical and electrical protection |
| Battery UPS | Longer autonomy or established UPS load profile | Battery aging, maintenance, environment, motor compatibility |
| Static transfer switch | A truly independent alternate source is continuously available | Transfer time, source phase relationship, and downstream tolerance |
| Generator + bridge storage | Long outages must be covered | Start time, fuel, transition, and bridge duration |
Limitations / When Not to Use
- Do not treat an interruption as an ordinary sag without verifying residual voltage.
- Do not claim a storage-less compensator will support a zero-voltage event at full load.
- For long outages, flywheel-only systems may be uneconomic or insufficient; battery, generator, or hybrid backup is usually evaluated.
- Restart prevention must include auxiliaries and process controls, not only the largest motor.
Recommended Architecture
Record the worst credible residual voltage and duration, then define the minimum process boundary that must remain alive. Size the power path for the full protected load and the energy source for the complete transition and recovery window.
UTILITY / FEEDER ↓ event detectionPower conversion stage ← Short-duration energy source ↓Critical feeder or selected VFD DC busTraceable sources
Sources / References
- IEC 61000-4-30: Power quality measurement methods. Measurement and interpretation methods for voltage dips, interruptions, harmonics, transients, and other power-quality parameters. Accessed 2026-08-24.
- IEEE 1159-2019: Monitoring Electric Power Quality. Recommended practice for describing, measuring, and interpreting power-quality phenomena. Accessed 2026-08-24.
- U.S. Department of Energy — LIRR High-Speed Flywheel Demonstration. Public overview describing flywheels as capable of frequent fast charge/discharge cycles and high power for short durations. Accessed 2026-08-24.