Summary: how to choose voltage sag protection
Voltage sag protection falls into five categories: 1) capacitor-based sag protectors, which run in bypass normally and supply power from supercapacitors during a sag, for several seconds; 2) active voltage conditioners, which store no energy and restore the missing voltage from what remains on the grid, suited to shallow sags; 3) online UPS, which double-converts continuously and covers both sags and outages but draws power constantly; 4) offline and line-interactive UPS, which don't convert in normal operation and take a few milliseconds to switch; 5) flywheel storage, which stores energy in a high-speed rotor and is often used to bridge to a generator. The key to choosing is knowing how deep and how long the sags you need to cover are, and whether you also need to ride through outages.
3 questions first
How deep?
A drop to 70–80% and a drop below 20% call for completely different technologies. Equipment without energy storage handles deeper sags less well.
How long?
Typical sags in Taiwan's science parks last about 0.1 s. But if you also need to ride through outages until a generator starts, you'll need seconds to tens of minutes of stored energy.
How big a load?
Protecting only control circuits may take just a few kVA; protecting a whole tool or production line makes a big difference in capacity and cost (see "How to size it").
How the 5 work
1. Capacitor protectors
How it works: In normal operation, current passes through an SCR bypass straight to the load while charging supercapacitors. When a sag is detected, it switches within about 1 ms to sine-wave output from the capacitors via an inverter. Advantages: almost no standby power use, capacitors that don't need routine replacement, small size, and the option to protect only control circuits. Limitations: compensation lasts seconds, so it can't cover long outages.
2. Voltage conditioners
How it works: It stores no energy; instead, through a series injection transformer, it draws energy from the remaining grid voltage to restore the missing portion. For example, a model with 40% compensation can restore 100% when voltage drops to 60%, but only about 90% when it drops to 50%. Advantages: no energy storage components, and compensation time isn't limited by storage capacity, so it suits frequent, long, shallow sags. Limitations: the deeper the sag, the less it can restore; it can't protect when voltage approaches zero or during an outage.
This kind of series compensation equipment is often grouped with dynamic voltage restorers (DVRs); some DVRs also include energy storage, so read the specs carefully to see which type you're buying.
3. Online UPS (double conversion)
How it works: Power flows continuously through a rectifier and inverter, so the load is always fed by the inverter. During a sag or outage, the battery takes over with zero transfer time. Advantages: covers both sags and outages, and isolates all kinds of electrical noise. Limitations: double conversion running 24 hours a day loses about 3–6% of power continuously; lead-acid batteries need replacing every 3–5 years, and lithium batteries need fire safety management.
4. Offline and line-interactive UPS
How it works: The load runs directly on utility power normally and switches to battery only when something goes wrong. Advantages: low standby losses and a lower price. Limitations: switching takes about 2–10 ms, which may be too slow for sensitive control power supplies; battery maintenance applies here too.
5. Flywheel storage
How it works: A rotor spinning at high speed stores kinetic energy and converts it to electrical output when something goes wrong. It's commonly used to ride through seconds to tens of seconds while a generator starts. Advantages: no chemical batteries, and high power output. Limitations: as mechanical equipment, its bearings and vacuum system need regular maintenance; it's generally used in large systems.
Comparison table
| Item | Capacitor protector | Voltage conditioner | Online UPS | Offline / line-interactive UPS | Flywheel storage |
|---|---|---|---|---|---|
| Energy storage | Supercapacitors | None | Battery | Battery | High-speed rotor |
| Response time | Within about 1 ms | ms, continuous | 0 ms | About 2–10 ms | Milliseconds |
| Ride-through time | Seconds (~1–3 s) | Limited by sag depth | Minutes+ | Minutes+ | Seconds to tens |
| Deep sags (below 50% remaining) | Covered | Partial | Covered | Covered | Covered |
| Outage | Within hold time | Not protected | Yes | Yes | Until generator |
| Standby power use | Very low (bypass) | Low | ~3–6% | Low | Constant draw |
| Main maintenance | ~10-yr capacitors | Power electronics | Batteries every 3–5 yrs | Batteries every 3–5 yrs | Bearings, vacuum |
| Typical use | Controls, tools, lines | Whole plants, shallow sags | Servers, must-run loads | Small and office gear | Large systems, generators |
Figures in the table are typical ranges for each technology, compiled from public technical data and product specs. Actual specs vary by product and capacity; when buying, rely on datasheets and an on-site assessment.
Which to choose
- Brief sags; only controls reset→Capacitor protector on controls
- Frequent shallow sags; whole plant→Voltage conditioner
- Must survive outages→Online UPS
- Large, bridging to generator→Flywheel or large UPS
- Small office gear, tight budget→Offline UPS
Many factories end up combining them: UPS for critical data systems, capacitor-based sag protectors for production-line control circuits, and a voltage conditioner for the whole plant depending on the sag pattern. For capacitor-based sag protector specs and sizing, see "Voltage sag protector (VSP)"; for a detailed comparison with UPS, see "VSP vs. UPS" on the product page.
FAQ
What types of voltage sag protection are there?
There are five common types: capacitor-based sag protectors, active voltage conditioners, online UPS, offline or line-interactive UPS, and flywheel storage. They differ in whether they store energy, how long they ride through, how deep a sag they handle, and their standby power use and maintenance costs.
How is an active voltage conditioner different from a sag protector?
An active voltage conditioner stores no energy; it restores the missing voltage from what remains on the grid, so the deeper the sag, the less it can restore, and it can't protect during an outage. A capacitor-based sag protector stores energy in supercapacitors and can compensate even when voltage drops close to zero, but compensation lasts only seconds.
Is flywheel storage suitable for voltage sag protection?
Flywheel storage rides through seconds to tens of seconds and is commonly used in large systems to bridge to generator start-up. If you only need to protect control circuits from 0.1 s sags, it's usually oversized; a capacitor-based sag protector or UPS fits better.
Can an offline UPS protect against voltage sags?
Yes, but switching takes about 2–10 ms, which may be too slow for some sensitive control power supplies, so they may still reset during the sag. To guarantee no interruption, choose an online UPS with zero transfer time or a capacitor-based sag protector that switches within about 1 ms.