1. Find and fix the problem. Yes, it's an obvious suggestion. But it's the best place to start. Figure out exactly what is causing the problem with a sag generator equipped with a good data acquisition system. Once you know what the problem is (if you even have a sag problem!), it will be much easier to fix. Tradeoff: Takes time and money; if you're a good at guessing, you might skip this step.
2. Add a power quality relay. These small, simple devices detect voltage sags, and give you a simple relay contact when a sag occurs. The best ones (such as PSL's PQ1 also detect other disturbances, such as high frequency impulses and voltage swells. Your system can then adjust its behavior whenever a voltage sag occurs - for example, by resetting some of its components. Tradeoff: Can require some simple reprogramming of your system.

The best solutions are the simplest. Add an inexpensive power quality relay to your system, then adjust your software so it corrects any problems whenever a power disturbance occurs.
3. Switch power supply settings. Many power supplies can be set to accomodate different voltage ranges, and these ranges often overlap. Choose a range where your nominal voltage is near the top of the range, and you'll have more room for voltage sags. For example, if your power supply has Range #1, 95V-250V (accomodating Japan and Europe), and Range #2, 110V-270V (accomodating North America and Australia), and you have a 240V nominal voltage, you will have greater sag immunity on Range #1. Tradeoff: Less margin at the top end against voltage swells.
4. Connect your single-phase power supply phase-to-phase. If you can stay within your power supply's acceptable voltage range, and if you have three-phase power available, you can get a quick 70% boost in available voltage by connecting phase-to-phase. For example, if your power supply is rated as 90V-250V, and you are using it on a 120V circuit, you can only tolerate a sag to 75%. But if you connect it phase-to-phase, the nominal voltage will be 208V and you will be able to tolerate a sag to 45%. Tradeoffs: Less margin for voltage swells; sometimes inconvenient; sensitive to sags on two phases, instead of just one.
5. Reduce the load on your power supply. Lightly loaded power supplies always tolerate voltage sags better than heavily loaded power supplies. If you can determine that a particular power supply is causing your equipment to mis-operate during a voltage sag, consider moving some of its loads to another power supply. Tradeoffs: May be inconvenient to install; carefully consider effects of a shut-down on one of the power supplies.
6. Increase the rating of your power supply. If you can't move the loads, use a bigger supply for the same load -- relative to its rating, it will be more lightly loaded. Tradeoffs: cost and size tend to go up; there may not be room for a larger power supply.
7. Use a three-phase power supply instead of a single-phase supply. A properly-designed (and lightly loaded) three-phase power supply will effectively tolerate voltage sags on one or two phases that would shut down a single-phase power supply. Trade-off: Cost and size are larger; requires three-phase circuit breakers, shut-down circuits.
8. Run your power supply from a DC bus. Sometimes you can substitute a DC-operated power supply for an AC-sourced supply. If it does nothing else, this will narrow down your problems to supporting a DC bus, which can often be done with simple capacitors or batteries. (This is the approach that high-reliability telecommunications systems take, using a 48 Vdc supply as their power distribution system.) Trade-offs: protective devices (fuses, circuit breakers, etc.) need to be changed or rated for DC; may not be convenient.

The telecommunications industry operates equipment from a 48Vdc bus, making systems virtually immune to voltage sags and power interruptions -- the ac power system is only used to charge the batteries.
9. Change the trip settings. If you can identify an unbalance relay, an undervoltage relay, or an internal reset or protection circuit that is inadvertently tripping during a voltage sag, change its settings. Consider changing the threshold, and consider changing the trip delay; either or both might make sense. Sometimes this can be as simple as twisting a knob; sometimes it may take a component change or firmware adjustment. You can only use this solution when the trip settings were set too conservatively to begin with; trips are useful and important, so you don't want to eliminate them completely. Trade-offs: someone chose those set-points for a reason, so you don't want to change them arbitrarily; changing components and/or firmware can create service and repair problems later on.
10. Slow the relay down. If the equipment is misoperating because a relay in the EMO circuit is operating too quickly, consider slowing it down. You might use a relay with more mechanical mass (such as a contactor), or you might use a relay hold-in accessory. Trade-offs: possibly more complexity on the EMO circuit; you don't want to slow the EMO circuit down so much that it becomes unsafe.

You can add a simple "hold-in" device across the coil of a relay or contactor to increase its sag immunity. Suppliers include Power Quality Solutions, among others.
11. Get rid of the voltage sag itself. As a last resort, consider installing a quick-operating voltage regulator on your AC supply. There are a variety of technologies: ferroresonant transformers, solid-state voltage compensation, etc. But make sure that you aren't making the problem worse; if the original cause of the voltage sag is downstream from your voltage sag regulator, the voltage sags will actually get deeper and longer when you install the fix. Tradeoffs: size and cost.

For larger three-phase applications, the Dynamic Sag Corrector from Soft Switching Technologies can eliminate the sag itself.

More Tutorials

Sag sensitivity - Five ways equipment fails during voltage sags
CBEMA curve - voltage sag depth and duration at world-wide semiconductor plants
Sag sources - The sources of voltage sags
Semiconductor sag standards - Industry standards F47, F42 Alex McEachern, 8/2001
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