The Interaction of Power Quality and Your UPS

The common misconception that a UPS fixes all power quality problems can lead to an overload condition.

Where does a UPS fit into your overall power quality plan? The street that runs between power quality and a UPS runs both ways. A properly implemented UPS is the only way to provide a smooth ride over several types of power quality potholes. With the right information, you will avoid some of the fender-benders that can result from using a UPS incorrectly to resolve power quality problems.

Let's clarify what a UPS does. A UPS conditions power and provides stored energy. It doesn't fix infrastructure problems or resolve capacity issues. Its purpose is to provide continuous, transient-free, sinusoidal power.

Many facilities make no effort to manage their critical power — that is, the power normally protected by a UPS. In most facilities, even data centers, you can find evidence of poor critical power management. For example, you are likely to find unbalanced loads, critical power outlets in use by the cleaning crews for their floor polishers, and the occasional space heater plugged into a UPS-protected receptacle. Each of these situations can cause expensive problems for the UPS, including overloads and downtime for repairs.

Facilities managers rarely track these loads. When nobody knows why the UPS suddenly transferred off-line with an overload condition, such loads are often the culprits.

What leads to this kind of situation is a misconception that a UPS will fix all power quality problems — even if you ignore some key aspects of sizing, selecting, installing, and servicing it. In truth, the UPS is part of your critical power management plan. Let's look at some specific issues you must address if your UPS is going to be an effective addition to your power system. We'll begin with harmonics.

UPS and harmonics

Installing a UPS will not remedy infrastructure problems such as neutrals overheating due to load-generated harmonics. In fact, the UPS may even cause power problems upstream from where it connects into your distribution system. For example, one facility installed several megawatts of UPS. After placing the UPS into service, their engineers were unable to adjust any of the remotely set clocks in the facility to the master clock. What was the problem? The UPS rectifier section caused enough harmonic distortion to the incoming power lines to prevent the remote clocks from reliably receiving the master clock signal, which transmits over the internal building wiring.

Read enough maintenance logs, and you'll find many accounts of such things as microprocessor-controlled vending machines refusing to operate and electronically controlled production equipment randomly locking up. Input harmonic attenuation filters, now fitted to (or offered as an option on) many UPSs, can help solve most of these problems.

Of course, you still must pay attention to the basic infrastructure requirements of proper wiring methods (NEC Art. 310) and grounding (NEC Art. 250 and IEEE Standard 142). Surprisingly, many engineers design grounding systems that don't even make good use of Ohm's Law and Kirchhoff's Law. To paraphrase an old cliché: “Show me an unbonded grounding electrode, and I'll show you a facility with power quality problems.” That same facility has safety problems, too.

Because a UPS interacts with its supply and that interaction isn't always benign, take care where you install the UPS. You must also be able to measure the harmonics that exist when the UPS is under load and determine the effects of those harmonics. Many loads can tolerate a high harmonic content, so you could have UPS-produced harmonics present and not suffer any ill effects.

Before installing a new UPS, do a baseline study. Beyond establishing voltage and current profiles, you'll want to measure the harmonics, do a thermographic scan of the transformers and other equipment supplied by the service that would power the UPS, and take a close look at your grounding system.

Also, make note of any high-harmonics generating equipment. For example, you may have old 6-pulse SCR motor drives. If your motor drives are putting out significant harmonics, consider replacing them with today's harmonics-corrected, power factor-corrected insulated-gate bipolar transistor (IGBT) drives. What's the bottom line? If you bring your entire distribution system up to today's standards, the harmonics from a UPS should not have a noticeable impact on other equipment.

Taking care of the entire distribution system can also prevent the UPS from having problems of its own. For example, suppose you have a 480V service. Use 480V distribution where possible, and step voltage down to 208/120V near the point of use. Use either a 480/208V power distribution unit (PDU) or a 480/208V transformer with separate distribution. This keeps the distribution neutral closer to ground potential near the distributed loads. It can also keep load-generated harmonics (some of which the distribution transformer cancels or attenuates) from reaching the UPS.

Additionally, do not use a PDU with power-factor correction capacitors on its input. Most UPS systems do not work well with leading power-factor (capacitive) loads.

UPS and generator interaction

Many diesel generators fail to operate well with a traditional double-conversion UPS. The UPS manufacturers often recommend that you size the supporting engine generator two to three times the kW rating of the UPS. Some recent generations of line-interactive UPS advertise as little as a 1-to-1 ratio of UPS to engine generator.

There is no easy answer to the UPS vs. generator problem. The point here is you should not assume any particular UPS will function properly with any and all generators. You'll need to work with the vendors to arrive at a proper engineering design and product selection. If you have a UPS and generator interaction problem now, you may simply have incompatible equipment. Before addressing that issue, though, do a formal study of your infrastructure. Even properly selected equipment will appear faulty if you have grounding and wiring problems.

Batteries

Oversizing your battery can lead to excessive cost or maintenance problems, which can drain you of the resources you need to address power quality issues. The “If a little is good, more is better” attitude does not apply to battery sizing. Many data center managers ask for several hours of battery backup time when purchasing a UPS. This may initially sound like a good idea, but you must consider other factors. If you have an engine generator, any battery time beyond that required for the engine generator to start, stabilize, and transfer online is wasted money — and not just as a purchase cost. Excess battery means excess maintenance costs, lost floor space, and the additional cost of proper environmental control. This usually results in reduced maintenance. That means you won't have the battery power when you need it most.

If the lack of an engine generator makes you feel the need to purchase additional battery time, ask yourself, “How will I maintain the operational environment?” With no secondary power source such as an engine generator, you have no way to maintain such things as air conditioning for the critical equipment and human operators. So, because you can't run the process anyhow, oversizing serves no purpose. But the resource demand this places on your facility will invariably create resource conflicts. Management rarely resolves such conflicts in a way that promotes battery reliability or uptime in general.

Design with experience

If you want to add a UPS to an existing facility, have an engineering/consulting firm with proven critical-power/UPS experience do the design — or at least review the design. Inexperienced designers may make critical errors that undermine the functionality of the UPS.

For example, general engineering firms often neglect to include critical UPS features such as an external maintenance bypass. This feature can route power around the UPS during times of UPS maintenance or repair. You don't want to power down critical loads to repair, test, or maintain the UPS. A maintenance bypass that passes through the UPS cabinet in any way is inadequate for this purpose — for reasons that become clear when you think about electrically or mechanically isolating that cabinet. You also must remember what works for one UPS application may not work for another.

Putting it all together

Most people think of a UPS as a complete solution to power quality problems, but it isn't. The UPS is not the answer to power quality problems. It is part of a total approach to resolving power quality and power-reliability issues for specific equipment.

Stamper is senior vice president, Technical Services, DC Group, Minneapolis.

Seven Tips for UPS Success

1. Size your UPS to your load and emergency generator.

2. Size your battery to the UPS, not to a target that will cause you to perform reduced maintenance.

3. Use the UPS to provide clean and reliable power to specific loads — not to make up for facility-infrastructure problems.

4. The UPS is part of an overall approach to resolving power quality problems, not a cure-all — so plan accordingly.

5. Use an experienced UPS consultant to prevent or correct common design and installation errors.

6. Keep UPS outlets inaccessible to unauthorized use.

7. A baseline study will give you a snapshot of your system's condition. This can ease troubleshooting problems.