A UPS will solve certain power quality problems, if you apply it properly.

Because it supplies good power, it's easy to assume a UPS will cure all power quality problems — but it won't.

It's easy to assume a UPS will cure all power quality problems, because it supplies good power. Unfortunately, this assumption isn't accurate. Expecting a UPS to cure all PQ problems is a strategy doomed to failure.

Just like any other strategy — grounding, panel layout, and wire management — a UPS has its limitations. Applying a UPS or any other fix to problems it can't solve is a sure way to delay a solution. So, you must determine what PQ problems you are trying to solve, and then apply the correct cure for the disease.

Fortunately, a UPS can solve many PQ problems. A UPS provides a finite source of electrical power to support selected critical loads during a loss of normal power. This backup time ranges from seconds to hours. Nearly every UPS offers power conditioning and overvoltage protection. As you move up the quality scale, you'll find features such as those listed in the sidebar below. Beyond protection from sags, surges, and loss of power, a UPS can provide power quality protection and enhancement in several ways (photo). Let's look at some of the problems a UPS can handle.

Periodic voltage swings. These aren't the same as sags and surges. In most facilities with a heavy electrical load, the supply voltage will decrease (sag) because of the increased load during normal operation. So, if your facility has large loads (e.g., process heaters, large presses), a UPS figures predominantly into your coping strategy.

What if your loads are small? We all face the grim specter of today's stressed utility distribution networks. The supply “cushion” of our national grid is at a historic low. Even if we could add more generating capacity, the distribution capacity just isn't there.

One strategy is to raise transformer taps in the summer and drop them back down in the winter. But this method has drawbacks. For example, changing taps usually requires shutting down the building. Getting permission to do this is a major undertaking, to say the least.

What happens when the load drops down at night or during other times? Voltage may rise to levels that damage equipment. You could use a power conditioner to accommodate this voltage rise, but that still isn't going to help you with deeper momentary sags or interruptions. A UPS is a constant voltage device that will reduce or eliminate the need for a tap change, while providing protection for deep sags and outages.

Frequency changes. Generators can cause frequency synchronization problems with the load. This becomes a real problem when you've nominally sized the generator for the connected load and then it has to deal with large block load steps — whether related to a transfer switch or not. Depending on design and configuration, a UPS can provide a constant 60 Hz output with a changing input frequency.

If the UPS bypass source is from a generator with a fluctuating frequency, the UPS will have difficulty synchronizing its output to the bypass source. Therefore, the UPS may not allow you to make a manually initiated transfer to bypass. And, a fault transfer to bypass may be to a source that is “out of sync” with the UPS output. This risks dropping the critical load and damaging equipment.

Localized harmonics. If you have excessive harmonics in a branch circuit or at a specific point of use, a UPS may be the answer. While a UPS will typically add harmonics to the wider system, it will protect specific loads from excessive harmonics. A solid strategy for accomplishing this includes an isolation transformer as part of the UPS design.

Most people vastly overrate the negative effects of harmonics and overestimate the need to mitigate them. If you aren't experiencing problems because of the harmonics you have, their presence may not be reason enough to install a UPS at that location or for that particular circuitry. Other reasons, the most important of which is ensuring a reliable source of power, make more sense.

Power quality problems rarely have a single cure. In fact, the various cures you can apply to a given problem interact with each other. If you are applying a UPS, ensure your installation matches specific cures to specific problems.

Hartfiel is a senior engineer with Power Engineering, Lenexa, Kan.; Lamendola is a technical editor with EC&M.

Common UPS Features

The following features affect both the price and performance of a UPS. They are available at various levels of sophistication and usability.

  • Adjustable output
  • Automatic shutdown initiation
  • Battery monitoring
  • Ground system monitoring
  • Power monitoring
  • Constant voltage output
  • Local controls
  • Data logging
  • Load management
  • Network interface
  • Remote control
  • Security measures
Problems a UPS Won't Solve

High ground current. This is often due to bad grounding practices, and the cure begins with making the grounding system comply with NEC Art. 250. No power conditioner will solve a grounding problem.

Inductive noise. This is a direct result of improper wire routing. Mixing high-voltage power conductors with low-voltage control wiring usually results in erratic control and corrupted data. Always route control and signal wiring in separate dedicated wireways.

System harmonics. A UPS can protect only the load on its output — not the system that supplies power to it. Typically, a UPS will exacerbate system harmonics because of its switching power supply. Depending upon the type of UPS (off-line or online) and its input/output configurations (e.g., harmonic filters, isolation transformers), harmonics may pass right through the UPS with no mitigation.

Phase imbalance. This is usually due to improper allocation of single-phase loads. Solve this problem by balancing those loads across the phases or replacing single-phase equipment with 3-phase when practical.

Low power factor. While most UPSs have negligible drag on your power factor, they won't raise it either.

Susceptibility to lightning. A UPS will not protect you from a lightning strike. It will protect you from a common result of a lightning strike — transient spikes that make it past the outer zones of protection.

Hartfiel is a senior engineer with Power Engineering, Lenexa, Kan.; Lamendola is a technical editor with EC&M.