Don't Pardon the Interruption

The proliferation of electronic data processing, along with an increasing reliance on electromechanical tools and equipment, has made uninterruptible power supply systems (UPS) an essential requirement for almost any type of facility. The question is no longer whether such a system is a justifiable expense, but rather which UPS system will be most reliable and economical, and whether a centralized or decentralized strategy should be employed.

The bottom line is that a loss of power—even for a typical office building, let alone a major data center or a hospital—can be devastating. Even the most sophisticated computer networks are susceptible to harm from short interruptions in electricity. For some enterprises, these small outages can temporarily put them out of business, with considerable financial loss. A permanent loss of data might prove incalculable.

In many cases, utility outages may only last a matter of seconds—the time it takes to flick a computer switch on and off. Such a loss is normally covered by stand-alone UPS units, which are usually found on today's essential computers. These built-in, backup battery systems may be sufficient for short-term outages, but as electrical loads for equipment requiring uninterrupted service increase, a better solution may be warranted.

A computer fed by its own UPS will stay online as long as the battery can sustain the computer load. But greater reliability depends on the answer to an important design question: Should equipment that requires a UPS have its own dedicated unit, which must be tested and changed out on a per-unit basis, or should all essential equipment be fed from a central "house" system?

Opting for the latter presents the dilemma of putting all the eggs in one basket, and of course, keeping a constant watch on the basket. On the other hand, using multiple stand-alone UPS units means every basket must be watched—a difficult, if not impractical, task that often results in a poor maintenance program. Furthermore, only in emergencies will building staff become aware that units are not working. By then, it's too late.

All things considered, for most applications it is more manageable to take care of a single system. But this is a decision that needs to be made carefully by each business owner, based on individual needs and preferences.

A centralized UPS can provide reliable service in a number of configurations, each of which can provide a desired level of power-quality protection as defined by the International Electrotechnical Commission (IEC) standard 62040-3 (see "Three Levels of Power Quality," p. 43). All UPS systems require a source of direct current (see "The Instant DC Source"). The three basic UPS configurations consist of the following:

• Standalone. In this configuration—with either batteries or a rotary flywheel as the direct-current source—emergency power is supplied continuously to the load as long as the DC source has the capacity to supply power (figure 1).
• Battery/flywheel-backed. With this option—typically including a diesel-engine-driven generator—the DC energy source to the UPS provides power to the load until the generator is on-line (figure 2, p. 42). The generator may be able to support the load in as little as 10 seconds. A rotary flywheel source generally is sized to the load and can provide the power required until the generator assumes the load. Based on the types of loads, a battery system backed up by a genset may be smaller, since the time the batteries are expected to provide power is only seconds, as opposed to hours.
• Combined DC energy sources. Combining a rotary flywheel and batteries prolongs battery life, because the batteries are not required to support the large demand of initial current requirements, as the flywheel supplies this energy. Such a configuration will also minimize deep discharges of the battery and frequency of their use, which will add considerably to the life of the batteries (figure 3).

The configurations in figures 2 or 3 may prove more advantageous in terms of providing a reliable source of energy for the electrical loads, because they combine the advantages of two systems. Moreover, UPS units can be configured in series- or parallel-redundant systems for even greater protection, and multiple gensets can also be configured in parallel arrangements. The inclusion of diesel generators in any UPS system—depending on the amount of fuel available—can add hours of off-line operation, particularly by sequencing them in parallel. Of course, any of these overlapping configurations has an associated cost, and the decision must really be based on the desired system reliability—how much capacity is needed now, and how much may be required in the future.

The benefit of decentralized units is that the UPS is local, with no additional wire required. In contrast, a centralized system requires, by definition, a distribution plan.

Typically, feeders from a distribution panel in the UPS room will feed panels on each floor (figure 4), and 120-volt receptacles are fed out of each panel. The receptacle cover plate can be a different color—often yellow—to distinguish that only a UPS load should be fed from that receptacle.

When a yellow UPS-protected receptacle is available, it's notable to see how many people suddenly feel that their computer loads also need to be on UPS. Strict enforcement may be needed so that non-critical equipment is not plugged into these receptacles, which could possibly overload the UPS system.

If the decision has been made to proceed with a centralized system, a number of items need to be determined. First, the cost of a UPS increases with its associated power requirement, along with the length of time the power is required. Therefore, it must be decided what equipment is critical; that is, what needs to be on the UPS system. Once the total electrical load of this critical equipment is determined, the time requirement can be factored in. Some equipment may require one hour, whereas a data center may want continuous operation for as long as it takes to restore utility power. The type of configuration will be determined by the answer to these questions.

Finally, while a major advantage of a centralized UPS is single point of maintenance, the system is only as good as its maintenance. It is essential that a regular schedule be set up to check all components, including the UPS, batteries, rotary flywheel and generators. Trained personnel can bring their maintenance experience to the table, to test the equipment and identify potential problems. But maintenance contracts are available and should be considered. Also, a fresh fuel supply should not be overlooked where generators are involved.

Power is something building users often take for granted, but system engineers are responsible for making the right decisions to keep power running continuously when possible. In any case, when it comes to power, don't pardon an interruption.