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Part 1: Making Informed Decisions About Uninterruptible Power
By Mark Peckover, P.E.
December 2011 -
Protecting a facility's critical systems and operations in the event of a power failure is paramount. An uninterruptible power supply (UPS) provides instantaneous or near-instantaneous protection from input power interruptions during the time it takes to bring a standby generator online.
The process of selecting the best UPS for a particular institutional or commercial facility can seem daunting with so many options. Understanding selection criteria and conducting a careful evaluation — a cost and liability analysis — of a facility's needs will help maintenance and engineering managers make cost-effective decisions that mesh with their organizations' priorities.
Managers must answer important questions in seeking to protect critical systems. Will I lose the results of an experiment that has been in development for years? Will the facility lose the ability to care for patients? Will the interruption inconvenience occupants for a brief period of time? Answers to these questions will assist in answering questions about the type of UPS, its configuration, battery runtime, and the specific technology a manager ultimately selects.
When selecting a UPS, managers must understand the other power systems supporting the facility. If a facility is equipped with two utility power sources — each fed from an independent utility substation — the risk for utility power loss is much lower than a service facility where the local utility-distribution system uses an overhead distribution system.
Two indices that most utilities use to measure the reliability of their systems are system average interruption duration index (SAIDI) and system average interruption frequency index (SAIFI), and they normalize these numbers regionally and nationally. These indices give the operator an opportunity to transition from always feeling as though the utility source fails regularly to understanding in quantifiable terms the system's reliability. Many regulators require an electrical utility to maintain this information and justify any drops in the reliability of their system.
Managers also must pay close attention to the configuration of the standby power system.
Does the facility have a generator with a fuel source independent of the electric utility? If so, is the plant a paralleled-generator plant installed within a seismically reinforced building with days of onsite fuel storage? Or is the generator a single-engine system installed outside the facility in a sheet-metal enclosure that relies on natural gas?
Obviously, the second system is more susceptible than the first to influences outside the facility that might affect generator operation.
When making a decision about UPS configuration, managers also need to consider such issues as vandalism, exposure to vehicle traffic, weather, natural-gas reliability, and generator location related to local flooding risks. While these issues might seem second-tier, they all contribute to the risk of failure for the system.
UPS Technology: Power When You Need It
Part 2: UPS Technology Matters
Part 3: Power Source Alternatives for UPS Systems