Putting "Utility Grade" Power Quality in Perspective

Issue 4 and Volume 106.

Utility grade power reliability has historically been regarded as three nines – 99.9 percent – an antiquated benchmark and far from acceptable for many customers in today’s world where data, communication, and manufacturing processes require reliability expressed in five, six, seven or more 9s.

In discussing the Starshine 2200 kVA rotary UPS at the 6th Annual Distributed Generation and On-Site Power Conference in Atlanta, Gene Weaver of SatCon Technology, maker of the flywheel UPS that operates without batteries and incorporates an integrated diesel engine, gave a bit of perspective on traditional reliability versus today’s reliability needs. “Below six 9s, standby engine generator sets will suffice,” says Weaver. “Above six 9s, it almost has to be state-of-the-art uninterruptible power systems.

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“We hear a lot about the 9s of reliability but the simple math shows that three 9s translates into an enormous period of annual downtime,” he says. (Table 1)

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Weaver notes that an interesting aspect of looking at reliability is that 99.9 percent reliability, usually accepted as utility grade, equates to one outage lasting 8.7 hours, 522 one-minute outages, or 31,320 one-second outages. Outages of even much shorter duration can result in significant financial losses for some industries. (Table 2)

“What this does say, however,” says Weaver, “is that the development of mitigating equipment must include some way to create a bridge between the time that the outage occurs and power is restored, or be able to initiate action to provide continuing outage protection for seconds, minutes, or even days.”