Powervar ABCs: Electrical Reliability

Powervar Home — ABCs Home — Power Viruses — Electrical Reliability — Calculating VA and Watts — Glossary

How reliable is the electrical system in North America? For that matter, how about the rest of the globe? How does the power company define reliability? These are all good questions. Here are three different definitions of electrical reliability from three different utility companies.

“The degree to which an electrical system can deliver power to customers at contract specifications, or acceptable regulatory standards. Reliability may be measured by the frequency, duration, and magnitude of adverse effects on the electric supply. It is usually considered for two primary elements: adequacy of supply and security of supply.”

“The ability of a generation system and of a transmission and distribution system to deliver uninterrupted electricity to customers on demand, and to withstand sudden disturbances such as short circuits or loss of major system components. Reliability maybe evaluated by the frequency, duration, and magnitude of any adverse effects on consumer service.”

“RELIABILITY is the assurance of a continuous supply of electricity for customers at the proper voltage and frequency.”

It’s plain to see that power industry itself doesn’t agree on a single definition of reliability and that, at best, many of the definitions themselves are of limited usefulness.

Reliability is expressed in 9s. In other words, if something is reliable 99% of the time, the reliability level is said to be two 9s. Three 9s of reliability would indicate a reliability level of 99.9%. The electrical system is expected to operate 24 hours per day, 7 days per week or 8766 hours per year. The table below illustrates how many hours, minutes, or seconds per year the electrical system will be unavailable for each level of reliability.

Electrical reliability is more than just power outages. Because electricity is used to power some very sensitive, mission critical technology, the quality of the electrical power is just as important as its availability. The Electric Power Research Institute (EPRI) reported in a 2002 industry paper that, from an availability standpoint, the average “well managed” electrical system delivered power with a reliability level of about three or four 9s. Their research went on to say, however, that when “potentially disruptive power quality disturbances” were factored in, the actual reliability level was at least an order of magnitude worse – in other words, one 9 lower in reliability. And that’s true for outside of North America as well. At its 18th annual congress in 2001, The World Energy Council reported similar circumstances in most developed nations throughout the world. Undeveloped nations have electrical reliability levels that are considerably worse.

At four 9s of reliability, outages affect the average electrical system about 8.8 hours per year. Other invisible power disturbances, however, reduce the reliability to three 9s, which adds approximately 79 more hours per year in which the electrical system does not reliably deliver electrical power that meets the needs of high technology equipment. The power quality picture now looks like this:

In other words, for a mission critical computer system, power outages are 10% of the problem and other power disturbances are 90% of the problem. Like an iceberg, it’s what you don’t see that will kill you. This is the single biggest reason why power conditioning, even as part of a UPS solution is so important. Applied in the right manner, POWERVAR power conditioners and/or power conditioned UPS solutions can add much needed 9s of reliability to the electrical power delivered by the power company.