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Power line disturbances - such as voltage spikes, sags, and outages - cost electric utility customers billions of dollars each year. A primary reason is that minor power disruptions, which once would have been noticed only as a momentary flickering of the lights, may now interrupt whole automated factory assembly lines because of sensitive electronic controllers or make all the computer screens in an office go blank at once. Even one cycle of outage or two cycles of a 25% voltage dip can cause unprotected microprocessors to malfunction.
About 90% of outages affecting customers originate on the utility distribution system and are due to causes such as lightning and line faults. As a result, the ability to provide premium-quality power is becoming a distinct competitive advantage as utilities face increasing competition through deregulation of retail markets. Up to now, many customers with sensitive loads have installed their own uninterruptible power supply (UPS) to provide ride-through capability, but these battery-dependent devices are often expensive and energy inefficient, and they require maintenance that may exceed the owner's available in-house resources. Installing a UPS may also be difficult on some customer premises because of severe space constraints or changing facility requirements. In addition, a recent EPRI survey revealed that the majority of industrial and commercial customers would prefer a utility-provided solution to power quality problems.
Most of today's distribution system controller equipment - such as mechanical reclosers, which require six cycles to react to a line fault - is not fast enough to provide the virtually instantaneous switching needed to keep sensitive equipment operating properly. Fortunately, a new generation of power electronic controllers coming into use on distribution systems will enable utilities to provide premium-quality electricity to customers with sensitive loads. Through the use of these Custom Power devices, utilities will be able to provide their customers with a cost-effective alternative to installing their own power-conditioning equipment.
"First, EPRI pioneered the use of power electronic controllers to create the Flexible AC Transmission System, called FACTS, which enables utilities to maximize the use of their present high-voltage networks," says Karl Stahlkopf, EPRI's vice president for power delivery. "Now the cost of power electronics has declined enough for us to begin using such controllers to improve service for individual distribution system customers. With utility demonstrations just getting under way, I would say Custom Power is at about the same stage of development that FACTS reached some five years ago - but I expect even faster market penetration because of pressing demand and the rapid development of the technology involved."
First, the switches
The first Custom Power devices to enter utility service are two basic kinds of distribution system switches. The solid-state breaker (SSB) offers fast isolation of line faults and can be used in conjunction with other Custom Power devices to provide a variety of innovative applications. The solid-state transfer switch (SSTS) provides uninterrupted power to a customer by quickly transferring a load from a faulted feeder to an independent unfaulted feeder. In addition to being much faster than their mechanical counterparts, both of these switches can be used repeatedly with no degradation of performance.
The primary function of the SSB is to interrupt fault currents quickly enough to prevent them from affecting service on adjacent feeders. Since an ordinary thyristor (sometimes called an SCR, for silicon-controlled rectifier) does not have the ability to turn off until the line current goes to zero, a solid-state breaker requires a gate-turnoff thyristor (GTO) in order to stop current flow at subcycle speed. With EPRI sponsorship, Westinghouse Electric Corporation has developed a 13.8-kV SSB that can react to faults in as little as one-quarter of a cycle, using a combination of SCR and GTO technology. (For background information on thyristor development, see the side-bar on page 12.)
During ordinary operation of this SSB, current flows through the GTO and out onto a …