Static Transfer Switch: Ideal Solution Across a Spectrum of Applications
In automotive components assembly line in Detroit, a customer service operation in Florida, an R&D lab in Baltimore, telecommunications and E-commerce firms, manufacturers, office buildings, industrial parks – all stand to be crippled at any moment by a common, everyday event. All are threatened by a sag, a swell or a momentary aberration in the electric power supply.
Over the years, numerous power quality solutions have been developed to avert – or more to the point – to avoid the inevitable power incident. However, no solutions are more practical, more economical or more reliable in the broadest spectrum of applications than the new generation medium voltage static transfer switch (STS). Capable of transferring megawatts in microseconds, the STS is an essential power quality solution that protects critical operations, automated lines, telecommunications, computer networks and sensitive equipment from aberrations in the power supply.
Building on Proven Technology
The fundamental technology of an STS has been used for nearly thirty years in low voltage configurations, typically under 480V. The mid-1990s yielded new developments in silicon technology and, consequently, saw the introduction of larger static switches with the capability of handling larger loads while operating at voltage levels up to 38kV.
The function of the STS is to maintain electrical power to a critical load by switching between two relatively independent voltage sources. Whenever it senses a disturbance, the STS automatically transfers electric power supply from the primary to an alternate feeder. The programmable switch detects momentary interruptions, outages, sags and swells, and automatically transfers to the alternate source in as little as four ms. This operation of the switch is completely transparent to the user (Figure 1.)
Switch Components
While the STS may appear physically different in different applications, it is typically composed of three main components: (1) two ac thyristor switches, (2) mechanical bypass switches/breakers and (3) mechanical isolating switches/breakers. The thyristors enable rapid switching between sources by first stopping current flow from the preferred source, and then allowing current to flow from the alternate source. Thus, all transfers are open transitions and the two sources are never connected to each other. The total detection and transfer time can vary, from as short as 2.5ms for a loss of voltage to almost 12ms for voltage swells. This is because the sine wave must reach zero for a thyristor to stop conducting. In all cases, however, switching time is sufficiently fast enough to preserve the operation of even the most sensitive customer equipment loads during distribution system disturbances. Since the requirements for users vary, the STS is designed to be programmed by the user so that the transfer between power sources takes place only under the exact specifications and conditions determined by the user (Figure 2.)
The mechanical bypass and isolating switches/breakers enable the switch to be bypassed and isolated for routine maintenance, testing, emergency overload or other specialized system requirements. This is all done without interrupting service to the customer. The design of the STS also incorporates N+1 redundancy: a modular building block component design for easier maintenance and internal diagnostics.
Switch energy loss with an STS is considerably less than with many other power quality solutions. With a 15kV switch, the thyristors provide power dissipation efficiency between 99.7 to 99.9%. The switch offers on-line efficiency that is nearly thirty times better than that offered by many UPS systems, which can be as low as 70-80% efficiency end-to-end. A practical alternative to UPS and backup generator systems, STSs offer improved system reliability, reduced downtime and increased productivity.
STS Configurations and Applications
The new generation STS is designed for large loads, with ratings as high as 5,000A for 480V systems and 1,200A for 27kV systems. They also have ample fault current withstand ratings up to 100kA for 480V, and 18kA for 15kV and for 27kV. The STS is available in either a preferred/alternate or split bus configuration.
Electromechanical switchgear, which commonly takes anywhere from two to ten seconds to transfer between power sources, no longer provides the protection required for most users. Because businesses and industries are dependent upon computers, telecommunications devices, routers and servers, programmable logic controllers, and because adjustable speed drives continue to proliferate, the reliable and virtually instantaneous protection offered by the STS becomes imperative.
Automotive Plant Application A Ford Motor Company automotive component plant in Detroit was experiencing numerous electrical incidents – 17 in 1996 alone – that were shutting down lines and affecting Ford’s primary downstream automobile assembly lines. Because of this recurring loss of production time, the utility, Detroit Edison, negotiated a special manufacturer’s contract with Ford. The contract stipulated that any interruptions in power supply to the component plant would require monetary compensation from Detroit Edison to offset Ford’s estimated losses.
The utility realized that a comprehensive power quality solution was required, and a goal of reducing plant disturbances to no more than two per year was established. Various power quality solutions were investigated and the STS was selected as the most reliable and practical answer based on the criteria of cost, delivery and the probability of success. Working closely with the utility, the switch manufacturer custom tailored the STS to meet the application’s specific requirements. In November of 1996, an STS was tested and installed at the Detroit Edison Norway substation. In the first year of service ten disturbances were averted, saving the utility $360,000 in compensation penalties as stipulated in the contract. Over a two and a half year period, the Ford plant experienced only one disturbance, although the feeders serving the facility suffered over 30 interruptions or sags.
Customer Service Facility Application As part of planning for a new 250,000 square foot customer service facility in Florida, a financial services/insurance group wanted to prevent frequent disturbances caused by the Sunshine State’s almost daily thunderstorms. They could not afford to have vital communications and computer networks shut down during peak business periods. The developer of the commercial office park had the utility install multiple underground feeders at the site, so that a dual source of power was readily available. Conventional mechanical transfer switches could not provide the level of power quality needed. To prevent corruption of data and interruption of transmissions, switching had to occur in milliseconds. The utility determined that a 4,000A, 480V static transfer switch would be adequate for the facility (Figure 3).
Working with the STS supplier, the building architect incorporated the STS into the design of the main electrical equipment room. The bypass and isolating breakers in the STS system are part of the building’s overcurrent scheme, which eliminated the need for duplicate breakers. The utility installed two 15kV feeds to the building and two padmount transformers. Either of the sources can be established as preferred, giving the utility flexibility in the loading of the feeders in the park.
Since its installation in 1998, the STS has averted several line disturbances and interruptions, enabling continuous operation of the customer’s computer and communication network.
Electronics Facility Application The first 38kV STS, the largest currently in commercial service, was installed at the Visteon facility in Lansdale, Pennsylvania, on December 14, 1998. Visteon, a manufacturer of automotive electronics, was experiencing as many as 17 power quality disturbances per year. Because of the sensitivity of their automated control systems, even the slightest aberration in the power supply would shut down their operation, threatening delivery deadlines and productivity.
Working with Visteon, the local utility determined that an STS offered the most viable solution. The utility system serving the plant is a 34.5kV. Initially there was some concern about a 38kV switch, since there were no other switches in service at this voltage. However, based on previous successful STS installations at other distribution voltages, it was decided to move forward with the project. The system designed for this application includes a 38kV, 600A, 6kA fault duty STS in an outdoor enclosure, integrated with a Powercon metal enclosed bypass switchgear package. Since the switch was installed, the company has not experienced any further power supply interruptions.
Premium Power Park Application Perhaps the most innovative application of an STS can be found at the Premium Power Commercial Park in Ohio. The local utility and developer of the park felt that offering a high quality power supply to all potential residents would serve as a strong incentive to move businesses into the park. The installation of the STS was as ingenious as the premium power incentive. With the assistance of the utility, a 15kV STS was installed in its own building along the entrance boulevard, which blends in perfectly with the general landscaping and architectural theme of the park. With a custom brick exterior, tile roof and windows with shades, the building appears to be another small office building in the park. Few would guess the attractive structure houses a large piece of electronic equipment (Figure 4).
The first businesses to join the park were an international bank and a financial service center. Both businesses rely heavily on computer and vulnerable communications networks. Thus far, the STS has successfully protected the businesses, preventing all line disturbances from reaching their operations. Both the utility and the developer see that the Premium Power Park is proving successful and are considering plans for similar parks in other areas. As more utilities, businesses and manufacturers become familiar with the power quality benefits of STSs, there is certain to be a proliferation of these switches, and even more innovative applications.
