Intro4u2u

When an automated, oil filled, load-transfer switch, installed more than 20 years ago, failed and interrupted service to a platform-mounted transformer bank, we suspected a lightning strike as the culprit. The transformer fed a storm-drain pump, whose sole purpose was to drain flood waters off of an interstate highway system in a heavily traveled urban underpass. The circuit consisted of a primary selective system to provide two separate electrical circuits to ensure continued pumping in the case of a system failure. The switch, an externally mounted stored-energy mechanism with porcelain air bushings, was platform-mounted about 15 ft (4.6 m) above ground. After the failure, temporary repairs were made and one circuit was back on line in about 2 hr. Replacing the Switch Since the state in which we are located requires an automatic changeover switch, we advertised for a replacement. We expected that we would install the same kind of switch in the same circuit configuration. We received two bids, one from the company that provided the original oil switch and the other from Joslyn Power Products Corp. of Alsip, Illinois, U.S. for a SF6 switch. The SF6 switch was less expensive than the oil-filled switch. It also was smaller, easier to install and appeared to require less maintenance. Because we could eliminate the oil, which could be an environmental hazard, and because explosions were not a concern, we specified the gas-insulated switch. An Innovative Solution The installation resulted in some unexpected bonuses. The original transfer switch had three sets of bushings: one for the preferred source, one for the alternate source and one to serve the load. With this arrangement it was possible to switch between only two circuits. In addition, the oil switch had four potential transformers, three on the preferred feed and one on the alternate feed. The transformer primaries were unfused and connected line to ground. Donut-type current transformers ringed the load bushing and fed overcurrent relays, which latched to prevent operation in case of an overload. The PTs and CTs were inside the same tank as the switch and the operating mechanism was external and linked to the switch with a pipe linkage. A bonus in the new installation is that the SF6 scheme uses two separate switches installed on adjacent poles, which not only switches between two sources but will automatically sectionalize a down-stream fault. Voltage and current of all three phases of both the preferred and alternate feeders are monitored continuously without PTs or CTs using sensors developed for interfacing with remote terminal units of automated distribution systems. A self-contained local logic opens and closes the SF6 switches in the correct sequence to ensure reliable service to the pumping station. Pole mounting the SF6 switch scheme required creativity. The purchased switches were designed to be mounted on a crossarm. To simplify the installation, Aluma Form, Inc. of Memphis, Tennessee, U.S., constructed custom brackets that permitted installation of the switches directly to the pole. The switches were mounted to the brackets on the ground (Fig. 1) and then the assembly was hoisted using a block and tackle (Fig. 2). The installation was quick and simple. Making the connections was the most mechanical part of the job, since the switch and the motor operator mechanism were assembled at the factory (Fig. 3). The control cables were attached to the pole with standard staples (Fig. 4). The Future The experience has encouraged us to consider the SF6 as a standard since it is environmentally safe and it enhances design flexibility. TDW Clarence Wooddell, a graduate of Memphis State University, is supervisor of Electrical Distribution Engineering and has been with Memphis Light, Gas Water for 43 years. He is a senior member of IEEE and is a member of the Insulated Conductor Committee. Reggie Bowlin earned the BSEE degree from University of Tennessee - Knoxville and is design engineer in the Electrical Distribution Engineering Department. He is registered as a professional engineer in Tennessee