New high-impedance bus differential relay delivers microprocessor-based protection, monitoring, and control

PULLMAN, Wash., Aug. 8, 2002 — In response to the demand for microprocessor-based, single-zone bus protection for utility companies and bulk power users, Schweitzer Engineering Laboratories (SEL) has launched a new high-impedance bus differential relay.

The SEL-587Z, designed to be easily programmable, provides accurate monitoring, control, and protection while its high-impedance differential elements provide fast tripping for in-zone faults, and greater security during heavy through-faults and current transformer (CT) saturation.

Costing less than a system of traditional single-phase, electromechanical bus protection relays, the SEL-587Z relay incorporates three-phase protection, internally mounted stabilizing resistors, and metal oxide varistor (MOV) overvoltage devices into one hardware unit. The SEL-587Z minimizes retrofit and original installation costs by packing more value into a single piece of equipment.

“For the past fifty years, the typical high-impedance bus protection for utilities and bulk power users was electromechanical,” notes Greg Rauch, a senior product manager for SEL. “But when these mechanical relays trip, the only indication is a little orange or red target. This gives you precious little information about what happened. Whereas the SEL-587Z provides an oscillographic record of the event in question-enabling users to shorten the time necessary to analyze the fault and protection system performance. “

The SEL-587Z oscillographic recorder provides a wealth of information to speed analysis of the effects of CT saturation for out-of-zone and in-zone faults. The sequential events recorder (SER) provides a comprehensive, time-tagged view of the sequence of protective device element operations, and serial-port communication facilitates local and remote monitoring and control of the relay. Front-panel pushbuttons save the expense of separately mounted control switches.

Optimized for uncompromising bus protection, the SEL-587Z relay increases its value by also providing sensitive restricted earth fault protection for grounded wye-connected transformer windings.

“There appears to be a growing awareness throughout the world of the advantages of applying restricted earth fault protection,” observes Rauch. ” Standard current differential relays are insensitive to faults internal to a power transformer near the grounded neutral connection point. On the other hand, the high-impedance SEL-587Z relay, when applied with restricted earth fault connections, is very capable of sensing a turn-to-ground fault.”

The SEL-587Z offers high-impedance differential (ANSI 87) with a range of 20-800 V (120 V continuous; linear to 3000 V symmetrical), overcurrent (50), and inverse-time overcurrent (51) protection elements.

The SEL-587Z can be integrated with existing SCADA systems via Modbus, SEL ASCII, or SEL’s proprietary Fast Messaging protocol through the rear-panel communication port (EIA-232 or 485) that is isolated to alleviate ground loops in critical control systems. A front-mounted serial port allows direct human machine interface via a laptop or other tool.

Available mounting options include horizontal rack and horizontal panel.

“The SEL-587Z high-impedance differential relay builds upon SEL’s reputation for quality, features, innovation, and fast delivery,” adds Rauch. “With some of the longest documented MTBFs in the industry, power engineers can count on this relay to match the longevity of our other products. When you factor in our technical support and the fact that this microprocessor-based relay costs less than the electromechanical relays it replaces, power engineers will have a tough time justifying any other choice.”

For further information and specifications, contact SEL at 2350 NE Hopkins Court, Pullman, WA 99163-5603, USA; phone (509) 332-1890; fax (509) 332-7990; or visit

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