Cover Story

Best of Both Worlds

Digital Distance Relays Following Analog Principles

by Bogdan Kasztenny and Mangapathirao (Venkat) Mynam, Schweitzer Engineering Laboratories, Inc. USA

Most protection principles we use today have remained relatively unchanged since their introduction. A protection engineer from the 1920s would recognize transformer differential logic described in an instruction manual of a modern microprocessor- based relay. What has changed dramatically are the devices that implement these protection principles. Relays based on early semiconductors replaced electromechanical relays in the 1970s only to be replaced a decade later with microprocessor- based relays. We have witnessed improvements in reliability, dramatically expanded functionality, reduction of size and cost, and many other enhancements. This technological progress – including expansion of functionality adjacent to protection – does not change the fact that protective relaying became a mature field. More