Protection Redundancy – Main I and Main II Security and Reliability

Protection history and practice has long had two relays applied for protection of one line or piece of primary equipment. The traditional approach for these relays may be to use two relays with different or identical schemes or manufacturers. There are issues of human usage and common mode failure that are considered when making a choice of how to best select these two relays. In the case of electromechanical relays it was easy to identify the operating principle and production considerations of a particular relay or scheme. In modern microprocessor relays it is not as clear. There are questions on the best way to balance security and reliability when selecting dual protection of a system component. This paper examines the microprocessor system hardware, software, and firmware aspects of a complete system. Fault tree analysis as well as consideration of human interaction with the relay is included. The many aspects of protection included in a modern protective relay are included; protection elements and algorithms, logic, communications, protocols, as well as production aspects. Guidelines from coordinating councils are considered but specific recommendations are not given in this paper. Information on design and production practices are given to let the engineer applying relays understand the implications of relay selection on the overall security and dependability of the system.