Protection and Control Redundancy Considerations in Medium Voltage Distribution Systems

This paper is intended to raise some discussions on selected aspects of protection and control redundancy in distribution systems. The paper will present general considerations, common definitions, and redundancy methods in distribution systems across various utility and industrial installations in North America. Review of the redundancy practices and their economical and philosophical backgrounds in distribution systems will help understanding of the main objectives. The paper will also outline in detail advantages and disadvantages of redundancy considerations such as separate DC power supplies, dual trip coils, separate trip circuitry, main/backup (or SET A/SET B) protection concepts, redundant feeder controls, alarms and indications, and finally redundant communication channels. This paper will present a case study for typically used redundant schemes and demonstrates some common implementation errors, pseudo redundancy, and illustrations of two relays providing inadequate redundancy. Lastly, the paper will elaborate on some of the redundancy issues and their solutions based on new generation microprocessor relays, such as multiple setting groups, automatic reclosing and breaker failure protection cross-initiation, oscillography cross-triggering, etc. The intention of this paper is to initiate an industry-wide discussion and idea sharing on the subject of redundancy and implementations in the utility and industrial applications