A Novel Method of Executing Main-Tie-Main LV Secondary Selective Systems

Large industrial facility power distribution systems are typically designed as secondary selective schemes to increase system reliability and flexibility for the operator during maintenance. Existing Main-Tie-Main protection and control schemes utilize three microprocessor-based relays for the Automatic Transfer Switching (ATS), breaker control, Human Machine Interface (HMI) and electrical system protection. One protection relay is located at each of the three Main-Tie-Main (M-T-M) breakers, each communicating with the others over fiber or copper to perform the ATS operation. This paper proposes using only one (1) microprocessor relay to control all three (3) breakers as well as perform ATS, Electrical Control and Monitoring System (ECMS) Interface and HMI. Details of a Proof-of-Concept test proving this functionality is included. This scheme reduces the quantity of microprocessor relays by two-thirds for LV Main-Tie-Main schemes. Methods of retaining protection reliability and redundancy are discussed.