Relay logic programming explained

Users of protective relays apply these devices specific to their needs and applications. In order to perform this task, schemes are developed and applied to protective relays in the form of relay logic. These methods vary depending on the age of the relay as well as the manufacturer's standard of programming. This paper is developed as a tutorial to examine the methods used to develop relay logic schemes. It will look at past methods of discrete contact/switch logic as well as the methods used today such as Boolean and IEC 61131-3 mapping. Logical mapping methods and simplification are considered. A comparison of the various programming methods will ultimately educate the reader as to the tools available to perform the development task. Relay-to-relay logical bit transfer is a method by which automated and protection specific schemes are developed. An examination of these methods is performed as well. Testing relay logic is an additional subject that is examined. Critical to performance of any relay logic scheme are the comprehensive testing methods used to prove the functionality performs according to design intentions. The goal of the paper is to provide a user of any type of relay, electromechanical, solid state or microprocessor, the knowledge of how to properly develop logic schemes and the proper testing methods associated with these schemes.