Implementing back-up protection using microprocessor based multifunction relays

The use of microprocessor based multifunction relays is on the increase due to the advantages they offer such as communications, measurements, simplified wiring, and reduced space requirements. Since many protection functions are included in one device however, there has been some concern about providing adequate protection should a multifunction relay fail. A reliable protection scheme is very important in order to protect capital equipment and operating personnel, and reduce down time by selectively clearing faults. This paper analyses some of the different means that have been used to achieve satisfactory levels of back-up protection using microprocessor based multifunction protection relays featuring the protection functions normally provided in large petrochemical plants.