Applications of Distance Protection Schemes for a Forked Inter-Tie between Industrial Cogeneration and a Large Utility: Tips, Tricks and Traps

This paper describes an experience with the application of distance protective relaying schemes, where a cogeneration facility is added to an existing industrial plant. The single contingency inter-tie between the industrial plant and a third party utility is modified to allow bi-directional power flow and isolation (islanding) if required. Within the industrial complex boundary, state-of the-art, modern, multi-stage, numerical distance relays are utilized to provide a fast, reliable and stable protection scheme for the plant´s short, high voltage transmission line. There are two competing objectives that the Industrial Engineer must balance for the integrated system to function properly. On one hand, the integrated protection system must be fast enough, under fault conditions near the co- generation facilities, to isolate and protect the equipment and avoid generator instability. On the other hand, the protection system must be stable and secure to prevent nuisance tripping and to insure islanding for continued plant operation post event. The protection scheme is further complicated by the forked inter- tie arrangement. Selection, setting and coordination between the plant and utility protection scheme provide for an interesting case study from an industrial protection engineering point of view. The challenge is in studying, adapting, coordinating, setting and de-bugging the integrated protection scheme for both sides of the inter-tie. It requires cooperation of the utility and must fulfill both safety and operation requirements.