Switchgear Main-Tie-Main Closed-Transition Transfer Design Strategy for High Levels of Available Fault Current

Large offshore production platform designs are characterized by high levels of horsepower for driven equipment. It is common to design around an all-electric approach, where a central electrical power plant is installed and few (if any) turbine or diesel engine drivers are included other than for generators. This approach leads to the use of relatively large transformers and to higher levels of available short-circuit current. Due to the production throughput and the capital investment associated with these facilities, the availability of electric power is a prime economic factor in design. To enhance both maintainability and operability of the electrical plant itself, dual radial secondary selective systems with main-tie-main breaker switchgear configurations are frequently employed. If one of the main supplies is intentionally removed from service, it is usually preferable to first closed-transition transfer the affected loads to the other main supply through the tie breaker. During this closed-transition transfer period, the switchgear may be exposed to an excessive level of available short-circuit current. This paper discusses this closed-transition transfer scheme and addresses safeguards that may be taken to mitigate the excessive short-circuit current risks during the transfer period.