Bus transfer systems: requirements, implementation, and experiences

A bus transfer system is designed to provide process continuity to the loads attached to a motor bus while transferring the bus from one source to another. A successful bus transfer under contingent conditions provides immense value and benefits to continuous process operations that cannot afford an interruption of power supply to plant auxiliaries. This paper describes some real-world bus transfer requirements, implementations, and experiences in thermal power plants and continuous process industry plants. The fast, in-phase, residual voltage, and momentary paralleling transfer methods are described, compared, and evaluated. The spin-down characteristics for different motor buses are analyzed, and the feasibility of the different transfer modes is deduced. Auto-initiation criterion for bus transfer is explored, using a combination of bus undervoltage, underfrequency, and (df/dt) characteristics. Different integrated system requirements, such as monitoring of readiness conditions, breaker failure detection and corrective action logic, and online testing measures, are discussed. The results of the resultant "hot" load trials and their benefits to the system are explained and interpreted. The concept of islanded transfer for grid-free operations of captive generation-load systems is discussed and elaborated.