Design and Selection of High Reliability Converters for Mission Critical Industrial Applications: A Rolling Mill Case Study

In the metal industry, rolling is the most widely used steel forming process to provide high production and control of final product. Rolling mills must be able to change the speed of the strip at the same time that the speed is controlled within precise limits. Furthermore, this application has a severe load profile, with high torque variations during the lamination process. These characteristics include rolling mills among the classical mission critical industry applications (MCIA). In addition to the high cost/failure rate, rolling mills have a critical dynamic loading, making the design of a reliable system doubly challenging. Design for reliability (DFR) is the process conducted during the design of a component or system that ensures them to perform at the required reliability level. In the context of the power converters for rolling mills and other MCIA, the DFR should be known and adopted in the design of the converter proper (component level) well as in the specification of power converters (system level). This paper contributes to the knowledge in the field by proposing a methodology covering the necessary steps for decision making during the design (component level) and selection (systems level) of power converters for MCIA. A rolling mill system from a large steel plant in southeastern Brazil is adopted as the case study. The standard high power converter solution is compared with two high reliability converter topologies: the fault-tolerant active neutral point clamped and the triple-star bridge cells modular multilevel converter. The importance of DFR in mission critical applications is demonstrated.