Assessing the Benefits of Redundancy in Solid-State Transformers: An Extended Total Cost of Ownership Model

DC-DC solid-state transformer (SST) is becoming more and more popular for medium voltage applications. This topology, due to its modularity, has the potential to present high levels of reliability. However, there is a lack of a proper design method that ensures reliable converter operation with minimal total cost of ownership. In this paper, a reliability model of the DC-DC stage of the SST is presented that includes infant mortality, random failures, and aging failures while considering and comparing the 2-level full-bridge and the 3-level half-bridge as possible submodule configurations, which are conventional solutions in SST systems. Furthermore, an analytical expression on the total cost of ownership of the converter is developed which includes installation, operation, repair, and downtime costs. A minimization problem is then formed with objective function the total cost of ownership and the number of redundant submodules as degrees of freedom. A case study is given to demonstrate the analysis and conclusions that can be drawn using the proposed method. The analysis shows that despite the increase in initial investment costs, redundancy can lead to significant reductions in total cost of ownership over the lifetime of the converter. In addition, design decisions can be severely impacted by the cost of downtime and the ability to restore the operation of the converter quickly after a fault.