Energization and Start-Up of CHB-Based Modular Three-Stage Solid-State Transformers

This paper discusses the energization and start-up process of modular three-stage solid-state transformers (SSTs) based on a cascaded H-bridge topology. A key element of the SST is the auxiliary power supply (APS) feeding the auxiliary circuitry. Redundancy and simpler implementation can be achieved by powering the APSs from the dc-link capacitors of cells, but at the expense of a more challenging start-up process of the SST. The first step of this process must be performed before APSs are operative. This can result in undesired transient events (inrush currents, voltage sags in the capacitors voltages, etc.), which could prevent a proper start-up of the SST or even jeopardize the power devices or other elements. A simple strategy for the energization and start-up of SSTs using this APS structure is proposed. The method is valid for both grid-forming and grid-feeding operations of the SST, regardless of the energization port and without the addition of any extra circuitry. Simulation and experimental results are provided.