Comparison of Three-Level Cell Topology and Control for Solid State Transformer

This paper presents a solid state transformer (SST) consisting of a cascaded neutral-point-clamped (NPC) H-bridge ac-dc stage and a three-level LLC dc-dc stage. Two different three-level topologies, namely, diode-neutral-point-clamped (DNPC) LLC and series half bridge (SHB) LLC, are compared in terms of zero voltage switching (ZVS) condition and neutral-point (NP) voltage balancing performance. ZVS conditions are analytically derived and their ZVS regions are explicitly given, showing that SHB LLC is more efficient than DNPC LLC with easier ZVS conditions. The phase-shift-based three-level operation mode is proposed for the SHB LLC with not only strong NP balancing capability but also symmetric operation of all switches, which is superior to its DNPC counterpart. Experimental evaluation is done on a 15-kW silicon carbide (SiC) device based three-level cell operated at around 200 kHz and a multi-cell 10-kV SST system.