3-D Space Modulation With Voltage Balancing Capability for a Cascaded Seven-Level Converter in a Solid-State Transformer

The solid-state transformer (SST) is an alternative for existing power transformer due to its advantage in low volume, bidirectional power flow, power factor control, and fault management capability. In this paper, a three-stage cascaded-type SST, which utilizes the cascaded seven-level rectifier as the interface with a 7.2 kV distribution system, is presented. In the described system, dc voltage balancing of the cascaded seven-level rectifier is a necessity for stable operation. Previous PI-based voltage balancing control has intrinsic disadvantage in compromise between fast regulation and PI saturation. Besides, the addition of a voltage balancing controller may also change the system loop and bring unexpected performance. This paper aims at giving an alternative view of voltage balancing mechanism in a cascaded multilevel converter. A novel 3-D space modulation technique with voltage balancing capability is proposed for a cascaded seven-level rectifier stage of SST. By choosing the most suitable switching pairs for dc voltage balance, this method has a very good voltage balancing capability that can realize fast regulation of dc voltages in all operation modes, including unity power factor operation, capacitive operation, and inductive operation. Simulation and experimental results in a scaled-down system are provided to verify the effectiveness of the proposed method.