Analysis and Control of a Modular Multilevel Cascaded Converter-Based Unified Power Flow Controller

This article presents a novel configuration for a unified power flow controller (UPFC) comprising a modular multilevel cascaded converter (MMCC) with a full-bridge inverter. The MMCC has one end of each phase-leg shunt-connected to the transmission line. The other end connects in parallel to the primary terminals of a series line transformer, and the ac output terminals of a full-bridge dc–ac inverter. The submodules in the MMCC are full-bridge flying capacitor converters. This UPFC is compared to another type of MMCC-UPFC which uses double-star configuration, and submodules of half-bridge chopper type; this is referred to as the Double Star Chopper Cells UPFC (DSCC-UPFC). The comparison is in terms of footprint, cost, and performance. The new topology is lighter, more efficient, and cheaper than the DSCC. Its operating principle and control scheme, which combines the regulation of voltage and of transmission line power flow, are presented. Simulation studies for the proposed MMCC-UPFC realizing power flow control in a dual voltage sourced power network are presented and show good performance under varying operation conditions. The article also evaluates the power control margins of this device.