A Back-To-Back (BTB) System Using Modular Multilevel Cascade Converters Based on Double-Star Chopper-Cells (MMCC-DSCC): Verification of Operation by Experiment and Simulation

Abstract

This paper presents a detailed discussion on the modular multilevel cascade converter based on double-star chopper-cells (MMCC-DSCC). Hereinafter, it is simply referred to as the DSCC. A couple of DSCCs are used to form a three-phase 6.6-kV back-to-back (BTB) system installed between two power distribution feeders. Each DSCC, using 3.3-kV IGBTs, consists of 16 cascaded chopper-cells per leg. Low voltage steps bring about significant reductions in the harmonic voltage and current to the BTB system. A conventional power conversion system employs bulky line-frequency transformers and dc-link capacitors, whereas this system does not require either of them, thereby reducing the overall physical size and weight. Although no voltage sensor is installed on the common dc link of the BTB system, an indirect feedback loop in the control system makes it possible to regulate the mean dc-link voltage to its reference. This paper describes the design, construction, and evaluation of a three-phase 200-V, 10-kW downscaled model, with focus on operating principles and performance. Finally, experimental waveforms are compared with simulated ones obtained from a software package of PSCAD/EMTDC under the same operating conditions, including steady and transient states. The experimental and simulated waveforms agree well with each other.