A parallel-redundant BTB (back-to-back) system using modular multilevel cascade converters for power transmission grids

This paper presents a parallel-redundant BTB (back-to-back) system using the modular multilevel cascade converters based on double-star chopper-cells (MMCC-DSCC). A couple of MMCC-DSCCs form a basic BTB unit rated at 132 kVdc and 200 MW. Parallel connection of six BTB units results in the 1.2-GW BTB system for an asynchronous intertie between two grids. Each DSCC consists of multiple cascaded chopper-cells and a center-tapped inductor per leg. Low voltage steps bring significant reductions in harmonic voltage and current to the BTB system. Neither dc-link capacitor nor voltage sensor is installed on the common dc terminals of the two MMCC-DSCCs because an indirect feedback control loop enables to regulate the mean dc-link voltage to its reference. A three-phase 200-V, 10-kW, 50-Hz downscaled BTB unit is tested to verify its operating principles and performance. Experimental and simulated waveforms agree well with each other not only in steady states but also in a transient state. Experimental results confirm the effectiveness of a self-starting/restarting procedure.