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This paper focuses on the zero-voltage ride-through (ZVRT) capability of a transformerless back-to-back (BTB) system intended for installation on a power distribution system with many distributed power generators. The BTB system consists of two modular multilevel cascade converters based on double-star chopper cells (MMCC-DSCC) and their ac sides are connected to a common ac mains via ac-link inductors. This paper provides an experimental discussion on the DSCC-based BTB system under voltage sags by using a three-phase 200-V 10-kW downscaled system. Experimental results show that the BTB system is equipped with ZVRT capability without causing any overvoltage and/or overcurrent even under the most severe voltage sags.

This paper presents an intensive discussion on a transformerless back-to-back (BTB) system using modular multilevel cascade converters based on double-star chopper cells (MMCC-DSCC). It is expected to be applied to a power distribution system with many distributed power generators. This paper designs, constructs, and tests a three-phase 200-V, 10-kW DSCC-based BTB system installed on a 200-V, 40-kW power distribution simulator consisting of two radial feeders. Experimental results show that the BTB system can mitigate power-flow imbalance between the two feeders. Moreover, the zero-sequence current circulating between the two feeders can be suppressed to be as small as 10 mA ( $0.02\%$ ) in rms by connecting a common-mode choke to the common dc link of the BTB system.

This paper provides an intensive discussion on analysis, simulation, and experiment of a back-to-back (BTB) system unifying two modular multilevel cascade converters based on double-star chopper cells (MMCCs-DSCCs). Each of the two DSCCs connected back-to-back 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 required for regulating the dc-link voltage and controlling the dc-link current. A three-phase 200-V, 10-kW, 50-Hz downscaled BTB system with phase-shifted PWM is designed, constructed, and tested to verify its operating principles and performance. Analytical, simulated, and experimental results agree well with each other in steady and transient states. Experimental waveforms confirm the effectiveness of a self-starting/restarting procedure.