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This paper presents bidirectional grid-connected modular multilevel converters (MMC) with direct digital control and division-summation processes to achieve power-injection and rectification functions. A direct digital control is developed to track current references, regulate dc-bus voltage and cell voltage, and balance the upper and lower dc-bus voltages. It can accommodate wide filter-inductance variation and take care of source voltage harmonics to achieve tight current tracking and low distortion output current. Based on the power-balance principle, the commands of arm currents can be determined directly. In this paper, the MMC configuration is introduced first, and then the control laws for current tracking and voltage regulation are derived in detail. Experimental and simulated results obtained from two 50 kW MMCs operated in power-injection and rectification modes have verified the analysis and discussion. [ABSTRACT FROM AUTHOR]

This paper presents a three-phase back-to-back transformerless inverter for online uninterruptible power supply applications. The inverter consists of a three-phase three-wire input rectifier stage and a three-phase four-wire output inverter stage with their ground wires tied together. Both of the two stages are with sinusoidal pulse width modulation based division-summation (D-∑) digital control. The input stage controls the power flow between utility grid and dc link, regulates the dc-link voltage, and fulfills power factor correction. While the output stage tracks three-phase ac reference voltages. The proposed control laws derived based on the D-∑ digital approach take into account the effects of dc-link voltage fluctuation, grid-voltage distortion, and inductance variation due to different current levels. Thus, distortion of input current and output voltage, and filter inductor core size can be reduced significantly. Moreover, circulating current through their common ground can be suppressed effectively. Simulated and experimental results from a three-phase back-to-back transformerless inverter have confirmed the analysis and discussion of the proposed control approach. [ABSTRACT FROM PUBLISHER]

This paper presents two-phase modulated digital control for a three-phase bidirectional inverter with wide inductance variation in dc distribution systems. The bidirectional inverter can fulfill both grid connection and rectification with power factor correction. With the proposed control, the inverter can track its sinusoidal reference currents, and it is allowed to have wide inductance variation, reducing core size significantly. The control laws based on space-vector pulsewidth modulation are first derived with either an accurate approach or an approximated one, and the gate signals were then derived based on two-phase modulation for lower switching loss and switching noise spectra. Determination of control parameters and stability analysis are also presented. In the design and implementation, the inductances corresponding to various inductor currents were measured and tabulated into a single-chip microcontroller for tuning loop gain cycle by cycle, ensuring system stability. Measured results from a 10 kVA 3φ bidirectional inverter have been presented to confirm the feasibility of the discussed control approaches. [ABSTRACT FROM AUTHOR]