Capacitor voltage ripple and capacitance evaluation in a direct three-phase to single-phase ac/ac MMC

This paper introduces the capacitor current and voltage ripple evaluation of a direct three-phase to single-phase ac/ac modular multilevel converter with full-bridge sub-modules. Based on a desired sub-module capacitor voltage ripple, the required capacitance is calculated, which is valuable to dimension sub-modules’ energy storage in many applications. Simulations and measurements using a scaled-down prototype validate the analysis

Integrated local control of active power and voltage support for three-phase three-wire converters

The derivation of a robust control algorithm is presented to provide decoupled active power regulation and local grid voltage support in three-phase three-wire grid-connected converters (GCCs). Unlike conventional control schemes, the proposed strategy is designed to be harmonic sequence asym-metric for the purpose of local voltage unbalance correction. A frequency-domain Norton equivalent model is derived to illustrate the working principle of the strategy. Accordingly, by following a frequency-domain decoupled method, the funda-mental positive-sequence, the harmonic symmetrical sequences and the fundamental negative-sequence components are regu-lated independently. Consistent to the model analysis, simulation results validate reduction of local voltage unbalance and total harmonic distortion. Since no external sensors are required for the implementation of the strategy, it is a local approach, applicable to already-existing GCC systems. Moreover, in view of the higher switching frequencies as attainable by devices from the next SiC generation, the accuracy and dynamic behavior of the control algorithms can be much enhanced, improving therefore the quality of the processed energy

Square wave operation to reduce pulsating power in isolated MMC-based ultrafast chargers

This paper presents an application of modular multilevel converters to reduce pulsating power, and therefore sub-modules in ultrafast chargers. The converter’s analysis and a control scheme were presented to realize bidirectional power transfer between a three-phase medium-voltage grid and a single-phase medium-frequency transformer with square-shaped voltage, successfully reducing power fluctuation

Switching Loss Reduction for an MMC-Fed AC/DC Converter

Medium-voltage connected ultra-fast chargers are getting more popular for charging electric vehicles with large battery capacities. Here, the solution based on a modular multilevel converter is more promising, since the isolation stage can be realized as a single medium-frequency transformer interconnecting the modular multilevel converter to a single-phase ac/dc converter. A new operating scheme is proposed for this converter, enabling zero-voltage switching and nearly zero-current switching across the entire load range. In contrast to the conventional phase-shift control method, the proposed scheme effectively reduces the reactive power through the ac/dc converter, leading to decreased turn-off switching losses in the ac/dc converter and a lower RMS current stress in the power path. A control scheme, integrating the operating principle, is developed for the modular multilevel converter. The method is verified through simulation and measurements on a scaled-down prototype. The results validate the theoretical analysis and practical feasibility of the proposed operating principle and the developed control scheme

Switching Loss Reduction for an MMC-Fed AC/DC Converter

Medium-voltage connected ultra-fast chargers are getting more popular for charging electric vehicles with large battery capacities. Here, the solution based on a modular multilevel converter is more promising, since the isolation stage can be realized as a single medium-frequency transformer interconnecting the modular multilevel converter to a single-phase ac/dc converter. A new operating scheme is proposed for this converter, enabling zero-voltage switching and nearly zero-current switching across the entire load range. In contrast to the conventional phase-shift control method, the proposed scheme effectively reduces the reactive power through the ac/dc converter, leading to decreased turn-off switching losses in the ac/dc converter and a lower RMS current stress in the power path. A control scheme, integrating the operating principle, is developed for the modular multilevel converter. The method is verified through simulation and measurements on a scaled-down prototype. The results validate the theoretical analysis and practical feasibility of the proposed operating principle and the developed control scheme

Extended operating region of modular multilevel converters using full-bridge sub-modules

This paper presents an application of modular multilevel converters to remove line-frequency transformers from ultrafast charging stations, reducing cost and volume. The converter analysis with full-bridge sub-modules enables an operating region, that converts a medium-voltage grid into a lower voltage DC-bus, ideal for charging batteries rapidly

Virtual Oscillator Control as Applied to DC Microgrids with Multiple Sources

Simple reference-frame transformations allow to implement the method of Virtual Oscillator Control, originally conceived for AC microgrids, to parallel-connected converters in a DC microgrid network. As a result, based only on local measurements and without recourse to a communication link, the DC-DC converters in the microgrid share the load power in proportion to their power ratings without circulating currents, and may be readily interleaved. Simulations validate the proposals for a set of three half-bridge converters serving a common DC load

Square wave operation to reduce pulsating power in isolated MMC-based ultrafast chargers

This paper presents an application of modular multilevel converters to reduce pulsating power, and therefore sub-modules in ultrafast chargers. The converter’s analysis and a control scheme were presented to realize bidirectional power transfer between a three-phase medium-voltage grid and a single-phase medium-frequency transformer with square-shaped voltage, successfully reducing power fluctuation

Square wave operation to reduce pulsating power in isolated MMC-based ultrafast chargers

This paper presents an application of modular multilevel converters to reduce pulsating power, and therefore sub-modules in ultrafast chargers. The converter’s analysis and a control scheme were presented to realize bidirectional power transfer between a three-phase medium-voltage grid and a single-phase medium-frequency transformer with square-shaped voltage, successfully reducing power fluctuation

Switching Loss Reduction for an MMC-Fed AC/DC Converter

Medium-voltage connected ultra-fast chargers are getting more popular for charging electric vehicles with large battery capacities. Here, the solution based on a modular multilevel converter is more promising, since the isolation stage can be realized as a single medium-frequency transformer interconnecting the modular multilevel converter to a single-phase ac/dc converter. A new operating scheme is proposed for this converter, enabling zero-voltage switching and nearly zero-current switching across the entire load range. In contrast to the conventional phase-shift control method, the proposed scheme effectively reduces the reactive power through the ac/dc converter, leading to decreased turn-off switching losses in the ac/dc converter and a lower RMS current stress in the power path. A control scheme, integrating the operating principle, is developed for the modular multilevel converter. The method is verified through simulation and measurements on a scaled-down prototype. The results validate the theoretical analysis and practical feasibility of the proposed operating principle and the developed control scheme