IEEE Transactions on Power Delivery · 2011 · 1.4K citations · 21 references
EngineeringElectrical EngineeringEnergy SystemsPower ElectronicsRenewable Energy SystemsAc-dc ConversionAc-dc ConvertersDc-dc ConversionDc-dc ConvertersMultilevel ConvertersPulse-width ModulationCarrier-based ModulationFrequency ModulationModular Multilevel ConvertersModular Power ConversionModular Power ElectronicsMultilevel Dc-dc ConverterPower Converter DesignSwitching Strategies
The authors propose a modified phase‑shifted carrier PWM scheme for modular multilevel converters that incorporates a reduced‑switching‑frequency voltage‑balancing algorithm and a circulating‑current‑suppressing controller to lower switching frequency and circulating currents. The scheme decomposes the three‑phase circulating currents into two DC components using a double line‑frequency negative‑sequence rotational frame and minimizes them with proportional‑integral controllers, while the voltage‑balancing algorithm adjusts the carrier to reduce switching frequency. Simulations confirm that the modified PSC‑PWM and voltage‑balancing algorithm effectively reduce switching frequency, the circulating‑current controller eliminates inner circulating current and improves AC output voltage quality, and both techniques lower converter power losses.
This paper describes a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC). In order to reduce the average device switching frequency, a reduced switching-frequency (RSF) voltage balancing algorithm is developed. This paper also proposes a circulating current suppressing controller (CCSC) to minimize the inner circulating current in an MMC. Based on the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a pair of proportional integral controllers. Simulation results based on a detailed PSCAD/EMTDC model prove the effectiveness of the modified PSC-PWM method and the RSF voltage-balancing algorithm. The proposed CCSC not only eliminates the inner circulating current but also improves the quality of the converter ac output voltage. A simple loss evaluation demonstrates that the RSF voltage-balancing algorithm and the CCSC reduce the converter power losses.
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