Abstract
Medium voltage hybrid bridge multilevel converter (MMC) usually has a lower number of links. Therefore, the traditional Nearest Level Modulation (NLM) method has the problems of low equivalent switching frequency and poor output quality. And the traditional Carrier Phase-shifted Pulse Width Modulation (CPS-PWM) modulation has problems such as difficulty in hybrid bridge control, difficulty in redundant configuration, and large amount of calculation. In response to the above problems, this paper proposes a new high-quality NLM method. Based on the traditional NLM modulation method, PWM modulation output and module voltage equalization are realized through the time difference between one module input and one module exit in each control cycle. It achieves the purpose of improving the equivalent switching frequency and output power quality, and at the same time has the advantages of the traditional NLM modulation method with small calculation amount, redundant configuration and hybrid bridge modulation. This article describes the three modulation methods and compares their effects. Finally, the theoretical analysis is verified by PSCAD simulation.
Highlights
In recent years, renewable energy has become the focus of development of all countries in the world
Traditional Nearest Level Modulation (NLM) modulation and Carrier Phase-shifted Pulse Width Modulation (CPS-PWM) have their own advantages and disadvantages when they are applied to low- and medium-voltage Medium voltage hybrid bridge multilevel converter (MMC) with a small number of modules
This article combines the advantages of the traditional NLM modulation method and the traditional CPS-PWM modulation method to propose a high-quality NLM modulation method
Summary
Renewable energy has become the focus of development of all countries in the world. The NLM method has the advantages of a small amount of calculation and simple redundancy settings. The medium voltage MMC used in the distribution network has a small number of bridge arm modules, and when the traditional NLM modulation method is adopted, the number of output levels is small, the power quality is poor, and the filtering is difficult. The CPS-PWM method has the advantages of fixed switching frequency and equivalent switching frequency. This article proposes a high-quality NLM modulation method by combining the advantages of the NLM modulation method and the CPS-PWM modulation method This method has many advantages such as small calculation amount, fixed switching frequency and equivalent switching frequency, significant voltage equalization effect, convenient redundancy design and accurate modulation output, and can be effectively applied to medium voltage MMC in AC/DC hybrid distribution network. This article describes the modulation method in detail and verifies the effectiveness of the modulation by means of PSCAD simulation
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