A Discrete-Time Low-Frequency-Ratio Nearest Level Modulation Strategy for Modular Multilevel Converters With Small Number of Power Modules

There has been increased usage of modular multilevel converters (MMCs) in the medium-voltage traction drives for increased efficiency and reduced switching losses. One of the disadvantages of utilizing the conventional nearest level modulation (NLM) scheme is the poor performance of output voltage distortion and dynamic response. The control design of low-frequency ratio for the MMC system with a small number of power modules should, therefore, be considered due to the industrial operation requirements. This paper focuses on the voltage state switching mechanism of a discrete-time low-frequency-ratio NLM strategy. A five-segment voltage state allocation guideline based on voltage state smooth transition is analyzed in the discrete-time domain. The proposed strategy features a linearly synthesized output voltage by means of simple arithmetic calculations to determine the switching sequences and the duration times. The overall strategy benefits from reduced dv/dt in the rising/falling edges of the output voltage and the unnecessary switching actions are highly constrained. The simulation analysis and the experimental results validate the proposed modulation strategy and establish its effectiveness.