Improved NLM modulation method without measuring capacitor voltage applied to Modular Multilevel Converter

Abstract

The NLM modulation method is particularly effective for Modular Multilevel Converter (MMC) systems with a large number of Submodules (SMs). This approach enables the generation of an output voltage on the AC side of the MMC, achieving up to 2N+1 voltage levels, where N represents the number of SMs in each branch. This article focuses on enhancing the NLM modulation technique for MMC converters by introducing an algorithm that eliminates the need for voltage measurements from the capacitors. Instead, it utilizes simple logical operations to determine which SMs to activate or deactivate during each duty cycle. This innovation significantly reduces the requirement for capacitor voltage and current sensors within the branches, streamlining the complexity of signal connections. Consequently, the hardware design becomes more compact, which is advantageous for MMC converters deployed in medium and high voltage applications. The outcomes of this study were simulated using Matlab/Simulink and were compared against the traditional NLM modulation method, which relies on capacitor voltage measurements to assess the performance of the proposed approach.