Impact of Sampling Frequency on Harmonic Distortion for Modular Multilevel Converter

TLDR

In high‑voltage MMCs, insufficient uniform sampling frequency delays submodule triggering, thereby degrading converter‑output voltage levels and increasing voltage harmonics. The study applies nearest‑level control to MMCs and proposes a systematic method to analyze voltage harmonics and compute two critical values for selecting an appropriate sampling frequency. The authors develop a systematic analysis of voltage harmonics that calculates two critical values influencing output voltage levels and total harmonic distortion to guide sampling frequency selection. Simulations in PSCAD/EMTDC confirm the modulation scheme’s validity and demonstrate the importance of selecting an appropriate sampling frequency.

Abstract

This paper applies nearest level control (NLC) to the modular multilevel converter (MMC). Since there are a number of submodules (SMs) in high-voltage applications of MMC and all SM voltages are required to be measured and sorted, if the uniform sampling frequency is not high enough, the SM will not be triggered as quickly as possible. Thus, the converter-output voltage levels and voltage harmonics will be affected. A method to systematically analyze the voltage harmonics is presented, considering the uniform sampling frequency. In order to select a proper sampling frequency, two critical values, which significantly influence the output voltage levels and voltage total harmonic distortions, are calculated. Simulation results based on PSCAD/EMTDC proved the validity of the proposed modulation scheme and the importance of a properly selected sampling frequency.