A capacitor voltage self‐balancing method of modular multilevel converters based on switching state matrix construction

Abstract

This study presents a submodule capacitor voltage self‐balancing method for modular multilevel converters (MMCs) based on switching state matrix construction, which has an advantage over eliminating massive sensor demanding and alleviating computational burden for a large number of submodules. It is mathematically proved that MMC has only one static equilibrium operating point to which the submodule capacitor voltages will converge naturally by the evaluation of capacitor voltage deviation index. A novel switching state matrix of submodules is constructed off‐line according to the mathematical proof, and the switching state vectors are cyclically selected from the matrix and distributed to the switching gate signals among the submodules to realize capacitor voltage self‐balance, avoiding real‐time capacitor voltage sampling and sorting. The proposed method is compatible with the conventional double closed‐loop control of MMCs. Theoretical conclusions are verified by simulations and experiments.

A selection principle of submodule switching state vectors for switching frequency reduction in voltage self‐balancing half‐bridge modular multilevel converters

“…Control strategies including optimized sorting algorithms, modulation strategies, auxiliary hardware circuits, new MMC topologies, and so on can contribute to the solutions addressing the problems above, but there still exist certain limitations [25]. Thus, a voltage self‐balancing control strategy for HB‐MMCs based on a full‐rank switching state matrix is proposed in [26, 27], which is not restricted by the system scale and its voltage level. No additional control rings or auxiliary hardware circuits are required, and heavy occupation of computing resources is effectively alleviated.…”

“…T 0 , ε 0 , and δ 0 being all positive numbers [30–33]. Meanwhile, it is verified that $${U_{\bm{pa}\_1}} = \cdots = {U_{\bm{pa}\_N}} = {U_{\bm{na}\_1}} = \cdots = {U_{\bm{na}\_N}} = {U_{\bm{dc}}}/N$$ is the only theoretical steady‐state capacitor voltage value in [27] based on the existence of unique static equilibrium operating point of HB‐MMC and the boundedness of submodule capacitor voltage deviation vector. That is to say, submodule capacitor voltages tend to converge to $${U_{\bm{dc}}}/N$$ automatically.…”