Steady-State Analysis of Interaction Between Harmonic Components of Arm and Line Quantities of Modular Multilevel Converters

TLDR

The fundamental frequency component in the arm currents of a modular multilevel converter is essential for operation, and its presence inevitably induces alternating components in the submodule capacitor voltages. This paper investigates how arm currents and capacitor voltages interact when submodules are connected and bypassed sinusoidally. The authors derive equations for the circulating current caused by variations in the total inserted voltage and identify resonant frequencies, which are confirmed experimentally. Least‑squares fitting of the analytical expressions to measured data yields effective arm resistance and submodule capacitance values, and the derived arm‑current expression is confirmed by experiment.

Abstract

The fundamental frequency component in the arm currents of a modular multilevel converter is a necessity for the operation of the converter, as is the connection and bypassing of the submodules. Inevitably, this will cause alternating components in the capacitor voltages. This paper investigates how the arm currents and capacitor voltages interact when the submodules are connected and bypassed in a sinusoidal manner. Equations that describe the circulating current that is caused by the variations in the total inserted voltage are derived. Resonant frequencies are identified and the resonant behaviour is verified by experimental results. It is also found that the effective values of the arm resistance and submodule capacitances can be extracted from the measurements by least square fitting of the analytical expressions to the measured values. Finally, the analytical expression for the arm currents is verified by experimental results.