Exploiting the Hidden Capacity of MMC Through Accurate Quantification of Modulation Indices

TL;DR

This paper compares direct and indirect modulation techniques in modular multilevel converters (MMC), revealing that direct modulation generally offers higher output capacity, but both can be equivalent under certain control conditions, supported by simulations and experiments.

Contribution

It introduces quantitative metrics and computational methods to evaluate and compare the performance of MMC modulation schemes, clarifying their capabilities and conditions for equivalence.

Findings

Direct modulation has higher ac voltage output capability.

Indirect modulation is limited in linear modulation regions.

Steady-state equivalence occurs under specific control conditions.

Abstract

The modular multilevel converter (MMC) has become increasingly important in voltage-source converter-based high-voltage direct current (VSC-HVDC) systems. Direct and indirect modulation are widely used as mainstream modulation techniques in MMCs. However, due to the challenge of quantitatively evaluating the operation of different modulation schemes, the academic and industrial communities still hold differing opinions on their performance. To address this controversy, this paper employs the state-of-the-art computational methods and quantitative metrics to compare the performance among different modulation schemes. The findings indicate that direct modulation offers superior modulation potential for MMCs, highlighting its higher ac voltage output capability and broader linear PQ operation region. Conversely, indirect modulation is disadvantaged in linear modulation, which indicates inferior output voltage capability. Furthermore, this paper delves into the conditions whereby direct and indirect modulation techniques become equivalent in steady-state. The study findings suggest that the modulation capability of direct modulation is the same as that of indirect modulation in steady-state when additional controls, including closed-loop capacitor voltage control and circulating current suppression control (CCSC), are simultaneously active. Simulation and experiments verify the correctness and validity.