Improved current reference calculation for MMCs interal energy balancing control

TL;DR

This paper proposes an improved method for calculating inner current references in modular multilevel converters, enhancing energy balancing during balanced and unbalanced grid conditions by considering network and arm impedances.

Contribution

It introduces a novel current reference calculation approach that accounts for AC/DC voltage components and impedance effects, improving converter control during faults.

Findings

Enhanced energy balancing during unbalanced faults

Effective current reference calculation considering impedance effects

Potential for improved converter stability in fault conditions

Abstract

The paper addresses an improved inner current reference calculation to be employed in the control of modular multilevel converters operating during either balanced or unbalanced conditions. The suggested reference calculation is derived based on the AC and DC additive and differential voltage components applied to the upper and lower arms of the converter. In addition, the impacts caused not only by the AC network's impedances but also by the MMC's arm impedances are also considered during the derivation of the AC additive current reference expressions. Another issue discussed in this article regards that singular voltage conditions, where the positive-sequence component is equal to the negative one, may occur not only in the AC network but also internally (within the converter's applied voltages). Several different inner current reference calculation methods are compared and their applicability during the former fault conditions is analyzed. The paper presents a detailed formulation of the inner current reference calculation and applies it to different unbalanced AC grid faults where it is shown that the presented approach can be potentially used to maintain the internal energy of the converter balanced during normal and fault conditions.