Modular Multilevel Converter Circulating Current Control with Single Active Filter Module per Phase

Abstract

Phase-legs of DC/AC modular multilevel converters (MMC, M2LC) transfer power from the DC input to the corresponding phase output terminal by means of a current that circulates through the DC input and the phase-leg. In addition to the load-dependent DC component required for the power transfer, this circulating current contains significant harmonics if no countermeasures are implemented. Prominently, a large second harmonic appears, essentially because each converter arm performs a single-phase power conversion. This results in higher RMS values of the arm currents and ultimately in higher-than-necessary losses. One option to mitigate these undesired harmonics and the associated losses extends each arm of the MMC by an active filter module that controls the circulating current by injecting a common-mode component into the arm voltage. In this paper, we propose a new variant of an MMC topology with such active filter modules. In contrast to the state of the art, the proposed realization shows lower realization effort: it uses only a single active filter module per MMC phase-leg instead of two, which corresponds to a reduced effort in terms of both, power hardware components and also control and communication electronics. Furthermore, a single active filter module can operate fully self-contained if desired, i.e., without an external communication interface, thus simplifying system integration.