Modular Multilevel Converter with Inductor Parallel Branch Providing Integrated Partially Rated Energy Storage

This paper investigates the Modular Multilevel Converter topology, enhanced with an extra branch featuring Energy Storage units, in order to provide ancillary or market services and serve as an energy buffer between the dc and ac sides. The new branch is installed in parallel with the arm inductors, and it includes a partially rated dc-ac converter which enables power exchange between the ES and the arms of the MMC in the form of a higher harmonic current. The topology adds new functionalities to the MMC substation without affecting the existing configuration; the ES branches are connected to the MMC in the existing busbars. The solution enables an addition up to 0.4 pu power from the ES for a typical rated high voltage MMC. A study of the main operational features is presented, showing that parameters as the circulating current magnitude and frequency can be adjusted to optimise the operation of the overall system in terms of efficiency and current/voltage ratings.