Simulation Framework for DC Grid Control and ACDC Interaction Studies Based on Modular Multilevel Converters

Modular multilevel converter (MMC) topology is discussed intensively as a crucial part of future dc grids, especially to integrate offshore wind power. While generalized voltage-source converter models are usually utilized for multiterminal investigations, conclusions on transient dc grid behavior and on ac-dc interactions during distorted ac grid conditions are hard to draw or are limited in their significance. In this paper, a MMC-based dc grid framework with a tripartite focus on an internal converter as well as dc and ac grid control is presented. Besides, an evaluation of the implemented MMC model that incorporates the available arm sum voltage, a decoupled ac and dc current system control concept that is suitable to handle unbalanced voltage conditions is derived for HVDC applications. Combined with wind farm arrays, this collocation allows a meaningful examination of hybrid ACDC structures and the implemented dc grid control methodologies. To provide proof of the universal applicability of this framework, simulations are carried out on a five-terminal system.