A Co-Simulation Platform for Modeling and Testing Modular Multilevel Converters and Their Controls in Large Networks

This paper introduces a method to represent the dynamics of modular multilevel converters embedded in a large power system using a multi-rate co-simulation platform. The co-simulation platform comprises dynamic phasor (DP) and electromagnetic transient (EMT) solvers. It uses large simulation time-steps in the DP subsystem for accelerated computations. The specialized dynamic phasors used in the co-simulator allow retention of a large portion of the harmonic spectrum of the waveforms and as such usage of average-value Modular Multilevel Converter (MMC) models with an enriched harmonic spectrum becomes possible. The paper develops a model for the MMC including the internal dynamics of the arm currents and embeds it in the DP-side of the co-simulator. Special provisions for modeling the converters control system, which may include non-linearities, are also discussed. This approach is useful in the study of large systems with a large penetration of MMCs. The developed model allows representation of MMCs in parts of the network where average-value models suffice and where conventional EMT models are not necessary. Extensive studies of the performance of the model in terms of its accuracy and computational advantage are presented to establish its merits.