Impact of Centralized and Distributed Control Structures on the Harmonic Stability of Modular Multilevel Converter Based on DQ Reference Frame Impedance Assessment

The Modular Multilevel Converter (MMC) is one of the suitable converter topologies for medium to high-voltage industry applications. Despite the inherent modularity and redundant structure of the MMC topology, most MMC controllers are designed based on a centralized controller structure. Even though a centralized controller inherently limits the hardware flexibility of an MMC, it is relatively faster than distributed control algorithms due to less communication and control delays. Both control structures have pros and cons, but the design of the controller may cause instability at specific frequencies when interfaced with an AC or DC grid due to control gains, delays etc. This paper investigates the effects of the control delays of the distributed controller on system stability and compares it with the centralized controller. An MMC and its controller are modeled in a Hardware-in-Loop environment with the Real Time Digital Simulator to analyze potential DC resonant frequencies in the dq reference frame. Passivity theory isapplied to the extracted frequency response to predict possible passive and non-passive regions based on control structures.