Active Filtering With MMC-HVdc Systems: Design and Small-Signal Stability Analysis

This paper studies the design and small-signal stability analysis of active harmonic suppression (AHS) controllers in modular multilevel converter (MMC)-based high-voltage direct current (HVDC) systems. When compared to traditional two-level voltage-source converters (VSCs), the increased effective switching frequency of MMCs makes active filtering feasible in high-voltage systems. A proof-of-concept is presented for an example AHS control structure working to suppress the harmonics in the MMC output current. Using a linear time-invariant (LTI) state-space MMC model based on the dynamic phasor (DP) theory, an eigenvalue-based stability assessment is carried out to study potential interactions between the active filtering action and regular MMC-HVDC controllers. The operation of the proposed AHS controller is demonstrated using time-domain simulations in MATLAB/Simulink, and guidelines are given for the design of AHS controllers in MMC-HVDC systems.