SDBC-MMC STATCOM Control Under Unbalanced Grid Conditions Based on Different Sequence Extraction Methods

This work analyzes the operation of a Modular Multilevel Converter (MMC) Single Delta Bridge Cell (SDBC) Static Synchronous Compensator (STATCOM) under unbalanced grid conditions based on different sequence extraction methods: Decoupled Double Synchronous Reference Frame (DDSRF), Dual Second Order Generalized Integrator (DSOGI), and Fortescue matrix-based filter in αβ frame. The conventional Voltage Oriented Control (VOC) cannot eliminate the 2ω components caused by unbalanced grid disturbances, which leads to the obstruction of the system along with its control operations. Moreover, under such conditions, the phase-locked loop (PLL) fails to provide an accurate frequency and phase angle of the AC voltage resulting in synchronization failure and non-accurate current injections. Sequence extraction aids the PLL to maintain the synchronization with the grid by following the fundamental component of the voltage positive sequence and allows for the appropriate injection of active and reactive currents. In this paper, the system behavior based on each sequence extraction method is investigated and its impact on the dynamic performances is highlighted through time-domain simulations performed in MATLAB/Simulink.