Enhanced STATCOM, Supercapacitor, Energy storage system, Conditioned Based Maintenance, Weibull failure modelling

The increasing penetration of inverter-based resources (IBRs) reduces the amount of kinetic energy available in the grid. As these resources replace conventional synchronous generators, the grid's natural inertial support decreases, making fast and controllable power support even more important. Modular Multilevel Converter-based High Voltage Direct Current (MMC- HVDC) systems are attractive because they offer fast response and control of active and reactive power. However, the internal energy stored in the MMC submodule capacitors is limited and insufficient for grid-support functions. The integration of external energy storage (ES) is investigated to extend the converter's capability for short-term ancillary service support.

This thesis compares two external ES integration methods for an MMC-HVDC system. The first connects the ES branch to the DC-link at full DC-link voltage, while the second connects a partially rated ES branch in parallel with the AC-side arm inductors. The comparison considers the ES branch requirements and converter response. The ES branch voltage, current, and energy utilization are compared with changes observed in the main MMC quantities, including arm currents, arm voltages, and submodule capacitor voltage ripple. The study uses MATLAB and PSCAD for different stages of the analysis. MATLAB provides the analytical model, power capability curves, and a look-up table for the arm-inductor topology. The selected operating points are then tested in PSCAD, where the converter schematic, modulation, ES-branch control, and time-domain behavior are represented in more detail. The results of the studies are then used to compare the ES-side rating requirements, the impact on MMC internal quantities, and the suitability of both methods for short-duration grid support operation.