A new enhanced synthetic inertia system for stability improvement of hybrid AC/DC grids using MMC integrated with batteries

The widespread expansion of renewable energy sources (RESs) has reduced the inherent inertia in modern power grids, increasing vulnerability to power fluctuations. In hybrid AC/DC grids with limited inertia, disturbances can drive the AC grid frequency and DC grid voltage beyond permissible limits, resulting in undesirable generation tripping or load shedding. To tackle this issue, this paper proposes an enhanced synthetic inertia (ESI) system that emulates both inertia and damping properties to support low-inertia hybrid AC/DC grids. The AC sub-grid is interconnected with a DC sub-grid via a modular multilevel converter (MMC)-based interlinking converter (IC). By exploiting the inherent capacitive storage capability of MMC integrated with a battery energy storage system (BESS), the MMC-based IC can provide synthetic inertia and damping support. Specifically, inertia is emulated through the augmented sub-module capacitance of the MMC, while damping is provided by the integrated BESS. A comprehensive parameter design and control strategy for the proposed ESI system are developed to enhance grid stability. Simulation results show that the ESI system reduces frequency deviation by 49 % compared to systems without SI and by 31.3 % compared to advanced damping-based controls. The rate of change of frequency (RoCoF) is improved by 36.4 %, and frequency overshoot is reduced by up to 74 %. Similar improvements are also observed in the DC grid, where the ESI limits peak voltage deviations within the permissible limit. The proposed strategy also ensures faster recovery times and avoids the need for additional voltage recovery schemes. These outcomes confirm the ESI system's capability to enhance frequency regulation and DC bus voltage stability, strengthening the resilience of low-inertia hybrid AC/DC grids.