An improved fault ride-through strategy for back-to-back MMC-based VSCs in an inverter-dominated renewable energy system

By integrating back-to-back voltage-source converters (VSCs) between the main grid and the regional grid, asynchronous interconnection of these grids can be established, enabling the regional grid to operate entirely on variable inverter-dominated renewable energy in islanded mode. This study first examines low voltage grid code requirements for different types of converters. It then investigates a common AC fault ride-through strategy for back-to-back VSCs for building 100% renewable electricity systems. By comparing the field test results of islanded and parallel operations of the regional grid, it is revealed that the conventional LVRT strategy fails to support the system under unbalanced grid conditions consistently. To improve low voltage ride-through capabilities, the study introduces an advanced approach that employs negative sequence voltage suppression, an adaptive AC current amplitude control of positive and negative sequence current composite amplitude control, combined with DC bus voltage stabilization. Finally, the effectiveness of the proposed approach is validated through hardware-in-the-loop simulations and empirical field testing. The results show a substantial reduction in the regional grid's negative sequence component and harmonic content by 18.5 and 81.7%, respectively. Moreover, the DC voltage oscillation is effectively suppressed following the initiation and clearance of a single-phase-to-ground fault.