Grid-Connected Converter With Grid-Forming and Grid-Following Modes Presenting Symmetrical and Asymmetrical Fault Ride-Through Capability
Diego Ríos-Castro, Diego Pérez-Estévez, Jesús Doval‐Gandoy
Abstract
Grid-connected converters must remain coupled to the grid during a fault. They have to control the reactive currents injected to the grid because of its limited overload capacity. Particularly, grid-forming (GFM) converters ride through the fault either preserving grid-forming operation or switching from grid-forming to grid-following (GFL) mode. In GFM mode the current cannot be controlled therefore mechanisms as virtual impedances become necessary. This article proposes a converter with grid-forming and grid-following capabilities that demonstrates effective voltage tracking in GFM mode and current tracking in GFL mode, good dynamic disturbance rejection response and, wide stability range and robustness in both modes. During a fault, the proposed converter switches from grid-forming to grid-following mode in order to precisely control the injected currents. The current control is able to limit the current at the inception of severe faults. This characteristic is preserved for a wide range of grid impedances, thanks to its good robustness and wide stability margin. As a result, the controller can operate in stand-alone mode, connected to a weak grid, or connected to a stiff grid with a short-circuit ratio up to 50. The fault-ride through capability of the converter is tested under severe symmetrical and asymmetrical faults.