Variable Structure Control of a Virtual Synchronous Generator for LVRT in presence of Local Load

Virtual synchronous generators (VSGs) are used in place of droop-controlled inverters for better inertial response in distributed generation. Distributed generators are more susceptible to voltage dips and other disturbances than high-voltage systems which increases the need for Low Voltage Ride Through (LVRT) capability in low-voltage systems. The presence of local loads between VSG and the grid adds operational constraints to the LVRT capability of VSG. This paper proposes a variable structure control strategy for incorporating LVRT capability into a grid-connected VSG in a low-voltage system. The LVRT capability is tested for light, medium, and deep voltage sags and abides by the Indian Grid Codes (IGC). The effect on the LVRT capability is analyzed for different types of local loads namely constant impedance, constant power, and constant current with the objective to determine the maximum local load which can be fed by VSG during sags. A comparison between the maximum reactive current injection with and without a local load is also done which shows the decreased reactive current support by VSG due to the presence of a local load for a specific range of symmetrical voltage sag levels.