A Current-Limiting PI Control Scheme for Voltage Source Converters

In modern power systems, the good and safe operation of voltage source converters (VSCs) is of principal importance. The aim is twofold: to regulate the active and reactive power injected to the grid and simultaneously to protect the VSC from overcurrent situations usually occurred after some abnormal grid conditions. A novel solution to this open problem is provided in this paper. Specifically, we present a novel current-limiting control scheme that enables to accurately limit the RMS current value of the VSC, even under abrupt faults. The controllers are implemented in the well-known d-q synchronously rotating reference frame for a VSC that is connected to a stiff bus through an LC filter. The synchronization with the grid-bus is carried out by a robust synchronization unit (RSU), whereas a modified PQ controller that efficiently accommodates the nature of the RSU and the inner-loop control scheme is implemented. The proposed design is analyzed through large signal analysis; it is established that the system is locally-input to state stable (I-ISS). Also, asymptotic stability and convergence to the equilibrium, is proven, in the domain of attraction as it is determined by the limiting current controller operation. Finally, the studied system is tested under various simulation scenarios where in conventional situations large current values are normally expected. In our case, the current-limiting property is fully verified and a smooth response to equilibrium is observed.