Voltage dip mitigation using shunt-connected voltage source converter

In this paper, a voltage source converter (VSC) connected in shunt with the grid to mitigate voltage dips for sensitive processes is presented. The VSC maintains the magnitude of the grid voltage at the connection point constant by injecting reactive power to compensate for the voltage dip. This is achieved by using a cascade controller, constituted by an inner vector current-controller (VCC) and an outer voltage controller, which calculates the current references for the VCC. The paper shows that using an inductor/capacitor/inductor (LCL)-filter instead of the simpler L-filter in between the VSC and the grid yields high performance and robust controller. Furthermore, in order to compensate for unbalanced dips, both positive- and negative-sequence components of the grid voltage must be controlled separately. This is done by using two independent controllers for the two sequences, with the same cascaded structure described above. Simulation results under balanced and unbalanced dips are presented to show the performance. Also, stability analyses are done to determine the robustness of the system against grid parameter variation.