Advanced control of a high power converter connected to a weak grid

This paper addresses the stability problems of a high power converter connected to a weak grid. The wide range values that grid impedance can take, challenges the stability and the performance of the controllers, which are responsible of regulating the current injection in such converters. In this work, a control strategy based on stationary reference frame controllers is selected and implemented using a proportional resonant (PR) controller, with capacitor voltage feedforward and a phase shifter. As it will be demonstrated in this paper, although the feedforward contributes to enhance the transient response of the converter, it may cause also deep unstable dynamics near to the medium frequency and decreases the phase margin in low frequency ranges. Therefore, it can be used to damp the unstable dynamics near to resonance frequency range and the LCL-filter can be adopted for the high frequency one. In order to improve the controller performance, a new phase shifter is added to the control scheme to enhance the phase margin at low frequency ranges. Simulation and experimental results considering weak grid conditions are shown to validate the proposed method.