Grid voltage control with distributed generation using online grid impedance estimation

Distributed generation (DG) units usually do not participate in the grid voltage control and inject power regardless of the grid voltage. Only when the grid voltage level surpasses a certain threshold, these units shut down to avoid any damage. In this paper, a method is presented to control the grid voltage with both active and reactive power with a DG unit. The effect of active and reactive power exchange on the grid voltage depends on the grid impedance ratio R/XR/X of the whole electrical power network the converter is connected to. Therefore, the presented grid voltage control method uses a grid impedance estimation method. In an online manner, a current harmonic is injected into the network and the harmonic response is analysed. This way, the DG unit provides improved grid voltage control, automatically adjusting to changes in the grid voltage and impedance. The end result is that the DG unit can still generate power under sub-optimal grid voltage conditions and more renewable energy is harvested. The presented method is generally applicable to DG units of any rated power. The effectiveness of the method is demonstrated by means of both simulations and experiments, showing the positive effect on the grid voltage.