Distributed control method for improving renewable energy penetration rate and maintaining low-bus-voltage fluctuation in DC local grid

Abstract

In this paper, a droop-based distributed control method is proposed to enhance the penetration rate of renewable energy sources and minimize low-voltage fluctuations within the DC local grid. The proposed method designs droop gains for state of charge (SoC)-based droop control considering line impedance and other parameters in the DC local grid, thereby facilitating precise SoC equalization among different energy storage systems (ESS) considering the dc bus voltage fluctuation and power sharing of each component. Further, the proposed technique applies droop control to the grid-connected converter considering the SoC of ESS, enabling effective power distribution between the AC grid and ESS based on the magnitude of SoC. Secondary voltage compensation control is performed by integrating the structure of ramp rate control into this control method for DC bus voltage control, which helps alleviate excessive output fluctuations of renewable energy sources. The proposed method can be applied based on the SoC of ESS, thereby achieving a 56.56 % improvement in the penetration rate of renewable energy. In addition, the DC bus voltage fluctuation is controlled within 1 %, and SoC equalization is achieved when operating more than two ESS units in DC local grid system. The validity of the proposed technique is verified using PSCAD/EMTDC simulations and shows improvement on both renewable energy penetration rate and voltage fluctuation.