Power Smoothing Control of Energy Storage System for DC Systems Connected to Weak Grids

Grid-connected dc systems with high penetration of renewable power generations (RPGs) are prone to dc voltage impact and stability issues, especially when connected to a weak ac grid. In order to address these problems, a novel cascaded power and dc voltage control loops based power smoothing control for energy storage system (ESS) has been proposed in this paper. With this method, transient power fluctuations can be compensated, then transmission power of the grid-connected converter (GCC) from dc system to ac grid and dc voltage can be both smoothed. After transient power supporting, the output of ESS can be recovered according to the pre-scheduled power instruction. Moreover, a detailed small signal model of a dc system based on state-space equations has been derived for dynamic and stability analysis. Finally, the effectiveness of the proposed control method is verified by PSCAD/EMPTC based simulation results in a weak grid-connected dc system with GCC, ESS and constant power load (CPL, to simulate RPG or real loads).