Balance Control of SOC for MMC-BESS With Power Fluctuation Suppression, PCC Voltage Regulation, and Harmonic Mitigation in Grid-Connected Wind Farm

With the rapid increase in the penetration rate of renewable energy, the power quality of grid-connected wind farm is deteriorated seriously with power fluctuation and point of common coupling (PCC) voltage instability. The conventional solution is to use the energy storage device and the reactive power compensation apparatus to adjust separately, which complicates the system structure. Besides, an active power filter is needed additionally when harmonic pollution caused by local nonlinear load is serious. To solve these problems, this paper investigates the modular multilevel converter-based battery energy storage system (MMC-BESS), which can realize fluctuating power suppression, PCC voltage regulation, and harmonic mitigation simultaneously in grid-connected wind farm. Firstly, the topology and energy flow of MMC-BESS have been studied. Then, three-layer balance control of the state of charge (SOC) is proposed, which is essential to maintaining the normal operation of the device. Finally, the overall control of MMC-BESS applied in the grid-connected wind farm is given. Plenty of experimental results are obtained from a downscaled MMC-BESS prototype, which validate the effectiveness of the proposed control strategy.