Control Strategy Of Hybrid Cascaded Power Conversion System Based On Battery Energy Storage For PV Power Plant

In recent years, as the growing global demand for electricity and the abatement of traditional fossil resources, new energy technology has rapid developed and applied. However, the stability and controllability of distributed energy connected to the power system has brought great challenges. Energy storage system provides new techniques and tools to solve these problems. Battery based on Hybrid Cascaded Power Conversion System (HC-PCS) is a comprehensive compensation device combining STATCOM with BESS mode, including active and reactive power regulation in four-quadrant regulations, which is the ideal solution to solve the problems of distributed energy grid-connected.This thesis is supported by the Beijing Science and Technology Commission "Research and demonstration application of large-scale photovoltaic power plants based on the unified power control equipment and DC microgrid technology (E13H00041)". Based on the analysis of the structure and characteristics of HC-PCS topology, this thesis proposed the battery self-balance control and the compound power control strategy of reactive and active power compensation, meeting the requirements of photovoltaic (PV) connected to grid. The main contents are as follows:First, there is a detailed explanation for the battery energy storage system model. Used battery energy storage system based HC-PCS architecture. The basic principle, control and power transmission characteristics of the two converter parts are analyzed. Concerned the practical engineering data, selected the main circuit topology of battery energy storage system model and the main parameters. according to the operational requirements of the battery energy storage system based on HC-PCS, designed PQ decoupling control strategy to achieve its four-quadrant operation characteristics, so that it can operate in two modes BESS and STATCOM.Second, in terms of State of Charge (SOC) imbalance existed in the cascade structure for battery between two groups, this thesis proposed two self-balance control strategies:(1) two-stage balancing control based on centre-of-gravity shifted in SOC triangle;(2) differentiated charging and discharging method based on the Selective Harmonic Elimination PWM (SHE-PWM). Analyzed the advantages and disadvantages of these two balancing control strategies comparatively, and gived the start/quit conditions and procedures of SOC self-balance control. Through theoretical analysis and PSCAD/EMTDC simulations, compared the traditional phase-shifted SPWM control and these two SOC self-balance controls in harmonic performance respectively.Finally, according to the State Grid requirements on PV power station, proposed complex control strategies of HC-PCS on active and reactive power compensation for PV station grid-connected. Briefly described the use of PV system and the structure running in parallel with battery energy storage system. The hybrid control strategy includes not only protecting the battery pack, improving the effective utilization of the battery, to avoid frequent battery charge and discharge, but also combining smoothing active power output of PV power station and meeting the Low Voltage Ride Through (LVRT) needs to determine the HC-PCS the output reference value, so as to coordinate its active and reactive power compensation. At the same time, added an additional rate control, using its fast power regulation characteristics to better compensate the irregular PV fluctuations. Model in PSCAD/EMTD of HC-PCS and the proposed hybrid control strategy to verify its validity.