Vanadium Flow Battery Grid-connected Energy Storage Converter And Research On Its Control System

As the energy shortage of the world, solar, wind and other distributed generationtechnologies are being used more widely. However, distributed generation technologies havethe shortcomings such as intermittent and volatility. This will pose an adverse effect on thestability and quality of the power system. Battery energy storage technologies can improvethe power output characteristics of the distributed power, thus effectively solving theproblems brought by distributed generation. As a key part of the energy storage system,grid-connected energy storage converter (ESC), the control platform of it is directly related tothe reliability, accuracy and response speed of the energy storage system.This paper takes vanadium redox flow battery (VRB) as the energy storage devicebecause of its advantages such as a long life and analysis the working mechanism of thevanadium redox flow battery and system equivalent loss model, and builds a simulationmodel based on PSCAD/EMTDC. According to the simulation model, voltage characteristics,charge and discharge characteristics and energy efficiency of vanadium redox flow battery isanalyzed.According to the actual needs, three-stage topology is applied to the ESC. The first stageis an industrial frequency transformer, the second stage is a PWM rectifier with LC filter, andthe third stage is a single-phase half bridge BUCK/BOOST circuit. A mathematical model ofthe ESC is built based on the topology. A quasi direct power control method based on quasiP+resonant (quasi-PR) control for PWM rectifier in stationary frame is presented in thispaper. In the proposed method, zero steady-state error of output current and fast-track ofpower is obtained without the need of rotating transformation. With the quasi-PR control,harmonic currents can be effectively suppressed in a simple way, which would reduce thevolume of the filter inductor and cost savings. In order to avoid the effects of fundamentalcurrent flowing through the filter capacitor on the grid current, a capacitive currentcompensation link is added, which improves the power factor. Double-loop control method isused to off-grid control. A voltage loop based on quasi-PR controller and current loop basedon load current feed-forward control and inductor current feedback control is proposed, whichobtains zero steady-state error of output voltage, limit of inductor current and the ability ofanti load current disturbance. According to the above control method, a simulation model isestablished to analysis the entire energy storage system.By designing the main circuit of ESC and the peripheral circuits of core control broadand programming the control program, an experimental prototype of ESC is built based on the platform of DSPF2812. In order to test the performance of the experimental prototype, thegrid-connected and off-grid experiments are completed. Finally, a human machine interface isdesigned to monitor and automatic control the ESC.