Research On Stand-alone And Active Off-grid Transfer Techniques Of Two Stage Power Conversion System

With the rise of distributed generation systems and smart grid, energy storagetechnology is becoming the important tool for power system to have security and qualityrunning. Power conversion system is an important component of the energy storage system.The two-stage PCS, consists of DC/DC converter and DC/AC converter, can flexiblyconfigure battery pack and accurately and reliably manage battery charge and discharge,widely used in large-scale power battery energy storage system. Based on two-stage PCS,this paper presents a detailed analysis and research on stand-alone mode control strategy,controller design and active off-grid smooth switching control strategy.Based on the principle of PCS in stand-alone mode, concluded a coordinated controlstrategy for DC/DC converter and DC/AC converter. Based on this, derive the averageDC/DC converter mathematical model by means of switch network model, choose theoutput voltage instantaneous value as outer loop and average inductor current as inner loop,and design controller parameters by use of Bode diagram, and simulation results verifieddesigned parameters. And combined with grid-collected mode, proposed a dual closed-loopwhich consisting of outer output voltage closed-loop and inner filter capacitor currentclosed-loop for DC/AC converter under αβ coordinate. In order to achieve littlesteady-state error, choses proportional resonant controller, on the basis of discussingparameters affect the characteristics of this system, proposed a decoupling-simplifiedanalytic design method. Finally, simulation results verify the effectiveness of controlstrategy and designed controller.Introduced dual-mode control strategy for two-stage PCS, and simulation verify thereliability of the cascade system in stand-alone mode. In order to ensure reliable operationof the local critical load, introduced seamless transfer technology. Combination transferrequirements and steps of off-grid, proposed an active off-grid smooth transfer controlstrategy for two-stage PCS. Analyzed transient process based on circuit model, andconcluded control mechanism of the control strategy proposed in this paper. Compared with the simulation results of natural zero-crossing transfer, concluded the superiority of thecontrol strategy proposed in this paper.Designed experimental platform for two-stage PCS, analyzed the static transfer switchand designed driving circuit. Base on this, completed related experiments: single-stageDC/DC converter with resistive load, single-stage DC/AC inverter with resistive load,two-stage PCS in stand-alone mode, two-stage PCS active off-grid smooth transferexperiment. Experimental results demonstrate the validity and accuracy of the proposedcontrol strategies in this paper.