Research On Scheme Of High Density Photovoltaic Integration In DC Way

Nowadays domestic and foreign scholars have less research on photovoltaic(PV)integration in DC way.In order to improve access capability,power quality and control flexibility,this paper takes high-density PV integration in DC way as the research objection.Using theoretical analysis and PSCAD/EMTDC,a combination of simulation and verification methods is designed to obtain a complete scheme for high-density PV integration in DC way.Besides,the advantages of DC access methods are demonstrated by comparison with AC integration methods,and combined with the actual DC power plant in Baoding,the PV integration scheme is designed.First,a complete scheme of high-density PV integration in DC way is designed,including architecture design,voltage level determination,operation mode selection,equipment selection and control strategy.The PV power generation generates electric energy,confluence,boost,access branch and other links adopt DC mode.On the basis of specifying the transmission capacity range and transmission distance of each voltage level,considering the matching degree between AC and DC voltage and the manufacturing capacity and level of the relevant equipment of the converter station,the medium voltage DC voltage level is determined.The main circuit of the PV integration in DC way is selected as a unipolar symmetrical or bipolar symmetrical connection according to the requirements of reliability and economy.With high modularity,easy expansion,and less harmonic content,the modular multi-level converter(MMC)is used as the grid-connected converter.At the same time,the PV plant adopts block power generation and centralized grid-connecting scheme,and the series mode is adopted between the PV subsystems to reduce the application of power electronic devices to achieve the purpose of outputting a higher DC voltage.The grid-connected converter control system adopts a layered control structure,and adopts incremental conductance method for DC/DC to MPPT control.Secondly,in view of the limitation of the actual grid's PV capacity,combined with the large number and denseness of PV power plants in Baoding,two schemes of PV integration in DC way are proposed,and the optimal scheme is determined through economic and technical comparison.Further theoretical calculation and simulation verification prove that the optimal scheme meets the requirements of power flow,power quality,short-circuit current and stability.There is no need to add specific equipment and to specify a specific mode of operation.Finally,comparing the PV integration in AC and DC ways from power supply capability,economy,control flexibility and power quality,it shows that the PV integration in DC way has the following advantages: access more powerful,ensuring power quality meets requirements without filtering,flexible control,easy access to other energy sources,saving line corridors,reducing line loss and improving transmission efficiency,reference value for wide application of DC grid-connected.