| With the increase of distributed generation system, especially that based on renewable energy,affected by temperature, light intensity, wind and other natural factors, it may not cause voltageimpact, voltage flicker or harmonic pollution, but may affect the system power factor and stability,the power quality, and may even lead to a large area power grid paralysis. Multilevel inverter asconnection point between distributed power plant and grid,once a effective control stagergy isproposed, can improve the power quality, reduce the harmonic, increase the output capacity, In acertainextent,it can solvetheeffectof distributedgeneration onpowernetwork.The study aims to improve the power factor and reduce the current harmonics, multilevelinvertercontrol technologies of connected-gridsystem are studied.Firstly, introducing the application of multilevel inverter in PV,wind power and other fields.The domestic and foreign research background and connected-grid power factor correct strategy onthemultilevel inverterwereanalysed.Secondly, introducing the work principle of various topologies multilevel inverter,especiallythe diode clamped three-level inverter.The mathematical model of the inverter is established byanalyzing the relationship between the output voltage and the switching function and the positiveand negative DC bus and neutral point voltage.A multilevel inverter power factor dual-correctioncontrol strategy is proposed through the analysis and summary of previous studies.Thedual-correction control strategy,the multilevel inverter power factor self-correction and grid voltageit-corrected, reduce the harmonic and improve power factor, so that the inverter has a high qualitygrid current and meet the grid requirements.A simple principle, good real-time performance andeasy realizationof voltageand current phasedifferencedetection method wasdesigned.Then, the designed control strategy and the models validated by the Matlab/Simulink; Thevoltage and current phase difference detection method and multilevel inverter power factordual-correction strategy was modelled and Simulated. Finally, the three-phase three-level2KVAprototype and the connected-grid control system experimental platform built to demonstrate correctandeffective of theproposed dual powerfactorcorrection strategy. |
