Research On Three-phase Inverters In Photovoltaic Desalination Systems

As the water shortage and environmental pollution is becoming more and more serious,it is a good solution to combine the technology of the new energy power generation with that of the seawater desalination.Photovoltaic(PV)power generation has some advantages including abundant resources and cleanest power generation process,so it is developing rapidly due to its characteristics such as high efficiency and no pollution.The reverse osmosis(RO)desalination technology is widely used in ships and coastal areas due to its characteristics such as low cost,easy auto-control.The inverter can convert the DC power generated by PV cells into AC power,which could be used as the power of the RO seawater desalination installations.Currently,it becomes the future development trend to combine the technology of PV power generation with that of RO seawater desalination.In the PV seawater desalination system,as a key device linking the photovoltaic array to the desalination installations,the three-phase inverter has become the focus of current research.In this paper,both the off-grid inverter for small marine desalination installations and the grid-connected inverter for land desalination installations are researched respectively.In the off-grid PV inverter system,its topological structure and control strategy are analyzed in detail as well as PI control,coordinate transformation and SVPWM modulation.In the grid-connected PV inverter system,its topological structure,mathematical model and control strategy are introduced,and the current inner loop and voltage outer loop are also analyzed as well as the phase-locked loop(PLL)control method.The locked loop control adopts the single synchronous reference frame software phase locked loop(SSRF-SPLL).The three-phase PV inverter is designed in two aspects of the hardware and software.The hardware design includes the calculation and selection of the components in the power circuit and the control circuit.Components in the power circuit mainly contain capacitors at the DC side,IGBT,output filter LCL.The control circuit mainly includes the driving circuit,sampling circuit,protection circuit and the master control circuit with DSP28335.In order to eliminate the influence of LCL resonance peak,the method of the series passive damping in the capacitive branch is adopted in the design of the output filter LCL.The AD7606 chip is used in the sampling circuit for improving the sampling accuracy.The software design includes the main program,interrupt service routines,data acquisition procedures and so on.The flow charts of main program and interrupt service routines are also given.To verify the feasibility of the proposed theoretical analysis and system design,two aspects are considered.First,the simulation platform in Matlab/Simulink for the three-phase invertersystem is set up,and the corresponding simulation results are given.Second,the experimental platform is built,and the corresponding experimental results are given.