| As an important renewable energy source,solar energy is widely exploited and utilized,among which photovoltaic grid-connected power generation technology is an significant means of solar energy exploitation.With China’s strong support for domestic photovoltaic industry and gradual growing power of photovoltaic power stations,it is of great necessity to study high-power photovoltaic inverter.In grid-connected photovoltaic system,the maximum power point tracking part of photovoltaic cells and the grid-connected inverter are mainly studied core technology.In this paper,the input-output characteristics of photovoltaic cells,the maximum power point tracking technology,the grid-connected inverter control strategy,the phase-locked loop technology and island detection technology are deeply studied.Firstly,the research background and significance of photovoltaic power generation technology are clarified,and the appropriate structure of grid-connected photovoltaic generation system comes to a conclusion after analysis.A mathematical model of the photovoltaic cells is structured and then analyzed in simulation.To process the typical jamming from large current ripple in the maximum power point tracking circuit,the staggered parallel buck circuit is adopted,the results show that this method has good ripple cancellation effect.This paper illustrates three commonly used maximum power point tracking algorithms,the incremental conductance method is selected as the maximum power point tracking algorithm and verified by simulation.Secondly,the coordinate transformation of Clark and Park are derived,and the mathematical model of three-phase inverter in abc,αβ and dq coordinate systems is established under the condition of ideal grid voltage,voltage outer loop and current inner loop of double-loop control strategy is decided to be adopted in the dq coordinate system.Voltage loop and current loop are designed to work according PI-controlling algorithm,in the meantime,the parameters of proportioning element and intergrating element are calculated.In addition,the principle and implementation of space vector pulse width modulation are introduced in detail.A 30 KW three-phase grid-connected inverter is built and verified by simulation.Thirdly,this paper analyzes the basic working principle of phase-locked loop,and a phase-locked loop model of three-phase grid-connected inverter is established.The advantages and disadvantages of single synchronous reference frame phase-locked loop and single synchronous reference frame phase-locked loop based on symmetric component method and dual second-order generalized integrator phase-locked loop are compared and analyzed.Basing on the conclusion of previous work,the phase-lock loop using dual second-order generalized integrator phase-locked loop is considered as an optimal option of grid-connected phase locking.After that we also verified it in simulation.Finally,a pattern of the hardware section and software section of the three-phase grid-connected inverter is designed.In this pattern,the hardware section includes the design of staggered parallel buck circuit and inverter,where the photovoltaic arrays sample circuit,the switching tube,the inductor and the freewheeling diode are designed as components of the staggered parallel buck circuit.Also,the DC-link capacitor,the switching tube,the LC Filter,the DC sampling circuit and the relative protection circuit,AC sampling circuit and the relative protection circuit are designed as the components of the interver.The other part,software section includes the design of the initialization of the system and the configuration of related registers,and the program of system MPPT,SVPWM,PLL program,over voltage protection and under voltage protection,over frequency protection and under frequency protection are designed.The experiment of three-phase grid-connected photovoltaic inverter on a sample machine with 30 KW is logged,which turns out a result that exactly corresponds to expectation of the conclusion in this paper. |
PDF Full Text Request