Photovoltaic Array Under Partially Shaded Conditions: Configuration,MPPT Methods And Impedance Matching Converter

Photovoltaic(PV)power generation is one of the main ways of utilizing solar energy.When PV array is partially shaded,the maximum output power will be reduced,and the output power characteristic will be changed,especially,multiple peak power points will occur.In order to maximize the utilization of the output power of PV array under partially shaded conditions,the optimization of PV array structure,advanced maximum power point tracking(MPPT)methods,and suitable impedance matching converter are studied systematically in this dissertation.The output characteristics of PV cells,PV modules,PV arrays(including the parallel-connection,PV string,series-parallel-connection and full-connection)under partially shaded conditions are analyzed in this dissertation,and the output characteristic curves of the PV string under partially shaded conditions are obtained.For the PV string under partially shaded conditions,the output current-voltage(I-V)curve presents step-shaped monotone decrease features,the current at the peak power point is approximately proportional to the corresponding segmented short-circuit current,and the voltage at the peak power point is approximately proportional to the open-circuit voltage of constituent PV modules in the PV string.The output characteristics of PV arrays are tested by simulating the partially shade conditions,and the experimental results verify the correctness of the theoretical analysis.These features provide a theoretical basis for the design of new MPPT methods.Based on the output characteristic curves of the PV string under partially shaded conditions,two kinds of global MPPT methods are proposed,namely,the “search-skip-judgment” three-step global MPPT(SSJ-GMPPT)method and the rapid global MPPT(R-GMPPT)method.The SSJ-GMPPT method makes full use of step-shaped monotone decrease features to skip and the quantitative characteristic of the output power-voltage curves to go on skipping or searching.The more times of the skipping and the larger of the skiped voltage range,the more the time saving and the faster the speed are for achieving the MPPT.The R-GMPPT makes full use of the quantitative characteristics of the voltages at the peak power points,and uses calculation instead of actual search process.Thus,the search speed is extremely fast.The operation process of the two global MPPT methods is analyzed in detail with a variety of partial shadow modes.The two global MPPT methods are tested and compared with the other two existing MPPT methods.The experimental results show that the two proposed global MPPT methods are effective and have fast speed and good accuracy.Multi-input buck converter is chosen as the impedance matching converter in this dissertation.The pseudo continuous current mode(PCCM)is adopted for the multi-input buck converter,which realizes independent regulation of the input voltages,which are the output voltages of the PV strings,and the inductor current ripple is reduced compared with the discontinuous current mode(DCM).The method of reducing the current threshold of the freewheeling power switch and the method of dynamically adjusting the working time of the multiple inputs are proposed,which make all the converter operated at PCCM,and thus reduce the conduction loss and improve the efficiency of converter.A 1 kW prototype is fabricated and tested in the lab,and the experimental results verify the validity of PCCM and the feasibility of the optimization strategies.