Study On Parallel Mismatch And Power Optimization Of Photovoltaic System

The output characteristics of photovoltaic?PV?installations are slightly different due to manufacturing differences and variations or differences in the operating conditions of PV modules.Therefore,in an actual array,the operation of each module or array will deviate from its maximum power and lead to changes in output voltage and current.Different output voltages of these modules connected in parallel can cause voltage mismatch and system power loss.That is,the parallel mismatch can result in power drop and loss of the PV system.In the development of PV generation technology,the parallel mismatch is the primary cause for the power loss of the entire PV System.So solving this problem in PV plants is of great research significance and practical value for the planning,investment and operation&maintenance of the entire PV system.By analyzing the equivalent circuits of solar cells,this paper provided an accurate,reliable and easy-to-use mathematical model for silicon solar cells.Under the standard test conditions?STC?,five parameters were extracted using the output characteristic formula of single-diode PV cells to simulate the output characteristics of PV array under complex conditions.In the model,manufacturers were required to provide data only under STC to determine five performance parameters(I_ph,Is,n,R_s and R_sh)iteratively,and the effects of R_s,R_sh and n on the output characteristic curve were taken into consideration.The parameters under all working conditions were corrected,and an electrical model for researching the working status of PV modules under different conditions was established.Then an simulation model of PV modules and arrays was established through Matlab/Simulink simulation to analyze the output characteristics of modules under the conditions of local shadings of a single cell,multiple cells and with uniform distribution,showing that different shading degrees had different effects on PV parallel mismatch;it focused on analyzing the results of parallel voltage mismatch in PV parallel array under the conditions of short-circuit fault and local shading through simulation.A solution based on DC/DC booster circuits as well as its optimization measures was proposed for overall power drop caused by parallel voltage mismatch in PV System array.A system optimizer was also constructed as required by the optimization measures,and its software and hardware were optimized.The optimizer improved and eliminated the power loss caused by parallel mismatch by boosting the mismatched branch after detected the voltage of parallel branch,and judged and determined the voltage of mismatched branch.Finally,the mismatch characteristics of parallel circuits in PV array was tested and analyzed,showing that the loss rates of parallel mismatch caused by shading and short-circuit fault were about 2%-15%and over 20%,respectively,and that dust deposition and attenuation had few effects on parallel mismatch in the array.The comparative analysis on the parallel mismatch tests of PV modules and arrays with and without optimizers showed that,the generation could be increased by 5%and 5.76%if optimizers were adopted for the parallel mismatch caused by shading in the module,and by the short-circuit fault in the array,respectively.Due to the response speed and boosting loss of the optimizer,no satisfactory optimization effect of the system optimizer proposed and constructed in this paper was achieved under the low irradiation,low power and fast irradiation change rate.Further optimization design is therefore needed.