Study On MPPT Constraint Conditions And Control Method Of Isolated PV System

With the continuous progress of society and the rapid development of economy,energy crisis and environmental crisis are becoming more and more serious.Photovoltaic(PV)power generation has attracted wide attention due to its advantages of low cost,wide distribution and no geographical restrictions.PV cells always have a maximum power point(MPP),but because of mismatches between PV system circuit parameters,weather conditions,and system structure,MPP may not be successfully tracked.In addition,the introduction of isolation transformer in the basic PV system can make the duty ratio of the converter moderate and achieve electrical isolation.However,at present,there is no research on MPPT constraint conditions for isolated PV system,which seriously hinders the application of isolated PV system and the development of related linear control theory.Therefore,this dissertation firstly compares 14 commonly used PV cell models,and focuses on the analysis of PV cell engineering model and MPP linear model.Based on these two PV cell models,the isolated PV system model is established and its key parameters are solved.Then,the isolation PV system is studied deeply,and the constraint conditions for their successful implementation of MPPT are found.These MPPT constraint conditions detail direct mathematical relationships between PV cell parameters,weather conditions,and circuit parameters.Finally,based on the MPP linear model and MPPT constraint conditions,two new MPPT control methods are designed for isolated PV system.The research content of this dissertation is as follows:(1)Fourteen modeling methods of PV cells are compared and analyzed,which can be used by PV system researchers and users to select the most suitable model for research and calculation according to the accuracy requirements of the model,computational complexity and environmental conditions before selecting the PV cell model.(2)The mathematical models and Simulink models of twenty isolated PV systems are established.A series of key parameters based on the PV cell engineering model and the linear MPP model are solved by linear fitting method,and the mathematical relationship between the output power of the PV system and the weather conditions is found.(3)Based on the PV cell engineering model and the linear MPP model,the MPPT constraint conditions of the isolated PV system are found.The relationship between MPPT constraint conditions and weather conditions,circuit parameters and system structure is shown.(4)The practicability of MPPT control algorithm is enhanced.In the design and improvement of MPPT algorithm,the MPPT constraint conditions should be fully considered to avoid the failure of MPPT.Therefore,two MPPT methods for different PV system structures are proposed to improve the stability,applicability and rapidity of MPPT control.In addition,simulation experiments are carried out for each research site,and the experimental results verify the correctness of the mathematical model of the isolated PV system,the correctness of the MPPT constraint conditions based on the engineering model and the linear MPP model,and the practicability of the two MPPT methods.Considering the MPPT constraints proposed in this paper,the appropriate load and transformer ratio range can be estimated through the measured data of irradiance and temperature in a certain area,and the range of MPP in PV systems with different structures can be estimated,which has a good guiding significance for the circuit design,theoretical derivation and product selection of PV systems.At the same time,the comparison experiment confirms the rapidity and accuracy of the proposed two MPPT methods,and has the advantages of simple program,small computation and low hardware cost.