Research On The Control Method Of High-Efficiency Energy Conversion In PV System

With the exhaustion of the traditional resources and environmental pollution and climate change,the renewable energy resources,such as solar energy,wind energy and so on,have been studied and applied more and more widely.It is great of significance to improve the energy structure and construct a pollution-free energy system by improving the energy conversion efficiency of photovoltaic power generation.Energy efficient conversion always is a hot topic in the field of photovoltaic power generation.This thesis focuses on the research of the theory and technology related to the efficient conversion of photovoltaic power generation.?1?Firstly,a study on the theoretical basis of the photovoltaic system's maximum power output is carried out.Based on the solar cell model and the structures of the photovoltaic?PV?systems,the theoretical analysis on the output characteristics of the solar cell is conducted.According to the maximum power output condition of the solar cell,the expression of the maximum power load R_mLL of the solar cell is obtained;the parameter equations of the maximum power point voltage V_m and current I_m on the R_mLL is derived;and the effects of solar irradiance and temperature on the R_mL is analyzed.The constraint conditions on the maximum power output of PV systems with different structures are studied from the viewpoint of load and power.And then,the mathematical relationship between the output power,load and the duty cycle is established.Based on the relationship,the factors affecting the output characteristics of PV system are analyzed.?2?In order to reveal the working mechanism of Perturb&Observation?P&O?method and incremental conductance?INC?method,the steady state oscillation mode and dynamic regulation characteristics of them are studied theoretically in this paper.Under the irradiance sudden change condition,the dynamic regulation characteristics of the P&O method are analyzed according to the sudden change direction,the position of the working point and the direction of motion.The oscillation modes and the dynamic regulation characteristics of P&O method and INC method are analyzed theoretically and geometrically.Meanwhile,pointing out the difference of the criterion of steady state and dynamic regulation.The mathematical relationship of criterions between the P&O method and the INC method is established.And,contrast analyzing the dynamic characteristic of them.?3?In order to eliminate the steady state oscillation and misjudgment phenomenon existing in the traditional maximum power point tracking?MPPT?control algorithm,a novel MPPT control model is proposed which selecting the maximum power point?MPP?operation voltage or current as the control target.This paper presents a method for computing environment parameters based on homeomorphism mapping;builds the nonlinear relation between variable environmental parameters and the output voltage of the solar cell at MPP.Then,a maximum power tracking strategy based on optimal voltage control is proposed which can eliminate the oscillation and misjudgment.?4?With partial shading,the PV system usually exhibits multiple-peak output power characteristics,but single-peak is also possible under special partial shading conditions.Based on establishing the mathematical model of the solar cell series circuit,analyzing the output characteristics of series circuit under the serious shading and slight shading conditions;demonstrating the necessary and sufficient condition which caused the multi peaks.Then,this paper establishes the rule for determination of the relative position and the peak number of the maximum power points.Based on the mathematical analysis,the upper and lower bounds of dynamic range of the global maximum power point of the series circuit are determined.In order to realize the energy efficient conversion of the PV system under partial shading condition,a global maximum power point tracking control strategy based on the increment conductance control with optimal initial value is proposed.