Study On The Analysis Of Mismatching And Optimal Control Of PV System Under Shading Conditions

The development and utilization of solar energy and other renewable energy sources are the fundamental way to solve the problem of environmental pollution and energy supply, and to realize the sustainable development of society. Distributed photovoltaic (PV) power generation technology, as an effective way to develop and utilize solar energy, has achieved rapid development in recent years. However, distributed PV power generation systems is extremely vulnerable to partial shading, dust covering, PV cells aging and other conditions, which will result in the inconsistency of the output characteristics of PV array called mismatching problem. Mismatching problems will not only seriously affect the efficiency of distributed PV power generation systems, while the hot-spot effect caused by mismatching will seriously degrade the reliability and safety of PV system.Research concerning on the analysis and optimal control of mismatching problem in distributed PV system has become a hot topic in PV field.At present, surrounding the analysis of mismatching problem and the optimal control of distributed PV system, there are some literatures studied on several specific contents, such as the mismatching characteristics analysis and the optimal design of PV array, and the global maximum power point tracking (MPPT) technique, and the module level distributed optimal control method which is called distributed MPPT technique or balancing and matching technique. However, in terms of the present research situation, there is still a lot of research work needing to be further studied and improved, and literatures which systematic and comprehensive study the analysis of mismatching and the optimal control of PV systerm is still relatively rare. Therefore, this paper, entitled "Study on the Analysis of Mismatching and Optimal Control of PV System Under Shading Conditions", will systematically study the mismatching problem in solar power generation system. The main research work and results are summarized as follows:(1) In order to improve the accuracy of the system analysis, considering the difficulty in explicitly solving parameters (Rs, A,I0 of the PV cells physical model, a explicit method for solving parameters (RS,A,I0) is established by analyzing the correlation of the output characteristic parameters and the physical model parameters in this paper. In view of the significant modeling error in the existing engineering model and the problem of the derivative dP/dU≠0 at the maximum power point, an improved engineering model is put forward by improving the calculating and selecting methods of parameters in engineering modeling process. In addition, an improved PV array small signal model is established which can reflect the voltage controlled characteristics of the internal resistance of PV array.(2) Analysis of mismatching features of PV array is the prerequisite and basis for optimizing the control method of PV systerm. In this paper, a piecewise mathematical model for analyzing the multi-peak phenomenon is established, considering the PV cells string (submodule) as the basic structure, based on the simulation and test results, the judgment rules for judging the most right extreme value point is the global maximum power point is concluded. Based on the analysis of power loss in PV modules under mismatching condition, this paper established the mathematical model of the hot-spot effect based on the temperature rise model of PV cell, the model parameter is identified with tests of the hot-spot effect. In order to improve the output power of PV module under mismatching condition, the paper studied the optimal configuration method of bypass diodes, and proposed a stacked configuration mode of bypass diodes.(3) In order to improve the efficiency of MPPT algorithm under the multi-peaks condition, based on the comprehensive analysis of the strengths and weaknesses of the existing global MPPT method, a new global MPPT method based on the voltage and power double closed-loop control is proposed, which utilizes the conditional stability characteristics of the power closed-loop control on a multi-peaks curve. After that, the effectiveness of the proposed method is verified by simulation. The main improvement and innovation of the proposed MPPT method is that an improved global scanning strategy is formed by introducing the power closed-loop control into the global scanning process of global MPPT method. In addition, a dynamic environment judgment rule is presented based on the voltage and power cumulative change amount in steady-state, simultaneously, a restarting method based on power closed-loop control is proposed, and some improving measures are put forward in order to overcome the defect of the misjuding and working point perturbing in P&O method.(4) In order to furtherly improve the efficiency of PV system under mismatching condition, the distributed optimal control technology of PV system is studied in this paper. Firstly, the distributed MPPT technology based on full power processing(FPP) and the power balancing optimization technology based on differential power processing(DPP) is comparatively analyzed. Considering the equivalent processing power under mismatching condition as index, the advantages and disadvantages of three different topologies achieving DPP is comparatively analyzed. Secondly, for the realization of the DPP optimization method and the stability analysis of the cascaded working mode of the system, the paper put forward a implementation scheme of the balancing optimizer based on the anti-parallel structure with forward circuit considering the "Cutting Peak and Filling Valley (CPFV)" balancing optimization topology, at the same time the paper analyzed the stability of the cascaded working mode in CPFV balancing optimization system based on the "Middlebrook impedance ratio criterion". Finally, the simulation method was used to verify and quantitatively analyze the power promoting efficiency of the CPFV balancing optimization method under mismatching condition.(5) In this paper, the optimal control method of the mismatching problem is realized and tested. Firstly, the optimal design of the main circuit and the system controller is discussed in combination with the particularity of the realizition process of the global MPPT method. Then, in order to test the feasibility and validity of the global MPPT method in different mismatch cases, some comparative tests of global MPPT method are carried out on a rooftop PV power gerneration systerm and a 1500W testing platform in lab. In order to verify the power promoting efficiency of the CPFV balancing optimization method, the paper gives the specific design and implementation of the balancing optimizer and establish a 500W test platform in the laboratory. Finally, some tests and quantitative analysis are carried out for verifing the power promoting efficiency of the CPFV balancing optimization method with different mismatch conditions.