光伏发电系统的MPPT控制策略研究

摘 要 

面对全球日趋严重的能源危机问题，可再生能源的开发和利用得到了人们的高度重 视。其中辐射到地球太阳能资源是十分富饶的，绿色清洁的太阳能不会危害我们的生存 环境，因而受到了人们的广泛利用。光伏发电作为当前利用太阳能的主要方式之一，得 到了不断的应用与发展。为了提高光伏发电系统的光电转换效率，需要系统实时的进行 最大功率点跟踪。 
在了解当前光伏发电技术背景、现状的基础上，对光伏发电系统做了简要的概述， 指出当前制约光伏发电产业发展的两个重要因素，即成本高和效率低。因此，需要寻找 一种合适的最大功率跟踪控制(MPPT)算法，不仅可以有效的提高系统的输出功率，还可 以降低单位面积的生产成本。然而全球大量燃烧化石能源带来的一系列环境问题，造成 了当前环境条件的恶劣变化，在一定程度上也影响到了光伏发电系统的高效稳定运行。 
在这种环境条件下，本文从光伏电池的工作原理出发，建立数学求解模型，分析光 照均一和局部阴影下光伏电池的输出特性，分别提出两种环境条件下的最大功率跟踪 (MPPT)控制算法，使系统能够时刻跟踪最大功率点，保证最大功率的输出。 
(1)．光照均一条件下 
为了更大程度的提高光伏发电系统的输出功率，通过分析光伏电池的P-U特性曲 线，需要对光伏电池进行数学建模。然后在增量电导法的基础上，总结出了一种优化的 MPPT控制算法——微元步长自适应增量电导法。该算法需要引入一个变步长控制因子， 使系统以微元步长自适应跟踪最大功率点。在Matlab/Simulink环境下对该算法进行了仿 真验证，表明能够自适应的选择步长，并且在外界环境变化的情况下，该算法明显加快 了跟踪的速度，有效地减弱了系统在最大功率点(MPP)周围的振荡现象，使系统具有较 高的跟踪速度和精度。 
(2)．局部阴影条件下 
为了防止局部阴影带来的热斑效应损害光伏电池，常常需要在光伏电池上并联若干 个旁路二极管。然而由于旁路二极管的作用，使系统输出的P-U特性曲线呈现出多个峰 值点，传统的单波峰MPPT算法容易受到局部峰值的影响，无法跟踪在最大功率点附近。 通过分析局部阴影下光伏电池的数学模型，提出了聚拢峰值扫描判别法。该算法通过控制系统分别从功率曲线的两端向中间依次扫描临近极值点的功率值，通过比较分析，当 两个极值点的功率相等时，这个点就是系统的最大功率点。经过仿真测试，该算法能够 有效的抵抗局部阴影效应，避开局部峰值点的干扰，并快速跟踪真正的最大功率点，明 显提高了输出功率。 
 
关键词：光伏发电，光照均一，局部阴影，MPPT 

ABSTRACT 

Facing the problem of increasingly serious global energy crisis，The solar energy resources is very rich  on earth，and green clean solar energy will not harm our environment，so it has been widely used by human.  Photovoltaic power generation is one of the main ways of using solar energy, and it has been continuously  applied and development. For the sake of improve the conversion efficiency of PV system, the system  needs to make a maximum power point tracking. 
Based on know the current background and status of photovoltaic technology, and make the  photovoltaic power generation system a brief overview. It points out two important factors that restrict the  development of PV industry, the high cost and low conversion efficiency. Therefore, we need to find a  suitable MPPT algorithm, which can improve the system's output power effectively，and reduce the cost of  production per unit area. However, the burning large amounts of fossil energy brings a series of  environmental problems to the global word, causing  the environment changes worse, and also affected  the photovoltaic power generation system's effective and stable operation in certain extent. 
In this situation, the paper from the working principle of photovoltaic cell, establishing the  mathematical model. Analysising the output characteristics of PV cells under illumination uniformity and  partial shade, putting forward two kinds of MPPT control algorithms, making the system  track the  maximum power point , and output the maximum power.  1. Under illumination uniformity conditions 
For improve the maximum output power of the photovoltaic power generation system, through  analysis the P-U curve of photovoltaic cells, and the mathematical modeling of PV array. On the basis of  the incremental conductance method, propose a new maximum power point tracking control algorithm， infinitesimal step size adaptive incremental conductance method. Which needs to introduce a variable step  size control factor , make the system track the maximum power point with infinitesimal step size adaptively.  By Matlab/Simulink environment simulation, when external environment changes, the new algorithm  reduces the time of maximum power tracking, and effectively suppress the oscillation of the system in the  maximum power point (MPP), make the system has high tracking speed and accuracy. 

2. Under partial shade conditions 
In order to prevent the hot spot effect which is caused by the local shadow to damage the photovoltaic  cell , the photovoltaic cells  need  some bypass diodes in parallel. However, due to the addition of bypass  diodes, the P-U characteristic curve of the system output shows multiple peak, traditional single peak  MPPT algorithm can fall into local peak so that it can not track the real maximum power point. By  analyzing the mathematical model of photovoltaic cells under partial shade, propose the gather peak  scanning discrimination method. The algorithm can control the system from both ends to the middle on  power curve to sequentially scanning every power value, through comparison and analysis, if the two  extreme point’s power is equal, then this point is the maximum power point. After the simulation test, the  algorithm can effectively resist the effects of partial shade, avoid the interference of local peak point, and  track the real maximum power point, improves the syetem's output power obviously. 
 
KEY WORDS：photovoltaic power generation，illumination uniformity，the local shadow，MPPT 

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