Since the PV characteristic of photovoltaic modules is a single-peak nonlinear curve, the maximum power point of the photovoltaic array composed of its series and parallel connections will also be affected by the light intensity and the surface temperature of the photovoltaic array. Therefore, how to track the maximum power point of the photovoltaic array is important for improving The overall efficiency of the system is extremely important.
⑴ The essence of maximum power point tracking
Under different light intensity and temperature, the photovoltaic array has a corresponding maximum power point. The external environmental factors are usually unable to be changed artificially. Temperature and light intensity always change throughout the day, so the output characteristics of the photovoltaic array also change accordingly. If the photovoltaic array can always output the maximum power, it must be controlled in time.
Figure 1 is the relationship curve between the various electrical parameters and the load when the photovoltaic array is directly connected to the load resistance R. It can be seen from the figure that the output power P, current I and voltage V and load resistance R of the photovoltaic array are not linear relationships. If the size of R is gradually increased from a smaller resistance value, I will gradually decrease, V will gradually increase, and the output power P will gradually increase. When R increases to match the load, that is, R=Rm, the output power P reaches the maximum value. If you continue to increase the size of R, although the trend of I and V has not changed, the output power P will gradually decrease. In general, the output power P of the photovoltaic array presents a single peak shape. At the peak point, the external load resistance and the internal resistance of the photovoltaic cell are equal, which is the characteristic of the photovoltaic array when it outputs the maximum power. Therefore, the essence of PV array maximum power point tracking is to add an impedance converter between the PV array and the load, and use related algorithms to control the impedance converter in real time, so that the transformed equivalent load impedance is always equal to the internal resistance of the photovoltaic array. Ensure that the photovoltaic array is always working at the maximum power point.
⑵ Maximum power point tracking method
There are many methods to achieve the maximum power point tracking of photovoltaic arrays, which can be roughly divided into two categories: one is the indirect maximum power point tracking method, which is called the quasi maximum power point tracking method; the other is the direct maximum power point tracking method. The tracking method is called the true maximum power point tracking method. This chapter uses the general circuit diagram in Figure 2 as an example to explain and analyze two types of methods. Among them, Vo And Io It is the output voltage and current of the DC/DC converter.
① Quasi-maximum power point tracking method
Ⅰ. Curve fitting method
Ⅱ. Look-up table method
Ⅲ. Constant voltage method
Ⅳ. Open circuit voltage proportional coefficient method
Ⅴ. Short-circuit current proportional coefficient method
Ⅵ. Finite period current perturbation method
②True maximum power point tracking method
Ⅰ. Disturbance observation method
Ⅱ. Incremental conductance method
Ⅲ. Intermittent variable step size search method
Ⅳ. Power step method
Ⅴ. Fuzzy control method
Ⅵ. Neural network control method
⑶MPPT method comparison
Table compares the MPPT methods in terms of detecting physical quantities, tracking speed, tracking power fluctuations in steady state, adaptability to environmental changes, and difficulty in execution. Throughout the various methods, each has its own advantages and disadvantages. Therefore, only the organic combination of several methods, complementing the short and long, can obtain the best maximum power point tracking effect.
|name||name||Detect physical quantity||Tracking speed||Tracking the degree of power fluctuation at steady state||Adaptability to environmental changes||Difficulty of execution|
|Quasi-maximum power point tracking method||Curve fitting||Voltage||quick||Small||Poor||easy|
|Quasi-maximum power point tracking method||Look-up table method||Voltage and current||quick||Small||Poor||easy|
|Quasi-maximum power point tracking method||Constant voltage (CVT) method||Voltage||quick||Small||Difference||easy|
|Quasi-maximum power point tracking method||Open circuit voltage proportional coefficient method||Voltage||Faster||Small||Poor||Easier|
|Quasi-maximum power point tracking method||Short-circuit current proportional coefficient method||Current||Faster||Small||Poor||Easier|
|Quasi-maximum power point tracking method||Finite period current perturbation method||Current||slow||Big||better||easy|
|True maximum power point tracking method||Perturbation and Observation (P&O)||Voltage and current||Faster||Bigger||better||easy|
|True maximum power point tracking method||Incremental conductance method (IncCond)||Voltage and current||Faster||Smaller||better||Easier|
|True maximum power point tracking method||Intermittent variable step size search method||Voltage and current||Faster||Smaller||better||Easier|
|True maximum power point tracking method||Power Step Method||Voltage||slow||Smaller||it is good||Easier|
|True maximum power point tracking method||Fuzzy control method||Voltage and current||Faster||Smaller||it is good||Harder|
|True maximum power point tracking method||Neural network control||Voltage and current||Faster||Smaller||it is good||difficult|