Abstract : In view of the unsatisfactory tracking speed and accuracy of the PV power system's maximum power point tracking (MPPT) algorithm, a novel variable-step-length hysteresis loop comparison method was proposed. The contradiction between speed and accuracy is optimized. System modeling and simulation under Matlab/Similink and experimental analysis. The results show that this method can significantly improve the speed and accuracy of MPPT tracking.
Key words: maximum power point tracking speed accuracy variable step size
Abstract:For the character is ticsofunsati factory of solar photovoltaic power generator system for the past, the maximum powerpoint tracking(MaximunPowerPointTracking,MPPT) algorithmto track the speed an daccuracy.It propose danovel variabl estephyster esismethod tooptimize the contradiction soft hespeedand precision of the areas traditional methods.Make modeling and simulation, and experimental analysisin Matlab/Similinksystem. There sults show that the methodcan significantly improve the speed andprecision of MPPT tracking.
Keyword: MPPTspeedaccuracyvariablestep
1 Introduction
With the development of society, energy and the environment have become a critical issue for people. Solar energy has a good application prospect as a widely distributed clean renewable energy source [1]. Among them, solar photovoltaic power generation technology has been researched and applied in a large amount. In photovoltaic power generation systems, the utilization of photovoltaic cells is in addition to the interior of photovoltaic cells. In addition to characteristics, it is also influenced by factors such as irradiance, load, and temperature. In different external environments, photovoltaic cells can operate at different and unique maximum power points (Maximun PowerPoint, MPP). Therefore, for photovoltaic power generation systems, the optimal working state of photovoltaic cells should be sought to maximize the conversion of light energy into electrical energy [2].
The commonly used methods for implementing MPPT include admittance increment method and perturbation observation method. The admittance increment method uses the PV cell to track the MPP with zero admittance to the voltage at the MPP. This method has good control effect and high control stability, but the control algorithm is complex and requires high precision in sampling. [3]. Disturbance observation method is based on the change trend of the output power of the photovoltaic cell to disturb the output voltage of the photovoltaic cell so that the photovoltaic cell eventually operates at the maximum power point. However, the existence of oscillations and misjudgment problems make the system unable to accurately track the maximum power point, resulting in energy loss [2].
In summary, the variable-step-length hysteresis loop comparison method based on the traditional disturbance observation method not only has good performance in overcoming the oscillation and misjudgment in the process of tracking the maximum power point, but also can take both speed and precision into consideration. The requirement to more accurately track the maximum power point of the photovoltaic cell, thereby increasing the photovoltaic system's power generation efficiency.
2 Analysis of Photovoltaic Cell Characteristics 2.1 Equivalent Circuit of Photovoltaic Cell The photovoltaic cell is actually a large-area planar diode and can be described by the single-diode equivalent circuit in Fig. 1. In the figure, RL is the external load of the photovoltaic cell, the load voltage (ie, the output voltage of the photovoltaic cell) is UL, and the load current is IL.
(1)
In the formula: ISC is the photon excitation current in the photovoltaic cell; IDO is the saturation current of the photovoltaic cell in the absence of light; q is the electronic charge, 1.6×10-19C; RS is the series resistance (by the battery's bulk resistance, surface Resistor, electrode conductor resistance, and resistance between the electrode and silicon surface); A is a constant factor (A value is 1 when the positive bias voltage is large); K is Boltzmann constant, 1.38×10-23J/K; Rsh Parallel resistance (due to the edge of the silicon chip is not clean or caused by defects in the body).
2.2 Mathematical Model and Output Characteristics of Photovoltaic Cells In general photovoltaic cells, the series resistance, Rs, is small and the parallel resistance, Rsh, is large and can be neglected when calculating the ideal circuit. The characteristics of the ideal photovoltaic cell are:
, (2)
Based on the reference conditions, Isc is its short-circuit current, Uoc is its open-circuit voltage, Im, Um is the maximum power point current and voltage, and when the voltage of the PV array is Vpv, the corresponding current is Ipv.
The output characteristics of the photovoltaic cell can be determined according to parameters such as Im, Um, Isc, Uoc provided by the photocell factory, but these parameters provided by the manufacturer are generally at the standard temperature (Tref=25°C) and the standard solar power (Sref=1000W). The test results under /m2) need to be compensated in practical applications [4], which can be obtained according to [2]:
among them:
Based on equation (3), we use Matlab to build a simulation model of photovoltaic cells as shown in Figure 2. The parameters are: current change temperature coefficient a=0.015Amps/°C, voltage change temperature coefficient b=0.700V/°C,
Isc=5.45A, Uoc=22.2V, Im=4.95A, Um=17.2V.
Figure 2 PV array Matlab simulation module internal structure The encapsulated PV module is as follows:
The output performance of the system when the simulated light intensity was changed from 1000 W/m2 to 200 W/m2 is shown in Fig. 4.
Figure 4 shows the output characteristics of the photovoltaic array under a given light curve. It can be seen from the figure that the photovoltaic device output power Ppv is nonlinearly related to the output voltage Upv and the current Ipv. Under different environments, ie temperature and sunshine intensity, the photovoltaic device has the only maximum output. Power point. When the photovoltaic device is placed in an outdoor environment, the maximum power of the photovoltaic device changes due to the external environment.
3 Variable step length hysteresis loop comparison method to achieve the maximum power point tracking technology When the working point reaches the maximum power point, for a given step perturbation method, the working point will cross the maximum power point, but after changing the direction of the disturbance, work The difference between the point voltage and the maximum power point voltage is still smaller than the step length, and the maximum power point cannot be reached. This kind of oscillation caused by the perturbation observation method is caused by the reciprocating movement of the working point on both sides of the maximum power point caused by the constant disturbance step length. phenomenon. When the external environment changes, the output power characteristic curve of the photovoltaic cell also changes, and there will be situations where the operating point sequence is located on different Ppv-Upv characteristic curves in a period of time. In this case, if the operating points on different Ppv-Upv characteristic curves continue to use the criterion for the fixed characteristic curve, the situation where the disturbance direction is opposite to the actual power variation trend will be the misjudgment phenomenon of the disturbance observation method.
3.1 Principle of variable step length hysteresis loop comparison From the control point of view, the hysteresis control strategy with nonlinear characteristics can be used to suppress the oscillation. For the Ppv-Upv characteristics of photovoltaic cells, the hysteresis control loop is shown in Figure 5.
When the power fluctuates within the set hysteresis loop, the photovoltaic cell's operating point voltage remains unchanged. Only when the power fluctuation exceeds the set hysteresis, the operating point voltage is changed according to a certain rule. It can be seen that the introduction of hysteresis loops can effectively suppress the oscillation phenomenon of the disturbance observation method. In fact, the misjudgment can be regarded as a dynamic oscillation process when the external environment changes, so this method can also overcome the misjudgment phenomenon of the disturbance observation method.
3.2 The realization of system function Although the hysteresis comparison method can largely avoid oscillation and misjudgment, if the step size is too large, the working point may stop in the area farther from the maximum power point, if the step length is too small, When the search begins in the new round, the work point will be searched in the area far away from the maximum power point for a long time. Therefore, the contradiction between speed and accuracy still exists, and the contradiction can be solved by the variable-step-length hysteresis comparison method. Figure 6 shows.
The principle of the variable-length hysteresis loop comparison method in Figure 6 is as follows:
Take three points C, A, B from left to right in the vicinity of the vertex of the PV characteristic curve of the solar cell, set the change flag m, the perturbation amount is n, and the PC, PA, and PB correspond to the power of the three points A, B, and C. When PB≥PA, m=1, PB