Abstract : To address the shortcomings of previous Maximum Power Point Tracking (MPPT) algorithms for photovoltaic power generation systems, which suffer from unsatisfactory tracking speed and accuracy, a novel variable step-size hysteresis comparison method is proposed to optimize the trade-off between speed and accuracy inherent in the traditional perturbation observation method. System modeling and simulation were performed in Matlab/Similink, and experimental analysis was conducted. Results show that this proposed method can significantly improve the tracking speed and accuracy of MPPT.
Keywords : maximum power point tracking speed accuracy variable step size
Chinese Library Classification Number: TK519 Document Identification Code: A
Abstract : For the characteristics of unsatifactory of solar photovoltaic power generation system for the past, the maximum power point tracking (Maximun Power Point Tracking, MPPT) algorithm to track the speed and accuracy.
Keyword : MPPT speed accuracy variable step
1 Introduction
With the development of society, energy and environment have become urgent issues for people. Solar energy, as a widely distributed clean and renewable energy source, has a good application prospect[1]. Among them, solar photovoltaic power generation technology has been extensively studied and applied. In photovoltaic power generation systems, the utilization rate of photovoltaic cells is not only related to the internal characteristics of photovoltaic cells, but also affected by factors such as the operating environment, such as irradiance, load and temperature. Under different external environments, photovoltaic cells can operate at different and unique maximum power points (MPP). Therefore, for photovoltaic power generation systems, it is necessary to seek the optimal working state of photovoltaic cells in order to maximize the conversion of light energy into electrical energy[2].
Currently, commonly used methods for achieving MPPT include the admittance increment method and the perturbation observation method. The admittance increment method uses the zero admittance of the photovoltaic cell's output power with respect to voltage at the MPP to track the MPP. This method has good control effect and high control stability, but the control algorithm is complex and requires high sampling accuracy [3]. The perturbation observation method perturbs the output voltage of the photovoltaic cell based on the trend of the photovoltaic cell's output power change, so that the photovoltaic cell eventually works at the maximum power point. However, the existence of oscillation and misjudgment problems makes the system unable to accurately track the maximum power point, resulting in energy loss [2].
In summary, the variable step-size hysteresis comparison method proposed based on the traditional perturbation observation method not only has good performance in overcoming oscillations and misjudgments in the maximum power point tracking process, but also takes into account the requirements of speed and accuracy, and tracks the maximum power point of photovoltaic cells more accurately, thereby improving the power generation efficiency of photovoltaic systems.
2. Photovoltaic cell characteristic analysis
2.1 Equivalent Circuit of Photovoltaic Cells
A photovoltaic cell is essentially a large-area planar diode, which can be described by the single-diode equivalent circuit shown in Figure 1. In the figure, RL is the external load of the photovoltaic cell, the load voltage (i.e., the output voltage of the photovoltaic cell) is UL, and the load current is IL.
From Figure 1, the load current IL can be obtained as follows:
Figure 1 Equivalent circuit of photovoltaic cell
In the formula: is the current excited by photons in the photovoltaic cell; is the saturation current of the photovoltaic cell in the absence of light; q is the charge of the electron; is the series resistance (composed of the bulk resistance of the cell, surface resistance, electrode conductor resistance, and resistance between the electrode and the silicon surface); A is a constant factor (A is 1 when the forward bias voltage is large); K is the Boltzmann constant, 1.38×10-23J/K; Rsh is the parallel resistance (caused by unclean edges of the silicon wafer or defects in the wafer).
2.2 Mathematical Model and Output Characteristics of Photovoltaic Cells
In a typical photovoltaic cell, the series resistance Rs is very small, while the parallel resistance Rsh is very large. These can be ignored in ideal circuit calculations. The characteristics of an ideal photovoltaic cell are:
Under reference conditions, let Isc be its short-circuit current, Uoc be its open-circuit voltage, and Im and Um be the current and voltage at the maximum power point, respectively. Then, when the voltage of the photovoltaic array is Vpv, its corresponding current is Ipv.
The output characteristics of a photovoltaic cell can be determined based on parameters such as Im, Um, Isc, and Uoc provided by the photovoltaic cell manufacturer. However, since these parameters provided by the manufacturer are generally test results under standard temperature and standard solar power (=1000W/m2), compensation is required in practical applications [4]. It can be obtained from [2]:
Based on equation (3), we built a simulation model of the photovoltaic cell using Matlab, as shown in Figure 2. The parameters are: temperature coefficient of current change a = 0.015 Amps/℃, temperature coefficient of voltage change b = 0.700 V/℃.
Isc=5.45A, Uoc=22.2V, Im=4.95A, Um=17.2V.
Figure 2 Internal structure of the photovoltaic array Matlab simulation module
The encapsulated PV module is as follows:
Figure 3. Packaging of the Matlab simulation model of the photovoltaic array
The output performance of the system when the simulated light intensity changes from 1000W/m2 to 200W/m2 is shown in Figure 4.
Figure 4 shows the output characteristics of the photovoltaic array under the given illumination curve.
As shown in the figure, the output power Ppv of the photovoltaic device has a non-linear relationship with the output voltage Upv and current Ipv. Under different environmental conditions, such as temperature and solar radiation intensity, the photovoltaic device has a unique point of maximum output power. When the photovoltaic device is placed in an outdoor environment, its maximum power changes accordingly due to the constantly changing external environment.
3. Maximum Power Point Tracking Technology Achieved by Variable Step Size Hysteresis Comparison Method
When the operating point approaches the maximum power point, with a fixed-step perturbation method, the operating point may cross the maximum power point. However, after changing the perturbation direction, the difference between the operating point voltage and the maximum power point voltage remains smaller than the step size, preventing the operating point from reaching the maximum power point. This back-and-forth movement of the operating point around the maximum power point due to a fixed perturbation step size is the oscillation phenomenon of the perturbation observation method. When the external environment changes, the output power characteristic curve of the photovoltaic cell also changes, resulting in a situation where the operating point sequence lies on different Ppv-Upv characteristic curves for a period of time. If the criteria for a fixed characteristic curve are continued to be applied to operating points on different Ppv-Upv characteristic curves, the perturbation direction will be opposite to the actual power change trend, which is the misjudgment phenomenon of the perturbation observation method.
3.1 Principle of Variable Step Size Hysteresis Loop Comparison Method
From a control perspective, hysteresis control strategies with nonlinear characteristics can be used to suppress oscillations. For the Ppv-Upv characteristics of photovoltaic cells, the hysteresis control loop is shown in Figure 5.
Figure 5. Schematic diagram of hysteresis technique
When power fluctuates within the set hysteresis loop, the operating point voltage of the photovoltaic cell remains unchanged. Only when the power fluctuation exceeds the set hysteresis loop does the operating point voltage change according to a certain pattern. It is evident that the introduction of the hysteresis loop can effectively suppress the oscillation phenomenon of the perturbation observation method. In fact, misjudgments can be viewed as a dynamic oscillation process when the external environment changes; therefore, this method can also overcome the misjudgment phenomenon of the perturbation observation method.
3.2 Implementation of System Functions
Although the hysteresis comparison method can largely avoid oscillations and misjudgments, if the step size is too large, the operating point may stop in a region far from the maximum power point. If the step size is too small, the operating point will search for a long time in a region far from the maximum power point when a new round of search begins. Therefore, the contradiction between speed and accuracy still exists. The hysteresis comparison method with variable step size can solve this contradiction. The specific flowchart is shown in Figure 6.
Figure 6 Flowchart of the variable step size hysteresis comparison method
The principle is as follows:
4. Analysis of Experimental Results
To verify the correctness of the proposed method, the MPPT control system was integrated into the Boost converter circuit. In a simulation model built using Matlab/Simulink software, an S-function was written as the MPPT control module to track the maximum power point of the photovoltaic cell. The MPPT simulation model is shown in Figure 7. For comparison, simulations were performed using the perturbation-observation method and the variable-step-size hysteresis comparison method for MPPT control.
Figure 7. Matlab simulation model of photovoltaic grid connection
Based on the photovoltaic system simulation model in Figure 7, the perturbation-observation method and the variable-step-size hysteresis comparison method were applied to the MPPT module to simulate a specific instant after changes in external conditions, as shown in Figures 8 and 9. Figure 8 shows that the perturbation-observation method reaches the maximum power point in approximately 0.004s, with the waveform exhibiting significant oscillations in the subsequent range. In contrast, the variable-step-size hysteresis comparison method in Figure 9 reaches the maximum power point at 0.00025s with relatively small fluctuations. This indicates that the variable-step-size hysteresis comparison method indeed improves upon the original perturbation-observation method in both accuracy and speed, thereby increasing the power generation efficiency of the photovoltaic system.
Figure 8 Simulation waveform of photovoltaic cell output power based on perturbation-observation method
Figure 9. Simulation waveform of photovoltaic cell output power based on variable step size hysteresis comparison method.
5. Conclusion
This paper provides a detailed analysis of the characteristics of photovoltaic cells and studies the maximum power point tracking (MPPT) problem in photovoltaic systems, proposing a variable-step hysteresis comparison method based on the perturbation-observation approach. The greatest advantage of the variable-step hysteresis comparison method is that it ensures the reliability of the perturbation-observation method through bidirectional perturbation confirmation, thus avoiding misjudgments. Simultaneously, the continuous adjustment of the search step size during the search process effectively suppresses oscillations near the maximum power point. Simulation results show that the algorithm has excellent tracking speed and accuracy, making it a relatively ideal control method; however, its stability still needs improvement.
6 References
[1] Zhu Minglian, Li Chensong, Chen Xin, et al. A variable step size perturbation observation method applied to MPPT in government systems [J]. Power Electronics Technology, 2010, 44(1):20-22.
[2] Zhang Xing, Cao Renxian. Solar photovoltaic grid-connected power generation and its inverter control [M]. Beijing: Machinery Industry Press, 2011.1.
[3] Zhang Chao, He Xiangning. Application of short-circuit current combined with disturbance observation method in maximum power point tracking control of photovoltaic power generation [J]. Proceedings of the CSEE, 2006, 26(20): 98-102.
[4] Zhang Jianpo, Zhang Hongyan, Wang Tao, et al. Simulation study on maximum power point tracking algorithm in photovoltaic system [J]. Computer Simulation, 2010, 1(27): 266-269.
[5] Zhang Chao, He Xiangning, Zhao Dean. Research on variable step size MPPT control strategy for photovoltaic power generation system [J]. Power Electronics Technology, 2009, 10(43): 47-49.
About the author:
[1] Xu Gaojing (1982-), male (Han nationality), from Qidong, Jiangsu Province, Master, Nanjing University of Science and Technology, Power Engineer (Intermediate) engaged in research and development of power electronics, power supply, inverter and other related fields.
[2] Chen Ting (1986-), female (Han nationality), from Zhumadian, Henan Province, Master, Nanjing University of Technology, currently studying, research direction is power system automation.
[1] Xu Tao (1980-) Male (Han nationality), from Nanjing, Jiangsu Province, Bachelor’s degree, Power Engineer (Junior) engaged in the research and development of power system control and monitoring systems.
Contact Person: Xu Gaojing
Phone: 15250997872
Email: [email protected]
Detailed mailing address: 5th Floor, Jiangnan Building, No. 258 Central Road, Nanjing
