基于FPGA的太阳能充电系统的研究与设计.doc

Research and design of the solar charging system based on FPGA Keywords:BUCK-BOOST,photovoltaic,MPPT,Weighted Variable-Step Voltage-Hysteresis Perturbation&ObservationAbstract:This article introduces an efficient solar charging system based on FPGA. The charging circuit composed of BUCK-BOOST switch converter, realizes the function of maximum power point tracking(MPPT) of solar cells by using high speed controller-FPGA and a new MPPT technology-Weighted Variable -Step Voltage -Hysteresis Perturbation&Observation(WVSVH P&O) method. This paper explains the design by its circuit configuration and control system, and simulates it in the simulink. The simulation results show that, this topology is able to achieve the purpose of charging efficiently with solar cells; The control strategy used here, compared with the conventional MPPT methods, shows its superiority of fast and efficient.0 Introduction After entering the 21st century, how to develop the economy sustainable under the double restrictions of the limited energy and environmental protection, has became a enormous historical topic human faced with. The deficient of conventional energy and the destruction of environment from fossil energy are forcing people to develop and use green new energy on a large scale. Among the numerous new energy, solar has been developed vigorously by human for its abundant reserves, wide distribution, clean, economy, high availability, etc1. Charging technology combined with photovoltaic technology is in a rapidly developing stage , and has made great progress already. But charging efficiency of charger is still not ideal, how to improve utilization of solar cells by using MPPT technology reasonable is still a technical problem in this area. Switching power supply becomes the direction of development of future power supply for its small size, light weight, high efficiency, low calorific, stable performance. At the time of choosing the structure of the charging circuit, noticed that the requirements of the relationship between input voltage and output voltage of BUCK converter and BOOST converter,limit their application.In order to make the system meet the requirements of photovoltaic charging application, taking system costs account as well, instead of choosing the BUCK converter and BOOST converter segmented running-parallelly circuit structure, select the BUCK-BOOST converter and a new MPPT technology -WVSVH P&O method to achieve the goal of charging energy storage devices efficiently by tracking the maximum power point of solar cells quickly and accurately23.1 Circuit configuration: The efficient solar charging system designed in this research is mainly composed of solar cells, BUCK - BOOST converter, FPGA, energy storage load. The structure of the circuit configuration is shown in Figure 1.Figure 1 Main components of charging system Solar cells used in this experiment are tested under the conditions: 25, AM1.5, 1000W/m2. Its parameter information: Open circuit voltage 22.41V; Short circuit current 0.62A; Peak voltage 17.9V; Peak current 0.56A; Maximum power 12.1W. The type of switching power supply used in this design is BUCK-BOOST. For, BUCK converter can only work properly in applications that its input voltage is higher than its output voltage; BOOST converter can only work properly in applications that its output voltage is higher than its input voltage. In order to make the system able to achieve high charging efficient during the whole charging process, BUCK-BOOST converter which can not only work in the BUCK state but also work in the BOOST state becomes the choice of this design. The FPGA controller used in this design is Cyclone EP2C8Q208C8N from Altera. By using the concurrent statement processing capability of FPGA, the running speed of the control system can be highly improved, control effect becomes more accurate and efficient. In addition, advanced custom integration brings great potential to reduce system costs. There are kinds of energy storage devices, the common electrical energy storage device is battery. But here, choose the emerging energy storage device with high power density characteristicSuper capacitor, to verify if the charging system based on BUCK-BOOST converter has the ability of charging efficient during the whole process by experiment of charging a 1F super capacitor from 0V to 40V.2 System control As this is a charging system combined with PV technology, by analyzing and studying MPPT technology with practical applications on the basis of understanding the basic characteristics of solar cells can formulate the control strategy legitimately.2.1 Characteristics of solar cells Solar cells are non-linear power. Under the given external environment, output current and output power of solar cells will change non-linearly with the change of its output voltage. The output characteristic curve is shown in Figure 2 and Figure 3.(a) U-P characteristic curve (b)U-I characteristic curveFigure 2 Output characteristic curve of solar cells under light intensity changes(a) U-P characteristic curve(b)U-I characteristic curveFigure 3 Output characteristic curve of solar cells under temperature changes Figure 2 and Figure 3 show the curve of output current and output power of solar cells under the change of its output voltage. MPPT technology is mean to make solar cells work at maximum power point(MPP) steadily while both of light intensity and temperature are changing. Essentially, MPPT technology achieves the equal between internal impedance of solar cells and impedance of the load(Impedance matching). All kinds of converters achieve impedance matching by changing the equivalent impedance of load. When BUCK-BOOST converter is working normally, its input voltage uo, output voltage ui and duty factor(d) satisfy equation (1): (1) (2) In equation (2), Ri:the equivalent resistance seeing from the input of converter; RL:the load resistance; Pi:converters input power(Solar cells output power); Po:converters output power; :conversion efficiency. (3) From equation (3), get that, by changing the value of duty factor(d), impedance matching can be realized. This supports the feasibility of BUCK-BOOST converters using in MPPT technology.2.2 Control strategy The Perturbation&Observation method is a traditional MPPT technology and being used widely for its characteristics such as simple structure, fewer measurements, realizing easily. But, it will be shocking excessively near the MPP when the value of perturbation step is too large and this may lead to a waste of energy; It will take a very long time to track the MPP when the value of perturbation step is too small. That means it can not take both of the tracking accuracy and speed into account at the same time. In order to improve this deficiency, observe the P-U curve of solar cells firstly.Figure 4 P-U curve of solar cells Figure 4 shows that, as the voltage variation(V) is constant, the smaller the distance to the MPP is, the smaller the power variation(P) is. Thats because, the smaller the distance to the MPP is, the smaller the absolute value of the slope of the P-U curve-K is, and the value of K is 0 on the MPP. Using this characteristic, K can be introduced into the P&O MPPT method as weight value of fixed step. In addition, the U-P curve can be further divided into two parts-K is greater than 0.2 and K is less than 0.2, and set different fixed step sizes for them. Thus, the system can track the MPP quickly with large step when its far from the MPP,then strengthen the accuracy of MPPT method by using small step when its close to the MPP to a certain extent. The initial voltage of the super capacitor is 0V, and its vary widely from the input voltage value of the converter, in the viewpoint of control, voltage hysteresis control technology is used, aiming at realizing MPPT method more stably. It will produce a large fluctuation during the process of MPPT if PWM control technology is used, for, the PWM control technology is realized by changing the duty ratio which takes a relatively long period of time, may miss the optimal control time. Hysteresis control method can avoid this problem by adjusting the on-off time of switch timely, and ensure that the MPPT technology can work steadily with its rapid aging control. This design uses a novel MPPT technology - Weighted Variable-step Voltage-hysteresis Perturbation&Observation (WVSVH P&O) method. Thats by setting the input voltage of the converter as a variable hysteresis value, making the hysteresis value perturb, observing the change of the output power of solar cells. The perturbation step of hysteresis is decided by consolidating the weight of the current operating point and the fixed step size corresponding to the slope of the curve. This MPPT method, which takes balance between speed and accuracy reasonable, makes the entire system completes the charging task more efficiently. The control flow chart of the WVSVH P&O MPPT method is shown in Figure 5.Figure 5 Control flow chart of the WVV P&O MPPT method In the flow chart, VC:voltage of super capacitor; VN:present state input voltage of converter (The upper limit of voltage hysteresis); IN:present state input current of converter; PN:present state input power of converter; PN-1:former state input power of converter; :voltage perturbation step; VN+1: after state input voltage of converter (The upper limit of hysteresis voltage); V(N+1)-:after state input voltage of converter (The lower limit of the hysteresis voltage); VW:width of the hysteresis; KHLIMIT:given upper limit of weight. What should be pointed out is that, during this control process, the consistency of switch duty ratio and the relationship between input voltage and output voltage of switch converter dont need to be concerned about. That is mainly because, if the duty ratio and the relationship between input voltage and output voltage contradict each other, the MPPT function will not be realized for the BUCK-BOOST converter can not work normally. Since output power begins to decline, the controller will change the on-off time of switch in order to maintain the maximum power output of the system, and that is to say, the system itself will not allow the situation that the duty ratio and the relationship between input voltage and output voltage contradict each other to happen4.3 Simulation results and analysis(a) Curve of MPPT with large fixed-step size(b) Curve of MPPT with small fixed-step sizeFig 6 Curve of MPPT with fixed-step sizeFig 7 Curve of MPPT with weighted variable step size By comparing figure 6 and figure 7. The curve of MPPT with large fixed-step size shows that it can track the MPP quickly, but not accurately and may waste energy for shocking excessively near the MPP; The curve of MPPT with small fixed-step size shows that, though an improvement in accuracy, the time used to track the MPP becomes much longer. Curve of MPPT with weighted variable step size shows that, it can take both of the tracking accuracy and speed into account at the same time and achieve the request of tracking the MPP quickly and accurately.Fig 8 Ripple comparison between MPPT methods with different step sizes A problem of the MPPT with weighted variable step size was found during the simulation. As is shown in Fig 9, the power point will jump from one power curve to another when sudden change occurs. V is very small, P is relatively great, so, as the weight value(P/V) tends to infinity, the perturbation step size becomes so large that the output power turns to 0W in a sudden.Fig 9 Problem of the MPPT with weighted variable step By regarding weight values out of this range as invalid values, this problem can be solved. Fig 10 shows the curve of output power of solar cells during the charging process. The system can track the MPP quickly and accurately in initial, and keep working at the MPP steadily till the end. Fig 11 shows the voltage curve of the super capacitor during the charging process, and the curve is very smooth without trembles and interruptions.Fig 10 Output power curve of so