63 CHAPTER 3 APPLICATION OF THE CIRCUIT MODEL FOR PHOTOVOLTAIC ENERGY CONVERSION SYSTEM 3.1 INTRODUCTION The power output of the PV module varies with the irradiation and the temperature and the output characteristics are nonlinear, as was presented in Chapter 2. Figure 3.1 shows the I-V characteristics with maximum power points (MPP) for varying irradiation. Figure 3.1 I-V characteristics with MPP for varying irradiation Figure 3.1 shows that the point of maximum power output for the module varies with irradiation. Hence, for the overall optimal operation of the
66 i.) Better utilization: Each converter module can independently control and optimize its power flow and independently perform the maximum power point tracking (MPPT) for its PV panel. It further offers the advantage of allowing the panels with different orientations and opens up new possibilities in architectural applications. ii.) Good data gathering: Each power source/power converter module will have an inherent data collection capability and most likely a control network connection, so that data gathering and reporting will add minimal additional complexity or cost. Individual identification of the PV panels that require inspection or replacement is easy. iii.) Safety during installation and maintenance: Depending on the design, each converter module may be able to isolate its connected power source, so that the wiring of the series or the parallel connections of these modules is safe. Thus, the power source converter connection is a safe low voltage connection. iv.) Cost savings: In most applications, many small converter modules, one per source, will displace one large converter. The total volt-ampere (VA) power rating of the power switching devices will remain the same, but the cost of the distributed solution could be more attractive in the future because of the following. Standardization of the hardware functionality of the modules, advantages of greater efficiency and reliability would increase the return on the installation investment and advantages of per module data acquisition, intelligent protection and control could displace additional hardware that might otherwise be required.
67 In this Chapter, the converter per panel approach is taken due to the above advantages and the developed circuit model of the PV module used for obtaining the MPPT operation and experimentally verified. 3.3 DESIGN OF THE MAXIMUM POWER EXTRACTOR FOR THE PV SYSTEM With the variation of the irradiation and the temperature, the power output of the PV module varies continuously. The MPPT algorithm is used for extracting the maximum power from the solar PV module and transferring that power to the load. Figure 3.5 shows a DC-DC converter (step-up/stepdown) that serves the purpose of transferring the maximum power from the PV module to the load and acts as an interface between the load and the PV module. Figure 3.5 Operation of the MPPT with the DC-DC converter There are various types of DC-DC converters. i. Buck (step-down) converter ii. iii. Boost (step-up) converter Buck-Boost (step-down/ step-up) converter
68 Figures 3.6 to 3.8 show the simplified diagrams of these three basic types of converters. Figure 3.6 Buck or step-down DC-DC Converter Figure 3.7 Boost or step-up DC-DC Converter
69 Figure 3.8 Buck-Boost or step-down/ step-up DC-DC Converter 3.4 SELECTION OF THE SUITABLE CONVERTER TOPOLOGY Of the above three types, the buck-boost DC-DC converter is used for battery charging purposes. For battery charging, the rated voltage of the PV module is chosen to be higher than the battery voltage. The buck mode is used under full irradiation, when the rated voltage from the PV module is higher than the battery voltage. The control circuit sets the boost mode when the PV output voltage is reduced due to the climatic conditions. Non-isolated buck and boost converters are widely used in photovoltaic power systems for the MPPT operation because of their simplicity and efficiency. Table 3.1 compares a buck converter with a boost converter.
70 Table 3.1 Comparison of a buck converter with a boost converter S. No. Buck (step-down)converter Boost (step-up) converter 1 A buck converter has a discontinuous input current and a continuous output current. 2 Inductance value used in the buck converter is lower. 3 In considering the input capacitors, the buck converter requires a larger and more expensive capacitor to smooth the discontinuous input current from the PV module and to handle significant current ripples. 4 The buck converter also requires higher current rating MOSFET driver, which is more complex and expensive. 5 In the buck interface, the blocking diode is an additional component that is needed to conduct the full PV current. This results in an increase in cost and additional power loss due to the forward voltage drop. The boost converter has a continuous input current and a discontinuous output current. To achieve the same ripple of inductor current, the boost converter needs more inductance than the buck converter. On the other hand, the PV current in the boost converter is as smooth as its inductor current, without any input capacitor. A smaller and cheaper capacitor can further smoothen the PV current and voltage. For the selection of the power MOSFETs and driver, the current rating is lower in the boost topology In the boost topology, the freewheeling diode can serve as the blocking diode to avoid the reverse current.
71 Table 3.1 (Continued) S. No. Buck (step-down)converter Boost (step-up) converter 6 In the buck interface, there are four time-varying parameters namely the switching duty cycle, the PV voltage, the inductor current, and the dynamic PV module resistance 7 The buck topology shows a light damping factor of 0.266 and a lightly damped system is a difficult control problem. In the boost interface, there is only one time-varying parameter, the dynamic PV module resistance and thus there are less time-variant factors in the mathematical model. The boost topology demonstrates a well-damped characteristic, where the damping factor is equal to one 8 The buck interface has less dynamic characteristics than the boost interface. The boost interface shows better dynamic characteristics, such as un damped natural frequency than the buck interface. The boost topology has several advantages over the buck converter for PV applications such as cheaper implementations due to the small input capacitance and better dynamic response such as wider bandwidth and lesser resonance when compared to the buck converter. So the DC-DC boost converter is used in this work for transferring the maximum power from the PV module to the load.
72 3.5 DESIGN OF THE DC-DC BOOST CONVERTER Figure 3.9 shows the basic circuit for the boost converter. To ensure that the solar module operates at the MPP, the input impedance of the DC-DC converter must be chosen properly. Figure 3.9 Basic circuit of the boost converter The boost converter consists of a DC input voltage source V s, a boost inductor L, a controlled switch S, a diode D, a filter capacitor C, and a load resistance R. Figure 3.10 presents the boost converter waveforms in the continuous conduction mode (CCM). When the switch S is in the ON state, the current in the inductor increases linearly and the diode D is OFF at that time. When the switch S is turned OFF, the energy stored in the inductor is released through the diode to the output RC circuit. The switch operates in two states. In the conduction mode (ON), the output of the module is connected to the inductor while in the cut-off (OFF) mode; the output of the module is disconnected from the inductor.
73 Figure 3.10 Boost converter waveforms in the continuous conduction mode (CCM) The ON time of the switch is related to its time-period such that T ON = DT, where D is the duty cycle. In the ON state, the current flows from the module through the inductor causing the inductor to store energy. In the
74 OFF state, the energy stored in the inductor is released through the diode to the output RC circuit and the OFF time is given by T OFF = (1-D)T. The duty cycle can be adjusted to set the output voltage of the converter to the desired value. For an ideal DC-DC converter, the duty cycle is the ratio between the output voltage and the input voltage, D = V o /V i. The duty cycle is set by means of a pulse width modulation (PWM) signal that is used to control the ON and OFF states of the switch. In this work, a MOSFET is used as control switch. Using the Faraday s law for the boost inductor gives Equation (3.1). V DT s V 1 o Vs D T (3.1) Equation (3.2) gives the DC voltage transfer function. M v V V o s 1 1 D (3.2) The boost converter operates in the CCM for L>L b, where L b is the critical value of inductance given by Equation (3.3). 2 1 D L b 2 f DR (3.3) The current supplied to the output RC circuit is discontinuous. Thus, a large lter capacitor is required to limit the output voltage ripple. The lter capacitor C min must provide the output DC current to the load when the diode D is OFF. Equation (3.4) gives the minimum value of the lter capacitance that results in the desired ripple voltage V r. DVo Cmin (3.4) V RF r
75 The design of the component values of the boost converter is described by Equations (3.1) to (3.4). Table 3.2 gives the designed component values of the DC-DC boost converter used for simulation. Table 3.2 Component values of the DC-DC boost converter Description Rating Inductor 120 µh MOSFET Power Diode IRFP460 IN5408 Capacitor 330 µf Switching frequency 20 khz 3.6 SIMULATION OFTHE DC-DC BOOST CONVERTER WITH DC SUPPLY The DC-DC converter, with the configuration given in Figure 3.9 and the component values in Table 3.2, is simulated in MATLAB/ Simulink with the battery supply, as shown in Figure 3.11. For the input DC voltage of 19.7 V, the output voltage obtained is 56.2 V, as shown in the display. Figure 3.12 shows the input and output voltages for the converter.
Figure 3.11 Simulation of the DC-DC boost converter with the DC supply 76
77 Figure 3.12 Input and output voltage waveforms for the boost converter 3.7 SIMULATION WITH THE DEVELOPED CIRCUIT MODEL The battery supply in the circuit shown in Figure 3.11 is replaced by the developed circuit model of the PV module. To connect the PV module with the boost converter an input capacitor is required, as shown in Figure 3.13. Figure 3.13 PV module connected with the input capacitor
78 3.7.1 Selection of the Input Filter The output voltage and output current of the PV module depend on the environmental conditions, which are varying continuously. Capacitor C in is provided as the input filter at the input to the boost converter to reduce this variation. The C in also reduces the effects of the input voltage fluctuations on the operation of the DC converter, and helps to reduce the ripple on the PV source. If the current has ripples, the required capacitance for a given ripple voltage can be found by rearranging the standard capacitor equation given in Equation (3.5). I T PV ON C in (3.5) VC Where I PV is the discontinuous current and V C is desired ripple voltage. The calculated value of C in used in the simulation circuit is 250 µf, as shown in Figure 3.14. Figure 3.14 Simulation of the boost converter circuit with the PV input for various values for irradiation
79 For the design of the MPPT, the developed circuit model is fed with the variable irradiation at a constant temperature of 25 ºC. The simulated data are collected and the results are tabulated in Table 3.3. Table 3.3 Duty cycle variations Duty Input Input Input Output Output Output Cycle Voltage Current Power Voltage Current Power Irradiation 1000 W/m 2 Temp 25 C 0.4 17.82 2.059 36.69 40.08 0.8106 32.13 0.41 16.7 2.303 38.46 40.45 0.8089 32.72 0.5 15.05 2.44 36.72 39.06 0.7812 30.52 Irradiation 700 W/m 2 Temp 25 C 0.3 17.6 1.375 24.21 32.46 0.6492 21.08 Irradiation 500 W/m 2 Temp 25 C 0.2 17.64 0.8661 15.28 25.76 0.515 13.27 From Table 3.3, it can be seen that for lower values of irradiation and constant load, the duty cycle is reduced from 0.41 for 1000 W/m 2 to 0.2 for 500 W/m 2. This variation coincides with the graph shown in Figure 3.15 (Hussein et al 1995), where the duty cycle variation with respect to irradiation is plotted.
80 Figure 3.15 Duty cycle variation with respect to the variable irradiation 3.8 SELECTION OF THE MPPT CONTROL ALGORITHM Tracking the MPPT of the PV module the essential part of a PV system. Figure 3.16 shows the characteristic curves with the maximum power point of a PV module. The MPPT technique automatically finds the V MPP and the I MPP at which the PV module should operate to obtain the maximum power output P MPP under a given temperature and a given irradiance.
81 Figure 3.16 Characteristics of the PV module for the MPP operation As such, many MPPT methods have been developed and implemented. These methods vary in complexity, requirement of sensors, convergence speed, cost, range of effectiveness, hardware implementation, popularity, and other aspects. They range from the almost obvious but effective to the most creative but not necessarily the most effective. MPPT techniques proposed in the literature are the perturb and observe (P&O), the incremental conductance (IC), the fuzzy logic etc. Different MPPT techniques will suit different applications. Esram et al (2007) analyzed various MPPT techniques and found that the P&O technique is able to continuously track the true MPP in minimum time and does not require periodic tuning. Further, the P&O algorithm is very popular and simple. So it is used in this application.
82 3.9 PERTURB AND OBSERVE (P&O) MPPT ALGORITHM Figure 3.17 Working of the P&O MPPT In the P&O method, a perturbation in current is made resulting in perturbation of the operating voltage of the PV module. In the case of a PV module connected to a power converter, perturbing the duty ratio of the power converter perturbs the PV module current and consequently perturbs the PV module voltage. From Figure 3.17, it can be seen that incrementing (decrementing) the voltage increases (decreases) the power when operating on the left of the MPP and decreases (increases) the power when on the right of the MPP. Therefore, if there is an increase in the power, the subsequent perturbation should be in the same direction to reach the MPP and if there is a decrease in the power, the perturbation should be in the opposite direction. Table 3.4 summarizes this algorithm.
83 Table 3.4 Summary of the P&O algorithm Perturbation Change in Power Next Perturbation Positive Positive Positive Positive Negative Negative Negative Positive Negative Negative Negative Positive It can be shown that the algorithm also works when the instantaneous (instead of the average) PV module voltage and current are used, as long as sampling occurs only once in each switching cycle. The process is repeated periodically until the MPP is reached. In the P&O MPPT algorithm, a slight perturbation ( D = 0.01) is introduced in the system. This perturbation causes the power of the solar module to change. If the power increases due to the perturbation, then the perturbation is continued (D + D) in that direction. After the peak power is reached, the power at the next instant decreases and after that the perturbation is carried out in the opposite direction (D D). Figure 3.18 shows the flowchart of the MPPT algorithm.
84 Figure 3.18 Flowchart of the P&O MPPT algorithm algorithm. Figure 3.19 shows the Simulink model for the P&O MPPT
85 Figure 3.19 Simulink model for the P&O MPPT algorithm 3.10 DEVELOPED CIRCUIT MODEL OF THE PV MODULE WITH MPPT CONTROL UNIT Figure 3.20 shows the detailed Simulink model for the control of the developed circuit model of the PV module with the MPPT control unit. V in and I in are taken as the inputs to the MPPT unit and the duty cycle D is obtained as the output. Figure 3.24 shows the simulation output of the MPPT
86 control circuit for irradiation of 500 W/m 2 and 1000 W/m 2 along with experimental results for 1000 W/m 2. Figure 3.20 MPPT control circuit 3.11 HARDWARE IMPLEMENTATION system. Figure 3.21 shows the block diagram of the proposed hardware
87 Figure 3.21 Block diagram of the proposed hardware system The maximum open circuit voltage of the existing module is 21.4 V and the short circuit current is 2.55 A. These values are stepped down and given as the inputs to the PIC microcontroller. A step-down circuit is used to step-down the voltage to a range of 0-5 V and given as one of the inputs to the PIC microcontroller. The input current is sensed with the help of a current sensing resistor and given as the second input to the PIC microcontroller. These two analog inputs are received inside the PIC 18F4550 and converted into corresponding digital signals. These two digital signals are used in the MPPT algorithm to calculate the error between the power at the present instant and the power in the previous instant. The PWM technique is then implemented to generate the pulses for the varying duty cycle. These PWM pulses are given to the gate of the MOSFET through an isolation circuit. The optocoupler acts as an interface between the PIC microcontroller and the power semiconductor device of the power circuit. A high-speed switching optocoupler of appropriate bandwidth must be selected for proper frequency response. Figure 3.22 shows the detailed circuit description of the entire MPPT system. It should be ensured that the components selected for the system must have proper power ratings.
88 Figure 3.22 Detailed circuit description of the entire MPPT system Figure 3.23 shows the hardware setup of the proposed system. The microcontroller program should be fed with the required range of duty cycle so that the circuit works properly.
89 Figure 3.23 Hardware set-up of the MPPT with the microcontroller unit and the pulse output for variable duty cycle
90 The experiment is carried out for both the P&O and the incremental conductance (IC) MPPT algorithms at the insolation of 700 W/m 2 and the temperature of 30 ºC. In Figure 3.24, the experimental results of IC MPPT algorithm are included. The improved performance of IC MPPT algorithm is seen that coincides with the results of existing literature. Experimental values of the PV current and power are lower by around 2 to 5 percent with respect to simulation values, as shown in Figure 3.24. Thus, the performance of the developed model in closed loop control circuit follows the simulation values with reasonable accuracy. Figure 3.24 Comparative simulation and experimental results for P&O and INC MPPT algorithm
91 3.12 CONCLUSION This Chapter presented the simple circuit-based PV model that is used in the MPPT design. The circuit model is used with DC - DC converter circuit and simulated for various values of irradiation. It was found that for constant load, the duty cycle was reduced from 0.41 for 1000 W/m 2 to 0.2 for 500 W/m 2.This variation coincides with the already available results in the literature. Thus, the performance of the developed circuit model in closed loop is tested in simulation and verified experimentally with P&O and incremental conductance (IC) MPPT algorithms. The simulation model has reasonable accuracy. Hence, it can be used for other applications of the PV module. This Chapter also demonstrates that the developed circuit model of PV module is a very useful tool for various simulation studies.