CHAPTER-3 Design Aspects of DC-DC Boost Converter in Solar PV System by MPPT Algorithm

Transcription

1 CHAPTER-3 Design Aspects of DC-DC Boost Converter in Solar PV System by MPPT Algorithm 44

2 CHAPTER-3 DESIGN ASPECTS OF DC-DC BOOST CONVERTER IN SOLAR PV SYSTEM BY MPPT ALGORITHM 3.1 Introduction In the recent past years, due to the energy crisis and environment pollution, the electrical generation system using photovoltaic cells become more significant. Photovoltaic (PV) power generation systems can substantially reduce environmental issues such as the green house effect and air pollution. In a photovoltaic system the solar energy can be converted directly into electrical energy. Photovoltaic cells are the basic component of a photovoltaic system. Cells may be grouped in series and parallel to form a solar module. Again modules may be grouped in series and parallel to form photovoltaic arrays. Cells are connected in parallel to increase the output current and connected in series to increase the output voltage. The major problem in PV power generation systems is that the amount of electric power generated by PV module is always changing with weather conditions, i.e., irradiation. Therefore, Maximum Power Point Tracking (MPPT) algorithms is implemented which has led to the increase in the efficiency of operation of the solar modules. 3.2 Description The main components of a photovoltaic system consist of PV panel, converter and the controller to control the converter operation. Therefore the efficiency of a PV system depends on the efficiency of its components i.e. the efficiency of the PV panel (8-15% in commercial PV panels), the efficiency of the inverter (95-98 %), and the efficiency of the maximum power point tracking (MPPT) algorithm (about 98%). For the improvement of the efficiency of the PV panel suitable semiconducting material is required which depends on the manufacturing technology used, whereas the efficiency of the converter can be increased by using suitable control strategy for their operation. Due to non linear voltage-current characteristic, the PV system should be operated at a point on the I-V curve where maximum power is produced. This maximum power point depends on the temperature of the cell and on the irradiance conditions. Both conditions change during the day and are also different depending on the season of the year. Furthermore, irradiation can change rapidly due to 45

3 changing atmospheric conditions such as clouds. Hence it is very important to track the MPP accurately under all environmental conditions so as to obtain maximum power available. In this chapter, advantages, shortcomings and execution of efficiency for two power-feedback type MPPT methods, including perturbation & observation (P&O) and incremental conductance (INC) methods are studied and compared. Matlab/Simulink is used in this paper to implement the modeling and simulations tasks. The performance of a PV array system depends on the the solar cell and array design as well as the operating conditions. The output voltage, current and power of a PV array are the functions of solar irradiation level, temperature and load current. Therefore in the design of PV arrays, the effects of these three quantities must be considered so that any change in temperature and solar irradiation levels should not adversely affect the PV array output power to the load or utility. To overcome the effects of the variable temperature and solar irradiation on the output power of PV systems, two control strategies have usually been applied: a) Controlling the solar irradiation to the PV array, and b) Controlling the electrical power output from the PV array. In both the systems either electrical or thermal energy storage systems or auxiliary power sources are incorporated which can supply electricity during the time of no solar irradiation. Solar energy available to the PV systems is kept as high as possible either by rearranging the solar cell configurations of PV arrays or by designing and controlling the position of sun tracking solar collectors with respect to the changes in weather conditions. Consequently, during the design process a simulation must be performed for the system analysis and parameter settings. When a photovoltaic (PV) array is connected directly to the load, the operating point of PV solar array is seldom at the maximum power point (MPP). The Maximum Power Point Tracking (MPPT) combined with a dc-dc power converter allows a PV generator to produce maximum continuous power, regardless of the environmental conditions (solar radiation, temperature). There are various different algorithms of MPPT control with different ways on implementation and performance. The best known MPPT classic algorithms are perturbed-and observe (P&O) and incremental conductance (IC). These algorithms are based on the same technology, regulating PV array voltage by adjusting the optimal set point that represents the voltage at maximum power point (MPP). 46

4 3.3 DC-DC Converter for Solar PV System The DC-DC converter used to supply a regulated DC output with the given DC input. These are widely used as an interface between the photovoltaic panel and the load in photovoltaic generating systems. The load must be adjusted to match the current and voltage of the solar panel so as to deliver maximum power. DC/DC converters are described as power electronic switching circuits since they convert one form of voltage to other. These may be applicable for conversion of different voltage levels. Generally three basic types of converters are accountable as per their use. They either step up by boosting voltage at output known as Boost converter or by stepping down by reducing voltage known as Buck converters. There is another class of converters used for both stepping up or down the voltage output described as Buck-Boost converters. Buck-Boost converters reverse polarity of output voltage, as such they are sometimes known as inverters Boost Converter and its Mode of Operation A simple boost converter consists of an inductor, a switch, a diode, and a capacitor. Figure 3.1 represents the circuit diagram of DC-DC boost convertor and figure 3.2 show the mode of operation of boost converter. Figure 3.1: Circuit diagram of boost converter 47

5 Boost converter circuit operation can be divided into two phases. Phase 1 begins when the switch SW is turned on at t = T on. The input current which rises flows through inductor L and switch SW. During this mode, energy is stored in the inductor. Phase 2 begins when the switch is turned off at t = T off. The energy stored in the inductor causes its voltage to swap polarity and maintain current flow in the circuit, which is now directed through inductor L diode D, capacitor C, load R, and the supply of V in. Figure 3.2: Circuit diagram of mode of operation of boost converter The inductor current falls until the switch is turned on again in the next cycle. The reversing of the inductor voltage polarity in phase 2 allows the V out to be greater than V in. Where V out is the output voltage, D is duty cycle, and V in is the input voltage which in this case will be the solar panel voltage. 48

6 3.3.2 Waveforms Figure 3.3 shows the different characteristics of boost converters. It shows the source voltage, source current, inductor current, capacitor current with respect to time for a complete duty cycle. Figure 3.3: Waveforms of boost converter It is assumed that the switch is made ON and OFF at a fixed frequency and let the period corresponding to the switching frequency is T. Given that the duty cycle is D, the switch is on for a period equal to DT, and the switch is off for a time interval equal to (1 - D)T. The inductor current is continuous and is greater than zero Modeling of Boost Converter The DC-DC boost converter circuit consists of Inductor (L), Diode (D), Capacitor (C), load resistor (R L ), the control switch(s). These components are connected in such a way with the input voltage source (V in ) so as to step up the voltage. The duty cycle of the control switch controls the output voltage of the boost converter. Hence by varying the ON time of the switch, the output voltage can be varied. Thus, for the duty cycle D the average output voltage can be calculated using 49

7 V o /V in =1/(1-D) (3.1) where V in, V o are the input and output voltage of the converter respectively and D is the duty cycle of the control switch. In an ideal circuit, the output power of the converter is equal to input power which yields. P o = P in (3.2) i.e. V o I o =V in I in (3.3) A. Selection of Inductor: The inductor value of the Boost converter are calculated using L=V in /(f s I L ) (3.4) Where f s is the switching frequency and I L is the input current ripple. Current ripple factor (CRF) is the ratio between input current ripple and output current. For good estimation of inductor value CRF should bound within 30%. The current rating of inductor should be always higher than that of the maximum output current I L /I o =0.3 (3.5) The current rating of inductor should be always higher than that of the maximum output current. B. Selection of Capacitor The capacitor value can be obtained from C = I out /(f s V o )D (3.6) Where V o is the output voltage ripple which is usually considered as 5% of output voltage which yields, V o /V o = 5% Block diagram Model of Boost Converter The DC-DC boost converter is designed for V in = 12V,V out = V, I out =1.05 amp and Using these values the components values are calculated as follows L=200µH, C=50µF and R L =20Ω. The block diagram is shown in figure

8 Figure 3.4: Block diagram model of boost converter Simulation Results The MATLAB simulation model for the above block diagram is given in figure 3.5. It consist of one switch input voltage source, inductor, DC load and scopes to observe the output. Figure 3.s shows the input voltage curve of boost converter whereas figure 3.7 shows the simulation results of boost converter. Figure 3.5: Simulink model of boost converter 51

9 Figure 3.6: Input voltage curve of boost converter It is observed from figure 3.6 that the input voltage is always constant for boost converter. The output voltage and currents of boost convertor for this input is given below. 20 Voltage across Inductor 30 Voltage across Load Voltage 0 Voltage Time Time 8 Capacitor Current 10 Diode Current 6 0 Current 4 2 Current Time Time Figure 3.7: Simulation results of boost converter 52

10 The above results show that the voltage across the inductor remains within 10V to +10V during switching ON and OFF position. The load current remains constant after seconds. The capacitor current remains stable within 1A to +1A whereas the output voltage of the boost converter is stable at 20A after seconds. The diode (D 1 ) current is 2A and diode (D 2 ) current is 20A after seconds. The output voltage comes constant to 21.5 volts for various levels of inputs and current is 19.3 ma which is shown in figure 3.7. Whenever output voltage becomes more than 22 volts the differential amplifier will generate a negative saturation level voltage signal which will increase the duty cycle and so output would come down to 22 volts. And when output becomes less than 22 volts the differential amplifier will generate a more positive level of voltage signal which in turn would reduce the duty cycle and thereby increasing the voltage to 22 volts. Thus how output voltage is kept constant. For the specified input variation, a regulated dc output voltage of 21.5V has been obtained resulting in an efficiency of 95%. 3.4 Modelling of PV Array and Performance Enhancement by MPPT Algorithm This section proposes modelling and simulation of photovoltaic model. Taking into account the temperature and sun's irradiance, the PV array is modelled and its voltage current characteristics and the power and voltage characteristics are simulated. This enables the dynamics of PV system to be easily simulated and optimized. It is noticed that the output characteristics of a PV array are influenced by the environmental factors and the conversion efficiency is low. Therefore a maximum power tracking (MPPT) technique is needed to track the peak power to maximize the produced energy. The maximum power point in the power - voltage graph is identified by an algorithm called perturbation & observation (P&O) method or Hill climbing. This algorithm will identify the suitable duty ratio in which the DC/DC converter should be operated to maximize the power output. 3.5 MPPT Techniques Used in Solar PV System The principal drawback of the PV Systems is their low efficiency. The typical efficiency of a solar cell is around 8-15%. In case of the solar panels, it becomes hardly 30-40%. That means the panels are capable to convert only 30-40% of the incoming solar irradiations into electrical power. So the fundamental aim of this part of the thesis is to increase the efficiency. There are several methods available, by which we can improve the efficiency by matching the source and load properly. The Maximum Power Point Tracking (MPPT) is one such 53

11 method, which has a huge importance in the era of Photovoltaic Technology. Now-a-days this technique is vastly used to develop maximum possible power from a varying source under a variable temperature and irradiance conditions. We know, the Maximum Power Transfer Theorem tells that the output power of a circuit is maximum, when the Thevenin impedance of a circuit i.e. the source impedance matches with the load impedance and complex conjugate to it. So, MPPT problem is one kind of impedance matching problem. Solar cells have a very complex relationship between solar irradiation, temperature and the total resistance that develops a non-linear output efficiency which can be analyzed based on the I-V curve. So the main function of MPPT is to sample the output of the cells and apply the proper load to obtain the maximum power for any given location, time, season and environmental conditions. The MPPT not only enables an increase in the power delivered from the PV module to the load, but also enhances the operating lifetime of the PV system. Various types of MPPT methods can be differentiated based on various features including the types of sensors required, convergence speed, cost, range of effectiveness, implementation of hardware requirements, popularity etc. The operating characteristics of a solar cell consist of two regions i.e. the current source region and the voltage source region. In the current source region, the internal impedance of the solar cell is high and this region is located on the left side of the current-voltage curve. The voltage source region, where the internal impedance is low, is located on the right side of the current -voltage curve. As per Maximum Power Transfer Theorem, Maximum Power is delivered to load when source internal impedance matches load impedance. For determining MPP appropriate Tracker is introduced between PV system and load. It is to be designed that gives good performance, fast response, and less fluctuations. Since the efficiency of the PV is affected by the panel s irradiance and temperature which are stochastic and unpredictable. For this reason, it is not possible to connect the load directly to the PV to obtain the maximum power, so it is necessary to include a balance of system (BOS). Typically this BOS is a DC-DC converter to adjust the properties of the load. This converter has the advantage of managing the power delivered to the load. A DC/DC converter (step up/step down) serves the purpose of transferring maximum power from the solar PV module to the load. A DC/DC converter acts as an interface between the 54

12 load and the module. By changing the duty cycle the load impedance as seen by the source is varied and matched at the point of the peak power with the source so as to transfer the maximum power. This acts as adjustment to match impedance of source & load. MPPT is normally operated with the use of a DC-DC converter (step up or step down). The location of the MPP is not known, but can be located, either through calculation models or by search algorithms. Figure.3.8: Block diagram of MPPT system The typical block diagram of MPPT system considered for simulation study which is derived from the concept of basic block diagram are given in figure 3.8 and 3.9 respectively. Figure 3.9: Block Diagram of Typical MPPT System. There are several MPPT method exists in order to maximizing the output power. The existing methods are a) Perturb and observation method. 55

13 b) Incremental conductance method. c) Parasitic capacitance method. d) Voltage based peak power tracking method. e) Current based peak power tracking method. Among all the MPPT methods, Perturb & Observe (P&O) and Incremental Conductance (IC) are most commonly used because of their simple implementation and lesser time to track the maximum power point and also other economic reasons. Table 3.1. Characteristics of different MPPT techniques MPPT Convergence Implementation Periodic Sensed technique Speed complexity Tuning parameters P & O Varies Low No Voltage INC Varies Medium No Voltage, current Fractional V OC Medium Low Yes Voltage Fractional I SC Medium Medium Yes Current Fuzzy Logic Fast High Yes Varies Perturb and Observe MPPT Method The P&O algorithms operate by periodically perturbing (i.e. incrementing or decrementing) the array terminal voltage or current and comparing the PV output power with that of the previous perturbation cycle. If the PV array operating voltage changes and power increases (dp/dv > 0), the control system moves the PV array operating point in that direction; otherwise the operating point is moved in the opposite direction. In the next perturbation cycle the algorithm continues in the same way. A common problem in P&O algorithms is that the array terminal voltage is perturbed every MPPT cycle; therefore when the MPP is reached, the output power oscillates around the maximum, resulting in power loss in the PV system. Perturb & Observe (P&O) is the simplest method and is widely used. In this technique we generally use only one sensor, that is the voltage sensor, to sense the PV module voltage and hence the cost of implementation is less and hence easy to implement without any complexity. 56

14 The time complexity of this algorithm is very less for calculating the maximum power but on reaching very close to the Maximum Power Point (MPP) it doesn t stop at the MPP and keeps on perturbing on both the directions so for that reason it have multiple local maximum at the very same point. First of all the algorithm which reads the value of the current and voltage from the photovoltaic module from that power is calculated the value of voltage and power at that instant is stored. Hence slight perturbation is added in the increasing direction. The next values at the next instant are measured and power is again calculated. Hence, by adjusting the maximum power duty cycle can be obtained based on it. In certain situations like changing atmospheric conditions and change in irradiance the maximum power point shifts from its normal operating point on the PV curve. In the next iteration it changes its direction and goes away from the maximum power point and results in multiple local maxima at the same point as shown in figure So the maximum power point deviates from its original position. Figure 3.10: PV curve Algorithm for Perturb and Observe Technique a) Read the value of current and voltage from the solar PV module. b) Power is calculated from the measured voltage and current. c) The value of voltage and power at k th instant are stored. d) Then next values at (k+1) th instant are measured again and power is calculated from the measured values. e) The power and voltage at (k+1) th instant are subtracted with the values from kth instant. f) In the power voltage curve of the solar PV module, it is inferred that in the right hand side curve where the voltage is almost constant and the slope of power voltage is negative (dp/dv<0) where as in the left hand side, the slope is positive 57

15 (dp/dv>0).therefore the right side of the curve is for the lower duty cycle (nearer to zero) where as the left side curve is for the higher duty cycle (nearer to unity). g) Depending on the sign of dp i.e. (P(k+1) - P(k)) and dv i.e. (V(k+1) -V(k)) after subtraction the algorithm decides whether to increase the duty cycle or to reduce the duty cycle. The above algorithm is shown represented in a flow chart which is given in figure Flow Chart of Perturb and Observe MPPT Algorithm START Measures of V(k) and I(k) P(k) = V(k) I(k) P = P(k) P O (k 1) No P > 0 Yes Yes Decrease Module Voltage V(k) V(k 1)>0 No No V(k) V(k 1)>0 Yes Increase Module Decrease Module Voltage Voltage Increase Module Voltage Updates V(k 1) = V(k) P(k 1) = P(k) k = k+1 Figure 3.11: Flow chart of Perturb and Observe MPPT algorithm Simulink Model of Perturb and Observe MPPT Algorithm To know the behaviour of Perturb and Observe MPPT Algorithm, a simulation model is developed which is given in figure The block diagram for this model is given in figure

16 Figure 3.12: Simulink model of Perturb and Observe MPPT algorithm Simulation Results D u t y c y c l e ( k ) time t(in sec) Figure 3.13: Simulink result of duty cycle of Perturb and Observe MPPT algorithm Figure 3.13 shows the duty cycle of the boost converter which is obtained from the MPPT by Perturb and Observe technique. Figure shows the ON and OFF period for the converter. 59

17 Limitations of Perturb and Observe Algorithm Figure 3.14: Curve showing wrong tracking of MPP by P&O algorithm under rapidly varying irradiance In a situation where the irradiance changes rapidly, the MPP also moves on the right hand side of the curve. The algorithm takes it as a change due to perturbation and in the next iteration it changes the direction of perturbation and hence goes away from the MPP as shown in the figure However, in this algorithm one sensor is used as voltage sensor, to sense the PV array voltage and so the cost of implementation is less and hence easy to implement. The time complexity of this algorithm is very less but on reaching very close to the MPP it doesn t stop at the MPP and keeps on perturbing in both the directions. When this happens the algorithm has reached very close to the MPP and an appropriate error limit is set or a wait function can be used which ends up increasing the time complexity of the algorithm Incremental Conductance (IC) Method The disadvantage of the perturb and observe method to track the peak power under fast varying atmospheric condition is overcome by IC method. The IC can determine that the MPPT has reached the MPP and stop perturbing the operating point. If this condition is not met, the direction in which the MPPT operating point must be perturbed & can be calculated using the relationship between dl/dv and I/V. This relationship is derived from the fact that 60

18 dp/dv is negative when the MPPT is to the right of the MPP and positive when it is to the left of the MPP. We Know, P= V I (3.7) or or or But for maximum power the slope should be zero. or or (3.8) In this equation, left hand side represents PV array s incremental conductance, while right hand side represents the opposite of its instantaneous conductance. It is clear that, at MPP, these two values should be equal. di dv > I V with dp dv 0 > (3.9) And di dv I dp < with < 0 V dv Equation 3.9 decides the direction in which the operating point should move towards the MPP. Under sudden changing condition of weather, the right direction is tracked in incremental conductance algorithm, which is not possible in P&O algorithm and also the point is not oscillated around the MPP as it is happened in case of P&O algorithm. Figure 3.15 shows the shifting of MPP under varying atmospheric condition and MPP is achieved when dp/dv = 0. This algorithm has advantages over P&O is that, it can determine when the MPPT has reached the MPP, where P&O oscillates around the MPP. Also, incremental conductance can track rapidly increasing and decreasing irradiance conditions with higher accuracy than perturb and observe. 61

19 Figure 3.15: PV characteristics showing MPP and operating points A and B. One disadvantage of this algorithm is the increased complexity when compared to P&O. The flowchart for incremental conductance algorithm is given in figure Start Measure V(k) and I(k) di = I(k) I(k 1) dv = V(k) V(k 1) No V 0 Yes (di/dv + I/V) 0 Yes No No (di/dv + I/V) > 0 V(k) =V(k) + dv Yes No No Error! Error! Yes Yes V(k) =V(k) dv V(k) =V(k) dv V(k) =V(k) + dv k=k+1 Figure 3.16: Flowchart for Incremental Conductance Algorithm. 62

20 3.6 Simulation and Result: The MPPT algorithm is simulated in MATLAB environment. The block diagram and the simulation diagrams for the MPPT algorithm are given below. Figure 3.17 shows the block diagram of PV System with MPPT Algorithm. Figure 3.17: Block diagram Model of PV System with MPPT Algorithm. Figure 3.18 shows simulation model of PV System with MPPT Algorithm.The MATLAB simulation model of the PV system with MPPT algorithm is shown in figure below. The simulation model is based on the block diagram model shown above. It consists of two sub systems that is PV panel and MPPT controller, a boost converter and several scopes to show the simulation results. Figure 3.18: Simulation Model of PV System with MPPT Algorithm. The simulation diagram for solar PV cell is shown in figure 3.19, which is a sub system of PV panel in the main simulation diagram given in figure

21 Figure 3.19: Simulation Diagram of Solar PV Cell There are several MPPT algorithm available in literature whereas, in this thesis two type of MPPT controller algorithm is used for simulation study that is IC algorithm and P&O algorithm. The simulation model of MPPT controller (which is a subsystem for the main simulation model given in figure 3.18) based on IC algorithm is shown in figure This model developed based on the flowchart which is given in figure Figure 3.20: Simulation Model of IC algorithm. The simulation model of MPPT controller (which is a subsystem for the main simulation model given in figure 3.18) based on P&O algorithm is shown in figure This model developed based on the flowchart which is given in figure The sub sytems areshown in subsequent figures. 64

22 The comparison result of output voltage, current and power for the above two MPPT controller algorithm is given in section 3.7. Figure 3.21: Simulation Model of P&O Algorithm. Figure 3.22: Simulation of Load Current. Figure 3.22 represent the subsystem of the main Simulink diagram for load current 65

23 Figure 3.23: Simulation of Shunt Current. Figure 3.23 represent the subsystem of the main Simulink diagram for shunt current Figure 3.24: Simulation of Diode Current. Figure 3.24 represent the subsystem of the main Simulink diagram for diode current Figure 3.25: Simulation of Photo Current. 66

24 The subsystem of the main Simulink diagram for photo current is given in figure 3.25 Figure 3.26: Simulation of Diode Leakage Current. The subsystem of the main Simulink diagram for diode leakage current is given in figure 3.26 Figure 3.27: Simulation of Reverse Saturation Current. The subsystem of the main Simulink diagram for reverse saturation current is given in figure Simulation Results The IGBT is used in boost converters and the GATE signal for triggering the IGBT is given in figure

25 Figure 3.28: Gating Signal in IC Method. The simulated results of output voltage, current and power are shown below. To understand the behaviour of P&O and IC methods, the simulated results of both the method are plotted in same plot for comparison purpose. Figure 3.29: Output Voltage in P&O and IC Method. Figure 3.30: Output Current in P&O and IC Method. 68

26 Figure 3.31: Output Power in P&O and IC Method. From the figure 3.29 to 3.31 it is observed that in MPPT controller based on P&O algorithm gives better result that is more voltage, current and power compare to IC algorithm. After the initial transient period the P&O algorithm gives the smooth variation of the outputs. The results show that the best MPPT technique is the modified P&O method. The logic turned out to be effective in both the situations which always provides the highest efficiency. P&O technique shows its limit in the response to the irradiance variation at low irradiance level. The IC technique has efficiency lower than the P&O techniques, but its response time is quite independent to the irradiation values and its efficiency increase with the irradiance level. This technique can be a good alternative to the P&O techniques in applications characterized by high, fast and continuous radiance variations, e.g. the PV applications in transportation systems. 3.8 Summary: In this chapter, the photovoltaic system with DC-DC boost converter and maximum power point controller has been designed and constant voltage of 21.5V is maintained at the output side of the converter. For the specified input variation, a regulated dc output voltage of 21.5V has been obtained resulting in an efficiency of 95%. It is concluded that Perturb & Observe method has better efficiency compared to Incremental Conductance method at low power. In this case, Perturb & Observe method gave an increase of 2.6% in voltage, 5.3% increase in current and 7% increase in power at low power output, but is inefficient in case of sudden change in irradiance level. From the modelling of boost converter, it was also observed that the output voltage of the boost converter increases along with the increase in duty cycle. 69