A Fast Converging MPPT Technique for PV System under Fast Varying Solar Irradiation and Load Resistance

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1 A Fast Converging MPPT Technique for PV System under Fast Varying Solar Irradiation and Load Resistance P.Jenopaul 1, Rahul.R 2, Barvinjegan.P 3, and Sreedevi.M 4 1,2,3,4 (Department of Electrical and Electronics Engineering, St. Thomas College of Engineering and Technology Kerala, India) Abstract: Under fast-varying solar irradiation and load resistance, a fast-converging maximum power point tracking (MPPT) system is required to ensure the photovoltaic (PV) system response rapidly with minimum power losses. Traditionally, maximum power point (MPP) locus was used to provide such a fast response. However, the algorithm requires extra control loop or intermittent disconnection of the PV module. Hence, this project proposes a simpler fast-converging MPPT technique, which excludes the extra control loop and intermittent disconnection. In the proposed algorithm, the relationship between the load line and the I V curve is used with trigonometry rule to obtain the fast response. Results of the experiment using single ended primary-inductor converter showed that the response of the proposed algorithm is four times faster than the conventional incremental conductance algorithm during the load and solar irradiation variation. Consequently, the proposed algorithm has higher efficiency. Keywords: Fast converging, incremental conductance, maximum power point tracking (MPPT), photovoltaic (PV) system single-ended primary-inductor converter (SEPIC) I. INTRODUCTION One of the major concerns in the power sector is the day-to-day increasing power demand but the unavailability of enough resources to meet the power demand using the conventional energy sources. Demand has increased for renewable sources of energy to be utilized along with conventional systems to meet the energy demand. Renewable sources like wind energy and solar energy are the prime energy sources which are being utilized in this regard. The continuous use of fossil fuels has caused the fossil fuel deposit to be reduced and has drastically affected the environment depleting the biosphere and cumulatively adding to global warming. All forms of energy are stored in different ways in the energy sources that we use every day. These sources are divided into two groups renewable (an energy source that we can use over and over again) and non renewable (an energy source that we are using up and cannot recreate in a short period of time). Renewable and non renewable energy sources can be used to produce secondary energy sources including electricity and hydrogen. Renewable energy sources include solar energy which comes from the sun and can be turned into electricity and heat. Wind, geothermal energy from inside the earth, biomass from plants, and hydropower and ocean energy from water are also renewable energy sources. Non- conventional sources of energy acquire growing importance due to its enormous consumption and exhaustion of fossil fuel. Solar energy is the most readily available source of energy and it is free. Moreover, solar energy is the best among all the renewable energy sources since, it is non-polluting. Energy supplied by the sun in one hour is equal to the amount of energy required by the human in one year. Photo voltaic arrays are used in many applications such as water pumping, street lighting in rural town, battery charging and grid connected PV systems. Solar energy is abundantly available that has made it possible to harvest it and utilize it properly. Solar energy can be a standalone generating unit or can be a grid connected generating unit depending on the availability of a grid nearby. Thus it can be used to power rural areas where the availability of grids is very low. Another advantage of using solar energy is the portable operation whenever wherever necessary. In order to tackle the present energy crisis one has to develop an efficient manner in which power has to be extracted from the incoming solar radiation. The power conversion mechanisms have been greatly reduced in size in the past few years. The development in power electronics and material science has helped engineers to come up very small but powerful systems to withstand the high power demand. But the disadvantage of these systems is the increased power density. Trend has set in for the use of multi-input converter units that can effectively handle the voltage fluctuations. But due to high production cost and the low efficiency of these systems they can hardly compete in the competitive markets as a prime power generation source. The constant increase in the development of the solar cells manufacturing technology would definitely make the use of these technologies possible on a wider basis than what the scenario is presently. The use of the newest power control mechanisms called the Maximum Page 1

2 Power Point Tracking (MPPT) algorithms has led to the increase in the efficiency of operation of the solar modules and thus is effective in the field of utilization of renewable sources of energy. Photovoltaic System Photovoltaic (PV) generation is gaining increased importance as a renewable source due to its advantages like absence of fuel cost, little maintenance, no noise and wear due to absence of moving parts etc. In particular, energy conversion from a solar cell array (SCA) received considerable attention in the last two decades. Since the PV generator exhibits a nonlinear characteristic, its maximum power (MP) point(1,2) varies with the solar insolation and temperature. At a particular solar insolation, there is a unique operating point of the PV generator at which its power output is maximum. Therefore, for maximum utilization efficiency, it is necessary to match the PV generator to the load such that the equilibrium operating point coincides with the MP point of the PV source. Figure 1.1 Working principle of a PV cell However, since the Maximum Power point varies with insolation and seasons, it is difficult to maintain MP operation at all solar insulations without changes in the system parameters. To overcome this problem, use of an intermediate dc-dc converter is proposed which continuously adjusts the voltage, current levels, and matches the PV source to the load. The PV energy is increasing interest in electrical power applications. It is crucial to operate PV energy conversion systems near maximum power point to increase the output efficiency of PV. However, the nonlinear nature of PV systems is apparent the current and power of PV array depend on the array terminal operating voltage. In addition, the maximum power operating point is changing with insolation level and temperature. Therefore, the tracking control of maximum power point is the complicated problem. To overcome these problems, many tracking control strategies have been proposed such as perturb and observe, incremental conductance, parasitic capacitance, constant voltage, neural network and fuzzy logic control. These strategies have some disadvantage such as costly, complexity and non-stability. The general requirements for maximum power point tracker are simple, low cost, quick tracking under condition change, and small output power fluctuation. A more efficient method to solve this problem becomes crucially important. Hence, this paper proposed the method to track power maximum power point by using adaptive fuzzy logic control. Fuzzy logic control is appropriate for nonlinear control and it has no complex mathematical. However, the fuzzy logic controller behavior depends on the membership functions, their distribution, and the rules that influence the different fuzzy variables in the system. There is no formal method to determine accurately the parameters of the controller. However, choosing fuzzy parameter to yield optimum operating point and good control system is up to experience from control engineer. For this reason, adaptive fuzzy logic control can solved this problem because it can re-adjust fuzzy parameter to obtain optimum performance. Maximum Power Point Tracking A typical solar panel converts only 30 to 40 percent of the incident solar irradiation into electrical energy. Maximum power point tracking technique is used to improve the efficiency of the solar panel. According to Maximum Power Transfer theorem, the power output of a circuit is maximum when the Thevenin impedance of the circuit (source impedance) matches with the load impedance. Hence our problem of tracking the maximum power point reduces to an impedance matching problem. In the source side we are using a boost convertor connected to a solar panel in order to enhance the output voltage so that it can be used for different applications like motor load. By changing the duty cycle of the boost converter appropriately we can match the source impedance with that of the load impedance. The efficiency of a solar cell is very low. In order to increase the efficiency, methods are to be undertaken to match the source and load properly. One such method is the Maximum Power Point Tracking (MPPT). This is a technique used to obtain the maximum possible power from a varying source. Page 2

3 In photovoltaic systems the I-V curve is non-linear, thereby making it difficult to be used to power a certain load. This is done by utilizing a boost converter whose duty cycle is varied by using a MPPT algorithm. A maximum power point tracker (MPPT) is a system that directs the converter to track the maximum power of a solar panel and deliver it to load. MPPT is an electronic system that varies the electrical operating point so that it will deliver a maximum output power. MPPT Techniques-Perturb and Observe Method The P&O algorithm is also called hill-climbing (3), but both names refer to the same algorithm depending on how it is implemented. P&O involves a perturbation in the operating voltage of the DC link between the PV array and the power converter. In the case of the Hill-climbing, perturbing the duty cycle of the power converter implies modifying the voltage of the DC link between the PV array and the power converter, so both names refer to the same technique. In this method, the operating voltage is sampled and the algorithm changes the operating voltage in the required direction. If dp/dv is positive, then the algorithm increases the voltage value towards the MPP until dp/dv is negative. This iteration is continued until the algorithm finally reaches the MPP. The major drawbacks of P&O or hill-climbing are occasional deviation from the maximum operating point in case of rapidly changing atmospheric conditions, such as broken clouds. Also, correct perturbation size is important in providing good performance in both dynamic and steady state response. Incremental Conductance (IC) Technique: The incremental conductance algorithm (4,5,6) is based on the fact that the slope of the curve power vs. voltage (current) of the PV module is zero at the MPP, positive (negative) on the left of it and negative (positive) on the right. This method uses the PV array's incremental conductance dp/dvto compute the sign of dp/dv. When di/dv is equal and opposite to the value of I/V (where dp/dv = 0) the algorithm knows that the maximum power point is reached and thus it terminates and returns the corresponding value of operating voltage for MPP. This method tracks rapidly changing irradiation conditions more accurately than P&O method. One complexity in this method (8,9,10)is that it requires many sensors to operate and hence is economically less effective. When the maximum power point is reached the slope dp/dv = 0. Thus the condition would be; dp/dv = 0. It is the same efficient as P&O, good yield under rapidly changing atmospheric conditions. Here, also the same perturbation size problem as the P&O exists and an attempt has been made to solve by taking variable step size. But, it requires complex and costly control circuits. II. PROPOSED INCREMENTAL CONDUCTANCE ALGORITHM Figure 2.1: proposed PV system with MPP controller A few modified algorithms have been introduced to improve converging speed during the variation in solar irradiation level and load. The relationship between the load line and the MPP locus is used in to present a fast-converging algorithm. The MPP locus is a line which approximately connects all the MPP for all levels of solar irradiation, a control loop is introduced to ensure the PV system operates in accordance with the MPP locus and thus the MPP searching time is reduced. However, tuning is required in the control loop, and it further complicates the MPPT system., the control loop is eliminated, but the short circuit current and open circuit voltage are required. Thus, disconnection of PV module is required to collect the data and the power is wasted during disconnection. Page 3

4 Although the aforementioned algorithms can provide fast response, the complexity of the systems is greatly increased. Therefore, this project proposes a modified MPPT algorithm, i.e., able to provide fast response without the requirement of an extra control loop. Other than that, the proposed system also does not require the intermittent disconnection. The proposed PV system simply consists of a dc dc converter which connected in between the PV module and load. Then, the current and voltage of the PV module are sensed by a PIC controller, which is also used to execute the modified MPPT algorithm. An inverter and a rectifier are connected at the output of the dc dc converter to validate the proficiency of the proposed algorithm under a nonlinear load. Characteristic of PV System with DC DC Converter A PV module consists of numbers of solar cell connected in series or parallel and the total power generated is the sum of the power contributed by all of the individual solar cells. A few methods exist in modeling the PV cell, and the model in is used to model the PV cell in this paper. Under different levels of solar irradiation, the PV module produces different levels of power. A load line is generated and it can be imposed on the I V curve when the PV module supplies power to the load. The power generated by the PV module is the product of the voltage and current of PV module at the intersection point between the load line and the I V curve. Therefore, the output power of PV module varies according to the solar irradiation (I V curve) and the resistance of the load (load line). Generally, a dc dc converter is connected in between the PV module and the load. Then, the MPPT controller is used to regulate the duty cycle of the dc dc converter to ensure the load line always cuts through the I V curve at MPP. Thus, the variation in the voltage and current of PV module during the variation in solar irradiation or load as shown in Table I must be considered by the MPPT controller. If the duty cycle of dc dc converter is fixed, the variation in solar irradiation will either increase or decrease both the voltage and current of PV module simultaneously. Meanwhile, load variation will increase (decrease) the voltage and decrease (increase) the current of PV module. Variations in the voltage and current are always in the opposite direction under load variation. The MPPT controller should only regulates the duty cycle of dc-dc converter after the variation in solar irradiation or load is determined. Proposed MPPT Algorithm Figure 2.2: The flowchart for the proposed algorithm Page 4

5 A flag value is used to indicate that the PV system is operating at the MPP if it is set to 1. Therefore, the flag is set to 0 initially. Then, the conventional incremental conductance algorithm is used to track the MPP. A permitted error of 0.06, issued in the proposed algorithm to eliminate the steady-state oscillation in the system after the MPP is reached. The permitted error is chosen based on the duty cycle step size (0.005), and the accuracy. Incremental Conductance (IC) The disadvantage of perturb and observe method to track the peak power under fast varying atmospheric condition is overcome by IC method. The fig.2.3 shows the power verses voltage graph of 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 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. This algorithm has advantages over P&O in 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. One disadvantage of this algorithm is the increased complexity when compared to P&O. Figure 2.3: Power versus voltage graph of IC method Figure 2.4: IC Algorithm The figure 2.4 shows IC algorithm. From this algorithm first of all find out the current and voltage of particular instant. Then find out the difference in voltage and current of the particular instant and the previous instant. Then check the change in voltage is zero, if this condition is true then check the change in current is zero, if this condition is true update the algorithm. otherwise check the sum of current and the product of incremental conductance and voltage is zero, if it is false,then again check it is greater than zero, if it is not Page 5

6 satisfy reduces the change in voltage from the reference voltage,otherwise add the change in voltage and the reference voltage and update the algorithm. Hardware System Description The system consists of a PV panel connected to a boost converter to enhance and regulate the output voltage. It drives the DC load by using the power tracked from the solar panel. The MPPT controller is used to track the maximum power from solar panel. The block diagram of the system is shown in Figure 2.5 Figure 2.5: Block diagram of the system PV Array Model A solar panel cell basically is a p-n semiconductor junction. When exposed to the light, a DC current is generated. The generated current varies linearly with the solar irradiance. The equivalent electrical circuit of an ideal solar cell can be treated as a current source parallel with a diode shown in figure 2.6. Usually a number of PV modules are arranged in series and parallel to meet the energy requirements. PV modules of different sizes are commercially available, For example, a typical small scale desalination plant requires a few thousand watts of power.a PV array consists of several photovoltaic cells in series and parallel connections. Series connections are responsible for increasing the voltage of the module whereas the parallel connection is responsible for increasing the current in the array. Figure 2.6: Solar cell model The cell can be modeled by the voltage-current characteristic equation I = N p I ph -N p I s [exp{ (2.1) Where, I ph is a light-generated current or photocurrent, I s is the cell saturation of dark current, q (= C) is an electron charge, k (= J/K) is a Boltzmann s constant, Tc is the cell s working temperature, A is an ideal factor, Rs is a Series resistance. The photocurrent mainly depends on the solar insulation and cell s working temperature, which is described as I ph =I sc +K I (T c -T Ref )H (2.2) I sc is the cell s short-circuit current at a 25 C and 1kW/m2, K I is the cell s short-circuit current temperature coefficient, T ref is the cell s reference temperature, and Page 6

7 H is the solar insulation in kw/m 2. On the other hand, the cell s saturation current varies with the cell temperature, which is described as (2.3) Where I RS is the cell s reverse saturation current at a reference temperature and standard solar radiation, Eg is the bang-gap energy of the semiconductor used in the cell and A is the ideal factor, dependent on PV technology. A PV array is a group of several PV cells which are electrically connected in series and parallel circuits to generate the required current and voltage. The equivalent circuit for the solar module arranged in NP parallel and NS series cells. The shunt resistance R sh is inversely related with shunt leakage current to the ground. In general, the PV efficiency is insensitive to variation in R sh and the shunt-leakage resistance can be assumed to approach infinity without leakage current to ground. On the other hand, a small variation in R s will significantly affects the PV output power. The unknown parameters of the module can be found out by the equations, IRS(Vt) = (2.4) V t V V I I 2 MP oc sc MP ( ISC I ln I ( I I ) I MP SC MP SC MP ) (2.5) DC-DC Boost converter The fig.2.7 shows the equivalent circuit of Boost converter. The Boost converter (step-up converter) is a DC-to-DC power converter with an output voltage greater than its input voltage. A boost converter is sometimes called a step-up converter since it steps up the source voltage. Since power must be conserved, the output current is lower than the source current. Boost converter steps up the input voltage magnitude to a required output voltage magnitude without the use of a transformer. Figure 2.7: Equivalent circuit of Boost converter The energy stored in the inductor gets discharged through opposite polarities which charge the capacitor. The load current remains constant throughout the operation. The maximum power point tracking is basically a load matching problem. In order to change the input resistance of the panel to match the load resistance (by varying the duty cycle), a DC to DC converter is required. Figure 2.8: Waveform of Boost Converter Page 7

8 Figure2.8 represents the boost converter and the waveform of boost converter.the main components of a boost converter are an inductor, a diode and a high frequency switch. These in a co-ordinate manner supply power to the load at a voltage greater than the input voltage magnitude. The control strategy lies in the manipulation of the duty cycle of the switch which causes the voltage change. SL NO III. EXPERIMENTAL RESULTS Table3.1 Experimental results OUTPUT VOLT WITH P&O OUTPUT VOLT WITH IC Figure.3.1: Output voltage with I C Figure3.2: Output voltage with P&O The above two graph shows the experimental results of the MPPT technique to achieve the incremental conductance method. The figure3.2 shows the output voltage with P&O. Here we are using the solar panel of rating 12V, 20W. The fig.3.1 shows the output voltage with I C. The two graph readings are obtained corresponding to the time instant. The table3.1 gives the overall experimental result of the IC algorithm of the MPPT technique. The table3.1 shows the readings of output voltage without MPPT and output voltage with MPPT. The output voltage with MPPT can be increased with respect the output voltage without MPPT corresponding to the different time instant. Page 8

9 Figure3.3: Comparison of I C and P&O The fig 3.3 represents shows the comparison of I C and P&O algorithms. The blue line represents the output voltage of with P&O. Here we can show that the output voltage is low. The red line represents the output voltage with I C algorithm; here the output voltage is higher than the P&O algorithm. Schematic Diagram Figure3.4: Gate control circuit of IGBT The schematic diagram of proposed MPPT algorithm is shown in the above figures. The figure3.4 shows the overall circuit layout of proposed system. This circuit mainly consists of a solar panel and a SEPIC convertor. Here we introduced solar panel having rating of 12V,20 W. Single ended primary inductor coupled convertor is type of DC-DC convertor allowing the electrical potential at its output to be greater than, less than or equal to that at input. The output of the SEPIC is controlled by the duty cycle of the IGBT The SEPIC is connected in between the solar panel and load. The SEPIC exchanges energy between the capacitors and inductors in order to convert from one volt to another. The amount of energy exchanged is controlled by the IGBT. SEPIC is said to be in continues conduction mode. If the current through the inductor L never falls to zero, during a SEPIC s steady state operation. The average voltage across capacitor is equal to input voltage. Because the capacitor, C36 blocks direct current, the average current across it (I36) is zero making inductor, L2 the only source of load current, the average current through inductor L2 is same as the average load current and hence independent of the input voltage. Figure3.5: The circuit layout of proposed system The figure3.5 shows the gate control circuit of IGBT. The voltage divider helps to sense the voltage. The voltage divider circuit which connect to the gate control circuit of IGBT. This circuit consist of a Page 9

10 differential amplifier..the difference in voltage is given to the differential amplifier.thus the output voltage is detected. Hence the current can be determined using the known value of the resister. The input and output terminal capacitor helps to filter the DC. By varying the duty cycle of the converter, the voltage of the PV module is able to operate at the peak pf the PV curve. IV. CONCLUSION The MPPT control algorithms, namely incremental conductance and P&O were discussed. The project presents a simulation& real time implementation and comparison of the maximum power produced for Incremental conductance and P&O algorithms. The IC algorithm is the simplest method, which results in low cost of installation and it may be competitive with other MPPT algorithms. On the other hand, the P&O algorithm is highly depends on the users knowledge of the process operation for the parameter setting of P&O Controller. The IC method track maximum power at 0.25s and P&O based algorithm track maximum power at a time of 0.28 s. The power curve obtained with IC is smoother when compared to Incremental Conductance algorithm. The MPPT algorithms using IC are capable of tracking maximum power rapidly under varying atmospheric conditions. V. REFERENCES [1] Bidyadhar Subudhi, RaseswariPradhan: A Comparative Study on Maximum Power Point Tracking Techniques for Photovoltaic Power Systems, IEEE Trans. Sust. Energy. Jan. 2013, vol. 4, no. 1,pp [2] HairulNissahZainudin, SaadMekhilef, Comparison Study of Maximum Power Point Tracker Techniques for PV Systems, Cairo University, Egypt, December 19-21, 2015, Paper ID 278. [3] T. T. N. Khatib, A. Mohamed, N. Amin, and K. Sopian, An efficient maximum power point tracking controller for photovoltaic systems using new boost converter design and improved control algorithm,wseas Transaction. Power System, vol. 5, no. 2, pp , [4] M. A. S. Masoum, H. Dehbonei, and E. F. Fuchs, Theoretical and experimental analyses of photovoltaic systems with voltage and current-based maximum power point tracking", IEEE Transaction on Energy Conversion,vol. 17, no. 4, pp , December [5] J.-A. Jiang, T.-L. Huang, Y.-T. Hsiao, and C.-H. Chen, Maximum power tracking for photovoltaic power systems, Science and Engineering, vol. 8, no. 2, pp , [6] Y. Chen, K. Smedley, F. Vacher, and J. Brouwer, A new maximum power point tracking controller, in Proceedings 18th Annual IEEE Conference Applied Power Electronics Conference Expo, Florida, [7] O. L-Lapea, M. T. Penella, and M. Gasulla, A new MPPT method for low-power solar energy harvesting, IEEE Transaction on Industrial Electronics, vol. 57, no. 9, pp , Sep [8] V. Salas, E. Olias, A. Lazaro, and A. Barrado, Evaluation of a new maximum power point tracker applied to the photovoltaic stand-alone systems, Solar Energy on Material Solar Cells, vol. 87, no. 14, pp , [9] B. Amrouche, M. Belhamel, and A. Guessoum, Artifical intelligence based P and O MPPT method for photovoltaic systems, in Proceedings Rev.EnergiesRenouvelables, Tlemcen, Algeria, Sep. 57, 2016, pp [10] A. Iqbal, H. A. Rub, and S. M. Ahmed, Adaptive neuro-fuzzy inference system based maximum power point tracking of a solar PV module, in IEEE International Energy Conference, December 18-20, Page 10