MPPT BASED GRID CONNECTED SYSTEM WITH P&O ALGORITHM

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1 MPPT BASED GRID CONNECTED SYSTEM WITH P&O ALGORITHM 1 Anuradha, 2 Satish Kumar 1 Scholar, RPIIT, Karnal, Haryana, India. nehraanu23@gmail.com 2 Assistant Professor, RPIIT, Karnal, Haryana, India. satishhctm@gmail.com Abstract The world is running out of conventional fuel resource with no alternate option other than to enhance the generation of clean and resourceful energy from renewable energy sources. This led to increased attention towards solar system because it offers maximum generation capacity among all renewable energy sources. Solar power has numerous advantages, but they do not show advantageous efficiency. Solar cell efficiency depends up on various factors i.e. Temperature, Insolation, shadow, spectral characteristics of sunlight, dirt etc. A rapidly changing irradiance on panels due to change in climate also reduces the result or output power of photovoltaic (PV) array. The utilization factor of irradiants and capability of photovoltaic systems can be improved to a large extent by employing various control techniques and algorithms. In this paper we present Perturb & Observe (P&O) to trace maximum power point with changing irradiations. The simulation presents a 200 kilo-watt PV array modeled and connected to a 25 kilo-volt (kv) grid. The grid and the array are connected by using two boost converters (DC-DC) and a single three-phase voltage source converter. This model allows using much larger time steps 50 μs, resulting in a much faster simulation. Keyword Solar Cell, grid, renewable energy. 1. INTRODUCTION Total installed power capacity is the crucial factor which decides the growth rate of any country. In order to achieve and maintain the expected economic growth rate (8-9%) India needs to generate more and more of electric power. Renewable energy sources have major contribution in electric power generation. These days Renewable Energy resources and technologies are gaining popularity and importance in the world. Solar energy, geothermal energy, wind energy, etc. are among the numerous renewable sources available for generation of electric power. Solar energy is gaining popularity in the field of electricity generation due to the ease it provides by directly converting solar irradiance in to electricity or electric energy. The easiness and the cleanliness make the solar energy a good choice for electric power generation. Silicon is commonly used in the fabrication of solar cell and a series of these cells is called a PV solar module. The current rating of these solar PV-modules depends on the area of the each cell and increases as area of cell increases. A parallel and series combinations of PV modules results in solar PV arrays, gives more power output. A typical solar cell generally converts 30-40% of solar energy to which it is exposed, into electrical energy. Based on the type of semiconducting material that is used in a module its efficiency varies. Several types of semiconductor materials like C-SI, P-SI, A-SI, CIS etc. are available. Cells that are connected in series satisfy higher voltage requirements and that of parallel which gives higher current requirements. The maximum amount of power that can be extracted from a solar panel depends on the solar irradiance, temperature, load so the input to the solar cell is basically irradiance and temperature of that particular area. Based on the isolation level and temperature maximum power output from a panel varies. A typical solar PV cell generates very low power and for extracting maximum power output from a solar cell we are using MPPT technique. MPPT techniques automatically find the voltage or current at which a PV module should operate to generate maximum power is called maximum power point. A particular MPPT technique is chosen based on the factors like, cost, simplicity quick tracking under varying power output locations, atmospheric conditions, small etc. Most MPPT techniques would automatically respond to changes in both temperature and irradiance. This thesis analyzes Perturb & Observe technique through results under normal and varying atmospheric conditions. Here, irradiance can be changed as per requirements. Grid connected systems are common now-a-days. It is very important to extract maximum power output from PV modules, for which MPPT is employed. The benefits of MPPT technique are that it automatically finds out the maximum current and voltage from the PV panel such that it operates under maximum power point. The most common and useful MPPT algorithms are Perturb and Observe algorithm, Fuzzy logic algorithm, Incremental Conductance algorithm. For a grid connected system there are two stages: First stage: Boosting stage which boosts the output from a solar PV module as per the requirements. Second stage: It is a dc to ac conversion stage. When a PV system is interconnected to the utility network system the main demands to that type of system are power quality and power system stability. For a PV grid connected system there are certain protection schemes that are implemented which particularly provides protection against islanding, voltage sag/swell, overvoltage and over current protection. Through proper analysis and experiments the quality of material and methods has been increased in due course of time, making solar cell more efficient and productive. The efficiency of collection process is based on various factors including PV cell efficiency, energy storage process and source radiation intensity. The efficiency of a PV-cell is affected by materials used in fabrication. It is hard to 55 P a g e

2 make advances in the overall operation of the PV cell, and hence the efficiency of overall process is limited. Therefore, the other possible method is to increase the intensity of solar radiation to improve the performance of solar cell module. Three major approaches are there for extraction of maximum power in solar systems. They are: (i) Sun tracking (ii) Hybrid (maximum power and sun tracking) (iii) Maximum power point tracking (MPPT) Solar energy has emerged as a major alternative over conventional energy, but its low efficiency and high initial cost are some constraints in its usage as a primary source of power. Solar PV generating system has to operate at the maximum power output point to utilize the maximum available solar power of the array and to obtain the highest energy conversion output from PV-array. Since the maximum power point varies with radiation and temperature, it is difficult to maintain optimum power operation at all radiation levels. The MPPT methods presented so far have been developed and implemented, differ from each other in several aspects such as complexity, required number of sensors, convergence speed, cost, range of effectiveness, ease of hardware implementation etc. Although different methods have been developed by different research groups, very little literature is available, where different MPPT techniques/methods are compared in terms of energy capture, conversion efficiency, response time and reliability. This research work comprises of the performance of Perturb & Observe algorithm MPPT methods that is currently used in a solar PV system and also advocates a new MPPT technique which offers better performance than the existing ones. The method used for analysis is as follows: Initially, a MATLAB based solar PV array model is first developed and validated; then, Maximum power point tracking techniques based on Perturb & Observe algorithm is employed on this PV array under varying insolation conditions and temperature to study the effectiveness of the particular Maximum power point tracking technique under consideration. The objective of this thesis is to design and simulation of MPPT controlled solar system using Perturb & Observe algorithm and track maximum power point from the characteristic curve of PV array. The maximum power depends up on various factors such as solar insolation, temperature, voltage and current etc. So, with the change in atmospheric condition, the power increases or decreases as its relation varies with that factor accordingly. So, it is essential to track maximum power point to draw more efficiency from PV panel. conductance, constant voltage, constant current and parasitic capacitance algorithm are various commonly used algorithms. Here we used P&O algorithm to track maximum power point as shown in results obtained from proposed model. 2. MAXIMUM POWER POINT TRACKING ALGORITHMS 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. There are different methods used to track the maximum power point (MPP). Some most popular techniques are: (i) Perturb and observe (hill climbing method) (ii) Neural networks (iii) Fractional open circuit voltage (iv) Fractional short circuit current (v) Incremental Conductance method (vi) Fuzzy logic The choice of the algorithm depends on the time complexity the algorithm takes to track the MPP implementation cost and the ease of implementation. MPPT Technique Perturb & Observe Table I Characteristics of Different MPPT Techniques Incremental Conductance Convergence speed Complexit y Tunin g Sensed Parameter s Varies Low No Voltage Varies Medium No Voltage, Current Fractional Medium Low Yes Voltage Fractional Medium Medium Yes Current Fuzzy Logic Fast High Yes Varies For this purpose, there are many MPPT algorithms are available. Perturb and observe (P&O), incremental 56 P a g e

3 3. PROPOSED PERTURB & OBSERVE ALGORITHM The Perturb & Observe algorithm states that when the operating voltage of the PV panel is perturbed by a small increment, if the resulting change in power is positive, then we are going in the direction of Maximum Power Point (MPP) and we keep on perturbing in the same direction. If P is negative, we are going away from the direction of MPP and the sign of perturbation supplied has to be changed. Figure. 2: Flowchart of Perturb & Observe algorithm 4. SIMULINK MODEL OF GRID- CONNECTED PV ARRAY Figure. 1: Characteristics of Solar panel showing MPP and operating points A and B Fig.1 shows the plot of module voltage for a solar panel at given irradiation versus module output power. The point marked as MPP is the Maximum Power Point, from the PV panel the maximum theoretical output can be obtained. Consider A and B are two operating points. The Point A is on the left hand side of MPP as shown in the figure above. Therefore, we can move towards the MPP by providing a positive perturbation to the voltage. On the other hand, point B is on the right hand side of the MPP. When we give a positive perturbation, the value of P becomes negative, thus it is imperative to change the direction of perturbation to achieve MPP. The flowchart for the P&O algorithm is shown in Fig.2. A Simulink model has been proposed of the solar system connected PV modules to form a 200-kW array which is connected to two DC-DC boost converters and a single three-phase voltage source converter in order to feed a 25- kv grid. The MPPT controller based on the Perturb and Observe controller is used to trace Maximum Power Point (MPP). The Perturb and Observe algorithm is coded in C-language which can also be simulated in MATLAB. The model contains: Two PV arrays delivering each a rating of 100 kilo- Watt at 1000 sun irradiance. Two boost converter each for Panel-PV1 and Panel- PV2 of rating 500 V DC. The two MPPT controllers use the Perturb and Observe technique. A Voltage Source Converter converting 500 V DC to 260 V AC at unity power factor. A Capacitor bank (20-kvar) used to remove harmonics introduced in the system by Voltage source converter. A three-phase coupling transformer (200-kVA) in step up mode 260V/25kV. A distribution feeder (25-kV) integrated with an equivalent transmission systems (120 kv) to form a complete utility grid. 57 P a g e

4 In the proposed Simulink-model the boost-converter and voltage source converter are modeled with a voltage sources producing the equivalent AC voltage averaged over one cycle of the switching frequency. This model is capable of doing faster simulation as it has much larger steps time of 50 s. In order to obtain iterations and accuracy, algebraic loops has been introduced in the PV models. 4.1 PV array: The single module of PV-array (PV-1) is having SPR (Sun-Power) of Watt. These modules are arranged in strings, each of having 66 modules. The array of these strings consists of 5 series-connected modules connected in parallel. Similar calculation can be done for Kyocera-DD205GX- LP used as solar panel-2 (PV-2). Module Specifications Number of seriesconnected cells Table II Specifications for One Module SunPower- SPR305 (PV1) Kyocera- DD205GX-LP (PV2) Open-circuit voltage: 64.2 V V Short-circuit current: 5.96 A A Voltage ( current( power ) and ) at maximum 54.7 V, 5.58 A 26.6 V, A (b) Figure. 3 : I-V and P-V characteristics of PV array (a) For a Single Cell (b) For the Array Red dots on blue curves indicate module manufacturer specifications (Voc, Isc, Vmp, Imp) under standard test conditions (25 degrees Celsius, 1000 W/m2). Any of the ten various array types can be selected from the Module type menu for the simulation. Here we used two 100 kilo-watt PV arrays. PV1 uses SunPower-SPR305 modules and PV2 uses Kyocera-DD205GX-LP modules. 4.2 Boost converter: In the detailed model, the boost converter boosts DC voltage from V to 500V. This converter uses a MPPT system which automatically varies the duty cycle in order to generate the required voltage to extract maximum power. Look under the mask of the Boost Converter Control block to see how the MPPT algorithm is implemented. The I-V and P-V characteristics for one module and for the whole array can be plot from the PV array block menu. The characteristics of the SunPower-SPR305 array are reproduced below. (a) 4.3 VSC Converter: The three-level Voltage Source Converter (VSC) regulates DC bus voltage at 500 V and keeps power factor to unity. The control system uses two control loops:1) Internal control loop 2) External control loop. An Internal control loop regulates the grid currents Iq and Id (reactive and active currents components) and an external control loop regulates DC link voltage to +/- 250 V. The output of the DC voltage external controller is reference Id current. Iq current reference is set to zero in order to maintain power factor unity. Vq and Vd are the outputs voltage of the current controller are converted to three modulating signals abc used by the Pulse Width Modulator (PWM) three-level pulse generator. The control system uses a sample time of 100 µs for voltage and current controllers as well as for the PLL synchronization unit. In the detailed model, pulse generators of Boost and VSC converters use a fast sample time of 1µs in order to get an appropriate resolution of 58 P a g e

5 PWM waveforms. Observe the performance of the two Perturb and Observe MPPTs under various irradiance changes. It can be seen that this type of MPPT controller tracks maximum power only while irradiance stays constant. Diode characteristic where: Id = diode current (A) Vd = diode voltage (V) Isat = diode saturation current (A) T = cell temperature (K), k = Boltzman constant = e-23 J.K^-1 q = electron charge = e-19 C Qd = diode quality factor Ncell= number of series-connected cells per module Nser = number of series-connected modules per string 5. RESULTS OF THE PROPOSED MODEL Figure. 4: Simulink model of PV array Figure. 5: Solar Irradiance Panel 1 & Panel 2 Figure. 6: Reference Voltage (Vref) & Average Voltage (Vmean) 59 P a g e

6 Figure. 7: Modulation Index Figure. 11: Voltage variation of Panel1 & Panel2 according to the solar Irradiance Figure. 8: Power on Bus B1 (kw) Figure. 12: Duty cycle of the boost converters (D1 & D2) All the above results outlined the variation of current, voltage and power at the grid/bus and the solar panels in accordance with change in irradiations in order to get maximum output using P&O integrated MPPT technique. 6. CONCLUSION & SCOPE Figure. 9: Grid Volatge(Va) and Grid Current(Ia) Figure. 10: Pmean1 (Panel1) and Pmean2 (Panel2) The investigation of results performance has been successfully demonstrated in Simulink. The developed model provides the optimal performance of the designed control algorithm for obtaining the maximum power under solar irradiations variation. The PV array has been mathematically modeled and its performance is tested on Simulink platform while varying the temperature and irradiations. The result waveforms obtained clearly presents the effect of temperature and irradiance on output voltage and power from the PV system. Finally the experimental study was carried out for extracting the maximum power from PV array using Simulink. The Perturb & Observe algorithm developed in Simulink model can be verified with the real time environment. The manual control of the MPPT in reference to the analysis using MATLAB-Simulink can replaced with a welldeveloped control algorithm so as to give a better performance. The experimental setup that was build can be used for studying the effects of partial shading of PV array. 60 P a g e

7 REFERENCES [1] TrishanEsram and Patrick L.Chapman, Comparison of Photovoltaic ArrayMaximum Power Point Tracking Techniques, IEEE Transactions on EnergyConversion, Vol. 22, No. 2, June [2] Hung-I Hsieh, Jen-Hao Hsieh, et al., A Study of High- FrequencyPhotovoltaic Pulse Charger for Lead-Acid Battery Guided by PI-INC MPPT. [3] K.H. Hussein, I. Muta, T. Hoshino and M. Osakada, Maximum photovoltaicpower tracking:an algorithm for rapidly changing atmosphericconditions, IEEEploc.-Gener. Transmission and Distribution, Vol. 142, No. 1, Jan [4] C.Thulasiyammal and S Sutha, An Efficient Method of MPPT TrackingSystem of a Solar Powered Uninterruptible Power Supply Application, 1stInternational Conference on Electrical Energy Systems, [5] NoppadolKhaehintung and PhaophakSirisuk, Application of MaximumPower Point Tracker with Self-organizing Fuzzy Logic Controller for SolarpoweredTraffic Lights, IEEE, [6] C. S. Chin, P. Neelakantan, et al., Fuzzy Logic Based MPPT for PhotovoltaicModules Influenced by Solar Irradiation and Cell Temperature, UKSim 13thInternational Conference on Modelling and Simulation, [7] PanomPetchjatuporn, PhaophakSirisuk, et al., A Solarpowered BatteryCharger with Neural Network Maximum Power Point Tracking Implementedon a Low-Cost PICmicrocontroller. [8] S. Yuvarajan and JulineShoeb, A Fast and Accurate Maximum Power PointTracker for PV Systems, IEEE, [9] Prof. Dr. Ilhami Colak, Dr.ErsanKabalci and Prof.Dr.GungorBal, Parallel DCACConversion System Based on Separate Solar Farms with MPPT Control, 8th International Conference on Power Electronics - ECCE Asia, TheShillaJeju, Korea, May 30-June 3, [10] [10] S. G. Tesfahunegn, O. Ulleberg, et al., A simplified battery charge controllerfor safety and increased utilization in standalone PV applications, IEEE,2011. [11] Yuncong Jiang, Ahmed Hassan, EmadAbdelkarem and Mohamed Orabi, Load Current Based Analog MPPT Controller for PV Solar Systems, IEEE,2012. [12] ArashShafiei, AhmadrezaMomeni and Sheldon S. Williamson, A NovelPhotovoltaic Maximum Power Point Tracker for Battery ChargingApplications, IEEE, [13] Ali F Murtaza, Hadeed Ahmed Sher, et al., A Novel Hybrid MPPTTechnique for Solar PV Applications Using Perturb & Observe and FractionalOpen Circuit Voltage Techniques. [14] Weidong Xiao, Nathan Ozog and William G. Dunford, Topology Study ofphotovoltaic Interface for Maximum Power Point Tracking, IEEETransactions on Industrial Electronics, Vol. 54, No. 3, June [15] Jun Pan, Chenghua Wang and Feng Hong, Research of PhotovoltaicCharging System with Maximum Power Point Tracking, The NinthInternational Conference on Electronic Measurement & Instruments ICEMI,2009. [16] Sandeep Anand, Rajesh Singh Farswan, et al., Optimal Charging of BatteryUsing Solar PV in Standalone DC System. [17] Mohamed Azab, A New Maximum Power Point Tracking for PhotovoltaicSystems, International Journal of Electrical and Electronics Engineering3:11, P a g e