A Hybrid Renewable Energy Generation System through a Multi Input DC-DC Converter Control

Transcription

1 A Hybrid Renewable Energy Generation System through a Multi Input DC-DC Converter Control Neeraja. G 1, Dr. Devi. V 2 1 M.Tech Scholar, Electrical & Electronics Engg. Dept., NSS College of Engineering, Palakkad, Kerala, India, 2 Professor, Electrical and Electronics Engg. Dept., NSS College of Engineering, Palakkad, Kerala, India, 1 neeraja.g2109@gmail.com, 2 devinsspkd@gmail.com Abstract: This paper presents a new system configuration of the front-end rectifier stage for a hybrid wind/photovoltaic energy system. It allows configuration allows the two sources to supply the load separately or simultaneously depending on the availability of the energy sources. The inherent nature of this Cuk-SEPIC fused converter, additional input filters are not necessary to filter out high frequency harmonics. Harmonic content is detrimental for the generator lifespan, heating issues, and efficiency. Operational analysis of the proposed system will be discussed in this paper. Simulation results are given to highlight the merits of the proposed circuit. Key Words: Hybrid energy system, Renewable, Cuk- SEPIC, Photovoltaic, Wind energy. I. INTRODUCTION Energy is the most fundamental part of our universe. It is very much essential to ensure quality of life in our society. The escalation in cost and environmental concerns involving conventional electrical energy posed a great question in today s world. Since the oil crisis in the early 1970s much attention has been focused on the use of renewable energy sources for electrical power generation. Appetite for energy is increasing day by day across the world due to the increasing needs and growing technology. In the current scenario, almost all the electricity generation takes place at central power station which utilizes coal, oil, gas, water or fissile nuclear material as the primary fuel source. There are many problems relating to the further development of generating methods based on any of these conventional fuels. Thermal power plants, the main source of electricity is costly and it uses coal as raw material which is non renewable and hence depleting. Hydro-power generation is restricted to geographically suitable areas. The possible hazards of nuclear power have been much publicized, particularly those concerning the storage and military use of nuclear waste. Renewable Energy Sources are those energy sources which are not destroyed when their energy is harnessed. Human use of renewable energy requires technologies that harness natural phenomena, such as sunlight, wind, waves, water flow, and biological processes such as anaerobic digestion, biological hydrogen production and geothermal heat. Among these solar and wind energy with wind turbines appears to be the most promising source of renewable energy. The power electronics is changing the basic characteristic of the wind turbine from being an energy source to be an active power source. In recent years, hybrid PV/wind system (HPWS) has become viable alternatives to meet environmental protection requirement and electricity demands. With the complementary characteristics between solar and wind energy resources for certain locations, hybrid PV/wind system presents an unbeatable option for the supply of small electrical loads at remote locations where no utility grid power supply. The other advantage of solar / wind hybrid system is that when solar and wind power production is when used together, the reliability of the system is enhanced. Since they can offer a high reliability of power supply, their applications and investigations gain more concerns nowadays. Water supply is one of the principal problems in all parts of the world. With increasing population need for water has also increased. In India, electrical and dieselpowered water pumping systems are widely utilized for irrigation applications. The continuous exhaustion of conventional energy sources and their environmental impacts have created an interest in choosing RESs such as solar-photovoltaic, solar-thermal, wind energy, producer gas and biomass sources to power water pumping systems [1] which uses water in the most effective manner. A recently proposed solution to this problem is the use of autonomous wind generators to electrically propel centrifugal water pumps. Autonomous water pumping system based on wind generation and control by rotor frequency is possible [2]. The power flow control is performed through the rotor winding of the generator by changing the frequency and voltage of its excitation. The need for the optimum utilization of water and energy resources has become a vital issue during the last decade, and it will become more essential in the future. The reported investigations with renewable energy source water pumping systems are categorised into five major groups.(i) solar photovoltaic water systems(spwpss), (ii) solar thermal water pumping systems (STWPSs), (iii) wind energy water pumping systems (WEWPSs), (iv) biomass water pumping systems (BWPSs) and (v) hybrid renewable energy water pumping systems (HREWPSs). 42

2 II. 43 SOLAR ENERGY SYSTEM Solar energy is the easily available and has immense potential to generate energy. It is non-polluting and is for free. It is only through the photovoltaic effect that sunlight can be converted directly into electricity. This feature of directness of conversion has been largely responsible for making photovoltaic such a popular mode of generation of electricity. Earth surface receives 1.2x10 17 W of power from sun. Energy supplied by the sun in one hour is almost equal to the amount energy required by the human population in one year. On an average the sunshine hour in India is about 6hrs annually also the sun shine shines in India for about 9 months in a year. Solar Energy is a good choice for electric power generation. The solar energy is directly converted into electrical energy by solar photovoltaic module. Solar powered water pumping systems consists of PV array, motor and pump. Depending on the system design, it requires storage batteries and charge regulator. The motor is chosen according to power requirement and the type of current output of the system. Batteryless systems are considered to be more economical. The use of solar photovoltaic energy is considered to be a primary energy source for countries located in tropical regions with solar radiation upto 1000W/m 2. Fig 1 Layout of solar photovoltaic pumping system PV Array: The PV array is constructed by many series or parallel connected solar cells [3]. Each solar cell is formed by a junction semiconductor, which can produce currents by the photovoltaic effect. The main technical factors over the past decade that have led to improved PV system performance include Improved PV module Cell manufacturing techniques and scale that have lowered PV module costs and resulted in higher module efficiency. Improved inverter performance (better efficiency, reliability, lower cost, improved protection and monitoring features). More effective application, design and integration of PV systems and standardized interconnection requirements for grid interaction system. Subsidies provided by the governments. III. WIND ENERGY SYSTEM Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electrical power, wind mills for mechanical power, wind pumps for water pumping or drainage, or sails to propel ships. Large wind farms consist of hundreds of individual wind turbines which are connected to the electric power transmission network. For new constructions, onshore wind is an inexpensive source of electricity, competitive with or in many places cheaper than fossil fuel plants. Small onshore wind farms provide electricity to isolated locations. As of 31 Jan 2013 the installed capacity of wind power in India was MW, India ranking the 5 th largest country in terms of installed wind capacity. A wind turbine obtains its power input by converting the force of the wind into torque (turning force) acting on the rotor blades. The amount of energy which the wind transfers to the rotor depends on the density of the air, the rotor area, and the wind speed. There are basically two types of wind turbine available- 1. Vertical Axis Wind Turbine (VAWT) 2. Horizontal Axis Wind Turbine (HAWT) The mechanical power generated by wind turbine in order to describe the power characteristic is given by P m = 0.5 ρ A C p (λ, β)v w 3 Where ρ = air density A = rotor swept area C p (λ, β) = power coefficient function β = pitch angle V W = velocity of wind A wind powered rotor is coupled to a synchronous generator with permanent magnets, which convert the wind energy into electrical power energy. The generator is then coupled to a common induction motor, which drives a centrifugal pump for water pumping. Fig 2 Layout of wind powered pumping system

3 IV. HYBRID SOLAR WIND ENERGY SYSTEM Hybrid systems as the name says, is the integration of two or more sources, here wind and solar. Here a multi input converter is employed. It offers higher reliability and flexibility. Even if one source is unavailable, the other can provide the required or smaller power, thus ensuring continuous power supply. Multiple-input converter (MIC) for hybrid power systems is attracting increasing attention because of reduced components, compactness and centralized control [4-5]. The MICs proposed in [6]-[9] are essentially based on parallel connection at the output of a number of boost converters and buck-boost converters. Such MICs do not enjoy the advantage of reduced device counts. The objective of this paper is to propose a novel multiinput inverter for grid-connected hybrid PV/wind power system. The proposed multi-input inverter has the following advantages: 1) Power from the PV array or the wind turbine can be delivered to the utility grid or the load. 2) A large range of input voltage variation caused by different insolation and wind speed is acceptable. The fusion of the two converters is achieved by reconfiguring the two existing diodes from each converter and the shared utilization of the Cuk output inductor by the SEPIC converter[10]. This configuration allows each converter to operate normally individually in the event that one source is unavailable. Fig 5 illustrates the case when only the wind source is available. Fig 5 Wind source is operational (SEPIC) PV array Wind Turbine Multi Input Rectifier Stage Load Fig 3 Block diagram of multi input converter V. OPERATING PRINCIPLE OF PROPOSED MULTI-INPUT INVERTER A system diagram of the proposed rectifier stage of a hybrid energy system is shown in Fig 4, where one of the input is connected to the output of the PV array and the other input connected to the output of a generator. Fig 6 PV array is operational (CuK) In this case, D 1 turns off and D 2 turns on; the proposed circuit becomes a SEPIC converter and the input to output voltage relationship is given.on the other hand, if only the PV source is available, then D 2 turns off and D 1 will always be on and the circuit becomes a Cuk converter. In both cases, both converters have step-up/down capability, which provide more design flexibility in the system if duty ratio control is utilized to perform MPPT control V dc / V PV = d 1 / 1-d 1 Fig 4 Proposed rectifier stage for multi input system 44 V dc / V W = d 2 / 1-d 2 If the turn on duration of M 1 is longer than M 2, then the switching states will be state I, II, IV. Similarly, the switching states will be state I, III, IV if the switch conduction periods are vice versa. To provide a better

4 explanation, the inductor current waveforms of each switching state are given as follows assuming that d 2 > d 1 ; hence only states I, III, IV are discussed in this example. In the following, I i,pv is the average input current from the PV source; I i,w is the RMS input current after the rectifier (wind case); and I dc is the average system output current. The key waveforms that illustrate the switching states are shown. The mathematical expression that relates the total output voltage and the two input sources will be illustrated in the next section. State I ( M1 ON, M2 ON ) i L1 = I i,pv + (V PV / L 1 ) t i L2 = I dc + ( V C1 + V C2 / L 2 ) t i L3 = I i,w + (V W / L 3 ) t State III ( M1 OFF, M2 ON) I L1 = I i,pv + ( V PV V C1 / L 1 ) t i L2 = I dc+ (V C2 / L 2) t i L3 =I i,w + (V W /L 3 ) t State IV ( M1 OFF, M2 OFF) I L1 = I i,pv + ( V PV V C1 / L 1 ) t i L2 = I dc- (V dc / L 2) t i L3 =I i,w + (V W - V C2 -V dc /L 3 ) t VI. 0 < t < d1t s 0 < t < d1t s 0 < t < d1t s d1t S < t < d2t s d1t S < t < d2t s d1t S < t < d2t s d2t S < t < T s d2t S < t < T s d2t S < t < T s SWITCHING STATES IN A SWITCHING CYCLE Fig 9 State III Fig 10 State IV Fig 7 State I 45 Fig 8 State II Fig 11 Proposed circuit inductor waveforms

5 VII. ANALYSIS To find an expression for the output DC bus voltage, V dc the volt-balance of the output inductor, L 2, is examined with d 2 > d 1. Since the net change in the voltage of L 2 is zero, applying volt-balance to L 2 results in (1). The expression that relates the average output DC voltage (V dc ) to the capacitor voltages (V C1 and V C2 ) is then obtained as shown in (2), where V C1 and V C2 can then be obtained by applying volt-balance to L 1 and L 3. The final expression that relates the average output voltage and the two input sources (V w and V PV ) is given by (3) It is observed that V dc is simply the sum of the two output voltages of the Cuk and SEPIC converter. This further implies that V dc can be controlled by d 1 and d 2 individually or simultaneously. (V C1 + V C2 ) d 1 T s + ( V C2 ) (d 2 d 1 )T s + (1-d 2 ) (-V dc )T s = 0 (1) V dc = (d 1 / 1-d 2 ) V C1 + (d 2 / 1-d 2 ) V C2 (2) V dc = (d 1 / 1-d 1 ) V PV + (d 2 / 1-d 2 ) V W (3) The switching voltage and current characteristics are also provided in this section. The voltage stress is given by (4) and (5) respectively. As for the current stress, it is observed from fig 3.14 that the peak current always occurs at the end of the on-time of the MOSFET. Both the Cuk and SEPIC MOSFET current consists of both the input current and the capacitors (C 1 or C 2 ) current. The peak current stress of M 1 and M 2 are given by (6) and (8) respectively. L eq1 and L eq2, given by (7) and (9), represent the equivalent inductance of Cuk and SEPIC converter respectively. The PV output current, which is also equal to the average input current of the Cuk converter is given in (10). It can be observed that the average inductor current is a function of its respective duty cycle (d 1 ). Therefore by adjusting the respective duty cycles for each energy source, maximum power point tracking can be achieved. V ds1 = V PV ( 1+ (d 1 / 1-d 1 )) (4) V ds2 = V w ( 1+ (d 2 / 1-d 2 )) (5) I ds1,pk = I i,pv + I dc,avg +V PV d1ts /2L eq1 (6) L eq1 = L 1 L 2 / L 1 + L 2 (7) I ds2,pk = I i,w + I dc,avg +V W d2ts /2L eq2 (8) L eq2 = L 3 L 2 / L 3 + L 2 (9) I i,pv = Po / V dc ( d 1 / 1-d 1 ) (10) faster than with traditional programming languages such as C, C++, and Fortran. Simulink is an environment for multi-domain simulation and modelbased design for dynamic and embedded systems. It provides an interactive graphical environment and a customizable set of block libraries that enables to design, simulate, implement, and test a variety of timevarying systems, including communications, controls, signal processing etc. The simulation was performed considering wind and solar as fixed dc sources. The input and output voltage was obtained. Fig.12 Simulink diagram for Solar and wind as fixed dc Fig 13 Input voltage- Solar VIII. SIMULATION RESULTS The performance of the proposed converter and the control strategy are evaluated by conducting the Simulation analysis of the system using MATLAB/Simulink version R2013a. MATLAB is a high-level language and interactive environment that facilitates to perform computationally intensive tasks Fig.13 Input voltage- Wind 46

6 Fig.14 Output voltage As the sources are variable, we cannot represent the sources as fixed dc voltages. Hence simulation is performed using variable sources. Fig.15 Simulink diagram of solar and wind as variable sources Fig.18 Output voltage As the sources both the solar and wind are highly fluctuating, it affects the load. The solar irradiation levels vary due to sun intensity and unpredictable shadows cast by clouds, birds, trees, etc. Similarly wind energy is capable of supplying large amounts of power but its presence is highly unpredictable as it can be here one moment and gone in another. The intermittent nature of the two sources makes the system highly uneven and thus decreases the efficiency of the overall system. Thus there is a need to control the voltage and current parameters in order to achieve maximum efficiency. The variation in voltage and current causes the wear and tear of the system components and thus results in the malfunctioning of the whole system. Hence closed loop operation of the converter is proposed using a simple controller. Fig.16 Input voltage solar Fig.19 Simulink diagram of solar and wind as variable sources using controller 47 Fig.17 Input voltage Wind Fig.20 Output voltage

7 IX. CONCLUSION The greater potential of solar and wind energy availability in India means that a solar photovoltaicwind hybrid energy system for water pumping applications should be developed. It is each citizen s responsibility to go for green energy to save earth. In this paper a new multi-input CUK-SEPIC rectifier stage for hybrid wind/solar energy systems has been presented. The features of this circuit are: 1) Additional input filters are not necessary to filter out high frequency harmonics; 2) Both renewable sources can be stepped up/down (supports wide ranges of PV and wind input); 3) Individual and simultaneous operation is supported. X. REFERENCES [1] Renewable energy source water pumping systems -A literature review C. Gopal, M.Mohanraj, P. Chandramohan P. Chandrasekar, Elsevier 30 may [2] L. Jian, K. T. Chau, and K. T. Chau, A magneticgeared outer-rotor permanent-magnet brushless machine for wind power generation IEEE Trans. Ind. Appl., vol. 45, no. 3, pp , May/Jun [3] S. Rahmam, M. A. Khallat, and B. H. Chowdhury, A discussion on the diversity in the applications of photovoltaic system, IEEE Trans. Energy Conversion, vol. 3, pp , Dec [4] Y.M. Chen, Y.C. Liu, S.C. Hung, and C.S. Cheng, Multi-Input Inverter for Grid-Connected Hybrid PV/Wind Power System, IEEE Transactions on Power Electronics, vol. 22, May [5] Y. Li, X. Ruan, D. Yang, F. Liu and C. K. Tse, Synthesis of Multiple-Input DC/DC Converters, IEEE Trans. Power Electron., vol. 25, no. 9,pp , Sep [6] H. Tao, A. Kotsopoulos, J. L. Duarte, and M. A. M. Hendrix, Family of multiport bidirectional DC- DC converters, Electric Power Applications, IEE Proceedings, vol. 153, pp , [7] Kwasinski and P. T. Krein, A Micro grid-based telecom power System using Modular Multiple- Input DC-DC Converters, in Proc. Int. Telecommunications Energy conf., 2005, pp [8] Kwasinski and P. T. Krein, Multiple-input dc-dc converters to enhance local availability in grids using distributed generation resources, in Proc. IEEE APEC, 2007, pp [9] D. Napoli, F. Crescimbini, S. Rodo, and L.Solero, Multiple input dc/dc converter for fuel-cell powered hybrid vehicles, in Proc. IEEE PESC, 2002, pp [10] O. C. Onar, O. H. A. Shirazi and A. Khaligh, Grid Interaction of a Telecommunications Power System With a Novel Topology for Multiple-Input buckboost Converter, IEEE Trans. Power Del., vol. 25, no. 4,pp , Oct [11] P. Nema, S. Rangnekar and R. K. Nema, "Prefeasibility Study of PV Solar Wind Hybrid Energy System for GSM Type Mobile Telephony Base Station in Central India," in Proc. 2nd Int. Con! on Computer and Automation Engineering, vol. 5, pp , Feb [12] Naikodi, Solar-Wind Hybrid Power for Rural Indian Cell Sites in Proc. Energy Con., 2010, pp ACKNOWLEDGEMENT All glory and honour be to the Almighty God, who showered His abundant grace on me to make this project a success. I would like to express my deep sense of gratitude towards Prof. Sapna Gopal (Head of the Department, Dept. of Electrical and Electronics Engineering) for providing all the facilities for making my project a successful one. I extend my sincere thanks to Dr. Devi V (Professor, Dept. of EEE), for her guidance and support. I also convey my sincere thanks to the M-Tech Coordinator Dr. E. G Janardhanan and members of staff for wholehearted co-operation in completing the project. I express my sincere gratitude to all the members of staff who helped me with their timely suggestions and support. I also express my sincere thanks to all my friends who helped in all the conditions. 48