Design and control of grid connected PV / Wind Hybrid system using 3 level VSC

Transcription

1 2017 IEEE 7th International Advance Computing Conference Design and control of grid connected PV / Wind Hybrid system using 3 level VSC Divya Betha 1 Majji Satish 1 Dr. Sarat Kumar Sahu 2 1. P. G Student, MVGR College of Engineerning, Vizianagram, A.P, India, bethadivya16@gmail.com 1. P. G Student, MVGR College of Engineerning,Vizianagram, A.P, India, majji.satish486@gmail.com 2. Professor, MVGR College of Engineerning,Vizianagram, A.P, India, sahu.sarat@gmail.com Abstract The demand for the electricity was gradually increasing during the past years causing imbalance in the existing utility power grid which leads to several problems like load shedding and unbalanced voltage which ultimately affect the end consumers. In order to overcome this situation the usage of non conventional energy sources have become research hotspot during the past decade. In his context autonomous PV and wind integrated hybrid systems have been found economically viable alternative to supply energy continuously to the numerous deficit customers. Individual configuration of solar or wind power leads to fluctuations in system. When combined together will provide a reliable and sustainable source with constant flow energy. PV and Wind energy systems are combined at the DC bus and its voltage is magnified to required level by using DC- DC Step-Up converter. This paper deals with common grid perturbations like balanced voltage dips, unbalances voltages and harmonic distortions. The generated output power from the PV/wind system is fed to all connected loads. Under surplus power condition excess power is fed to the utility grid. This paper presents integrated hybrid system model in MATLAB/ SIMULINK environment. Keywords photovoltaic (PV); maximum power point tracking (MPPT); voltage source converter/ inverter (VSI); nonconventional energy sources(nces); MATLAB/Simulink. I. INTRODUCTION The fast degrading rates of conventional energy sources and the increasing rates of environmental pollution has strongly forced the researchers to concentrate on the available non-conventional energy sources (NCES). So Distributed generation(dgs) started playing a vital role in power production, which can decrease global warming due to their advantageous like eco-friendly, omnipresent, cost effective maintenance and have longer life. The integration of two or more non-conventional sources of energy together to develop a Hybrid system is an best option available for interfacing distributed generators (DGs) with the grid to deliver power to all connected consumers and also to the existing grid. These hybrid systems can be standalone or can be grid connected. The Grid integrated wind and PV configuration can provide reliable power to all the connected loads. During shortage of power from this hybrid system the loads are directly connected to the existing power grid. Wind and solar based power is normally irregular and can create technical challenges to the power supply in the grid particularly when the measure of solar and wind power integration increments or the grid is not sufficiently strong enough to handle rapid changes in generation levels. The power generated by PV array system is very low during bad weather conditions and zero during nights in order to overcome this drawback PV system has to be integrated with wind energy system and storage systems. In general the available wind is dynamically varies with reference to time. So, the output of wind energy system will not have fixed magnitude of voltage and frequency. The wind turbine generator which generates AC voltage can be transformed to DC by using uncontrolled rectifier and then, be regulated using a step up converter. Further, 3 phase 3 level VSI can be used to connect the systems to the grid through LC filter. The major challenge of integrating multiple energy system arises from its control and protection mechanisms. The active and reactive power flows from the converter can be regulated by controlling magnitude and phase of the converter output voltages relative with grid parameters. The active power flow can be controlled by regulating the phase difference and reactive power flow is by adjusting the magnitude of inverter output voltage with reference of grid voltage. The reliability to deliver continuous supply to load is more for grid connected mode since the utility grid always acts as a backup. High power losses are produced due to the usage of more power converter in the system. This paper deals with technical challenges and possible opportunities for integrated of multiple Renewable energy sources (RES). II. PROPOSED SYSTEM ARCHITECTURE The wind and PV array are considered as inputs to the proposed architecture. The major challenge of integrating wind/pv system is arises from its control mechanism. A dc-dc step up converter topology is used to connect the integrated energy system to dc link. Various energy sources are integrated to the DC bus to minimize the system complexity and its cost. The dc output voltage of step up converter is given as the input to the 3-phase 3- level inverter, to get required magnitude of ac voltage. The energy available from the sun is dynamically varies throughout the year. In cloudy days/night the magnitude of irradiance is not sufficient to generate the energy to the connected load. In such conditions the required energy is supplied from wind turbines or peak load power plants connected to the power grid. During normal operation integrated system plays a dominant role and utility grid acts as a backup. The 3 phase /17 $ IEEE DOI /IACC

2 inverter s AC output voltage is stepped up to required voltage magnitude using a transformer and fed to the grid via LC filter to give a ripple free voltage. This proposed methodology helps to improve the customer reliability to the required standards. The integrated system approach is validated under the resistive, Resistive & inductive (RL) and dynamic load. = rotor swept area The power ( developed by blades of the rotor from wind is the algebraic difference of up and downstream of wind powers..(i) = up and down wind velocity at the starting of the blades of the rotor Mass flow rate of the wind = Substitute the value of in equation (i) Now Fig. 1. Pictorial representation of grid connected PV/wind system The wind power generated from PMSG is given to the uncontrolled rectifier before integrating to the PV arrays. Integrated DC output is then step uped to the desired level using a dc-dc step up converter and controlled to obtain the maximum power then connected to a DC bus. The inverter placed at grid side is provided with PWM control mechanism which injects active power to the Utility grid with negligible reactive power. III. MATHEMATICAL MODELING OF WIND TURBINE Wind turbine system captures the wind s kinetic energy and gets its power with the help of aerodynamically structured blades and changes it to rotating mechanical energy which is coupled to an electrical generator. Mathematical representation of wind turbine plays a vital role in understanding the response of the wind turbine over its region of operating. Wind turbine s performance is dependent on Local weather conditions, actual installation weight and modeling of turbine blades. The magnitude of kinetic energy of wind striking the blades of a wind turbine can be represented by the equation, = mass of air striking the blades of wind turbine = speed of the wind in V = density of air = the power co-efficient of wind turbine. (Cp) is dependent on two parameters namely the tip speed ratio (λ) and blade pitch angle (β). The generated output power in watts ηw, ηg = efficiencies of turbine,generator. Here, λ, β being the tip speed ratio and pitch angle respectively. To get maximum power from the wind turbine pitch angle (β=0 ) is maintained at 0, power coefficient is kept at maximum value and rotor speed is adjusting to the optimum λ value. If the rotor speed is to be increased, the output power is kept lower than captured value and vice versa. IV. MODELING OF PHOTOVOLTAIC CELL In general PV cell is a semiconductor device. This diode has got a p-n junction which acts as a current source when exposed to light (irradiance) separates positive and negative charge carriers in the presence of an electric field and produces power. Power delivered by a solitary PV cell is insufficient commercial purposes. Thus, we connect a number of PV cells in arrangement (for voltage improvement) and in parallel (for current improvement) can get the desired power. A basic identical PV circuit model is given by an ideal current source in parallel with a real diode, a series and shunt resistance as shown in Fig. 1. The ideal current source develops output in accordance to the solar irradiance exposed on it. Fig.2 wind turbine Basic model The extracted output from wind in watts is expressed as

3 converter are utilized as step up to the source voltage to a higher level. Thus, DC-DC step up converter [1] is controlled to track maximum power output and to magnify the terminal voltage to a reasonable value of PV in order to incorporate with the grid. Fig.3. Equivalent model of a PV cell. The PV cell works on photoelectric effect principle which is characterized as a phenomenon in which an electron gets ejected out from the semi conducting material upon the exposure to electromagnetic radiation. the diode equation given as expression for the current through a diode as a component of voltage as = Diode current; = current in the cell; =shunt current (from Kirchhoff s current law) the electrical characteristics of an ideal diode, modeled by the Shockley equation Fig.4. DC-DC step up converter schematic diagram. A boost converter is a Up Converter which yields an average dc output voltage prominent than its input dc voltage. By applying Kirchoff s laws to the circuit topologies obtained by switching in between two distinct positions. a) when switch is closed, procreating an increase in the inductor current b) When switch is open, procreating a diminish in the inductor current through the diode to the load. The gain value of DC-DC step up converter varies in accordance with the duty cycle, which varies the operating voltage of PV module at maximum power point. n= diode identity factor close to 1.0; = diode saturation current; = diode voltage; = thermal voltage given by Ncell = no of series cells in a Photovoltaic module K (Boltzmann s constant) =1.381* ; Q (electron charge)= * C The current in the PV cell that results from sun irradiance is known as the photo current which flows in the opposite direction of the forward. Its value remains the same regardless to external voltage and subsequently it can be measured by the short circuit current (Isc = ). This current change linearly with the solar radiation based radiation as expanded radiation can isolate increased charge carriers. The complete governing equation for the PV cell model = Step up converter output voltage, = Step up converter input voltage, D= Duty cycle. B. Three level inverter analysis In a Multilevel inverter topology IGBT modules are attached in series configuration, taking into consideration of voltage ratings far larger than the reverse blocking voltages if IGBT. This unique structure allows them appropriate for high voltage and power applications. The objective of Inverter used in interfacing with the grid network is to inject pure sinusoidal current which can satisfy power quality and efficiency requirements of the load. Voltage source inverter (VSI) is utilized to convert a relatively steady DC voltage of the PV array to three phase AC power. V. MODELING OF CONVERTERS A. Boost converter analysis The power from Photovoltaic array is a non-linear function of the operating voltage and there is a need of boosting the voltage for supplying adequate load. Boost Fig.5. three-level, three phase VSI

4 In three-phase three level inverter each leg made up of four pairs of IGBT s. Each phase of the inverter will switch across different voltage levels (+V dc /2, 0, -V dc /2) at any instant of time. the maximum output voltage of the IGBT is exactly half of the DC voltage (V dc /2). The main advantage of this inverter is its waveform of output voltage is very close to sinusoidal waveform with less harmonic content and good efficiency which leads to reduction of the overall application cost. In grid-integrated PV system, distinctive inverter topologies and controllers governed with different suitable control strategies for interfacing the PV array to the utility grid. Pulse width modulation converter topology has been effective and proficient method for generating the switching pulses at the gate terminals of each IGBT for controlling the current waveform of the inverter to track a predetermined reference signal to acquire a desired square wave voltage. For effective reduction of harmonic distortion high frequency pulse width modulation techniques are more preferable and provide load voltage control by consolidating additional switches to the circuit. C. PWM control technology In general switching phenomenon of inverter is controlled in order to obtain the required voltage magnitude at a particular frequency. The most commonly used methods for controlling the inverter switching phenomenon are pulse width modulation techniques, space vector modulation, control of hysteresis. By changing the duration of voltage applying to the inverter we can have a good control over the fundamental components and harmonics In the simple sinusoidal PWM technique PWM signal is generated by continuously comparing the power frequency sine wave with high frequency carrier wave and then obtained PWM signal is used for switching of the inverter. The logic for switching of individual phases is given as follows The fundamental midpoint voltage is Where m is the modulation index m= Fig.6. Basic Sinusoidal PWM Topology The fundamental amplitude of output wave form can be changed by varying the modulation index and output frequency can be bought to desired value by changing the frequency of reference wave form. The ratio of carrier and reference wave frequency will have a significant impact on the magnitude of harmonics. Large number of lower order harmonics can be eliminated with high frequency carrier wave form ( ) but this will produce high switching losses and observable deteriorating of source voltage. In order to overcome this carrier wave frequency ( ) is kept at fixed value. VI. SIMULATION ANALYSIS For this simulation, sun irradiance(1000 W/m2) and wind speed (12 m/sec) constants are being used. It is an integrated model of a PV array /wind connected to a DC-DC step up Converter and 3-phase 3-level Voltage Source Inverter (VSI) to a 25-kV Utility grid through. Photovoltaic array delivering a maximum power at a constant of 1000 W/m2 sun irradiance is connected in parallel with the wind turbine connected along 5-kHz step up converter enhancing voltage magnitude to 500 V DC. 3-level 3-phase VSI converts the 500 V DC to 260 V AC maintaining unity power factor with LC filter across VSC provided for filtering lower order odd harmonics. A three-phase coupling step-up transformer of 260V/25kV being connected to 25- kv distribution feeder along with 120 kv generation system. PV array of the model uses Soltech modules (1 STH 215P). The array -connected modules connected in parallel (40*45* W= Kw) whose model parameters are listed below TABLE I. Parameters of a PV array 1 Soltech 1 STH 215P Parameter ratings Cells per module 60 OC voltage (Voc) V 36.3 SC Current (Isc) A 7.84 maximum power Voltage (Vm) 29 maximum power Current (Im)

5 Fig.8. Simulation model of Grid integrated PV/wind system In this paper the results of the PV/ Wind integrated system simulation model are presented and also the waveform of grid voltage and current is obtained. Fig.11. waveform of dc voltage given to the 3 level inverter The result suggests that the magnitude of voltage at the dc step uping stage in three levels generates about 200 volts.three level inverter based system provides reactive power compensation to the grid along with high active power. This DC-DC converter precisely tracks the duty cycle continuously to generate the desired voltage and extract maximum power. Fig.9. phase voltage waveforms at bus B1 The synchronized grid power along with the variations in, solar irradiation and load conditions are considered for the simulation analysis and the results are shown in Fig:7 Fig.12. waveforms of grid voltage and current The three levels inverter generates line voltage and currents shown in Fig:10 which are sinusoidal and balance with the grid Fig.10. waveforms of PV panel (V_PV,I_PV,I_d, irr, Temp)

6 renewable energy systems to extract optimum energy. Both wind PMS generator output is transformed to dc by using a uncontrolled rectifier and PV panel outputs are integrated and given to the inverter. The inverter placed at grid side is provided with PWM control mechanism which injects active power to the Utility grid with negligible reactive power. Excellent performance of the inverter with negligible fluctuation of the DC bus voltage is obtained. Fig. 13. waveform of the active power at the bus_b1 The total generation output of the grid integrated hybrid PV /wind system is about Kw. The simulated models were exact and are utilized to decide the voltage and current characteristics. The dynamic PV performances were examined under different operating conditions, the results demonstrated here shows that the DC voltage extracted from the PV array delivers an AC sinusoidal current at the output of the Voltage source inverter which are integrated to the PMSG. The simulation of grid systems was improved the voltage profile and the adequate performance of Hybrid PV/wind system was analyzed. The harmonic analysis of the grid voltage and current performed and the THD is obtained which comes out to be 0.25% and 16.70%. VIII. REFERENCES Fig.14. FFT analysis of grid voltage and current of proposed system The Fourier transfer analysis of grid voltage and current is done and the harmonics is calculated of both the grid voltage and current. [1] Dr P.S. Bimbhra (2012) Power Electronics, Khanna publishers, Fourth edition, pp [2] G.B.Hangaragi, Recent Integration Of A Pv-Wind Energy System With Enhanced Efficiency, Indian J.Sci.Res. 11 (1): , 2015 [3] Aditi, A.K.Pandey, Performance Analysis of grid connected PV Wind Hybrid Power System, International Journal of Applied Engineering Research ISSN Volume 11, Number 1 (2016) pp [4] Jitendra Kasera, Ankit Chaplot and Jai Kumar Maherchandani, (2012). Modeling and Simulation of Wind-PV Hybrid Power System using Matlab/ Simulink, IEEE Students Conference on Electrical, Electronics and Computer Science. [5] Chen et al., Multi-Input Inverter for Grid-Connected Hybrid PV/Wind Power System, IEEE Transactions on Power Electronics, vol. 22, May 2007 [6] F. Blaabjerg, R. Teodorescu, M. Liserre and A.V. Timbus. Overview of control and grid synchronization for distributed power generation systems, IEEE Transactions on Industrial Electronics, Vol.53, No.5, 2006: [7] T.A.Maynard, M.Fadel and N.Aouda, Modelling of multilevel converter, IEEE Trans. Ind.Electron., vol.44, pp Jun [8] Akagi H., Ito Y., Zhongqing Y., DC Micro-grid Based Distribution Power Generation System. The 4th International Power Electronics andmotioncontrolconference,2004.ipemc aug Vol. 3, Page(s): [9] J. M. Guerrero, F. Blaabjerg, T. Zhelev, K. Hemmes, E. Monmasson, S. Jemei, M. P. Comech, R. Granadino, and J. I. Frau, Distributed generation: Toward a new energy paradigm, IEEE Ind. Electron. Mag., Vol. 4, No. 1, pp , Mar [10] Green and M. Prodanovic, Control of inverter-based micro-grids, Electric Power Systems Research, Vol. 77, No. 9, pp , Jul VII. CONCLUSION This paper presents the modeling and simulation of a grid integrated PV /Wind Power System and work has been evaluated in Matlab/Simulink. The proposed system architecture provides an elegant integration of the two