INTEGRATION OF BATTERY ENERGY STORAGE SYSTEM BASED PV POWER PLANT INTO GRID

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1 INTEGRATION OF BATTERY ENERGY STORAGE SYSTEM BASED PV POWER PLANT INTO GRID MAHESHA G PG Student Power Electronics siddaganga institute of technology Tumakuru,India mahesha021@gmail.com Abstract With increase in the power demand, the energy generated by various renewable resources such as solar, wind, tidal energy sources is clean, cheap and has less environmental impact and requires meticulous attention. The renewable energy sources are the one of the most promising solution for today's energy crisis. Among the all renewable energy sources the photo voltaic(pv) system attracts more because it generate power with much lower level of carbon dioxide(co 2 ) emissions. This paper aims to investigate and emphasize the importance of the grid-connected PV system including the intermittent nature of renewable generation and the characteristics of PV generation with regard to the grid compliance. The integration of PV system to the grid requires models of PV array, SEPIC converter with maximum power point tracking(mppt) inverter control strategy, Bi-directional converter and battery energy storage system using matlab/simulink and compatible controller is used for MPPT(incremental conductance method),dc bus voltage regulation. battery charge and discharge control, protection of system. and the developed models are used for the validation of simulation studies. KEYWORDS PV array, SEPIC converter,maximum power point tracking,bidirectional converter, Battery energy storage. I. INTRODUCTION IN the recent power scenario energy generated from clean, efficient, and without affecting the environment such sources has become one of the major challenges for engineers and scientists. Among allrenewable energy sources, photo voltaic power systems attract more attention because they provide excellent opportunity to generate power with less carbon-dioxide(co 2 ) emission comparing DR.RASHMI Associate Professor EEE Dept siddaganga institute of technology Tumakuru, India rash_mysore@yahoo.com with the conventional electricity generated by burning coal and using natural gas. Regarding the endless aspect of solar energy, it is worth saying that solar energy is a only best prospective solution for today's energy crisis. Effective interconnection of always varying renewable energy resources like solar PV (photovoltaic) systems to the grid requires much more attention to ensure system stability, reliability and reducing complexity. The operating characteristics of such PV power plants is to be easily understood to power system designer and engineers. Such easy access can effectively be done only via system modeling, simulation and case studies. Grid interconnection of PV power generation[6-7]system has the advantage of more effective utilization of generated power. however the technical requirements from the utility power system grid is to be safe to the PV installer and the reliability of the utility grid. clarifying the technical requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems. Grid interconnection of PV systems is accomplished through the inverter, which convert dc power used for ordinary power supply to electric equipments. The problems with the most existing models are photo voltaic generation variability and intermittency on the power system due to ever changing solar insolation, temperature and shading in the atmosphere is not taken into account. The power which is generated by the existing PV modelis comparatively less and mainly the output of DC- AC converter is not constant. This paper presents the mitigation of the variability andintermittency of the photo voltaic power generating systemsby employing power electronic converter and energystorage system. The photo voltaic model output, 56

2 efficiencyisincreased by incorporating maximum power pointtracking using incremental conductance method, and regulation of the inverter output frequency and voltage can be achieved by robustness of the control system. II. PROPOSED MODEL The proposed model is implemented in steps, section A gives the model of the PV array while section B discuss the modeling and working of SEPIC converter. section C deals with the MPPT control technique while section D details the modeling of energy storage converter and battery energy storage system and section E discuss the control of voltage source inverter. A. PV ARRAY MODEL The basic structural unit of a solar module is the PV cells. A solar cell converts energy in the photons of sunlight into electricity called photoelectric phenomenon. PV module uses semiconductor materials such as silicon and germanium cells to convert solar energy into electricity when they exposed into light. A single PV cell generates only a small voltage. so many cells which are together to generate more voltage called PV module. such PV module are connected in series or parallel to make PV array[1-3]. The output voltage and output current (V-I) characteristics of the PV array is developed by the equations. The equation of output current of the PV module is fig 1. BLOCK DIAGRAM The block diagram of the proposed model(fig.1) consists of PV array, SEPIC converter with MPPT control strategy, battery energy storage system, DC-AC converter and controller. The PV array which converts solar energy into electric energy. The power generated by PV module is less and varying each time then generated power by PV module is boosted by SEPIC converter which as a capability of both increasing and decreasing the output voltage of PV array and incremental conductance MPPT is used to increase the PV efficiency and output power. The output of the SEPIC converter is fed into the DC bus. The battery energy storage system is also connected to DC bus, when during the light load, the excess power generated by the PV array is stored into the battery, when during peak demand the energy stored in the battery is fed into the DC bus through the bidirectional converter. The DC-AC converter(inverter) is required to convert DC power into AC power. The LC filter is connected after VSI to reduce the harmonics, The filtered output is fed to the grid. III. IMPLEMENTATION I o = n s I ph n p I rs [exp KoV Ns 1] where I o is the PV array output current; V is the PV output voltage; I ph is the cell photocurrent that is proportional to solar irradiation; I rs is the cell reverse saturation current that mainly depends on temperature; K o is a constant; n s represents the number of PV cells connected in series; n p represents the number of such strings connected in parallel. The photocurrent of the module is given by s I ph = [I scr + K i T T r ] 100 where I scr cell short-circuit current at reference temperature and radiation; k i short-circuit current temperature coefficient; T r cell reference temperature; S solar irradiation in mill watts per square centimeter. And the module reverse saturation current is calculated from I rs = I rr [ T T r ] 3 e ([qe G KA [ 1 Tr 1 T ]) where T r cell reference temperature; I rr reverse saturation at T r ; E G band-gap energy of the semiconductor used in the cell. 57

3 The above equations are used to develop the model of PV panel using matlab/simulink.the general single diode equivalent model of solar cell fig.2 is used to develop the equations. This paper the BP solar SX3190 was chosen to develop the model of the PV panel and same model is used for simulation studies. the technical specifications of the SX3190 module is tabulated in table.i, and the matlab/simulinkmodel of the single diode equivalent circuit fig.3 is developed and variations in the power output because of the environmental variables is taken into account. fig 2. single diode equivalent model of solar cell This result of SEPIC converter[8] is similar to that of the buck-boost and Cuk converter equations,with the important distinction that there is no polarity reversal between input and output voltages. although the buckboost converter is cheaper than the SEPIC and some of the disadvantages in buck-boost and cuk converter are high peak current in power components, and poor impulse response, makes less efficient. In the other hand the efficiency of the SEPIC converter is more compare to other converter and The SEPIC converter as an ability to have output voltage greater or less than the input with no polarity reversal makes this converter suitable for many applications.so the SEPIC converter is best converter to be employed in MPPT. fig.4 shows SEPIC converter module. The SEPIC converter which is interfaced between the PV array and DC bus to track the MPPT of the PV array to increase the efficiency and regulate the changing output of the PV panel to the DC bus requirements. The complete analysis of SEPIC converter is carried out the derived equations are listed below and the design specifications table.iiand calculated values are used in simulation L 1 = V in D/( i L1 f s ) C 0 = D/(R Vo Vo f s) C s = D/(R Vcs Vo L 2 = V in D/( i L2 f s ) f s) table I. specifications and parameters of BP SX3190 Fig.4 circuit of SEPIC converter fig 3. matlab/simulink model of PV panel B. SEPIC CONVERTER MODELING table II. SEPIC specifications and parameters 58

4 C. BIDIRECTIONAL CONVERTER AND ENERGY STORAGE SYSTEM Bidirectional DC-DC converters serves the purpose of stepping up or stepping down the voltage level between its input and output along with the capability of power flow in both the directions. Bidirectional DC-DC converters[5],[9] are employed when the DC bus voltage regulation has to be achieved along with the power flow capability in both the direction. In the solar power systems where there is a large fluctuation in the generated power because of the large variation and uncertainty of the energy supply to the PV panels by the primary source. These systems cannot serve as a standalone system for power supply because of these large fluctuations and therefore these systems are always backed up and supported by the auxiliary sources which are rechargeable such as battery units.therefore a bidirectional DC- DC converter is needed to be able to allow power flow in both the directions at the regulated level. Several types of bidirectional converter topologies exist. This paper uses half bridge DC-DC converter topology(fig.5) and design specifications(table.iii) is given.which operates in two modes depending upon the switching of MOSFET's. condition the current-voltage(i-v) and power-voltage(p-v) characteristics of PV module are changing throughout the day.photovoltaic modules have a very low conversion efficiency and on the other hand due to the temperature, radiation and load variations, this efficiency can be highly reduced. In order to ensure that the photovoltaic modules always act supplying the maximum power as possible and at the ambient operating conditions, a specific circuit known as Maximum Power Point Tracker (MPPT) is employed. There are so many MPPT [4] control algorithms are exist to track the maximum power point, some of the methods are simple and some were complicated. The comparison of several MPPT's is listed below(table.iv),however this paper uses incremental conductance(ict) method because it as advantage of high accuracy in tracking maximum power from PV module, medium implementation complexity and digital implementation can be done.fig.6the basic power-voltage(p-v) curve of the PV array The incremental conductance method uses this curve as key factor to track the maximum power point tracking. This technique is basedon the fact that the MPP can be tracked accurately bycomparing the incremental conductance (ΔI/ΔV) with the instantaneous conductance (I/V) of the PV array. This is implemented by taking power or current as a input variable and duty ratio as a output variable. both conductance's are equal at MPP this point need not to change the duty cycle, negative on the right side of MPP this point decrease the duty ratio of the converter and positive on the left side of MPP.The basic equations of this method are fig 5. Half bridge Bidirectional converter di = I, at MPP dv V di dv > I, left of MPP V di dv < I, right of MPP V table III. Design parameters of bidirectional converter D. MPPT CONTROLLER MODELING The power output from the solar panel is a function of insolation level and temperature. But for a given operating fig 6. P-V characteristics of BP solar SX

5 table IV. comparison of various MPPT techniques fig 7. matlab/simulink model of control of inverter E. INVERTER CONTROL The three phase full bridge inverter topology is the most widely used configuration in three phase systems. The output voltage and frequency of inverter should be same as that of grid frequency and voltage. The output of grid connected inverter can be controlled as a voltage or current source and pulse width modulated (PWM) voltage source inverters (VSI) are most widely use in PV systems. The control strategy applied for inverter[10] consists of two control loops. Usually there is a fast inner control loop which controls grid current and an external voltage loop which control dc link voltage. The current control loop is responsible for power quality issues like low THD and good power factor, whereas voltage control loop balances the power flow in the system. Synchronous reference frame control also called d-q control uses a reference frame transformation abcto dq which transforms the grid current and voltages into d-q frame. The transformed voltage detects phase and frequency of grid, whereas transformed current controls the grid current. Thus the control variables becomes dc values, hence filtering and controlling becomes easier. The matlab/simulink model (fig.8) shows the control strategy used to control of voltage source inverter. IV. RESULTS AND DISCUSSIONS The characteristics of developed BP solar SX3190 PV array model are shown in figs (8,9).The effect of temperature on PV array is shown in the fig.8.it is clearfrom the figure with the increase in ambient temperature the output current of the PV array slightly increases and fig 8. Effect of temperature on PV array 60

6 fig 9. fig 10. Effect of solar insolation on PV array Battery charging mode bidirectional converter, This mode is referred as buck mode. during the buck mode the fig.10 shows the state of charge(soc) of battery, current and battery voltage. when during the power generated by PV array is less or insufficient that means the load requires more power than the generated in that case the stored power in the battery is fed back to the dc bus called boost mode, this is shown in the fig.11. The output characteristics of the inverter is shown in the fig.12 it shows the rms current and voltage of the varying load. when load is suddenly varied within the fraction of seconds the output voltage and frequency comes to steady state, This shows the effective control strategy of the inverter. fig 11. Battery discharging mode V. CONCLUSION The simple method of modeling of PV array is implemented and it can be used for power flow in bulk study systems. and the effect of environmental aspects such as temperature and irradiation are taken into account to validate the performance of PV array. The efficiency and power output of PV generator is maximized by employing the incremental conductance MPPT control through SEPIC converter. The battery energy storage system minimizes the intermittency and variability in the PV generator via bidirectional converter and the inverter output is effectively controlled by inner current control loop and outer voltage control loop and in other hand the simulation results show the integration of PV power plant and battery energy storage system into the grid. fig 12. rms voltage and current of inverter the open circuit voltage of the PV array decreases from V is shown in the fig.9. with increase in the current and decreasing in the open circuit voltage the net power output of the PV array decreases, This conclude that with increasing the ambient temperature the PV array output decreases. The performance characteristics of battery energy storage system with half bridge bi-directional converter is shown in the fig,10and fig.11.when there is enough power generating from the PV array, generated power is fed to the load through inverter. if still excess power is generated it is used to charge the battery through REFERENCES [1] M. E. Ropp and S. Gonzalez, Development of a matlab/simulin model of a single- phase grid connected photovoltaic system, IEEE Trans. Energy Conversion, vol. 24, pp ,2009. [2] Y. Mahmoud, W. Xiao, and H. H. Zeineldin, A simple approach to modeling and simulation of photovoltaic modules, IEEE Translation Sustainable Energy, vol. 3, no.1, pp , Jan [3] A. Keyhani, Modeling of photovoltaic microgrids for bulk power grid studies, in Proc. IEEE Power & Energy Society General Meeting, Detroit, 2011, pp.1-6. [4] A. Safari and S. Mekhilef, Simulation and hardware implementation of incremental conductance mppt with direct control method using cuk converter, IEEE Trans. Industrial Electronics, vol. 58, pp , April, [5] N.Mohan,T. M.Undeland, and W.P.Robbins,Power Electronics, Third edition, India Willy 2010.pp [6] F. A. Farret, M. G. Simões, Integration of alternative sources of energy, Wiley-IEEE Press,

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