ADVANCES in NATURAL and APPLIED SCIENCES

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1 ADVANCES in NATURAL and APPLIED SCIENCES ISSN: Published BY AENSI Publication EISSN: Special 9(7): pages Open Access Journal Capacity and Location Effects of Photo Voltaic Power Generators on Power Systems Stability Angelin Grace Stella S., Mahendra Babu T.K. and 3 Gnanambal K. Affiliated To Anna University, Chennai,Electronics and Communication Department, K.L.N College Of Engineering, Pottapalayam,Pin:6306,Sivagangai District, TamilNadu, India Affiliated To Anna University, Chennai,Electronics and Communication Department, K.L.N College Of Engineering, Pottapalayam,Pin:6306,Sivagangai District, TamilNadu, India 3 Affiliated To Anna University, Chennai,Electrical and Electronics Department, K.L.N College Of Engineering, Pottapalayam,Pin:6306,Sivagangai District, TamilNadu, India Received February 05; Accepted 0 March 06; Available 5 March 06 Address For Correspondence: Angelin Grace Stella S., Affiliated To Anna University, Chennai,Electronics and Communication Department, K.L.N College Of Engineering, Pottapalayam,Pin:6306,Sivagangai District, TamilNadu, India, Tel: ; sagstella@gmail.com, Copyright 06 by authors and American-Eurasian Network for Scientific Information (AENSI Publication). This work is licensed under the Creative Commons Attribution International License (CC BY). ABSTRACT The main challenge in integrating the Solar Photo Voltaic Power Generator (SPVG) with grid is maintaining the voltage stability in the power system. The main objective can be achieved by the design of dynamic modelling of SPVG with MATLAB tool. Since the library model does not include the dynamic effect of temperature variation, a better dynamic model of MW SPVG is simulated and integrated with IEEE 9 bus system as test case. In order to keep the voltage profile within the limit, the optimum size of SPVG and optimum location is to be identified. Hence a suitable model is developed to analyse the impact of integrating SPVG with grid. The different sizes of SPVGs like 0 p.u, p.u and 4 p.u are injected into IEEE 9 bus system. The five parameter model is coded with open circuit voltage maximum power tracking technique. But the power generation from SPVG mainly depends on rapidly varying irradiance and temperature. The load curves for different irradiance and temperature is analysed with PSAT tool. The P-V curves are analysed with and without integration of SPVG. The simulation result shows that the voltage stability is improved by placing the SPVG of size 043 MW at 9th bus. KEYWORDS: Photo Voltaic Power generators, Voltage Stability, Optimum Location, Maximum Power Point tracking INTRODUCTION Mankind is always in search of renewable sources of energy. Most of the energy that is generated today is acquired as derivatives of fossil fuels like coal, oil and natural gas. The encroaching of nature has reached the extreme that requires reassessment of possible energy supply strategies with a focus on sustainability. Therefore, the effective utilization of the energy irradiated by the Sun is the potential key to a sustainable energy production in future. However, as more Solar Photo Voltaic Generators(SPVG) are connected to the grid, their impact on the utility with PV power generation is becoming more apparent, so it is necessary to analyze the voltage stability of power system including PV power plants. In power system studies, unlike other conventional power plant technologies, no standardized model is available today for SPVG. The PSAT (Power System Analysis Toolbox) is an open source software, MATLAB and GNU/Octave-based software package which include wind generator model but not the dynamic Solar Photovoltaic Power generator model. Many studies on various Photovoltaic panel models have been presented in literature. [] validated the modeling approach of PV module by estimating To Cite This Article: Angelin Grace Stella S., Mahendra Babu T.K. and Gnanambal K.., Capacity and Location Effects of Photo Voltaic Power Generators on Power Systems Stability, 05. Advances in Natural and Applied Sciences. 9(7); Pages:

2 337 Angelin Grace Stella S. et al., 05/ Advances in Natural and Applied Sciences. 9(7) Special 05, Pages: parameters. [] proposed the dynamic PV Model by Parametric identification. The papers discussed PV array modeling from the commercial datasheets with the single diode model of PV cell. Most of these studies are focused on V-I characteristics and Maximum Operating Point rather than its effect and applicability in power system studies. It is essential to analyze the power system stability by integrating SPVG with Utility network. The voltage stability can be analyzed by several methods and tools. One of the methods is to determine the maximum loading point from PV curve. Hence five parameter model of Photovoltaic module [4] is identified. The Maximum Power Point Tracking (MPPT) techniques are compared in the literature. In order to eliminate the P-I control loop, the direct control method of Incremental Conductance method is developed with MATLAB and linked with PSAT tool for power system stability studies. Instead of measuring voltage and current of PV module, irradiance and temperature are considered as inputs for MPPT [0]. The power processing stages can be single stage or two stages From the literature, the five parameter model of PV array is identified, in order to tie the SPVG with the utility network. Then irradiance is changed from 00 W/m to 00 W/m. Similarly temperature is varied from 5 C to 45 C to generate power. The algebraic relationship V oc the open circuit voltage and I sc the short circuit is also coded in MATLAB. The developed MW plant is considered as a basic unit. This paper deals with the SPVG with 9 bus IEEE test system. In the voltage stability analysis, PV curves, voltage profiles and the loading parameters are discussed. The organization of this paper is as follows: Section describes the modeling of SPVG and maximum output power with respect to irradiance and Temperature, Section 3 describes the simulation results for the voltage stability analysis and Section 4 describes the results and discussions.. SPVG Model: Reliable and accurate Photo Voltaic (PV) model is essential for simulation of power systems. Currently, standard library PV models are not available in PSAT. Hence, the physical behavior of a PV panel can be represented by its equivalent electrical circuit composed of linear and non linear components as one diode model or two diode model. Though the two diode model is accurate it suffers from complexity and low computational speed. The single diode provides trade off between simplicity and accuracy. Hence single diode PV models are analyzed. Lo Brano et al. 5&6 have proposed a equation which represents the electrical and thermal behavior of a PV panel as follows, =, /! "+$ %&' +( ) ( )* where =, -, denotes the ratio between the generic solar irradiance and solar irradiance at Standard %&' Test Condition (STC), is photocurrent, V voltage generated by the panel, current generated by the panel T is the temperature of the panel,t ref temperature of the panel at STC and K thermal correction factor (Ohm/deg C), R sh and R s are the shunt resistance and series resistance of the panel. The panels are connected in series and made in to PV string. The PV strings are connected in parallel in order to meet the power requirement. The default PV cell model does not include dynamic variation of reverse saturation current. Hence the experimentally validated five parameter model is coded. The block diagram of the MATLAB Simulink based PV module model is shown in Figure-. The mathematical model of single PV module is implemented by using five parameter model. For the sake of convenience all components are incorporated in to a subsystem block. The subsystem block will be a basic PV module. In the subsystem block, input ports are irradiance G and temperature T. The subsystem blocks can be connected in series/parallel according to the actual PV array; then it can be simulated in MATLAB-Simulink. In order to model MW SPVG, fifteen PV panels of 75W each is connected in series and made a PV string, such 4 strings are connected in parallel.dynamically irradiance is changed from 00 W/m to 00 W/m. Similarly temperature is varied from 5 C to 45 C to generate power. The maximum Power Point Tracking from V oc the open circuit voltage and I sc the short circuit is coded in MATLAB. The developed MW plant is considered as a basic unit. For simulation purpose, the KC75GT datasheet is chosen and shown in Table.The PV manufacturer datasheets 9 provide only limited information about the output electrical characteristics of their PV modules at Standard Test Condition (STC). Thus, only four equations can be written based on the datasheet information. Among MPPT techniques P&O method is not suitable for rapidly varying conditions.the Fuzzy and neural techniques depend on the technical knowledge of the engineer and the training pattern respectively. Hence Incremental conductance method is identified which overlooks the disadvantages of the previous methods. In maximum power extraction 9, Incremental conductance method has two control loops one for MPPT algorithm, the other for P or P-I controller to minimize the error. In order to reduce the cost and circuit complexity a small error is fixed as.0 and the duty cycle is directly adjusted. A high step up DC-DC converter is configured as first power processing stage, which can boost a PV

3 V Angelin Grace Stella S. et al., 05/ Advances in Natural and Applied Sciences. 9(7) Special 05, Pages: array up to a high dc-bus voltage and a full-bridge inverter which can stabilize the DC bus voltage and shape the output current. The Table describes the algebraic relation between PV module and PV array electrical parameters. This mathematical relation is helpful in sizing the SPVG as per the requirement. Figure- shows the relationship between PV power and irradiance at various temperatures. PS S 5 PS-Simulink Converter3 Ns V I Resistor Current Sensor PS S PS-Simulink Converter4 4 Np G G Iphoton I C Resistor - + Variable Resistor Voltage Sensor P S V Out I Out Subsystem Display T T Display4 PV module S PS Simulink-PS Converter f(x)=0 Solver Configuration Electrical Reference S PS PS-Simulink Converter Rvar Varying In R Divide P Ramp Fig. : MATLAB SIMULINK model of MW Power plant. Table : Electrical specification of the panel at STC ( KC75GT). Maximum Voltage (V mpp) 3.6V Maximum Current(I mpp) 7.4A Maximum Power (P max) 75W(+0%-5%) Open circuit Voltage(V oc) 9.V Short Circuit Current(I sc) 8.09A Table : Datasheet Values of an Array. Module Datasheet I sc V oc V mpp I mpp P mpp Equivalent Array Parameters I sc x N p V oc x N s V mpp x N s I mpp x N p P mpp x N s x N p 4 x 05 0 TEMP=5(DEGREE C) TEMP=30(DEGREE C) TEMP=35(DEGREE C) TEMP=40(DEGREE C) TEMP=45(DEGREE C) Power Vs Irradiance Power( W) --> Irradiance (W/m) --> Fig. : Power(W) Vs Irradiance W//m at various Temperature. Table 3 shows the maximum power for MW plant at various irradiance and temperature. It shows when irradiance increases the power also increases. When temperature increases the maximum power decreases which satisfies the mathematical equation of single PV cell [8]. Similarly 0 p.u, p.u,4 p.u SPV generators are modeled and fed into the IEEE 9 bus system. The Figure-3and Figure-4 shows the effect of change in temperature and irradiance. It can be studied by plotting the V-I and Power-Voltage (P-V) characteristics at different temperature and irradiances. A program is embedded in the MATLAB Simulink model to plot these curves. The program also estimates the values of Maximum Operating Power. The maximum Power can be calculated from the mathematical equations of Open circuit Voltage and Short Circuit Current.

4 339 Angelin Grace Stella S. et al., 05/ Advances in Natural and Applied Sciences. 9(7) Special 05, Pages: The P-V characteristics are obtained by taking the product of the voltage and current. Table 3 lists the MPP of the array under different environmental conditions as estimated by the mathematical equations. The shapes of the V-I and The P-V curves for a module and that of an array are exactly the same, the only difference in the plot is that the voltage and current get multiplied as mentioned in the Table Table 3: Maximum Power (MW) under different environmental conditions of MW SPVG. Irradiance Temperature ( C) (W/m ) x 05 0 Power Vs Voltage IRRADIANCE=00W/Sq.m IRRADIANCE=000W/Sq.m IRRADIANCE=900W/Sq.m 8 Power W --> Voltage V --> Fig. 3: P-V Characteristics of MW Power Plant VI Characteristic of MW Solar Power Plant Current A --> IRRADIANCE=00W/Sq.m IRRADIANCE=000W/Sq.m IRRADIANCE=900W/Sq.m Voltage V --> Fig. 4: V-I Characteristics of MW Power Plant. 3. Modelling using PSAT Tool: PSAT is actually the first free software project in the field of power system analysis.psat includes power flow, continuation power flow, optimal power flow, small-signal stability analysis and time-domain simulation. The toolbox is also provided with a complete graphical interface and a Simulink-based one-line network editor. Though PSAT permits several IEEE bus systems, an optimal bus system is to be identified for connecting SPVG. For 9 bus IEEE system, Single line diagram of the system is illustrated in Figure-5(a).The system data is given in PSAT manual 5.The load profile is increased in order to get the voltage stability from weaker system. The MW SPVG is considered as basic block and the required p.u generation is modelled.the Figure- 5(b) describes the block diagram of SPVG. The maximum power is calculated with help of Incremental Conductance MPPT algorithm with high step up DC-DC converter. The efficiency of inverter is analytically fixed as 9%.The SPVG power is injected in the

5 340 Angelin Grace Stella S. et al., 05/ Advances in Natural and Applied Sciences. 9(7) Special 05, Pages: buses where conventional generators are not located i.e. the SPVG power may be injected at the load buses.. Continuation power flow is run and the maximum loadability limit is identified. This maximum loadability point is the voltage stability point of the system. In voltage stability analysis, optimum location and SPVG size is determined. The Figure-6 shows PV curves for buses 5, 6, 7 and 9 without SPVG power. λ max was equal to 8.56 p.u. The SPVG is connected to the system at buses 4, 5,6,7,8 & 9 and act as load buses. The Figure-7 shows the variation of the λ max SPVG and the generated SPVG power at 000 W/m and 5 C and 30 C respectively. The irradiance varies from 00 W/m to 00 W/m at temperature 30 C and observed that 9 th bus shows the best loading point. RESULTS AND DISCUSSIONS Figure-8 shows maximum loading parameter with different irradiance levels at 30 C.Though λ max increases with increasing SPVG power at many locations; Bus 9 shows the best values of λ max..by analyzing the nose curve it is identified, in order to improve the voltage stability, the best location of SPVG is ninth bus. The optimum value of SPVG at Bus 9 is 0.43 p.u, after which λ max starts to decrease. Sizing of the SPVG is also considered, as it has great effect on the voltage stability. Different cases are identified to explain the effect of the size of the SPVG at Bus 9 on the PV curves and the voltage profiles of the buses. Fig. 5: (a)ieee 9 bus system with SPVG. Fig. 5: (b) Block diagram of SPVG Bus Voltage (p.u) Base case V-bus5 Base case V-bus6 Base case V-bus7 Base case V-bus Loading Parameter λ (p.u.) Fig. 6: P-V curves for load buses without SPVG for buses 5,6,7 & 9.

6 34 Angelin Grace Stella S. et al., 05/ Advances in Natural and Applied Sciences. 9(7) Special 05, Pages: Bus Voltage (p.u) Bus9 Bus7 Bus Loading Parameter λ (p.u.) Fig.7: Bus 6,7 & 9 at STC 000W/m and 5 C. Bus Voltage(p.u) Bus9 Bus6 Bus Loading Parameter λ (p.u.) Fig. 8: Maximum loading parameter λ max with SPVG at various irradiance levels at 30 C. These cases are: Case : Without SPVG power: The base case test shows that the loading at bus 9 is 8.56 p.u while SPVG power is not injected. Case : With SPVG at Bus 9, 0.0 p.u SPVG : The irradiance varies from 600W/m to 00W/m at 5 C temperature, the loading point increases from 856 MW to MW at 000 W/m i.e. 8% is increased compared to base case. Case3: With SPVG at Bus 9,.0 p.u SPVG: Similarly, the maximum loading point occurs at 800 W/m is 04.3MW and improved 7.8% Case4: With SPVG at Bus 9, 4.0 p.u SPVG: Similarly, the maximum loading point occurs at 600 W/m is 04.9MW and improved 7.9%. The Table 4 lists the voltage of Bus 9 for generators 0 p.u,p.u,4 p.u and 6 p.u. at irradiance 600 W/m to 00 W/m and Temperature 5 C. Table 4: Effect of sizing of SPVG on λ max at Temperature 5 C. Irradiance (W/m ) 0 p.u p.u 4 p.u 6 p.u It is clear that λ max decreases though the sizing of SPVG increases. Consider the irradiance level at 900 W/m beyond 0 p.u size the λ max starts to decrease. There is no improvement in λ max at the cost of SPVG size. Hence the optimum sizing is identified as 0 p.u.. It is clearly shown that the 0 p.u, SPVG exhibits the best voltage profile.

7 34 Angelin Grace Stella S. et al., 05/ Advances in Natural and Applied Sciences. 9(7) Special 05, Pages: Conclusion: This paper presents a detailed analysis of the optimal location and sizing of SPVG power for voltage stability improvement. Modelling of MW as basic unit and different size of SPVG is constructed and connected to the full bridge converter with Incremental Conductance MPPT algorithm in MATLAB tool. The generated power from MATLAB tool is fed to the PSAT tool, in order to analyze the power system stability. Simulations show that: (i)the maximum loading point λ max is p.u at Bus 9. (ii)the optimum sizing of SPVG is 043 MW Location of SPVG at bus 9 with 0 p.u SPVG is the optimum choice for voltage stability at IEEE 9 bus system. From the simulation results voltage stability has been improved by locating SPVG at the 9 th bus and the optimal size of SPVG is 043 MW. REFERENCES Mahmoud, Y.A., W. Xiao, H.H. Zeineldin, 03. Parameterization Approach for Enhancing PV Model Accuracy. IEEE Transactions on Industrial Electronics, 60(): Piazza, M.C.D., M. Luna, G. Vitale, P.V. Dynamic, 03. model parameter identification by least-squares regression. IEEE Journal of Photovoltaics, 3(): Chatterjee,A., A. Keyhani, D. Kapoor, 0. Identification of Photovoltaic Source Models. IEEE Transaction on Energy Conversion, 6(3): Villella, M.G., J.R. Gazoli, 009. Ernesto: Comprehensive Approach to Modeling and Simulation of Photovoltaic Arrays.IEEE Transaction on Power Electronics, 4(5): Brano, V.L., A. Oriolo, G. Ciulla, A.D. Gangi, 00. An improved five parameter model for photovoltaic modules ELSIVIER Solar Energy Materials & Solar Cells, 94(8): Brano, V.L., A. Oriolo, G. Ciulla, 0. On the experimental validation of an improved five parameter model for silicon photovoltaic modules. ELSIVIER Solar Energy Materials & Solar Cells, 05: Esram, T., P.L. Chapman, 007. Comparison of Photovoltaic Array Maximum power Point Techniques, IEEE Trans. On Energy Conversion, (): Bhatnagar, P., R.K. Nema, 03. Maximum power point tracking control techniques: State- of- the-art photovoltaic applications. ELSEVIER, Renewable and Sustainable Energy Reviews, 3: 4-4. Safari, A., S. Mekhilef, 0. Simulation and Hardware Implementation of Incremental Conductance MPPT with Direct Control Method using Cuk converter. IEEE Transactions on Industrial Electronics, 58(4): Stella, S.A.G., B.L. Prasanth, T.K. MahendraBabu, K. Gnanambal, 03. Low Cost Photometric sensing for Maximum PV power generation.ieee Conference Power, Energy and Control (ICPEC),India, Patel, H., V. Agarwal, 009. MMPT Scheme for a PV-Fed Single Phase Single stage Grid connected Inverter Operating in CCM with Only One Current sensor. IEEE Transactions on Energy Conversion, 4(): Yang, B., W. Li, Y. Zhao, X. He, 00. Design and Analysis of a Grid- Connected Photovoltaic Power System.IEEE Transactions on Power Electronics, 5(4): Ropp, M.E., S. Gonzalez, 009. Development of a MATLAB/Simulink Model of a Single-Phase Grid Connected Photovoltaic System. IEEE Transaction on Energy Conversion, 4(): Milano, F., 005. An Open Source Power System Analysis Toolbox. IEEE Transactions on Power Systems, 0(3): Power System Analysis Toolbox Version.. manual Chen, S.M., T.J. Liang, L.S. Yang, J.F. Chen, 0. A safety enhanced, High Step-Up DC-DC Converter for AC Photovoltaic Module Application. IEEE Transactions on Power Electronics, 7(4): Kjae, S.B., J.K. Pedersen, F. Blaabjerg, 005. A Review of Single Phase Grid-Connected inverters for Photovoltaic Modules. IEEE Transactions on Industry Applications, 4(5): Chetan Singh Solanki, 0. Solar Photovoltaics Fundamentals, Technologies and Applications. nd edition. Prentice Hall of India Pvt. Ltd. Datasheet of PV panel accessed on November 04.