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  • 1. International Journal Of Computational Engineering Research (ijceronline.com) Vo2. 2 Issue. 5 Load Flow Analysis of Distribution System Including Wind Turbine Generating System Models P.Srihari1, G.Srinivasa Rao2 1,2 ,Post Graduate Student , Department of EEE, V R Siddhartha Engg. College (Autonomous), Vijayawada-7.Abstract— The power world is sauntering towards eco- Equipment’s are underpinning this strategy. Wind energyfriendly distributed generation (DG), their integration into the is converted into mechanical power by wind turbines. Thisdistribution network postures challenges to existing load mechanical power is fed to the generator directly or throughflow techniques. This paper is concerned with developing gear system. This cumulative arrangement of turbine andmodels of various types of wind turbine generating systems Generator is called a wind turbine generating unit (WTGU).(WTGSs) used as distributed generation (DG) sources and A large number of wind turbine generating systemsdemonstrating their application for steady state analysis. As wind (WTGSs) are already in operation and many new systems areenergy is used as a DG power source, its impact on the being planned. The integration of WTGS with the powerdistribution system needs to be analyzed. This requires a systems is taking place at both the transmission and theload flow algorithm capable of simulating both radial and distribution voltage levels. This growth in wind generationweakly mesh systems along with Wind Turbine Generating has spurred investigations, to understand the behavior of theSystems (WTGSs). The compensation based distribution load wind turbine generating Systems (WTGSs) as well as theirflow algorithm is extended including WTGS modeling in the impact on the power grid.present work. The application of the proposed models for the As distributed generation penetrates into the distributionload flow analysis of radial systems having WTGS has been network its impact on the voltage profile and total powerdemonstrated. Based on these studies we evaluate the impact losses needs to be analyzed by appropriate load flowof wind based DG on the voltage profile and losses of radial methods. Due to high R/X ratio of distribution lines, thedistribution networks. Simulation studies have been conceded performance of load flow methods used for theout on radial distribution systems having WTGS as DG transmission network are inadequate for distribution systems.sources to illustrate the application of the proposed models. These methods do not make use of the structural advantage (radial / weakly mesh) of distribution system. This led to the Keywords— Distributed Generation, Distribution load flow, development of separate load flow methods for distributionWind Turbine Generating Systems, Forward and Backward system. These methods are broadly classified into threesweep, Voltage Profile, Losses. categories: direct methods, backward / forward sweep methods and Newton-Raphson based methods [1]. The I.INTRODUCTION backward/forward sweep methods have an edge over the The supply of electric power to homes, offices, schools, others as they are simple, flexible and the computational timefactories, stores, and nearly every other place in the modern requirement is low [1].world is now taken for granted. Electric power has become a The basic backward/forward sweep method described in [2]fundamental part of the infrastructure of contemporary society, is applicable for radial networks only. The practicalwith most of today’s daily activity based on the assumption distribution systems are weakly meshed. The compensationthat the desired electric power is readily available. The power based distribution load flow algorithm proposed in [3] addresssystems which provide this electricity are some of the largest this issue. It is improved to adapt the PV bus modeling in [4].and most complex systems in the world. They consist of three The method was further extended to the distribution networksprimary components: the generation system, the transmission containing DG’s, where these are modeled as constant PQ orsystem, and the distribution system. Each component is PV nodes [5].essential to the process of delivering power from the site Ii. Wtgs Modelingwhere it is produced to the customer who uses it. Wind energy The mechanical power converted from wind energy bycontinues to be one of the fastest growing energy technologies a wind turbine is described asand it looks set to become a major generator of electricitythroughout the world. The role of wind energy in electrical power generation is (1a)increasing day by day and new advances in power electronicIssn 2250-3005(online) September| 2012 Page 1521
  • 2. International Journal Of Computational Engineering Research (ijceronline.com) Vo2. 2 Issue. 5 (1b) From the equivalent circuit, given, in Fig.1,the reactive powerWhere, Pm is the mechanical power developed in watts, ρ is balance can be written as followsdensity of air (kg/ ), V is wind speed(m/s), is powercoefficient, is pitch angle, λ is tip speed ratio, is gear ratio (4)and A is the area swept by the rotor ω is the angularvelocity(rpm),R is the turbine rotor radius(m) and ,x are Substituting the voltage equation, Eq.(3) into Eq(4), we getconstants. (5) The variation of power coefficient with a variation of υ, Similarly, from the equivalent circuit, the active powerλ is non linear in nature. It can be estimated by the relation. balance can be written as (6) (1c) And, substituting voltage equation, Eq.(3), into Eq.(6), we Where, (2) get the active power demand of the machine as follows (7) The induction generators are generally used inwind turbine for electricity generation and modeledwith standard equivalent circuit from using the Equ.s (5) &(7) have a straight solution and depend on theconcept of a rotating transformer [6–9]. In the study terminal voltage magnitude, mechanical power input and theof [6], Feijoo and Cidras proposed two models based equivalent circuit [6-8].on the steady- state equivalent circuit of theinduction generator. In the study, wind farms withasynchronous generators are modeled as PQ or RXbuses in the load flow analysis. When the WTGS nodeis modeled as a PQ-bus, produced active andconsumed reactive power. The other model, in thatstudy, is called RX bus model in which active andreactive powers are calculated by usingequivalent circuit parameters of the inductionmachine. Both are solved iteratively using a Newton Fig.1. Induction motor equivalent circuit.type algorithm. Implementation of RX model intodistribution system load flow analysis is found to be Model-II for WTGS:problematic, therefore, excluded from this study Similarly, the referred rotor voltage (Vr) of theand the PQ model is used for the analyses. induction motor, given in Fig. 1, can also be obtainedModel-I for WTGS: by using modified bi-quadratic equation given in [12] Let us consider an induction motor equivalent as followscircuit referred to the stator side as given in Fig. 1. Inthe figure, Vs and Vr stand for the magnitude of thestator voltage and referred rotor voltage, (8a)respectively. Rsc and Xsc show short-circuitequivalent resistance and reactance, respectively. Rm (8b)resistance represents core losses and Xm showsmagnetizing reactance of the motor. Ps and Q sstand for the active and reactive power demand ofthe motor, respectively. From figure, recognizing (8c)the fact that Qr = 0, the referred rotor voltage (Vr)can be obtained by solving well-known bi- From Fig.1,recognizing the fact that =0,thequadratic equation given in [10] by neglecting the referred rotor voltage can be written as followsshort-circuit losses as follows. (9a) (3) (9b)Issn 2250-3005(online) September| 2012 Page 1522
  • 3. International Journal Of Computational Engineering Research (ijceronline.com) Vo2. 2 Issue. 5 power output of the WTGU for a given terminal voltage and wind speed. (9c) Substituting the voltage equation, Eq.(9c) into Pitch regulated fixed speed WTGU[7] Eq.(4), we get In this class of WTGU, the pitch angle controller regulates the wind turbine blade angle (ν ) according to the wind speed variations. This designed power output Pe of the WTGU with wind speed is provided by the manufacturer in the form of a power curve. Hence, for a given wind speed Pe can be (10a) obtained from the power curve of the WTGU, but Qe needs to (10b) be computed. With the slip known, the reactive power output Qe is calculated from the induction generator equivalent circuit. Pitch regulated fixed speed WTGU power output (Pe and Qe) Similarly, Substituting the voltage equation, Eq.(9c) into active power balance, Eq.(6),we get (12)(11a) (13) (11b) Semi Variable Speed WTGU [7] Likewise, Equations (10) and (11) have also a This class of WTGU consists of a pitch controlled windstraightforward solution and depend on terminal turbine and a wound rotor induction generator. The rotorvoltage magnitude, mechanical power input and the circuit of the generator is connected to an external variableequivalent circuit parameters of the induction machine resistance. There are two controllers, a pitch controller andexcept the rotor slip. It is noted that for the rotor resistance controller. These two controllers are designedinduction generators the mechanical power is to operate in a coordinated manner. This design guaranteesnegative due to the change on the flow of power that the active power output is equal to the maximum powerwhich is transferred from ‘‘rotor’’ (bus-r) to the at wind speeds below nominal and equal to rated power‘‘stator’’ (bus-s). Hence, the mechanical power above nominal wind speeds. For this class of WTGU also, theinput, Pm, must be taken in negative sign for both manufacturer provides the designed real power out-put versusdeveloped models. wind speed characteristics [7]. We note that the expression Since there are two solutions of real and reactive for Pe and Qe can be recast as a quadratic function of a newpower outputs of the developed Model-II[11], the variableR2/s. Hence, even when R2 and s are unknown therequired root of the equation, given in Equations quantityR2 /s (say Req) can be computed by solving the(10) and (11), must be determined. For this reason, quadratic equation in Req involving Pe. To compute Qe, thisvariation of the active and reactive power output of value of Req is used in the modified expression for Qe. Thethe generator with different mechanical power quadratic equation for Req is as follows:input (Pm) are calculated using both expressions,for a hypothetical machine. On the other hand, the (14)solution of Equations (10b) and (11b) remain on the Wherefeasible region. Therefore, the solution of (15)Equations (10b) and (11b) is unique for the reactiveand active power outputs of the induction generator,respectively. They can facilitate the computation ofreal and reactive powers of the induction generator (16)for a specified mechanical power input and terminalvoltage, in a simple way. Presently various types of WTGU have been installed andthey can be broadly classified into three categories, namely (17)fixed, semi-variable and variable speed types[7]. The models Given wind speed u w and the terminal voltage Vdeveloped here for the WTGU are intended to obtain theIssn 2250-3005(online) September| 2012 Page 1523
  • 4. International Journal Of Computational Engineering Research (ijceronline.com) Vo2. 2 Issue. 51. For the given uw obtain Pe from the power curve of the III. LOAD FLOW ANALYSIS USING WtgssWTGU (provided by the manufacturer). As the wind generator is of induction type the bus2. Compute (18) connected to wind generator can be modeled as PQ bus in load flow algorithm. The load flow algorithm3. Knowing Req , compute Reactive power Qe as including wind generator is explained through flow (19) chart. Initialize the bus voltages of the system, andThe Pitch regulated fixed speed WTGS model and Semi mechanical power of the WTGSs for avariable speed WTGS model considered here are the Vestas specified wind speed by using Equation (1)unit of 1MW rating .The Induction generator circuitparameters are given in Table I. Set the iteration counter (it=1) TABLE I Induction generator circuit parameters Calculate active and reactive powerType of Pitch Regulated Semi Variable outputs of WTGSs by using: Eqs: (5) andWTGS Fixed Speed Speed (7) for Model-IModel Eqs: (10b) and (11b) for Model-II (12) and (13) for PRFSR1(pu) 0.005986 0.005671 Calculate loads and WTGSs current asXl1(pu) 0.08212 0.15250R2(pu) 0.01690 0.00462 Calculate active and reactive powerXl2(pu) 0.107225 0.096618 transferred from each branch including losses and each line currents by using loads andXm (pu) 2.5561 2.8985 WTGSs powers and currents, respectively.Xc (pu) 2.5561 2.8985 Calculate each line receiving end bus voltages using forward voltage formulation TABLE II of particular load flow algorithm. Induction generator circuit parameters (Model I & II) 1. Generator short circuit resistance(ohm) 7.81 The system NO reaches it = it+1 2. Short circuit reactance(ohm) 18.76 convergence? 3. Open circuit resistance (ohm) 1230 YES 4. Open circuit reactance(ohm) 500.6 Print Results. 5. System rms base voltage(kV) 12.66 Stop Fig. 2. Flow chart for distribution load flow with WTGS models IV. Results And Discussion A 33-bus distribution system is used to evaluate the effect of WTGS on the power losses, voltage profile of radial distribution system, and on the performance of load flow algorithms. Sweep-based algorithms are used and they all have been coded inIssn 2250-3005(online) September| 2012 Page 1524
  • 5. International Journal Of Computational Engineering Research (ijceronline.com) Vo2. 2 Issue. 5 Matlab. Transformer parameters are taken from [7]. Bus WTGS Ps Qs Pl Ql Tables III & IV represents the load flow results of the No. systems with and without WTGS modeled by using Model (MW) (MVAr) (MW) (MVAr) both developed models, pitch regulated and semi variable speed W T G S m o d e l s are also depicted. 6 _ _ _ 0.9331 Similarly, in load flow analyses, initial voltage magnitude at all buses is considered to be the same 18 _ _ _ 0.8959 as the source bus (1.0 pu) for test system and a 0.369 0.247 25 _ _ _ 0.9648 tolerance of 10-4 on convergence criteria is used. M echanical power input is selected as Pm =1.0 MW. 32 _ _ _ 0.8788 From the tables, it is clearly seen that the inclusion of a WTGS significantly, alters the power intake 6 SVS 1 0.624 0.9614 from the main substation, and total power losses of the test systems are also decreased. Moreover, 18 SVS 1 0.640 0.9243 the voltage magnitude of the bus at which the WTGS 0.316 0.219 i s connected is increased due to injected active 25 SVS 1 0.616 0.9737 power. The Induction machine circuit parameters and the data is taken from [11] for Model I & Model II, 32 SVS 1 0.650 0.9246 whereas for the remaining models i.e., Pitch regulated fixed speed and Semi variable speed is taken from 6 PRFS 1 0.5665 0.9593 [7].It is clearly observed that from the Tables III & IV developed models are more advantageous than the Pitch 18 PRFS 1 0.5640 0.9218 regulated fixed speed WTGS and Semi variable speed 0.285 0.197 WTGS models. 25 PRFS 1 0.572 0.9731 is the WTGS bus voltage. 32 PRFS 1 0.564 0.9202 is the mechanical power input(1MW). is the WTGS active power generation. TABLE IV Load flow result of the 33-bus system with and without is the WTGS reactive power demand. WTGS models.(Model I & Model II) is the active power loss. Bus WTGS Ps Qs Pl Ql No. Model (MW) (MVAr) (MW) (MVAr) is the reactive power loss. 6 _ _ _ 0.9331Semi Variable Speed(SVS) WTGS. 18 _ _ _ 0.8959Pitch Regulated Fixed Speed(PRFS) WTGS. 0.369 0.247 25 _ _ _ 0.9648 ABLE IIILoad flow result of the 33-bus system for Pitch Regulated and 32 _ _ _ 0.8788 Semi Variable Speed WTGS models 6 II 0.919 0.923 0.9680 18 II 0.919 0.923 0.9295 0.181 0.129 25 I 0.992 0.017 0.9750 32 I 0.992 0.017 0.9449Issn 2250-3005(online) September| 2012 Page 1525
  • 6. International Journal Of Computational Engineering Research (ijceronline.com) Vo2. 2 Issue. 5 33-BUS RADIAL DISTRIBUTION SYSTEM the voltage profile is analyzed. The effectiveness of 1 linking two buses can be studied in terms of decrement Voltage Magnitude in losses and improvement in voltage profile. And the 0.95 same load flow analysis can be done for other models i.e., Variable speed WTGS and Stall regulated fixed 0.9 speed WTGS are also useful for the future planning of Without WTGS With WTGS the system. 0.85 0 5 10 15 20 25 30 35 Bus Number References [1] A. Losi, M. Russo, "Object Oriented Load Flow for RadialFig. 3 Voltage profile variation of the 33-bus system for PRFS and Weakly Meshed Distribution Networks," IEEE Trans. Power Syst., vol. 18, no.4, pp. 1265-1274, Nov. 2003. WTGS model [2] W.H. Kersting, “Distribution System Modeling and 33-BUS RADIAL DISTRIBUTION SYSTEM Analysis,” CRC Press ,New York,2002. 1 [3] D.Shirmohammadi, H.W.Hong, A.Semlyen, and Voltage Magnitude G.X.Luo,”A compensation based power flow for weakly 0.95 meshed distribution and transmission networks,” IEEE Trans. Power Syst., vol.3, pp. 753-762, may 1998. [4] C. S. Cheng, D. Shirmohammadi, “A three-phase power flow 0.9 method for real-time distribution system analysis,” IEEE With WTGS Without WTGS Trans. Power Syst., vol10, pp. 671–679, May 1995. 0.85 0 5 10 15 20 25 30 35 [5] Y. Zhu, K. Tomsovie, “Adaptive Power Flow Method for Bus Number Distribution Systems with Dispersed Generation,” IEEE Trans. Power Del, vol. 17, pp. 822-827, July 2002.Fig. 4 Voltage profile variation of the 33-bus system for SVS [6] Feijoo A, Cidras J. “ Modeling of wind farms in WTGS model the load flow analysis,” IEEE Transaction on Power Systems 2000;15(1):110–6. 33-BUS RADIAL DISTRIBUTION SYSTEM [7] Divya KC, Rao PSN. “Models for wind turbineVoltage Magnitude 1 generating systems and their application in load flow studies,” Electric Power Systems Researc2006;76:844-56. 0.95 [8] Cutsem TV, Vournas C. “ Voltage stability of electric power systems,” Boston: Kluwer 0.9 Academic Publishers Springer; 1998. [9] Hatziargyriou ND, Karakatsanis TS, Papadopoulos Without WTGS With WTGS M. “ Probabilistic load flow in distribution 0.85 systems containing dispersed wind power 0 5 10 15 20 25 30 35 Bus Number generation,” IEEE Transaction on Power Systems 1993;8(1):159–65.Fig. 5 Voltage profile variation of the 33-bus system (with and [10] Haque MH.“ Load flow solution of distribution systems with voltage dependent load models,” without WTGS models) Electric Power Systems Research 1996;36(3):151–6. [11] Ulas Eminoglu “Modeling and application of wind V. CONCLUSION turbine generating system (WTGS) to distribution systems,” Renewable Energy, vol.34,no.11, pp.2474- In this paper, two new models[11] for wind turbine 2483, Nov.2009.generating systems are provided. The main [12] Satyanarayana S, Ramana T, Sivanagaraju S, Rao GK.advantage of the developed analytical models for An efficient load flow solution for radial distributionWTGS is that they facilitate the computation of real network including voltage dependent load models.and reactive power outputs for a specified Electric Power Components and Systemsmechanical power input and terminal voltages, in a 2007;35(5):539-51.simple way such that it does not require computationof any system parameters (i.e., rotor slip).Whereas theother proposed models in [7] rotor slip and powercurves plays a crucial role. These developed models [11] can overcome thecomputational complexity and the computational timeproblem. The WTGS impact on the system losses andIssn 2250-3005(online) September| 2012 Page 1526