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1 Fuzzy ogic Impoved Static Synchonous Compensato to Contol Voltage of a Gid-connected Squiel Cage Induction Geneato Reza Shaifian Dastjedi 1, Ehsan Jafai 2, Fashad Kabii 3, ayam Ghaebi anah 4 1,2,3,4 Depatment of Electical Engineeing, enjan Banch, Islamic Azad Univesity, Isfahan, Ian shaifian@iauln.ac.i Abstact: In this pape a new model fo a wind geneation system is intoduced which includes induction machine, static synchonous compensato, excitation and load capacitos. Moeove, a new method is poposed fo voltage contol of a gid-connected squiel cage induction geneato. Unfotunately, induction geneatos equie a flexible eactive powe souce beside them since unde vaiable load conditions the output voltage cannot emain constant autonomously. In this pape, a compensato is used to contol the eactive powe and stabilize voltage fluctuations duing a thee-phase fault and geneato isolation. Futhemoe, a fuzzy contolle is developed to amelioate compensato's pefomance. Accodingly, simulations ae caied out in MATAB/Simulink envionment to confim the abilities of new contol scheme. [Reza Shaifian Dastjedi, Ehsan Jafai, Fashad Kabii, ayam Ghaebi anah. Fuzzy ogic Impoved Static Synchonous Compensato to Contol Voltage of a Gid-connected Squiel Cage Induction Geneato. ife Sci J 2012;9(4): ]. (ISS: ) Keywods: Fuzzy ogic, Squiel Cage Induction Geneato, Static Synchonous Compensato, Thee-phase Fault. 1. Intoduction Renewable enegy has been developed geatly duing ecent yeas especially when the pice of oil and subsequently elevant poductions inceased. Obviously, among diffeent souces, wind powe gabbed a huge attention. Hence, induction geneatos got a dominant ole in the wind powe geneation. Appaently, salient featues of induction geneatos ae small size, sepaate dc excitation and bush omission, and low maintenance cost [1-2]. On the othe hand, analyses of geneato's voltage and fequency ae easie when the machine is connected to the gid in compaison with the isolated situation [3-4]. The two main dawbacks of induction geneatos ae weak voltage egulation and eactive powe equiement. The needed eactive powe is povided without any paallel capacito and just fom the DC link capacito when it is chaged. Geneally, both geneato and load ae eactive powe consumes. Howeve, unbalanced eactive powe causes voltage fluctuation. Theefoe, a good choice fo capacitance would be when the minimum equied eactive powe is popotional to invese of squaed speed and maximum amount is equal to the magnetizing eactance [5]. ogically, constant capacitos could not povide the appopiate eactive powe. Theefoe, a combined set of capacitos is installed which includes constant and switching types. Fuzzy sets can be vey helpful in making decision duing diffeent situations and theeupon the pefomance of compensato is impoved. Off-gid geneatos sometimes suffe fom excess o lack of active and eactive poduced powes which can cause fequency deegulation. An effective solution is to change the blades angle [7]. Blades angle is simila to steam tubine's valves though which the speed is contolled. ote that vividly the esponse time in wind tubines is vey faste than steam types [8]. On the othe hand, in a squiel cage induction geneato, the magnetizing eactive powe in teminals is undesiable. Hence, capacitos ae installed to obviate the poblem. They can povide the magnetizing cuent and even load eactive powe as well. Besides, a powe electonic based compensato such as SVC, STATCOM, etc. is installed in paallel with the machine to make the esponse bette duing a fault. umeous publications ae dedicated to this issue. Some eseaches focused on modeling of an induction machine and a capacito next to [9]. This technique is useful in sliding mode contol, state feedback, output feedback, etc. [1, 2, and 10]. 2. Distibution System Model A single line diagam is illustated in Fig. 1 which epesents the wind enegy convesion system. This local system is fed by a geneato which is connected to a 132kv infinite bus [1]. A 133/33kv tansfome is a medium between local gid and infinite bus and similaly, a 33/0.kv tansfome connects the induction geneato to the local gid. Induction geneato is diven by a wind tubine which is contolled via otation angle (β). In ode to contol β, a efeence value is compaed with the geneato's injected powe (p *). Aftewads, the esulted eo is passed though a popotionalintegal contolle and theefoe, an acceptable value fo β is estimated. The capacito povides the 2955

2 equied eactive powe fo the magnetizing cuent. The system's specifications ae given in the appendix. Figue 1. System model 3. System Elements 3.1. Wind Tubine The output powe of the induction geneato is obtained as follows [1, 2, 3]: m R cp(, B) V w (1) 2 whee ρ is ai density, Vw is wind speed, R is blade length, and C p indicates the faction of aeodynamic wind powe extacted fom the tubine. ote that Cp vaies with wind speed and can be defined as Equation (2). whee R and R s ae equivalent oto and stato esistances espectively s and ae stato and oto leakage inductances espectively, m magnetizing is inductance, s and ae stato and oto inductances espectively. (The (3), δ and ρ values as elations (5) ae defined). d m m,, 1 (5) 2 dt s whee w is an optional speed and is the oto speed in ad/s. Figue (2) depicts the model of an asynchonous machine in which R d is the esistance that indicates the absobed powe fom the input. ( R ( 1 s) R s ). d / 5 3 C p (2) is the Blade tip speed and can be calculated using Equation (3). R m (3) Vw tubine. whee m 2 is angula otating velocity of 3.2. Induction Geneato Equation (4) pesents model of induction geneatos in an optional efeence fame [2-3]. In this equation, the oto's esidual flux is essential fo geneato to make the initial voltage. The uppe case indices ae elated to oto and convesely, the lowe case ones ae coesponding to stato values. s s iqx s idx q s d 0 vqs vds 0 0 s s 0 s s s s s s s Figue 2. Induction geneato model and the mechanical pats C is the field capacito which povides the magnetizing cuent flown though m. R1 and 1 ae loads. In geneation mode, the oto moves faste than the synchonous speed; theefoe, slip value is negative (s<0). The capacitance can be calculated using poduced eactive powe and voltage STATCOM Figue (3) shows a device which can compensate and contol the voltage o powe facto in the d-q coodinate [3-4]. V ae geneato teminal voltage, Iabc ae thee-phase cuents injected by the compensato to gid, V ms is the effective voltage, and V dc is the dc voltage of capacito. The contolle consists of a phase locked loop in ode to synchonize the thee-phase voltage in the output of convete accoding to zeo cossing point. Thus ceates θ which is used in d-q (4) convesion. (4) Figue (3), This figue includes fou I contolles. The fist block tunes teminal voltage i. ext I has with espect to the efeence cuent q the ole of keeping dc voltage constant via active powe exchange with gid. In othe wods, the efeence cuent i d is detemined by the second contolle. The next two blocks poduce base abc 295

3 voltages vq and vd which afte convesion into theephase system, the base voltage V abc is fomed and applied to invete switches accoding to WM scheme. In this section, a new analytical model is pesented fo the system's behavio [2-3]. Fo simplicity, let's assume: 1) ion loss is negligible 2) All paametes ae constant except the magnetic inductance. 3) All pats have thee-phase sta connected configuation. Table 1: Fuzzy linguistic vaiables Figue 3. compensato contol diagam The STATCOM which should contol powe facto opeates like the block esponsible fo voltage contol but the only diffeence is that eactive powe is tuned with egad to demand. Theefoe, Vms ef should be eplaced with Q ef in Figue (3). This value is usually assumed zeo to egulate the measued eactive powe in the consume's place. 4. Fuzzy Contol Den and en ae fuzzy block inputs and zetan is the output signal. K de, K e, and Kzn ae values which can vay and have an impotant ole in fuzzy contolle pefomance in both tansient and steady states. In this eseach, the latte values ae assumed constant and equal to 0.2, 500, and 0.05 espectively fo simplification and easie implementation. As it is shown in Figue (4), tiangula membeship functions with ovelap ae used in both input and output of fuzzy block. Fig. 4. compensato contol diagam Fuzzy ules ae defined using linguistic vaiables listed in Table (1) which include (egative), (eo), and (ositive). Mamdani max-min technique is adopted to poduce outputs fom the fuzzy vaiables. Figue (5) depicts complete fuzzy system though which the efeence voltage is obtained []. In ode to elevate modeling, the mechanical pats ae taken into account and modeled in Figue (2). In this model, the applied toque to the shaft ) is modeled by a cuent souce. Similaly, the ( shaft den en zeta n electomechanical toque ( em ) is shown as a cuent souce. Finally, the inetia of oto is modeled as a capacito. It should be noted that the mechanical model ( and em ) ae DC vaiables wheeas elecic pats ( V and V s ) includes ac vaiables in 50 o 0 Hz. R d is a negative esistance in geneation mode which poduces mechanical powe ( em ) and epesents the conveted powe fom mechanical to electical fom. Rs and s ae equivalent esistance and inductance of tansmission lines. In ode to contol the voltage, a cuent contolled voltage souce is used in the model as shown in Figue ( - a). In this model, anothe scheme ( - b) is employed to tune the voltage souce. In this method, bus voltage is compaed with a efence value and the subsequent eo passes though a I block to poduce the equied cuent fo eactive powe geneation. This cuent affects voltage accoding to equation (). KV s -V s = d I d () 5. System Modeling 2957

4 Figue 5. Fuzzy system diagam (a) (b) Figue. a) expanded model fo voltage contol b) contol diagam. Simulations Enegy convesion system consists of a 9 MW wind tubine connected to a 33 kv distibution system. This souce feeds a 133 kv load though a 25 Km tansmission line. Stato of the squiel cage induction geneato is diectly connected to a 0 Hz gid and the oto moves by a tubine with a vaiable angle pitch.). Angle pitch is useful fo the output powe estiction when the speed is highe than nomal (9 m/s). The output powe can be extacted just when the speed is highe than the synchonous value. The speed usually changes fom 1 pu in no-load and pu in full-load conditions. Moeove, the geneato has eo monitoing equipments fo cuent, voltage, and speed fom which if a fault epot eceived, the geneato goes off. Besides, a goup of capacitos, installed in the low voltage teminal bus, supply the equied eactive powe of geneato, equal to 400kVa. The equied eactive powe fo voltage stabilization is povided by a 3MVa compensato in the one pe unit voltage (33kV). The tubine delives ated powe (3 MW) at the ated speed (9 m/s). If wind speed changes, the output powe altes with egad to the speed-powe cuve. Hee, wind speed at t=2s is set to 8 m/s and inceases to 11 m/s in one second. At t=15s a tempoay fault occus in the bus b. Subsequently, both active and eactive powes of bus b ae given in Figues (7) and (8). The coesponding dc voltage is illustated in Figue (9). When simulation stats active powe ascends gadually until nominal point in 5 seconds. Duing this peiod, the geneato speed inceases fom to pu. The esulted voltage without b is sketched in Figue (10). The teminal voltage dops to aound 0.97 fuzzy contolle in due to wind speed change. If a fault is occued, voltage vaiations ae sevee and pehaps the steadystate eo would be emakable. Figue (11) illustates the bus voltage when fuzzy contolle is unning. ot only voltage vaiations in tansient condition (due to wind speed alteation and eos) ae impoved but also in steady state mode the efeence voltage is taced well. Futhemoe, the geneato's absobed eactive powe inceases with the poduced active powe gowth. The geneato demands eactive powe of 1.47 MVa in the nominal state. The output powe is 9 MVa in 11 m/s wind while the compensato keeps teminal voltage constant, equal to 0.984, via eactive powe injection of 1.2 MVa. At t=15 a phase to b and it is obviated at t= phase fault occus at The powe injected by compensato is depicted in Figue (12). The bus connected to geneato is encounteed with eactive powe shotage; theefoe, voltage dop is inevitable. At fist, tubine's angle pitch is zeo; howeve, when the output powe exceeds 3 MW, the angle pitch expands to eight degees fo the output powe compensation equal to the nominal value as it is shown in Figue (13). The paametes values employed in the simulation pocess ae listed in Table 2. Figue 7. Active powe of bus 2958

5 Figue 8. Ractive powe of bus Figue 9. vaiation of DC link voltage Figue 10. ine to line voltage without FC Figue 11. ine to line voltage with FC Conclusions In this pape a new model and a contol diagam fo a wind enegy convesion system composed of load, exciting capacito, and induction geneato ae intoduced. The equivalent cicuit makes a complete connection between electical and mechanical pats. As a esult, a deep insight into the behavio of both systems is achieved. Such these illustated knowledge could be extemely wothwhile fo contolle design and voltage egulation. The model aims to contol the bus voltage via a compensato in an induction geneato connected to the gid. To excel the compensation policy, a fuzzy contolle is designed to detemine the efeence voltage. Consideing simulation esults, fuzzy contolle amelioates both tansient and steady states voltage wavefoms. Coesponding Autho: Reza Shaifian Depatment of Electical Engineeing, enjan Banch, Islamic Azad Univesity, Isfahan, Ian. shaifian@iauln.ac.i Appendix Table 2. aamete of simulation paamete value m 50mH l 30mH ls 30mH R S 5.7Ω R 3.4 Ω 2 F 0Hz 440Kw V 40V RM 1450pm Figue 12. Reactive powe injecting by STATCOM Figue 13. itch angle 2959

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