e t International Journal on Emerging Technologie (): 3-36() ISSN : 975-8364 Etimation of aturation in grid connected induction generator Shelly Vadhera and KS Sandhu Deptt of Electrical Engineering, National Intitute of Technology, Kurukhetra, (HR) INDIA (Received 5 Nov, 9, Accepted Jan, ) ABSTRACT : Prediction of the aturation level and henceforth finding the value of magnetizing reactance X m i the firt need for accurate etimation of teady tate performance of grid connected induction generator (GCIG) Study reveal that mot of the reearch peron adopt the unaturated value for magnetizing reactance for the teady tate analyi of uch machine Wherea in thi paper an iterative technique i propoed to predict the aturated value of magnetizing reactance with change in operating condition The comparion between experimental and imulated reult prove the validity of technique propoed Further effort have been made to predict the operating limit of machine uing propoed modeling Analyi ha been extended to identify effectivene of the machine parameter to improve the operating performance of the generator The reult are preented and inference are drawn to ugget guideline for real deign problem in GCIG Keyword : Aynchronou Generator, Grid Connected Induction Generator, Iterative Technique, Steady State Analyi, Wind Energy Converion NOMENCLATURE Air gap voltage per phae I Stator current per phae I real Real part of I I imag Imaginary part of I I Rotor current per phae I real Real part of I I imag Imaginary part of I I m Magnetizing current per phae pf power factor P fw Friction and windage loe P g Air gap power P in Input Power P m Mechanical Power P out Output power P r Rotational loe (friction, windage and core loe) P rcu Rotor copper loe P core Stator core loe P cu tator copper loe Stator reitance per phae R Rotor reitance per phae referred to tator Slip V Terminal voltage per phae X Stator leakage reactance per phae X Rotor leakage reactance per phae referred to tator X m Magnetizing reactance per phae I INTRODUCTION Recently coniderable attention i being focued on environmentally clean and afe renewable energy ource like wind, olar, hydro etc The fat depletion of foil fuel and our over dependence on them i believed to oon bring the wheel of our civilization to creeching halt Henceforth at thi juncture the need i being felt for relevant technological effort in order to tap the vat potential energy of hydro, wind etc available in iolated location The ue of an induction generator [] in general and quirrel cage induction generator in particular for uch application in remote, unattended and maintenance free ite i earning more favor over that of the ynchronou alternator The reaon for the widepread popularity of induction generator i owing to it capability to generate the power from variable peed a well a contant peed prime mover, low unit cot, reduced maintenance, rugged and bruhle rotor, abence of a eparate dc ource for excitation, abence of moving contact, inherent overload protection, improved performance due to low tranient impedance, natural protection againt hort circuit etc Two mode of operation can be employed for an induction generator One i through elf-excitation and other i through external-excitation In firt mode, the induction generator take it excitation from VAR generating unit, generally realized in the form of capacitor bank With uitable capacitor connected acro the terminal and with rotor driven in either direction by a prime mover, voltage build up acro the terminal of the generator due to elf excitation phenomenon leaving the generator operating under magnetic aturation at ome table point Such generator i known a elf-excited induction generator (SEIG) [-6] In econd mode it draw the reactive power for it operation from the grid to which it i connected Such generator i known a externally excited generator or grid connected induction generator (GCIG) [7-] In GCIG the grid regulate voltage and frequency automatically Thu GCIG ha no voltage regulation problem and henceforth eliminate the need of voltage regulator a in cae of SEIG The ynchronou machine already connected to grid and running in parallel with an induction generator provide the reactive power, while generator output i decided by it operating lip Therefore a bottleneck in the application of GCIG remain the requirement of inductive
3 Vadhera and Sandhu VAR for both induction generator a well a the load, which ha to be fed by the generator The cutom deigned induction generator eem unneceary becaue an induction motor give correponding performance a induction generator The fact that induction machine are readily available from everal manufacturer make them very competitive for jut in time intallation However there i a cope to improve the performance by proper deigning of machine To incorporate deign modification, there i a need for prior etimation of teady tate performance of the machine Prediction of the aturation level and hence finding the value of magnetizing reactance of induction generator i firt tep in the teady tate analyi of GCIG A review of the available literature reveal that although a lot of work ha been reported on analyi of GCIG uing the philoophy of fixed value of magnetizing reactance (X m ) but no attempt eem to have been made in uing the aturated value of X m with correponding change in air gap voltage ( ) Computation of the magnetic reactance i done uing thevenin equivalent circuit [7], while in [8] the value of X m i found by carrying out variable voltage no load tet [], [9-] ue the fixed value of X m and [] calculate the value of X m correponding to induced electromotive force (EMF) by uing a oftware package of MathCAD In the preent paper it ha been hown that there i a coniderable variation in X m with load/lip and hence there i need to compute aturated value of X m correponding to any operating load Therefore, a new iterative technique ha been propoed to compute the correponding value X m with change in operating condition Further the analyi i extended to identify the effectivene of variou deign parameter in order to improve the performance characteritic of a machine working a a generator II STEADY STATE MODELING V j X R j X I I m I j X m R - Fig(a) Equivalent circuit repreentation of an induction machine Analyi of equivalent circuit repreentation of an induction machine a given by Fig(a), with any value of lip (negative for generator operation) reult in to the following mathematical expreion : R + jx ( jx m) Z + jx + R + jx ( + Xm ) () Unknown and aturated value of X m for generator operation may be obtained uing iterative technique, a explained in ection III Z Z real + jz imag () Where R X m Z real R (3) + ( X + Xm) Z real i negative for generator operation R Z imag X + V X + X X ( X + X ) m m m R + X + ( X ) m (4) I Z (5) I I real + ji imag (6) Where, VZ real Z + Z (7) I real ( real imag ) VZ imag I imag ( Zreal Zimag ) + (8) V I ( + jx ) (9) I m E jxm () I I I m () P out 3V I real () Thi reult in negative power for negative lip in cae of generator The input to rotor i P in P out + 3I + 3I R + P r (3) Equation () and (3) give efficiency (η) of the generator a; η P P out in (4) III ITERATIVE TECHNIQUE Iterative procedure for the computation of magnetizing reactance X m in generating mode i ummarized a following : Step Aume X m correponding to E a pu from the relationhip between and X m depicting the magnetic characteritic of induction machine Step Compute uing () to (9) Step 3 Find out the new value of magnetizing reactance Xm correponding to air gap voltage computed in Step Step 4 If X m X m Then the value of X m may be ued a the final magnetizing reactance needed for further computation the
Vadhera and Sandhu 33 performance of the induction generator Otherwie X m may be replaced by the new value of X m and the procedure may be repeated unle until the difference between ucceive value of magnetizing reactance come out a deired IV GENERATOR OPERATIVE LIMITS It i found that for generator operation I real in (7) become negative in contrat to motor operation Thi i the indication for reveral of active power in generating mode Now real power flow from machine to grid ytem, which i oppoite in cae of motoring mode However a indicated by I imag in (8), direction of reactive power flow remain ame irrepective of operating mode Such obervation lead to a new repreentation for the grid connected induction generator a hown in Fig(b) V j X R j X I imag I real I m j X m I imag R (-) I real Fig(b) Equivalent circuit repreentation of grid connected induction generator Further for generator operation I real i negative only if Z real i negative quantity Thi reult in the generating condition a given below; RX m R + + ( X Xm ) > (5) Equation (5) with boundary condition reult in a quadratic equation a + b + c Where, a (X + X m ), b R X m, c R Solution of quadratic equation give the operating lip of machine in generating zone a; 4 m ± m 4 ( + m) R( X + Xm ) RX R X R R X X (6) Out of the two olution a obtained by (6), only one feaible value can be elected a operating lip correponding to boundary condition ie, mg Therefore mg i the maximum value of lip up to which generator i poible However operating lip ( mt ) correponding to maximum value of torque generated ( ve in cae of generator) come out to be; R mt (7) R + ( X + X ) e e Where, R e X e [ ( + ) ] R X X X X X m m m R + ( X+ Xm) m + m + R + ( X+ Xm) R X X X ( X X ) Comparion of two expreion a given by (6) and (7) yield the following deduction; mt mg mt > mg 3 mt and mg are dependent upon aturated value of magnetizing reactance X m in addition to machine parameter 4 Operating range of the machine can be controlled by proper handling of magnetic circuit ie, X m and machine parameter Power flow diagram for the induction generator ha been hown in Fig P in -P fw -P rcu -(P cu + P ) core Shaft P m Rotor P g Stator Fig Power flow diagram of induction generator Fig3 give the flow chart for performance evaluation of the generator and thi ha been programmed in MATLAB START Read machine parameter Set initial value of lip Set iteration count K Aume air gap voltage E XK and compute correponding value of K m from given relationhip Compute new value for K + and find K + now the correponding value of Xm Ye Performance evaluation uing generator modeling Set S S + END No Fig3 Flow chart for performance evaluation P out Advance iteration count K K +
34 Vadhera and Sandhu V RESULTS AND DISCUSSIONS Propoed iterative technique i adopted to imulate the reult on Machine- [Appendix-I] P out (pu) 7 6 5 3 Unaturated X m 3 5 7 9 I (pu) Fig4 Variation of output power with tator current 8 7 6 5 The computed and experimental reult for the variation of output power and operating power factor with tator current and lip are hown in Fig4 to Fig7 The imulated reult uing the aturated value of Xm in contrat to unaturated value of Xm are far cloer to experimental reult Thi cloene of imulated reult with experimental one etablihe the validity of propoed iterative technique Thi paper lay an emphai on computing aturated value of Xm, accounting for aturation in magnetic circuit, which wa generally neglected by reearch peron earlier X m (pu) 3 5 5 5 3 Unaturated X m 3 5 7 9 I (pu) Fig5 Variation of power factor with tator current 7 6 8 Fig8 Variation of magnetizing reactance with lip Fig8 how the imulated reult for variation of magnetizing reactance with operating lip Thi variation in Xm i due to the aturation effect 4 3 P out (pu) pf 6 5 3 Unaturated X m 3 Fig6 Variation of output power with lip 8 7 6 5 3 Unaturated X m 3 Fig7 Variation of power factor with lip Torque (pu) Q (pu) 5 4 3 3 Fig9 Variation of torque with lip Unaturated X m Unaturated X m 3 Fig Variation of reactive power with lip The variation of torque and reactive power with lip for aturated and unaturated value of Xm are hown in Fig9 and Fig repectively It i oberved that accounting of aturation for analyi purpoe affect the generating torque and reactive power conumption of the machine
Vadhera and Sandhu 35 8 Efficiency 6 K K 75 K 5 3 Fig Variation of efficiency with lip, tator reitance K Fig5 Variation of power factor with lip, tator reitance K Fig Variation of efficiency with lip, rotor reitance KR Fig6 Variation of power factor with lip, rotor reitance KR Fig3 Variation of efficiency with lip, tator reactance KX Fig4 Variation of efficiency with lip, rotor reactance KX Fig to Fig4 how the effect of variation in machine parameter on the operating efficiency of the generator It i oberved; Variation in tator and rotor reitance will affect the operating efficiency of the generator Thi effect i more pronounced at low operating lip The tator or rotor reactance doen t play much role in the enhancement of efficiency of induction generator 3 Rotor reitance i more effective to control efficiency in comparion to tator reitance Similarly Fig5 and Fig6 indicate that any change in tator and rotor reitance affect the operating power factor of the machine However thi effect i large, for ame variation of reitance, in cae of rotor a compared to tator Thee dicuion lead to the concluion that tator and rotor reitance may be elected a main deign parameter for the induction machine to be operated a generator VI CONCLUSION For the analyi of grid connected induction generator, etimation of aturated magnetizing reactance i very important in contrat to motor operation (may be conidered
36 Vadhera and Sandhu a unaturated value), due to the fact that loading of generator hift the operation in to aturated region of magnetization curve wherea it i not o for motor operation Study reveal that mot of the reearch peron adopt the unaturated value for magnetizing reactance for the analyi of uch machine, which may lead to inaccurate performance etimation In thi paper an iterative technique i propoed to predict the aturated value of magnetizing reactance with change in operating condition The comparion between experimental and imulated reult prove the validity of technique propoed Further effort have been made to predict the operating limit of machine uing propoed modeling Analyi ha been extended to identify effectivene of the machine parameter to improve the operating performance of the generator It i found that operating performance of the machine may be improved by proper deign of tator and rotor parameter APPENDIX I Specification; Machine I Three Phae, KW\3HP, 3V, 86A, 5Hz, Delta connected, Squirrel Cage Induction Machine V bae 3 V I bae 496 A N bae 5 RPM The Machine parameter are: 335Ω, R 76Ω, X 485Ω, X 485Ω Variation of magnetizing reactance Xm with air gap voltage i 787 Xm 8, 787 75 Xm 35553-337, 75 99 Xm 56-35, 99 3444 Xm 399-6, 3444 < Xm, REFERENCES [] JE Barkle, RW Ferguon Induction Generator-Theory and Application AIEE Tranaction, 73: -9(954) [] SS Murthy, OP Malik, and AK Tandon, Analyi of elf-excited induction generator, Proc IEE, 9c(6): 6-65(98) [3] L Quazene and G McPheron, Analyi of the iolated induction generator, IEEE Tran Power Apparatu and Sytem, (8): 793-798(983) [4] TF Chan, Analyi of elf-excited induction generator uing an iterative method, IEEE Tran Energy Converion, (3): 5-57(995) [5] KS Sandhu and SK Jain, Operational apect of elfexcited induction generator uing a new model, Electric Machine and Power Sytem, 7(): 69-8(999) [6] KS Sandhu, Iterative model for the analyi of elf-excited induction generator, Electric Power Component and Sytem, 3(): 95-939(3) [7] SS Murthy, CS Jha, PS Nagendra Rao, Analyi of Grid Connected Induction Generator Driven By Hydro/Wind Turbine Under Realitic Sytem Contraint, IEEE Tran Energy Converion, 5(): -7(99) [8] AH Gorahi, SS Murthy, BP Singh, Bhim Singh, Analyi of Wind Driven Grid Connected Induction Generator Under Unbalanced Grid Connection, IEEE Tran Energy Converion, 9(): 7-3(994) [9] SSY Narayanan, BK Murthy, GS Rao, Dynamic Analyi Of A Grid Connected Induction Generator Driven By A Wave-Energy Turbine Through Hunting Network, IEEE Tran Energy Converion, 4(): 5-(999) [] MA Abdel-Halem, Solid State Control of a Grid Connected Induction Generator, Electrical Power Component and Sytem, 9(4): 63-78() [] T Fukami, K Nakagawa, Y Kanamaru and T Miyamoto, A Technique For The Steady State Analyi Of a Grid Connected Permanent Magnet Induction Generator, IEEE Tran Energy Converion, 9(): 38-34(4) [] MVA Nune, JAP Lope, HH Zurn, VH Bezerra, RGAlmeida, Influence of The Variable Speed Wind Generator In Tranient Stability Margin of The Conventional Generator Integrated In Electrical Grid, IEEE Tran Energy Converion, 9(4): 69-7, December (4)