Research Article POWER FLOW ANALYSIS OF SELF-EXCITED INDUCTION GENERATOR DRIVEN AT VARIABLE WIND SPEEDS Swati Devabhaktuni 1 *, S.V.Jayaram kumar 2 Address for Correspondence 1 Associate professor, Gokaraju Rangaraju Insitute Of Engineering And Technology,Hyderabad,A.P.,India 2 Professor, J.N.T.Ucollege of engineering,hyderabad,a.p.,india ABSTRACT This paper presents the active and reactive power analysis of self-excited induction generator. These models are used for steady state power flow calculation in electric power systems in which generating plant driven by renewable energy sources such as wind energy is connected to partially serve loads. No previous literature is available regarding the steady state power flow analysis using wind turbine. This research demonstrates the power flow in self excited induction generator by power (PQ) models, in order to predict the power flow distribution through feeder lines resulting from the grid connection of renewable power plant. In this paper the variable speed wind turbine equipped with a self excited induction generator is presented. And also the typical characteristics of the variable wind turbine with self excited induction generator are studied. This paper also presents the theoretical and experimental results of self excited induction generator under varying rotor speed operation of research. Three phase 3.7kW induction machine excited with symmetrical capacitor bank and loaded with symmetrical 440v bus, was the subject of investigation. Experimentally obtained results have been compared with calculated performance curves and very good agreement between them has been achieved. KEYWORDS: Wind turbine, self-excited induction generator, steady state analysis, power flow calculation, PQ model. 1. INTRODUCTION The use of induction generators can be traced back to the beginning of the 20 th century until they were almost disappeared in the 1960s [1]. The dramatic increase in gas prices during the 1970s, created the favorable situation for the revival of the induction generator. For its simplicity, robustness, and small size per generated kw, the induction generator is favored for small hydro and wind power plants [6][7]. If an induction generator is connected to the grid or to other sources or storage, it can easily approach 100 kw [1]. Typically, small renewable energy power plants rely mostly on induction machines, because they are widely and commercially available and very inexpensive [3]. It is also very easy to operate them in parallel with large power systems, because the utility grid controls voltage and frequency while static and reactive compensating capacitors can be used for correction of the power factor and harmonic reduction [2], [4], [5]. Self-Excited Induction Generator for its operation, the induction generator needs a reasonable amount of reactive power which must be fed externally to establish the magnetic field necessary to convert the mechanical power from its shaft into electrical power [1]. Therefore, the external reactive source must remain permanently connected to the stator windings responsible for the output voltage control.. When capacitors are connected to induction generator, the system is usually called a SEIG (a self-excited induction generator) When the shaft is rotated externally, such movement interacts with a residual magnetic field and induces a voltage across the external capacitor, resulting in a current in the parallel circuit which, in turn, reinforces the magnetic field and the system builds up an increasing excitation. Due to high cost of capacitors and maintenance needs, self-excitation of the induction generator is economically recommended only for small power plants [7]. In is paper, a simple method to compute the power flow of a self-excited induction generator is introduced. The proposed method includes the following aspects:.it is based on the nodal admittance method for steady-state analysis of the SEIG with considering the saturation. The load and excitation capacitance branches in the equivalent circuit are decoupled to facilitate the solution of the self-excited frequency. No trial-and-error procedure is involved, hence it may be regarded as a direct method. Reduced computational effort A self excited induction generator running at a particular steady state operating point is investigated to determine its significant effects and voltage profiles in power distribution systems. The motivation for the work presented in this paper was to identify how the variation of rotor speed affects voltage, frequency, stator current, active power, reactive power and shaft torque of the SEIG. Theoretical and experimental research was done assuming that SEIG was fed with three-phase symmetrical 440V bus and excited with symmetrical capacitor bank connected to the stator. Experimented results obtained on a laboratory machine are presented to verify the accuracy and validity of the present approach. 2. Three Phase Self-Excited Induction Generator Model When an induction machine operates as a SEIG, there is no external power grid that defines voltage and frequency on the stator terminals. Thus, both of them are unknown variables whose values change independently, being affected by rotor speed, capacitance of excitation capacitors and loading conditions. Saturation level of the magnetic circuit is also variable, which means that magnetizing inductance can not be considered constant. In such circumstances, the standard equivalent circuit of an induction machine is not suitable for analysis, and specific modifications have to be made. Since SEIG always operates in the saturated region of the magnetizing curve, it is clear that such simplification can diminish accuracy of prediction. On the other hand, simplifications of this kind were necessary, due to great mathematical difficulties that follow attempts to solve high-degree nonlinear equations obtained from equivalent circuit.for the modelling of the self-excited induction generators, the main flux path saturation is accounted for while the saturation in the leakage flux path, the iron and
rotational losses are neglected. Therefore in the following analysis the parameters of the induction machine are assumed constant except the magnetizing inductance which varies with saturation [5]. 3. Steady-state circuit model The steady state circuit of a self-excited induction generator under bus connection is shown in Fig.1 Fig.1.Equivalent circuit of SEIG Here the machine core losses are have been ignored. Considering these losses increases the mathematical work involved in obtaining the results, without increasing the accuracy of the analysis substantially. All the circuit parameters are assumed to be constant and unaffected by saturation. Machine parameters except capacitance and frequency all are known values and the machine is driven at variable wind speeds[6]. 4. Mathematical model Fig1. shows the per phase equivalent circuit of the SEIG. Rotor is driven by the prime mover. Before the synchronous switch is closed the speed of the rotor is so adjusted and such that E 1 and C varies according to the speed and slip of the machine. Rotor is driven by commonly used for the steady state analysis of the SEIG.For the machine, 1. By adjusting the rotor speed, we can adjust active power delivered by machine to bus. 2. By adjusting C, we can adjust reactive power delivered to bus. From the Fig.1, Applying loop analysis, (1) At synchronization,.adjusting C will not affect machine in anyway. Only adjusting rotor speed will effect machine. Now after synchronization by adjusting c or by adjusting slip,s we can expect E to vary.with all parameters known and also knowledge about the transmission line parameters,the terminal voltage of the induction generator can be calculated using the equation (2) Due to the relationship between real power exported and reactive power drawn, independent control of the power factor of the self excited induction generator by itself is not available. To achieve the power factor control, an appropriate capacitor is placed across its terminal. From the equivalent circuit,,, can be expressed as follows: (4) (5) From the above equations, the current injected by the induction generator is the summation of eq.(3)-(5),thus (3) cos sin sin sin cos sin cos cos (6) Complex power delivered by machine to bus (7) The complex power injected to the point of connection can be decomposed into real and imaginary parts as given by (8) (9) With Eqs.(8)-(9) the SEIG can be represented by the real power injection P to and the reactive power drawn Q from the connection point. This PQ process is repeatedly updated during the power flow process. This analysis is subjected to the positive, negative and zero slips. 4. Results And Discussions In this paper, the computed results are obtained by the procedures and calculations outlined above, number of experiments are conducted using three phase induction machine coupled with a wind turbine. Wind turbines can operate with either fixed speed (actually within a speed range about 1%)or variable speed. For fixed-speed wind turbines, the generator (induction generator) is directly connected to the grid. Since the speed is almost fixed to the grid frequency, and most certainly not controllable, it is not possible to store the turbulence of the wind in form of rotational energy. Therefore, for a fixed-speed system the turbulence of the wind will result in power variations, and thus affect the power quality of the grid. For a variable-speed wind turbine the generator is controlled by power electronic equipment, which makes it possible to control the rotor speed. In this way the power fluctuations caused by wind variations can be more or less absorbed by changing the rotor speed and thus power variations originating from the wind conversion and the drive train can be reduced. Hence, the power quality impact caused by the wind turbine can be improved compared to a fixed-speed turbine The rotational speed of a wind turbine is fairly low and must therefore be adjusted to the electrical frequency. This can be done in two ways: with a gearbox or with the number of pole pairs of the generator. The number of pole pairs sets the mechanical speed of the generator with respect to the electrical frequency and the gearbox adjusts the rotor speed of the turbine to the mechanical speed of the generator
Fig.2. Rotor speed as a function of wind speed Fig.6.Slip as a function of terminal voltage Curves typical of the typical variations of terminal voltage,e 1 with slip are as shown in Fig.6.,for positive(motor) and negative(generator)slips. At synchronous speed, s=0,e 1 is very nearly equal to V:it may be within 2 or 3 percent. Due to the excitation capacitance the voltage increases for the negative slips. Otherwise the voltage falls rapidly with the increase of the slip in either direction and thereafter tends to a constant value Fig.3. Slip as a function of wind speed An experiment is conducted to determine how the active and reactive powers depend on the slip, the results are as shown in fig.3. It can be seen in the Fig.4. that the power through the converter, given the mechanical power, is higher for positive slips. This is due to the factor 1/(1 s) in the expressions for the rotor power. For a wind turbine, in general, at low mechanical power the slip is positive and for high mechanical power the slip is negative, as seen in Fig. 3. Fig.4. The power flow for different slips of the SEIG system A3.2 MW wound rotor induction machine (WRIM) was used for testing on a laboratory-scale model. A Jung wind turbine was used as the prime mover and the wind turbine controls were performed with a power electronic converter comprised of IGBT switches and governed by a DSP board. It should be noted that the induction machine operates at a higher efficiency at supersynchronous speeds than at sub-synchronous speeds. Fig.7.Slip as a function of Current Typical variations of current with the slip is as shown in Fig.7.The current is zero at synchronous speed and increases rapidly with the variation of the slip and thereafter tends to a constant value. The stator current is the magnetizing current at synchronous speed, but soon reaches values very close to those of rotor current, since I m is comparatively small. Fig.5.Power flow under different operating conditions Where P R is the input rotor power Fig.8.Slip as a function of Active power The mechanical output is (1-s)M,while the fraction sm=sp 2 is the true I 2 R loss in the rotor. The equivalent circuit dissipates the whole of P 2 in its resistance,,and to get the real I 2 R loss in the actual machine the fraction s of P 2 must be taken, the remainder being the mechanical
output.the curve for the slip as a function Active power is as shown in Fig.8. Fig.9.Slip as a function of Reactive power The typical variation of the slip with the reactive power is as shown in Fig.9.The reactive power is positive for positive and negative slips which means that the machine is delivering reactive power to the bus when the slip is negative and it is taking the reactive power when the slip is negative. In both the cases the reactive power is less than the active power. At synchronization the reactive power is zero which means that the machine is neither taking nor giving the reactive power from the bus and to the bus respectively. Fig.11.Slip as a function of Torque Typical Variation of slip with power factor is as shown in Fig.12.At synchronous speed, s=0,the power factor is maximum. Fig.10.Slip as a function of Excitation capacitance The variation of the excitation capacitance with the slip,s is as shown in Fig.10.As the terminal voltage is increasing the excitation capacitance is decreasing. As the slip is increasing in the positive direction, the excitation capacitance is decreasing. As slip increases in negative direction, the maximum value is obtained at minimum value of the slip and as the slip increases in the negative direction, the capacitance value decreases at synchronization the value of the excitation capacitance needed for the machine is 15 µf.due to this excitation capacitance the SEIG characteristics differs from the characteristics of the conventional induction generator.the minimum capacitance required for the selfexcited induction generator to build up the voltage. Fig.11.shows the typical characteristics of the slip as a function of torque of the induction generator running at sub synchronous (motor) and super synchronous (generator) speed. For sub synchronous speed, operation, r 1 larger, x 1 and x 2 are reduced. The rotor resistance does not affect the speed at which the maximum torque occurs. The lower the rotor resistance, the nearer to the synchronous speed does the torque to attain a maximum at starting.for super synchronous or generator operation the maximum torque is independent of r 2 as for normal motor condition, and increase with reduction of both stator and rotor reactance. But an increase in stator resistance now increases the maximum torque. If the primary resistance is large, the maximum torque running super synchronously may be very high indeed.the equation for the torque is given in APPENDIX-A Fig.12.Slip as a function of Power factor As the slip is increasing in either direction the power factor falls rapidly and there after tends to a constant value in either direction. The power factor rises to a value between 0.8 to 0.9.For larger overloads both current and slip increases rapidly, until at maximum load the current may be about three times its full load value, at a power factor in the neighbourhood of 0.7.At this stage a small increase in overload will cause the motor to stop suddenly. The current will rise, the torque will be small, and power factor will be lower and is between 0.2-0.4.The power factor obtainable on full load naturally depending on the size of the machine. Fig.13.shows the typical characteristics of slip as a function of frequency. At synchronous speed, the slip s=0,the frequency is at 50.Hz.As the slip is increasing in the positive direction, when the machine is running at sub synchronous speed, the frequency is decreasing as the increase in the slip.however,when the machine is operating at super synchronous speed, the slip is varying in the negative direction, the frequency increases Fig.13.Slip as a function of Frequency Fig.14.shows the variation of speed with the slip..at slip s=0,the machine is running at synchronous speed of 1500r.p.m.As the slip is increasing in the positive direction the machine is running at sub synchronous speed and behaves as induction motor. When the slip is
increasing in the negative direction the machine is running at super synchronous speed and the machine behaves as induction generator. Fig.14.Slip as a function of Speed Fig.15.Power flow in terms of mechanical power The power flow in SEIG for different slips is as shown in Fig.15.From the table it can be confirmed that the stator power delivered by the machine when it is running as motor is greater than the stator power delivered by the machine when it is running as generator. In this analysis both saturation and losses are neglected. The core loss is difficult to assess with any accuracy, and stray losses tend to be large. If a load torque is applied to the shaft, the rotor tends to slow down. The rotor e.m.f consequently increases, and with it the rotor reactance has little effect, and the rotor current is nearly in phase with the rotor e.m.f.te slip increases so as to provide enough rotor current and torque, and, very nearly, the load torque and slip are proportional. Thus both the slip/torque and speed/torque relations are rectilinear. 5. CONCLUSIONS The classical definition of active and reactive powers leading to the complex power are discussed. Electrical power generation, specifically the induction generator and later the self -excited induction machine and the power flow were discussed. The simplified analysis showed the possible operating ranges of the self excited induction generator drive for production of active and reactive powers. The drive system topology allows for an independent control of the reactive power in the generator and mains side as well as the decoupling from the active power due to the mains voltage or flux orientation. The suggested optimization is based on the controlled distribution of the reactive power flow in the drive components. The reactive power can be defined the rate of magnetic and electrical energy exchange between the generator, the mains supply and the inverters. It is a function of the reactive current and of the voltage amplitude and, therefore, closely related to process losses. For this reason, it proves to be a very suitable optimization variable.the expressions discussed above are derived from the equivalent circuit,may be used to calculate the performence of an induction machine where the several impedance components are known. 6.REFERENCES 1. S.M.Alghuwainmen, Steady-state analysis of selfexcited induction generator including transformer saturation,ieee transactions on energy conversion, Vol.14,No.3,september1999. 2. T.F.Chan,L.L.Lai, Steady state analysis and perfoamance of a single-phase self regulated self excited induction generator, IEEEproc. generation and transmission distribution,vol.149,no.2,march2002. 3. S.M.Alghuwainmen, Steady-state analysis of an isolated self-excited induction generator driven by regulated and unregulated turbine,ieee transactions on energy conversion,vol.14,no.3,september1999. 4. K.S.Sandhu,D.Joshi, Steady state analysis of selfexcited induction generator using phasor-diagram based iterative model,wseas transactions on power systems,issue12,volume3,december2008. 5. A.I.Aloah,M.A.Alkanhal, Analysis of three phase self excited induction generator under static and dynamic loads IEEEProc.,1991. 6. T.F.Chan, Self-excited induction generators driven by regulated and unregulated wind turbines,ieee transactions on energy conversion, Vol.11, No.2, June 1996 7. K.S.Sandhu,S.P.Jain, Steady state operation of self excited induction generator with varying wind speeds,international journal of circuits,systems and signal processing,issue-1,volume2,2008. APPENDIX-A To compute the torque,consider the friction losses as 200W and I 2 1 R losses,then torque is given by Torque, T e = Air gap voltage: The piecewise linearization of magnetization characteristic of machine is given by: E 1 =0 X m 260 E 1 =1632.58-6.2X m 233.2 X m 260 E 1 =1314.98-4.8X m 214.6 X m 233.2 E 1 =1183.11-4.22X m 206 X m 214.6 E 1 =1120.4-3.9.2X m 203.5 X m 206 E 1 =557.65-1.144X m 197.3 X m 203.5 E 1 =320.56-0.578X m X m 197.3 Jung Data Acquisition Setup The experiments were made on a VESTAS V-52 850 kw WT, located at the inland ( 100 km from the west coast) in the southern part of Sweden. The wind turbine is located in a flat DATA OF VESTAS V-52 850 KW WT [104]. Rated voltage (Y) 690 V Rated power 850 kw Rotor diameter 52 m Rotor speed 14.0 31.0 rpm (26 rpm) Cut-in wind speed 4 m/s Nominal wind speed 16 m/s Maximum wind speed 25 m/s the current to 5 A and the voltage to 110 V. In addition, the stator currents are also measured directly using LEM modules. The induction machine was three, phase3.5kw, 415V, 7.5A, 1500r.p.m, star connected stator winding. The machine was coupled to a D.C.shunt motor to provide different constant speeds. A 3-Φ variable capacitor bank or a single capacitor was connected to the machine terminals to obtain self-excited induction generator action.the measured machine parameters were: r 1 =11.78Ω; r 2 =3.78Ω; L 1 =L 2 =10.88H. L m =227.39H