Excitation Process in Three Phase Squirrel Cage ... - IEEE Xplore

Excitation Process in Three Phase Squirrel Cage Induction Generator for Wind Mill Application Sachin Kumar and Kunal Gaur Department of Electrical Engineering Indian Institute of Technology, BanarasHinduUniversity Varanasi, India [email protected], [email protected] Abstract—This paper describes the results of a case study concerning role of capacitive excitation on voltage profile of three phase self excited induction generator (SEIG). In this paper, an attempt is made to analyze how an induction machine can be made to get self-excitation and thus be used in any stand-alone application; especially for windmill application. The most appropriate capacitive VAR required for self excitation has been computed in Matlab/Simulink environment. Keywords—Renewable Energy Sources, Wind Energy, Self Excited Induction Generator (SEIG), Self Excitation, Stand-Alone, Reactive Power, Total Harmonic Distortion(THD).

I.

INTRODUCTION

Wind energy is one of the most available and exploitable forms renewable energy. Wind blows from a region of higher atmospheric pressure to one of lower atmospheric pressure. The difference in pressure is caused by (a) the fact that earth’s surface is not uniformly heated by the sun and (b) the earth’s rotation. Essentially, wind energy is a by-product of solar energy, available in the form of that kinetic energy of air. Induction machineoperated as a generator isgood candidates for wind powered electric generation applications, especially in areas where, there is no grid or external power supply to produce the magnetic field. Self excited induction generator(SEIG) are increasingly being preferred over conventional alternator in isolated system due to its overall maintenance and operational simplicity, lower unit cost and ready availability in lower rating[7].possible application of interest are the small rural power resources for developing countries, using prime movers such as hydro turbine, wind turbine, diesel/kerosene/bio-gas engine. Among the alternative generating system self excited induction generator has drawn considerable attention by researchers to cater for the mentioned requirement due to its inherent advantages like rugged construction, brushless design, asynchronous operation etc. However, it requires a reactive power source to provide magnetization current, thus it needs a power grid or a VAR source, like capacitor or synchronous condenser. The process of self excitation of induction machine to operate as isolated induction generator by using exciting capacitors has been known for more than half a century.

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Basset and Potter [1] described tests to show that an induction motor could be operated as an independent or isolated generator, at a predetermined voltage and frequency, with excellent waveforms, by means of capacitive excitation. However, such generators suffered from poor voltage regulation, which becomes evident when load was applied to it. The recent advancement in power electronics has made it possible to regulate the self excited induction generator in many different ways .Berennen and Abbondati et al [2] proposed the use of static exciter to control the output voltages .The work emphasizes on the possibility of using the generator at constant speed. Al Jabri and Alolah et al [3] introduces a new simple and direct method of finding the minimum capacitance required for self excitation and exact values for minimum capacitance under different loading conditions. T.F.Chan et al[4]presents a simple method for obtaining the ,minimum value of capacitance required for initiating voltage build up in a three phase self excited induction generator, based on the steady state equivalent circuit model. Ref. [5] had presented the performance characteristics and optimum utilization of a cage machine as capacitor excited induction generator.an analytical technique has been developed to obtain the performance characteristics. The continuous variation in the excitation is necessary for regulating the voltage at different loads, no. of switched capacitors are required to load the machine up to its rated capacity and to maintain terminal voltage within desired limits for given speed Ref. [6] analysed the voltage build up in the SEIG using generalized machine theory and proposed the onset theory of self excitation. Ref.[8]presents the effect of capacitive power on performance of three phase self excited induction generator.in this paper the effects of reactive power were investigated using Matlab and optimize value of reactive power is found. II.

TYPES OF SQUIRREL CAGE INDUCTION GENERATOR

These are of two types: (a) In first type, when a squirrel cage induction motor is run over its synchronous speed, it starts functioning

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as a generator.In this type, it is alw ways necessary that the machine runs aboveits synchroonous speed and it should remain connected to poower supply for excitation.

Fig. 3. Modified steady-state T-forrm circuit model of induction machine.

Fig.1. Grid connected induction generator. (b)

In the second type, when a squuirrel cage motor running near to its synchronous speeed is switched off and simultaneously a capacitor bank b is connected across the motor terminals, it starts functioning as a generator. This type is characcterized by selfexcitation.

Fig. 4. Equivalent circuit of SEIG.

G with speed emf in the rotor circuit. Fig. 5. Modified circuit model of SEIG

Fig. 2. Self excited induction generator.

In this paper, the second type of generator onnly (self excitation type) has been discussed. III.

THE SELF EXCITATION PRO OCESS

The phenomenon of self excitation in induuction machines is nowadays well known.it is well known thaat when capacitors are connected across the stator terminals of an induction machine, driven by an external prime moverr, voltage develops at its terminals[1]. For the self-excitation to initiate a capacitor c bank of suitable size must be connected across the machine m terminals, the core of which retains some residual flux.In order to understand the basic self-excitation process, let us refer to the circuit shown in Fig.3 and Fig.4.Combining them after neglecting the stator leakage impedancee and the shunt resistance, a simplifiedcircuit model off the self-excited induction generator is obtained as shown inn Fig.5.the process of voltage build-up is explained with referencce to the Fig.5.

For any speed of the rotor, thee residual flux generates a small synchronous emf Er. The steaddy-state magnitude of the current through the LmC circuit is suchh that the difference between the synchronous saturation curve (voltage across Lm) and the capacitor load line, as shownn in Fig.4, at this value of the stator current equals Er. At thiss stage, the slip s being zero for no speed difference between thhe rotor and the air-gap flux, no induced rotor current flows and a the machine operates as a synchronous generator. If Er is less than Er1, the machine operates in the stable steady state in the synchronous mode over the region oa. An increase in current in this region demands more synchronous vooltage than the residual voltage Er. Consequently, the increasedd current is not sustained and the current comes back to its original value. By the same reasoning, if the Er is between Er1 and Er2, a stable synchronous mode is observed over the reegion cd. For , stable synchronous operation takes place p from the point f onward. The regions ac and df are unstaable, where, for the residual emf equal to Er1, or Er2, the machinee terminal voltage rises owing to synchronous self-excitation, before b entering the next stable region. In the stable regions, the t machine operates as a selfexcited synchronous generator.

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connected across it, a steady state voltage is generated at the motor terminals.The value of capacitance in the capacitor bank should neither be too low nor too high. If the value of capacitance is too low, self-excitation will not take place. If the value of capacitance is too high, there will be inadequate build-up of steady state voltage due to saturation of flux paths. Hence, there is a need to use optimum size of capacitor for proper build-up of voltage and also for keeping the cost of capacitor bank low [3]. IV.

Fig. 6. Building up of voltage in a self excited induction generator:(a) the capacitor load line and the saturation curve, (b) the difference between them.

The possibility of a changeover from synchronous generator operation to the self exited asynchronous generator mode occurs in the region where the saturation curve emf is greater than the capacitor voltage. While the machine operates in the synchronous mode, any disturbance initiates an oscillation in the LC resonance circuit formed by the machine terminal capacitance and the magnetizing inductance at the terminal . Only at the point’s b and e does angular frequencyω ωn equal the synchronous frequency ω1.between the point’s b and e, the synchronous inductive reactance greater than the capacitive reactance. Hence, the natural frequency ωn of oscillation is lower than the rotational (i.e., synchronous) frequency ω1.the air-gap associated with the oscillating current rotates at a speed lower than that of the rotor, implying a negative value of the slip. The corresponding rotational emf E (1-s), which exceeds E, drives a current into stator circuit, building up the terminal voltage. The machine now enters the asynchronous generating mode. An unstable oscillatory condition between the capacitor and the magnetizing reactance still persists owing to a continuous fall in the effective value of the magnetizing reactance as the terminal voltage rises. The natural frequency of oscillation progressively increases, and sustained oscillation is reached when the capacitive reactance is close to, but still less than, the magnetizing reactance near the point e. The small negative slip compensates the losses near the point e. The small negative slip compensates the losses in the stator circuit. With a resistive load connected across the capacitor, the circuit must be underdamped to initiate the asynchronous generation mode the induced emf and current in the stator windings will continue to rise until steady state is attained, which is influenced by the magnetic saturation of the machine. At this operating point, the voltage and current will continue to oscillate at a given peak value and frequency. This process can be observed in a laboratory also by connecting three-phase supply to an Induction machine driven by a prime mover, At the rated speed, when the supply to the motor is switched off and simultaneously a capacitor bank is

WIND FARM CASE STUDY

Nowadays, most countries are all over the world aware of the potential of wind energy. There is huge activity in wind power, pan-India with the installed capacity increasing to 10,000 MW. India today has the fifth largest installed capacity of wind power in the world with 11087MW installed capacity and potential for on-shore capabilities of 65000MW. As a case study, we consider, the wind farm situated at JAMGODRANI HILLS comes under M.P.WINDFARM LTD. (MPWL) near DEVAS city, MADHYA PRADESH, INDIA.

The wind farm which we consider has specifications as following: TABLE I.

Total installed capacity Induction generator specifications Height of tower Length of blade Average wind speed Total number of wind turbines V.

15MW 225kw,440V,grid connected 34-30m 3.4m 10m/s 58

SIMULATIONS RESULT AND DISCUSSION

The above mentioned case study of wind turbine generator is modelled and simulated in Power system toolbox of MATLAB 7.8.0(R2009a)/ SIMULINK, with different value of capacitor bank to analyze the effects capacitive excitation on the voltage profile.The SIMULINK model of the system is shown in fig.7. The simulation time is 50 sec. Machine parameters are given in

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Table I.The proposed Simulink model of SEIG consists of a 440 V, 50Hz, 225 kW, induction generator, driven by a wind turbine at a fixed inductive load of 0.7 pf lagging. A three phase star connected capacitor bank is connected to the terminals of the induction generator. The value of this capacitor bank is to be changed to study the effect of capacitive excitation on voltage profile and harmonics.

When the value of capacitor bank is 95 KVAR, no voltage builds up take place and the voltage across the generator terminal is almost zero. This variation in voltage is due to presence of inductive load. The inductance decreases the VAR requirement for self excitation in the induction generator. 3rd,5th,9th and 12th harmonics are seen in the output voltage and the variation is shown in the figure 9. It is observed that no noticeable increase is seen in the harmonic contents. The increase is not observed because of the inductance of the load, which acts as a sink of reactive power and suppresses the voltage buildup. B. Capacitor bank value = 115 KVAR.

Fig.7. Simulink model of SEIG.

A. Capacitor bank value = 95 KVAR.

Fig. 10.induction generator terminal Voltage VL and its FFT analysis.

Fig. 8.induction generator terminal Voltage VL, and its FFT analysis. Fig. 11.3rd, 5th, 9thand 12th harmonics in terminal voltage(VL) of SEIG.

Fig. 9.3rd , 5th, 9th and 12th harmonics in terminal voltage(VL) of SEIG.

A sudden change in voltage profile is recorded when the value of the capacitor bank at the terminal of induction generator is 115 KVAR (fig.-10). At the starting i.e. during the instant (0 sec to 2 sec), the voltage drops to very low value of 0 p.u. and remains at the same value up to 17 sec. After 17 sec, the voltage starts building up and reaches the maximum value of 1.5 p.u at 24 sec. After 24, it settles down at a value of 0.8 p.u, as seen in the harmonic components is observed and it is seen that the harmonic components are noticeable fig.11. When the capacitor value is changed to 115 KVAR, a large increase in all the harmonic components is observed and it is seen that the harmonic components are noticeable at this value. (Fig.-11)

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The 3rd harmonic component attains the maximum value of 0.32 at t=23 seconds, back to 0.05 at t=26seconds and finally settle to a value of 0.15 at t=35 seconds. The value of 5thharmonic component is low initially, but the value rises to 0.15 near time t=23 seconds and then a drop is observed and value reaches 0.08 at t=35 and becomes almost steady at a value 0.07. Similar variation is recorded in 9th harmonic, whichattains maximum value of 0.07 at t=24 then drops back to 0.04 at t=27, and again rises back to a value of 0.08 at t=33 and after a small dip becomes steady at 0.03. 12thharmonic maximum value is 0.05 and similar kind of variation is recorded in this. C. Capacitor bank value =215 KVAR. Fig. 14.induction generator terminal Voltage VL and its FFT analysis.

Fig. 12. induction generator terminal Voltage VL and its FFT analysis.


Fig.14 shows the voltage profile of the SEIG used, with a capacitor bank of 255 KVAR. A further increase in the voltage is observed, when the capacitor bank value is changed to 255 KVAR, as compared to 215 KVAR. When the capacitor value is changed to 255 KVAR, a prominent decrease in 3rd, 5th, 9th and 12th harmonic components is achieved. The basic nature of their variation,though, remains the same except the fact that they reach their maximum value earlier than when the capacitor value was 215 KVAR. E. Capacitor bank value =295 KVAR.


Fig.12 shows the performance of the SEIG used, with a capacitor bank of 215 KVAR. A further increase in the voltage is observed, when the capacitor bank value is changed to 215 KVAR, as compared to 115 KVAR. For capacitor bank of 215 KVAR, a considerable increase in 5th, 9th and 12th harmonics components is observed. The basic nature of the wave pattern remain the same as in the previous cases but it improves in reaching for its maximum value with respect to time. (Fig.-13) D. Capacitor bank value =255 KVAR. Fig. 16.induction generator terminal Voltage VL and its FFT analysis.

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VII. APPENDIX


The capacitor bank value when fixed at 295 KVAR, the characteristic shows that the voltage profile becomes smooth and there is a very small variation at the start. The terminal voltage is 1.0 p.u. With this value of capacitance, the parameters under consideration i.e. voltage profile and generator speed are well within operating limits. When the capacitor value is changed to 295 KVAR a large decrease in all the harmonic components is observed and it is seen that the harmonic components are now almost negligible. (Fig.-17) VI.

CONCLUSIONS

The experiment attempted to investigate the effects of threephase capacitor bank/reactive power source, on the performance of SEIG’s under general resistive-inductive load. Simulation results, as observed, indicate the importance of such attempt. For the particular machine (used for simulation), reactive power source with 295 KVAR gives smooth voltage profile and satisfactory operating results. Also it is observed that reactive power source with 295 KVAR gives negligible value of 3rd, 5th, 9th and 12th harmonic and better performance in terms of total harmonic distortion (THD). The main source of harmonics in the machine is due to magnetic saturation of the iron core and the level of generation of the wind turbine. This shows the need of appropriate technique to select the optimum rating of capacitor bank and the in turn improvement in the performance of the machine.

A. Three Phase Induction Generator a. Rotor type: b. Reference frame: c. Nominal power: d. Voltage: e. Frequency: f. Stator resistance (R1): g. Stator reactance(X1): h. Rotor resistance(R2): i. Rotor reactance(R2): j. Magnetising reactance(Xm): k. Inertial constant: l. Fraction factor: m. Pair of poles:

squirrel cage rotor 225Kw 440V(Line) 50 Hz 0.019Ω 0.18Ω 0.019Ω 0.345Ω 4.8Ω 2 0 3

B. Three Phase load a. b. c. a.

Load p.f: Voltage(line to line): Frequency: Configuration:

0.707 lagging 440 V 50Hz Y grounded.

VIII. REFERENCES [1] [2] [3] [4] [5] [6]

[7] [8]

Basset, E.D. and Potter, F.M., “Capacitive excitation of induction generators”, Trans. Amber.Inst. Elector, Eng., 54, 1935, pp.540-545. M.B.Berennen and A.Abbondati, “Static exciter for induction generator”, IEEE Trans.on industry applications, Vol. 13, No.5, 1977, pp. 422-428. A.K. Al Jabri and A.I. Alolah, “Capacitance requirement for isolated self excited induction generator”, IEE Proc., Part–B, Vol. 137, No. 3, 1990, pp. 154-159. T.F. Chan, “Capacitance requirements of self-excited induction generators”, IEEE Trans. on Energy Conversion, Vol. 8, No. 2, 1993, pp. 304-311. Bhim Singh, C.S.Jha, B.P.Singh, “analysis of self excited induction generator feeding induction motor”, IEEE Trans.on Energy Conversion, Vol. 9, No.2, 1994, pp. 390-396. L. Shridhar, Bhim Singh, C. S. Jha, B. P. Singh and S. S. Murthy, “Selection of Capacitors for the self regulated short shunt self-excited induction generator, ”IEEE Trans. on Energy Conversion, Vol. 10, No. 1, 1995. B.Singh, L. Shridhar, and C. S. Jha, “Transients analysis of self-excited induction generator supplying dynamic load”, Elect. Mach. Power syst., vol.27, 1999, pp.941-954. Sachin Kumar, Sovit Pradhan and Alok Yadav, “Effect of capacitive excitation on volatge stability of three phase self excited induction generator”, International Journal of Electrical and Electronics Engineering Research (IJEEER),Vol.3, Issue.1,March 2003, pp.87-96.

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