Energy Procedia 18 ( 2012 ) 349 – 358

1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of The TerraGreen Society.doi: 10.1016/j.egypro.2012.05.046

Current Control of the Isolated Self-Excited InductionGenerator using Shunt Active Filter

A.M. BOUZIDa, M. BENGHANEMb, B.HAMANEc, A.BELABBESd,M.BOUHAMIDAe,A.DRAOUf,a*

a,b,c,d,eLDDE laboratory members, Université Mohamed Boudiaf USTO 1505Bp El Mnaouer,Oran 31000, Algeriaf, Senior MIEEE, Department of Electrical Engineering,Hail University,Hail,Saudi Arabia

Abstract

The Self Excited Induction Generator (SEIG) is an isolated power source whose terminal voltage and frequency arecontrolled by the excitation of the capacitance or the load impedance. This paper presents a method for calculatingthe minimum excitation capacitance using the equivalent circuit approach for analyzing the steady state operation ofSEIG. A new strategy based on an active power filter (APF) for controlling the current and power quality of the selfexcited induction generator (SEIG) is also presented in this paper. The shunt active power filter was implementedusing a three phase PWM current controlled voltage source inverter (VSI) and connected to the wind generator andloads in order to compensate the current harmonics and reactive power. The PWM-VSI gate control signals arederived from hysteresis band current controller. The proposed active filter proved to play an important role and givegood dynamic response and robust behavior upon changes in load parameters. This investigation demonstrated thatpower average control strategy can facilitate the improvement of the power quality. The proposed control methodextracts fundamental (reference) components of the source current for the shunt active power line conditioners fornonlinear and unbalanced loads. The Power average approach additionally maintains the voltage of the capacitor (ofthe PWM inverter) nearly constant without any external control circuit. The shunt APF in conjunction with theproposed controller performs perfectly under different steady state and transient conditions. The simulation resultswith nonlinear loads and unbalanced loads have showed the effectiveness of the proposed scheme for harmonicreduction in Wind based Power Generation.

© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer]

Keywords: Shunt Active Filter, Self Excited Induction Generator (SEIG), Wind Energy, PWM Inverter, Renewable Energy.

* Corresponding author. Tel.:+213-550-023-222; fax:+213-41-560-328.E-mail address: allalbouzid@live.fr (A.M BOUZID) / mbenghanem69@yahoo.fr (M.BENGHANEM)

adraou@yahoo.com (A.DRAOU).

Available online at www.sciencedirect.com

© 2012 Published by Elsevier Ltd. Selection and/or peer review under responsibility of The TerraGreen Society.Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

350 A.M. Bouzid et al. / Energy Procedia 18 ( 2012 ) 349 – 358

1. Introduction

n recent years, the Self-Excited Induction Generator (SEIG) has emerged as the best electromechanicalenergy converter to replace the conventional synchronous generator in isolated power generators driven

by renewable energy resources: biogas, micro-hydroelectric, wind etc. The main advantages of the SEIGare: low cost, ruggedness, absence of a separate DC source for excitation brushless rotor construction(squirrel cage construction) and ease of maintenance. The fundamental problem with using the SEIG wasits inability to control the terminal voltage and frequency under varying load conditions. The analysis ofthe SEIG under steady-state conditions and imposed speed is already known [1]-[2]; however there arefew papers about transient state operation. Active Power Filters (APF) are often used in applicationswhere low current harmonics are desirable and/or improvement of quality of energy taken from the powergrid are needed. With the use of APF, it is possible to draw near perfect sinusoidal currents and voltagesfrom the grid or renewable distributed power sources. Moreover, it will be possible to balance loadcurrents in different phases which itself is important in stand-alone power generation like wind turbines asfor the case of unsymmetrical load currents, it could lead to torque pulsation in generator’s shaft and adecrease of reliability. With the use of APF it is also possible to control reactive power and keep unitypower factor that is why they are mainly used in industry where DC current is needed e.g. aluminumplants, train power substations, arc welders. The currents taken by household and office consumers haveusually high harmonic contents which is related to an increased number of non-linear loads such asrectifiers and capacitors, where the current is drawn at the peak of sinusoidal voltage. At last, it can besaid the APF could be used to prevent any kind of harmonic generation (computer’s power supply, energysavings lamp, etc.), to reduce: harmonic contents in the grid, peak value of the current drawn from thegrid, the inrush current taken from the grid, and to compensate the neutral line current, and correct theactive power factor correction, and thus transformers will not be necessary [3].

2. Description of the proposed system

A schematic of the proposed system is shown in Fig. 1. It consists of a three phase star-connectedinduction generator driven by an uncontrolled micro hydroelectric turbine. The generator is operated as an

load [4]. When SEIG supplies a non-linear load, the load draws a fundamental component of current andharmonic current from the generation systems, which are to be properly controlled. The shunt APF cancompensate the harmonic current by continuously tracking the changes in harmonic content. APF’sconsists of a voltage fed converter with a PWM current controller and an active filter controller thatrealizes an almost instantaneous control algorithm shown in Fig.1. As the input power is nearly constant,the output power of the SEIG must be held constant at all consumer loads. Any decrease in load mayaccelerate the machine and raise the voltage and frequency levels to prohibitively high values, resulting inlarge stresses on other connected loads.

Fig. 1. Block diagram of the APF with SEIG

I

Variable load

V

PWM APF Controller

SEIG

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3. Modeling of the SEIG

The dynamic model of the three-phase squirrel cage induction generator is developed by using astationary d-q axes references frame [5] and the relevant volt ampere equations are as follows:[ ] = [ ][ ] + [ ] [ ] + [ ][ ] (1)

Thus, the current derivative can be expressed as:[ ] = [ ] {[ ] [ ][ ] [ ][ ]} (2)

Where,[ ], [ ] , [ ] , [ ] and [ ] are defined in the Appendix (Section 9). The SEIG operates in thesaturation region and its magnetization characteristics are non-linear in nature. Thus, the magnetizingcurrent should be calculated at every step of integration in terms of stator and rotor currents as in [6]:= ( + ) + + (3)

Magnetizing inductance is calculated from the magnetization characteristics plotted as against ,as shown in Fig. 2 for the machine under test. The relation between and is obtained bysynchronous speed test.

Fig. 2. Variation of magnetizing inductance as a function of magnetizing current

The developed electromagnetic torque of the SEIG is:= (3 /4) (4)

The torque balance equation is:= + (2/ ) (5)

The derivative of the rotor speed from (4) is:= (2/ ) / (6)

4. Process of self-excitation

The process of self-excitation can be compared with the resonance phenomenon in an RLC circuitwhose transient solution is of the exponential form (Elder et al., 1984), (Grantham et al., 1989). Inthe solution, K is a constant, and root 1 is a complex quantity, whose real part represents the rate atwhich the transient decays, and the imaginary part is proportional to the frequency of oscillation. In realcircuits, the real part of 1 is negative, meaning that the transient vanishes with time. With the real part

0 5 10 15 20 250.02

0.04

0.06

0.08

0.1

Magnetizing current Im (A)

Mag

netiz

ing

indu

ctan

ceLm

(H)

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of p1 positive, the transient (voltage) build-up continues until it reaches a stable value with saturation ofiron circuit. In other terms, the effect of this saturation is to modify the magnetization reactance , suchthat the real part of the root 1 becomes zero in which case the response is sinusoidal steady-statecorresponding to continuous self-excitation of SEIG. Any current (resulting from the voltage) flowing ina circuit dissipates power in the circuit resistance, and an increasing current dissipates increasing power,which implies some energy source is available to supply the power. The energy source, referred to aboveis provided by the kinetic energy of the rotor (Grantham et al., 1989). With time varying loads, newsteady-state value of the voltage is determined by the self-excitation capacitance value, rotor speed andload [7] [8] [9]. The formula for calculation of minimum capacitance is:= 1 (7)

5. Reference current generation using average power method

The average power method gives accurate results even if the current is distorted. A PLL based unitvector template is used to obtain fundamental component of mains voltage. To get unit vector templatesof voltage, the input voltage is sensed and multiplied by a gain equal to 1/vpk where vpk is the peakamplitude of fundamental supply voltage. These unit vectors are then passed through a PLL forsynchronization of signals. Three phase fundamental components are multiplied by vpk to get fundamentalmains voltage. The Power average method needs reduced calculation, since it works directly with abc-phase voltage and line currents. The elimination of the Clark transformation makes this control strategysimple [10] [12]. The Power average method presents a minimum rms value to draw the same three phaseaverage active power from the source as the original load current. The control strategy principle for theshunt active power filter based on three-level inverter is illustrated in Fig.3.

Fig. 3. Block diagram of the proposed shunt active power filter control scheme

6. Analysis and modeling

The three phase instantaneous source current can be written as( ) = ( ) ( ) (8)

The instantaneous source voltage is given by( ) = sin (9)

Gating signals

Hysteresis basedCurrent Controller

Compute command

shunt APF currents

Compute AveragePower

Compute sourcereference currents

A.M. Bouzid et al. / Energy Procedia 18 ( 2012 ) 349 – 358 353

If a nonlinear load is applied, then the load current will have a fundamental component and harmoniccomponents, which can be written as( ) = ( + ) = ( + ) + ( + ) (10)

The reduction of current harmonics in the load current is achieved by injecting equal but oppositecurrent harmonic components at the point of common coupling, thus cancelling the original distortion andimproving the power quality. The system comprises an ac source, non-linear load, unbalanced load, theAPF and the new control scheme. The components of the system are analyzed separately and integratedto develop the complete model for the simulation [13].

6.1. Computation of the average power

The sensed load currents ( , , ) and bus voltages( , , ) through PLL are used to derivethe instantaneous power as given by( ) = ( ) ( ) + ( ) ( ) + ( ) ( ) (11)

The three phase instantaneous reactive power in each phase becomes [12]:== (12)=The instantaneous active and reactive power delivered to a nonlinear load must satisfy (12) and (13).= + = + (13)= , = , , (14)

Where - Instantaneous active power supplied by the source- Instantaneous active power supplied by the APF- Instantaneous active fundamental power of the load- Instantaneous harmonic power of the load- Instantaneous reactive power generated by the APF at phase k.

In order to ensure that the fundamental active power is supplied to the load from the source, theinstantaneous reactive power and harmonic power must be compensated by the APF. When consideringthe compensation of both harmonic and reactive power, is expressed as:( ) = ( ) ( ) + ( ) ( ) + ( ) ( ) (15)

6.2. Computation of source reference currents , ,From (14) and (15), the reference compensating currents are determined as:== (16)=

354 A.M. Bouzid et al. / Energy Procedia 18 ( 2012 ) 349 – 358

6.3. Computation of the command shunt APF currents , ,Finally the desired 3-phase references of the APF currents( , , ) are computed by taking the

difference between the three phase instantaneous reference source currents ( , , ) and the actualsource currents( , , ) as below:== (17)=6.4. Hysteresis current controller

The actual source currents are monitored instantaneously, and then compared to the reference currentsgenerated by the proposed algorithm. In order to get accurate and instantaneous control, switching of theIGBT devices should be such that the error signal approaches zero, thus providing quick response. Forthis reason, hysteresis current controller with fixed band is used to derive the switching signals of thethree phase IGBT based VSI bridge. The upper device and the lower device in one phase leg of VSI areswitched in complementary manner otherwise a dead short circuit will be take place [17]. The APFreference currents ( , , ) , compared with, the sensed source currents ( , , ) , and the errorsignals are operated by the hysteresis current controller to generate the firing pulses which activate theinverter power switches in a manner that reduces the current error. The switching logic for ‘phase-a’ isformulated as follows: If < ( ), then upper switch is OFF and lower switch is ON in the phase‘a’ leg then (SA=1). If > ( ), then upper switch is OFF and lower switch is ON in the phase‘a’ leg then (SA=0). In the same fashion, the switching of phase-b and c devices can be derived using

as the width of hysteresis band. The switching functions SB and SC for phases b and c are determinedin a similar manner.

7. Simulation results and analysis

The performance of the proposed control strategy is evaluated through simulation using SIMULINKtoolbox in the MATLAB. The system parameters values are: Line to line source voltage is 380 V; Systemfrequency (f) is 50 Hz; Source impedance of RS, LS

Rc, Lc L, LL

respectively; DC voltage (VDC) is 500V; = 1100 ; Power devices used are IGBT/Diode.

7.1. Performance of self excited induction generator

Fig. 4. Simulation results of stator voltage and current of the SEIG with saturation

0 0.2 0.4 0.6 0.8 1

-200

0

200

Time (s)

Volta

geVd

s(V

) Vds

0 0.2 0.4 0.6 0.8 1-20

0

20

Time (s)

Cur

rent

Ids

(A) Ids

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=1500 rpm, the generatedvoltage and current attain their steady state values of 380 Volts and 19 A in 0.8 sec as shown in Fig. 4.

7.2. Shunt active power filter system performance

The computer simulation results are provided to verify the effectiveness of the proposed controlscheme. The unbalanced load RL current before compensation is shown in Fig 5(a) and the six-pulsediode rectifier RL load current or source current before compensation is shown in Fig 5(b).

(a)Unbalanced load currents; (b) Non linear load current

Fig. 5. (a) Simulation results of the load currents

(a)Load currents or source current before compensation ; (b) Reference currents before APF

Fig. 6. (a) Simulation results of source and reference currents

(a) Reference currents by the power average control algorithm (b) Currents source after compensation

Fig. 7. Simulation results of source and reference currents

0 0.05 0.1-5

0

5

Time (s)

Iabc

-load

(A)

0 0.05 0.1-2

0

2

Time (s)

Ia-re

ctifi

er

0 0.05 0.1

-5

0

5

Time (s)

Isab

c(A

)

0 0.02 0.04 0.06 0.08 0.1-5

0

5

Time (s)

IFab

c(A

)

0 0.05 0.1-5

0

5

Time (s)

IFab

c*(A

)

0 0.05 0.1

-2

0

2

Time (s)

Isab

c(A

)

356 A.M. Bouzid et al. / Energy Procedia 18 ( 2012 ) 349 – 358

Fig. 6(a) shows the simulated results of the load currents. The harmonic currents of a nonlinear loadand unbalanced load are compensated by the shunt active power filter. The actual reference currents forthe three phases are shown in Fig. 6(b). This waveform is obtained from the proposed average powercontroller. The source current after compensation is illustrated in Fig. 7(b) which indicates that the currentbecomes sinusoidal. After active filter operation the AC-source current only supplies the activefundamental current to the load. The shunt APLC supplies the compensating current that is shown in Fig.7(a). The current after compensation shown in Fig. 7(b) would have taken a shape as shown in Fig. 6(b)without APF. It is clearly visible that this waveform is sinusoidal with some high frequency ripples.

7.3. Measurement of the total harmonic distortion measured

The total harmonic distortion is measured using the source current waveform and presented in Table 1both with and without APLC.

Table 1. Total harmonic distortion (THD %) of source current

ConditionTHD

Source current (Is)Without APLC

Source current(Is)With APLC

Steady state 23.08% 2.01%

The FFT analysis that was carried out confirms that the active filter brings the THD of the sourcecurrent down to less than 5% which is in compliance with IEEE-519 standards for harmonics.

In general, the THD values for both current and voltage in advanced aircraft electric power system inpresence of APF are lower than those for conventional aircraft system [18].

8. Conclusion

This paper has presented the implementation of a cage-rotor IG system completely isolated from theutility grid, in order to supply rural sites or isolated areas. It has been demonstrated that the system is ableto feed resistive and inductive loads with regulated current and satisfactory energy quality. In this paperwe also discussed the problem of terminal current stabilization of the self excited induction generator(SEIG) in standalone mode from which a new method of stabilization of the current is used to improvethe performance characteristics of the SEIG. This investigation demonstrated also that the generalizedPower average control strategy can facilitate the improvement of the power quality. Simulation results areincluded in order to validate the proposed control technique. The shunt APF has been implemented with athree phase PWM current controlled voltage source inverter and is connected to the AC mains in order tocompensate the current harmonics and reactive power. It has been shown that the Power averageapproach additionally maintains the voltage of the capacitor (of the PWM inverter) nearly constantwithout any external control circuit. Different types of linear and non linear loads for reactive power andcurrent harmonics compensation have been connected to the APF to analyse the steady-state and transientperformance of the system. The APF has been proved to remarkably eliminate the harmonic and reactivecomponents of load current resulting in sinusoidal and unity power-factor source currents.

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References

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conventional and advanced aircraft electric power systems’, Elsevier, Electric Power Systems Research 79, pp80–88,2009.[19] Bhim Singh, S. S. Murthy, and Sushma Gupta, “Analysis and Design of Electronic Load Controller for Self-ExcitedInduction Generators”, IEEE transactions on energy conversion, vol. 21, no. 1, march 2006.[20] Vargil Kumar E, Narasimham PVRL, Sarma AVRS, “Steady State Investigation of Self Excited 3 Phase InductionGenerator with Novel Leading VAR Controller and Mitigation of Harmonics Using Active Power Filter’, IEEE InternationalConference on Power and Energy (PECon2010), pp 495-500,Nov 29 - Dec 1, 2010.

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[21] Dheeraj Joshi, Kanwarjit Singh Sandhu, and Mahender Kumar Soni,’ Constant Voltage Constant Frequency Operation for aSelf Excited Induction Generator’, IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 1, pp 228-234,MARCH 2006.[22] A.M.Sharaf, Subramanian Kanthi Murugan,’ Dynamic Power Filter & Capacitor Compensator for Isolated Self-excitedInduction Generator driven by a Wind Turbine’, IEEE 11th International Conference on Harmonics and Quality of Power, pp46-49,sept 2004.

Appendix A. Modeling of SEIG

The matrices of (1) are defined as follows:[ ] = ; [ ] = ; [ ] = [ ][ ] = +0 0+ 0 0

0 0 +0 0+ [ ] = 00 00 00 000 0 0+ +0Here, suffixes and refer to and axis (in stator reference frame), and refer to stator and

rotor; refers to magnetizing component.

Appendix B. Parameters of SEIG

Table. Generator Rating and Parameters

Rated Power 3.5KWRated Line to Line VoltageRated line to line Current

380 V14 A

Rated Frequency 50 HzNumber of poles, PRated Rotor speed Nn

41410 rpm

Stator Resistance, RsStator Leakage inductance Lls 0.003mHRotor Resistance, RrRotor leakage inductance, LlrCapacitance for excitation C

0.003mH270 μF