* Department of Power Engineering, Science and Research branch, Islamic Azad University, Tehran,Iran, (E-mail: edu.rostami@gmail.com)

** Assistant Professor, Department of Power Engineering, Science and Research branch, Islamic AzadUniversity, Tehran, Iran, (E-mail: soodabeh_soleymani@yahoo.com)

***Assistant Professor, Department of Power Engineering, Science and Research branch, Islamic AzadUniversity, Tehran, Iran, (E-mail: mozafari_babak@yahoo.com)

IJPE, 4:1 (2012): 73-84Research Science Press, New Delhi, India

Effects of FACTS Devices on ImprovingQuality of Produced Energy from Self Excited

Squirrel Cage Induction Generator Due toDynamic Load Fluctuations

MEHDI MOHAMMADZADEH ROSTAMI*, SOODABEH SOLEYMANI** AND BABAK MOZAFARI***

Abstract: In this article in order to improve quality of produced energy fromself-excited asynchronous generator, with regard to the problems such as voltagecontrol, frequency and reactive power control under load variation andsymmetrical short circuit and issues related to power quality, the use of twoimportant FACTS devices namely STATCOM and SVC is evaluated and it isshown that these equipments have a significant effect on improving quality ofproduced energy from the generator, and they can also be effective in improvingthe generator loading.

Keywords: Quality Power; Induction Generator; FACTS.

1. INTRODUCTION

Using squirrel case inductive generator with capacitor stimulation insmall wind and water power stations has many privileges. Thesegenerators are inexpensive, firm, simple, and their maintenance is veryeasy due to not having ring, bush, commutator, battery and inverter.This But these generators in the form of SEIG aren’t used very much insmall water power stations due to not having a proper and inexpensivecontrol system. [1, 2, 14]

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When induction machine works as generator, it can supply itsmagnetic flux (reactive) in two ways. If it is connected to a grid, thisflux is supplied through the grid and where it is operated in self excitedform, this flux is supplied through capacitor banks [8]. Of courseexistence of capacitor even in the first case can lead to current reductionof transfer lines which this also leads to reduction of losses andimprovement of voltage regulation. The notable fact is that ingenerating mode the rate of drawn reactive power from grid by aninduction machine, for every specific slip, is greater than motor modeand this rate will be increased by increasing generating active powerand the minimum value would be at synchronous speed. [1]

Its noticeable that sometimes the reactive power the inductivegenerator absorbs from network, even exceeds the active power itproduces. This undesirable index is an unnecessary imposition on thenetwork and synchronization units connected to it. Therefore, it mayweaken the system in adjusting the voltage. To solve this situation, theneeded reactive power for each inductive generator must be preparedlocally. Since the reactive power produced by capacitor is dependenton generator’s terminal voltage and it’s not always possible to changeit, loading is done to stabilize the frequency and voltage amplitude indifferent situations, in variable amounts of reactive power like FACTSequipment. [3, 4, 5] Several articles have been published in this field sofar in most of which the performance of generator’s constant mode isreviewed and each of them has dealed with designing one of thesedevices and most of them have talked about designing and determiningdifferent parameters [4, 5, 11], and they didn’t compare generator’sbehavior besides each of these equipment and their privilegesover each other in different dynamic situation. In [6] we comparedSVC and DSTATCOM in improving inductive generator’s stability.Therefore, while reviewing the dynamic performance of generatorbesides SVC and STATCOM, we have tried to compare them to givereasons for choosing each of them over the other.

Later we have the dynamic model of inductive generator in dqaxes and generator and capacitor equation in these two axes, and alsowe talk about the models SVC and STATCOM and their controllers. Inpart 3 the way to simulate the above equipment will be presented. Inpart 4 the results of simulation by the software MATLAB are presentedand we deal with analyzing voltage profile and its frequency underthe situation of current change and symmetric connection, and in eachcase using SVC and STATCOM is assessed. In part 5 the final assessmentabout vitilizing these equipment is presented.

EFFECTS OF FACTS DEVICES ON IMPROVING QUALITY OF PRODUCED… / 87

2. MODELING

2.1.Generator

In this article we used machine model in dq device [7]. The volt-ampererelations are in [15] and proposed that the equations of voltage bewritten on flux per second for only one differential operator to appearin equations, because this is essential for solving differential equationsnumerically. These relations finally came to [1].

( )sqs b qs ds mq qs

b ls

RP

X

ωΨ = ω ν − Ψ + Ψ − Ψ ω (1)

( )sds b ds qs md ds

b ls

RP v

X

ωΨ = ω − Ψ + Ψ − Ψ ω

( )r rqr b qr dr mq qr

b lr

RP v

X

ω − ωΨ = ω − Ψ + Ψ − Ψ ω

( )r rdr b dr qr md dr

b lr

RP v

X

ω − ωΨ = ω − Ψ + Ψ − Ψ ω

The equation of torque speed is gained from relation in which H isthe constant for generator and turbine inertia. The equations for activeand reactive power are stated in (3):

2 re L

b

T Hp Tω= +ω

(2)

3( )

2e qs qs ds qsP V I V I= + (3)

3( )

2e qs ds ds qsQ V I V I= − +

2.2. SVC

Block Diagram of SVC is depicted in Figure 1 [9, 12). Since the 3-phaseused reactors of SVC don’t have considerable magnetic coupling, likecapacitor, their dq model is composed of two similar elements in dqaxes.

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Figure 1: Block Diagram of a SVC with Voltage Control

Figure 2: The Controller Block Diagram Fire Angle TCR.

2.3. STATCOM

Block diagram model of STATCOM are in Figures 3 and 4 [9].

Figure 3: Block Diagram of a STATCOM with PWM Voltage Control

EFFECTS OF FACTS DEVICES ON IMPROVING QUALITY OF PRODUCED… / 89

In Figure 3, m is the modulation index, δ is the phase angle ofnetwork’s voltage and α is the phase angle of STATCOM terminal.

Figure 4: The Block Diagram a Controller Voltage, PWM in a STATCOM

3. SIMULATION

Mode variables are the fluxes of dq and rotor speed and capacitor voltageis considered for generator along with the capacitor. Mode variables inSVC are the changes in fire angle and for STATCOM are DC bass voltage,PWM modulation index and the phase angle of inverters voltage.

Taking into account the phenomenon of saturation, the relationsfor fluxes of dg axis are presented in (4), [15].

( )qs qr aqmq aq mq

ls lr M

XX f

X X X

Ψ Ψ Ψ = + − Ψ

(4)

( )dqds drmd ad md

ls lr M

XX f

X X X

Ψ ΨΨ = + − Ψ

The function f is stated by relation (5), [10]f(Ψm) = Ψm – tan–1 (Ψm) (5)

The relation between voltage and capacitor current is:

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iqc = ωCvds + Cpvqs (6)

idc = ωCvqs + Cpvds

The utilized relations in simulating SVC are:

2

πα = ∆α +

0

(2 sin 2 )2eB B

α − α = − π (7)

( )pref

i i

kp V V

T T

∆α∆α = − −

STATCOM model is gained in each second according to the rule ofthe permanence of energy which is stated in relation (8). The relationof dc bass voltage in STATCOM is gained from (9).

ac dc lossP P P= + (8)

2

cos ( ) cdc dc

dc dc

GVI RIPV V

CV c CV= δ − θ − − (9)

The parameters used in this relation are depicted in Figure 3.Considering the control block shown in Figure 4, we can gaindifferential equation the changes of modulation index and phaseangle of STATCOM terminal voltage which are mentioned in (10) and(11), [9].

4. SIMULATION RESULTS INVESTIGATION

In this part we discuss simulation of generator in states of gridconnected and self excited respectively and in both of them weinvestigate employment of SVC and STATCOM under followingdynamic conditions:

1. 20 per cent load increase.2. Symmetrical short circuit.Taking Selective criteria for system Application Evaluation, is

addition results vector size of dq axis voltages. This value is very closeto voltage push. This criteria selection was due to Range small changesthat could be seen easily. In the following figure this criteria has beenshown.

Point: All horizontal axis were time axis and All figures are in termsof units in the Perunit.

EFFECTS OF FACTS DEVICES ON IMPROVING QUALITY OF PRODUCED… / 91

4.1. Self-Excited Induction Generator

This issue is so important about Self Excited Induction Generatorbecause this generator separated from network and its frequency isnot steady. Slipping coefficient Grid connected indicated stator voltagefrequency which calculated from origin frequency to be compared withconnected manner to the network and would be used in synchronizedmanner generator working frequency obtained directly from voltagewave form.

4.1.1. Study of Load Changes Over Voltage Profile

Since in Figure 6 Self Excited Induction Generator application obtainedin Figure 7 voltage range before overload was 0.449 and after overloadreached 0.333 therefore, in this manner 0.126 voltage loss per unitexisted. Also, before overload frequency was 32 and after overloadwas 32.85 Hz. So that, generator could not response this overload anddue to terminal voltage drop, load current (flow) decreased and finallystator flow and resistant torque flow decreased and caused frequencyincrease 0.85 Hz.

4.1.2. Study of Short Circuit Effect Over Voltage Profile

In Figure 8 generator application showed in short circuit mode.Figure 9 indicated that generator in short circuit 0.2s mode had velocityincrease 0.1243 per unit and after 2.45 seconds reached permanent

Figure 5: Phase Voltage Wave Figure and Addition Results Vector Size of dq AxisVoltage

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voltage level. In this manner, due to generator speed increase, recoveredvoltage had a peak in 7.25 by 0.526 size and then speed decrease in9.75 reached its previous value before connection.

4.2. Self Excited Induction Generator with SVC

4.2.1. Study of Load Changes Effect on Voltage Profile

In Figure 10 we have Self Excited Induction Generator application withSVC. In Figure 11 voltage range before overload was 0.427 and after

Figure 6: Voltage, Torque, Active and Reactive Power of SEIG Generator Diagram

Figure 7: SEIG Generator Voltage Push Wave Diagram in Load 20% Increaset = 6.

EFFECTS OF FACTS DEVICES ON IMPROVING QUALITY OF PRODUCED… / 93

Figure 8: SEIG Generator Active, Reactive Power, Torque, Voltage Diagramduring Short Circuit

Figure 9: SEIG Generator Voltage Push Wave Figure in Short Circuit Time t = 6.

Figure 10: SEIG Generator Active, Reactive Power, Torque, Voltage Diagram withSVC.

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overload reached the value 0.421. therefore, in this mode, the value0.005 per unit voltage drop could be seen. Also, frequency beforeoverload was 27.5 and after that 28 Hz.

4.2.2. Study of Short Circuit Effects Over Voltage Profile

In Figure 12 generator application showed in short circuit mode.Figure 13 showed that generator in short circuit time 0.25 increasedspeed to 0.051 per unit and after 2.5 s reached its permanent voltagelevel.

Figure 11: SEIG Generator Voltage Push Wave Diagram with SVC in 20% LoadIncrease in t=6.

Figure 12: SEIG Generator Active and Reactive Power, Torque, Voltage with SVCin Short Circuit Time.

EFFECTS OF FACTS DEVICES ON IMPROVING QUALITY OF PRODUCED… / 95

4.3. Self Excited Induction Generator with STATCOM

4.3.1. Study of Load Changes Effect Over Voltage Profile

In Figure 14 Self Excited Induction Generator application camealong with SVC. In Figure 15 voltage range before overload was 0.843and after overload reached 0.839 therefore 0.004 per unit voltage losswill be seen. Voltage frequency in this mode remained steady in 50 Hzmode.

Figure 13: SEIG Generator Voltage Push Wave Figure with SVC in Short CircuitTime t=6

Figure 14: SEIG Generator Active, Reactive, Torque, Voltage Diagram withSTATCOM.

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4.3.2. Study of Short Circuit Effect Over Voltage Profile

In Figure 16 generator application with STATCOM showed in shortcircuit manner. Figure 17 showed that generator in 0.2 s short circuittime increased speed to 0.0021 per unit and after 1 s reached itspermanent voltage level.

Figure 15: SEIG Generator Voltage Push Wave Figure with STATCOM in 10Per cent Load Increase in t=6

Figure 16: SEIG Generator Active, Reactive, Torque Diagram with STATCOM

EFFECTS OF FACTS DEVICES ON IMPROVING QUALITY OF PRODUCED… / 97

5. CONCLUSION

SVC caused voltage loss due to less load increase. (0.005 instead of0.126) Also, frequency changes could improve it 0.3 Hz. (0.5 instead of0/85) but SVC itself caused 0.022 per unit voltage and it is concludedthat SVC is not proper and suitable tool for SEIG.STATCOM increasedloading from SEIG, effectively. Whereas, in this mode, generator notonly response applied load in contrast with two previous mode, butalso accepted its 20% increase. Voltage value 1 per unit approached(0.843) and also voltage loss resulted from only load in 0.004 perunit.Also the most important effects for SEIG was its frequency fixationthat STATCOM supply this requirement considerable. In general, itcould be concluded that in Self excited mode, using STATCOM couldhave suitable explanation and using it could be a proper reviral foralternative system through productive voltage rectification andinverting, could be productive.

REFERENCES

[1] Al Jabri, A. K. & Alolah, A. I. (1990), “Limits on the Performance of the ThreePhase Self Excited Induction Generators”, IEEE Trans. On Energy Conversion,Vol. 5, No. 2, (June), pp. 350-356.

[2] B. singh, Murthy, S.S. & Gupta, S. (2004), “Analysis and Design of STATCOM-based Voltage Regulator for Self-excited Induction Generators”, IEEE Trans.on Energy Conversion, Vol. 19, No. 4, (December), pp. 783-790.

Figure 17: SEIG Generator Voltage Push Wave Figure with STATCOM in ShortTime t=6

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[3] Brazil John, “Stand-alone Induction Generators for Use in Small IsolatedHydro Plants”, Fourth International Conference on Small Hydro Water Power,1990.

[4] C. A. Canizares, “Modeling and Implementation of TCR and VSI Based FACTSControllers”, Internal Report, ENEL and Politecnico di Milano, October 1999.

[5] D. Jovcic, Pahalawaththa, N., Zavahir, M. & Hassan, H.A. (2003), “SVCDynamic Analytical Model”, IEEE Trans. On Power Delivery, Vol. 18, No. 4,(October), pp. 1455 -1461.

[6] E. G. Marra & Pomilio, J. A. (2000), “Induction Generator Based SystemProviding Regulated Voltage with Constant Frequency”, IEEE Trans. IndustrialElectronics, Vol. 47, No. 4, (August), pp. 908-914.

[7] Li. Wang & Ching-Chung Tsao, “Performance Analysis of Three-phase Self-excited Induction Generator Connected to a Utility Grid”, IEEE Trans.,0-7803-6672-7/01, pp. 1398.

[8] Li. Wang & Ruey-Yong Deng (1999), “Transient Performance of an IsolatedInduction Generator Under Unbalanced Excitation Capacitors”, IEEE Trans.Energy Conversion, Vol. 14, No. 4, (December), pp. 887- 893.

[9] M. B. C. Salles, Freitas, W. & Morelato, A. (2004), “Comparative Analysisbetween SVC and DSTATCOM Devices for Improvement of InductionGenerator Stability”, IEEE Melecon, Vol. 3, No. 2, (May), pp. 1025-1028.

[10] Malik, N.H. & Al-Bahrani, A.H., “Influence of the Terminal Capacitor on thePerformance Characteristics of a Self Excited Induction Generator”,Generation, Transmission and Distribution, IEE Proceedings C, Vol. 137, No.2, (March), pp. 168-173, 1990.

[11] P.C. Krause, Analysis of Electric Machinery, New York:McGraw-Hill BookCompany, 1986.

[12] Sung-Chun Kuo & Li. Wang, “Dynamic Eigenvalue Analysis of a Self-excitedInduction Generator Feeding an Induction Motor”, IEEE Trans., Vol. 3, No. 2,(March), pp. 1393-1397, 2001.

[13] T. Ahmad, Nishida, K. & Nakaoka, M., “Static VAR Compensator-basedVoltage Regulation Implementation of Single Phase Self-excited InductionGenerator”, IEEE Trans., Vol. 3, No. 2, (October), pp. 2069-2076, 2004..

[14] T. Ahmad, Noro, O., Hiraki, E. & Nakaoka, M., “Terminal Voltage RegulationCharacteristics by Static VAR Compensator for a Three-phase Self excitedInduction Generator” IEEE Trans. On Industry App., Vol. 40, No. 4, (July/August), pp. 978-988, 2004.

[15] W. Freitas, Morelato, A., Wilsun Xu & Sato, F., “Impacts of ac Generators andDSTATCOM Devices on the Dynamic Performance of Distributed Systems”,IEEE Trans. on Power Delivery, Vol. 20, No. 2, (April), pp. 1493-1501, 2005.