"Comparison Analysis ofAC Voltage Controllers Basedon Experimental and Simulated Application Studies"

Hamdy. A. Ashour and Rania. A. IbrahimArab Academy for Science & Technology

Department of Electrical & Computer Control Engineering, 1029 Miami, Alexandria, Egypthashourgaast.edu, rania assemgalexseeds.com

Abstract- This paper introduces a detailed comparison betweenpossible connections of AC voltage controllers. For eachconfiguration, the experimental setup is implemented and thecorresponding simulation program is presented using Simulinkunder Matlab. The simulated and experimental instantaneousvoltage and current waveforms in case of resistive and inductiveloads are matched well, validating the simulation comparisonfor analysis. The comparison analysis includes the requirednumber of devices and isolated gate signals, which determinesthe complexity and the size, hence the overall cost. Alsoharmonic spectrum, total harmonic distortion, effective rmsvalue, dc offset and the control range are compared to specifythe performance. The implementation of a fixed- capacitorthyristor controlled reactor (FC-TCR) and three phaseinduction motor starters (SOFT STARTER) as two applicationcase studies of AC voltage regulators has been discussed.Experimental and simulation results have been obtained andwell correlated, showing the effectiveness of such configurationsin the fields of control of reactive power flow and in the field ofcontrolling the starting performance of three phase inductionmotor.

LIST OF SYMBOLS

VOrms

VSn, mka

'30

IFC-TCR, VFC-

BFC-TCRBCBTCR

BmaxCoC,L

: rms output load voltage

: rms supply voltage: Number ofON & OFF cycles: Duty cycle: Delay angle: Extinction angleLoad angleConduction angle (8 =,8 - a)

-TCR FC-TCR current and voltageCompensator susceptanceCapacitor susceptanceInductor susceptance as a function of

delay angleMaximum inductor susceptanceAngular frequencyCapacitance & inductance value

I. INTRODUCTION:

The AC voltage controller can be considered as a voltageregulator device by which the root mean square load voltage(rms), hence the power flow, can be set and maintainedconstant at a certain desired value. The recent developmentsachieved in the field of power electronics, control techniquesand microprocessors have introduced such AC voltagecontrollers for the applications of power ranges from fewwatts up to fractions of megawatts, such as light dimmers,

heating, melting, arc furnace, transformer tap changing,cycloconverters, wind turbines, power factor improvement,flexible transmission systems (FACTS), static switches, ACmotors control and operation [1-13]. The operation of the ACvoltage controllers have been explained in literatures [5-8].This paper introduces a comparison between eleven possibleconfigurations of the AC voltage controllers based onexperimental and simulation analysis. Principle of operationis reviewed, experimental setup and software simulation areintroduced, a detailed comparison has been carried out andtwo different application case studies have been discussed forpractical validation.

II. AC VOLTAGE CONTROLLERS:

If a thyristor switch is connected between an AC supplyand load, the power flow can be controlled by varying therms value of the AC voltage applied to the load. This type ofpower circuit is known as an AC voltage controller(regulator). Since the input voltage is AC, the thyristor is linecommutated, so there is no need of extra commutationcircuitry and the circuits for AC voltage controllers aresimple and relatively inexpensive. AC switch can beimplemented either using a single triac with a single isolatedgate circuit but for lower power applications, using two backto back thyristors with two isolated gate circuits, using twodiodes and two thyristors with a single isolated gate circuit,or with four diodes and a single thyristor with a singleisolated gate circuit. The power flow to the load can becontrolled by the ON-OFF or phase angle control techniques.For the ON-OFF control, the rms output voltage for resistiveload can be expressed as [6]:

Vorms L22(n+) fm)s m+nt) =VSmn =VSk

While for the phase control VO can be expressed as:rm)

(1)

1 F1( Si 2- f 2f§ Sir?II 1 7-a/-v (2)V°rms 021- I

0i 4t)1 =V / (T_ 2il

The ON-OFF type of control can be applied in applicationshaving mechanical inertia and high thermal time constantsuch as industrial heating and speed control of small motors,while the phase control can be utilized in many industrialapplications such as motor starters, transformer tap changingand static VAR compensators. In case of inductive load, thecurrent will not be in phase with the controlled voltage. Inthis case, in order to ensure full control of the AC voltage, asingle gate pulse should be replaced with continuous train ofpulses and the range of delay angle a is limited within therange of:

1-4244-0272-7/06/$20.00 ©2006 IEEE

i:

79

O<a<cTWhile (2) maybe then re- written as:

(3)

1 1

F2/m=22 F1(sin2asin)Vs('+ 2,6+12 4

The AC voltage controllers can be configured to be usedeither in the single phase low power domestic applications, orin the three phase high power industrial applications.Different possible configurations of the AC voltagecontrollers are depicted in fig. 1 and will be comparedthrough the next sections.

III. EXPERIMENTAL SETUP AND SIMULATION SOFTWARE:

A general block diagram for the experimental setup is shownin fig. 2-a while an example of practical connections is shownin fig. 2-b. The setup is built in a modular form and consistsof a variable power supply, a variable RLC load bank, asynchronizing and isolating firing gate signals, a controlledset point (delay angle a) and a group of individual diodes andthyristors power electronic devices. The setup is reliable andflexible to be reconnected to get different configurationsshown in fig. 1.

(h) (i) (j) (k)

Figure 1: Different circuit configurations for the implementation ofAC voltagecontrollers

I~~~~~~~~~~~~~~-i4~~~~~~~~~~~~~~~~~~~~0 &I &6Ci t0f0Delay Syndnoui_ tIon

Angle Gatc

l0v

(a) General block diagram (b) Example of actual connections (conf. c)

Figure 2: Experimental set-up

(a) General block diagram (b) Example of simulation program (Conf. c)

Figure 3: Simulation software

The simulation analysis has been carried out using Simulinkunder Matlab version 6.5 which provides strong powerelectronics and analysis toolboxes. The system is simulated ina modular form, as shown in fig. 3, typical to theexperimental block diagram for clear comparison and has anadditional block for harmonic analysis. This simulation blockcalculates the Fourier coefficients, the total harmonicdistortion (THD), the effective rms value, the DC offsetcomponent and then plots the harmonic spectrum as will bedemonstrated in the next sections.

IV. COMPARISON ANALYSIS:

For each configuration in fig. 1, the experimental setup isreconnected and the experimental waveforms are obtainedusing storage scope, then the simulation is reconfigured toobtain the corresponding simulated waveforms forcomparison and analysis. An example of the experimentalconnections and the corresponding Simulink simulatedprogram for one of the possible three phase configurationsare illustrated in fig. 2-b and (3-b) respectively. For the singlephase configuration, the gate signals (1, 2) are shifted by 1800while for the three phase configurations the gate (1, 2, 3, 4, 5,6) are shifted by 600. For all configurations, the comparison iscarried out for load voltage waveforms at delay angle a=1080and in two cases: unity power factor (resistive load) and 0.6lagging power factor (resistive and inductive load).The comparison depicted includes the followings:

A. Experimental and Simulation Waveforms.These waveforms shown in fig. 4 are for clear comparison

and simulation validation. The scales of time, voltage andcurrent of the experimental waveforms are typical for theseshown in the corresponding simulation graphs. A goodagreement between the simulated and experimentalwaveforms can be seen in fig. 4 for different configurations.The slight differences noticed between waveforms,particularly for inductive loads, could be due to the differencebetween switching performance of the actual and simulateddevices.

B. Number ofDevices.Lower number reduces the cost; hence the cheapest

configuration is the single phase configuration (a) while thecheaper one in the three phase is configuration (k). However,the SCR current rating of configuration (k) should be XThhigher than the other three phase configurations. For threephase, since the devices are connected at the phases for

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Levt

configurations (i, j, k), and not at the lines like others, theycould have lower current and higher voltage ratings.

C. Number ofIsolated Gates.Increasing this number complicates the circuits, increases

the size and the overall cost. Configuration (a) is the bestfrom this point of view as it needs only one gate signal, whilethe three phase configurations (d, f, h, k) require only threeisolated signals rather than six required by the rest.

D. Number ofLoad Terminals.For the three phase configurations (g, h, i, j, k), the six

terminals of the load should be available and not connectedas star or delta. This could limit the applications of theseconfigurations according to the nature of the available loads.It is not the case for other configurations which require loadswith only three terminals and are suitable for most threephase loads (connected or not connected as star or delta).

E. Effective rms Value and the DC Offset.These values are calculated in per unit for the supply

voltage taken as a base voltage and for a certain delay anglea=1080 and the load impedance is the base for currentcalculation. Due to the presence of diodes in configurations(a, d, f, h) or the possible forced path through the ON SCR inconfiguration (k), the output voltage and input current areasymmetrical containing a DC component. This is very clearin the single phase configuration (a) which also has a limitedrange of control as the Vorms can be only varied from 0.7 p.u.to 1 p.u. If there is a magnetic element, such as a transformer,such DC component may cause saturation problems. Forthese reasons, these configurations, named as unidirectional,are more suitable for resistive loads, such as heating andlighting applications. Configurations (b, c, e, g, i, j) arebidirectional control and the waveforms are symmetricalalong the x-axis, containing no DC component. Theseconfigurations are most suitable for AC motor controls andpower system applications.

F. Harmonic Spectrum and Total Harmonic Distortion(THD):Harmonics may cause problems particularly for motors

(negative torque) and power systems (resonance and noiseinterference). Harmonics may be useful in some applications,such as heating, since the effective rms may be increased.Configurations (c, g) and (d, h) produce similar waveformsrespectively as seen from fig. 4-c & 4-d. From fig. 4-a & 4-bit can be seen that the unidirectional configurations introduceeven and odd harmonics while the bidirectionalconfigurations introduce only odd harmonics due to thesymmetrical positive and negative parts of the waveforms.Even harmonics may cause problems in motor applications. Itcan be also seen that the inductive load increases the value ofharmonic components and THD values due to the distortionin the waveforms. The triplen harmonics will be disappearedin the line values for the delta connected loads(configurations e, f, i). It should be noted that the controlrange of the delay angle may change the voltage wave shapesand fig. 6 depicts the different voltage wave shapes at

components and the THD, since the output waveform tendsto be sine wave.

(a) Conf. a

-----------

--G

2:~ ~~~---- -----;;;;;;;;_;;;; ;;;;;;;,;;;;

(b) Conf. b

(c) Conf. c &g

(d) Conf. d &h

(e) Conf. e

n 0v L 7FI~~~~~~~~---------------------------------------------------

O 04 0Oc ttreRsecf) ConfDo

--------------i000---00----- --- 00000T

------------i

-4

different delay angles using resistive load for clarity.Reducing the delay angle a reduces the value of harmonic

(g) Conf. i

81

-

I.1

,I

,I.1 .-

. r T T T T |~~

3[------------------I---------

--1----------------------------------- ;s,...

------------I -----------------------------------------------------

--------------------------------------------------------------------------

----------------------------------------------------------- -----------------

I-......_

I.-----------------------------

-------------- ------------------------

-

----------------------------------------------------------------------------

THD = 82.95% THD = 96.43%RMS= 82.95 p.u RMS= 1.25 p.u

DC= -7e-4 P.u DC= -2e-3 p.u

2 f9 10 13 t4 1S 1S

(f) Conf. f

(h) Conf. j.L

(g) Conf. i

(i) Conf. kFigure 4: Simulation and experimental load phase voltage and current waveformsfor different configurations. Graphs in sequence are:Left graphs: SIMULATION Right graphs: EXPERIMENTAL1st Voltage, R -load 1st Voltage, R -load2nd = Current, R- load 2nd = Current, R- load3rd= Voltage, RL- load 3rd= Voltage, RL- load4'- Current, RL- load 4'h Current, RL- load

Scales: Voltage: 50 V/ div, Current: 1 A/ div, Time: 0.01 sec/div

_THD = 105 .1%RMS= 0.33 p.u

DC=Op.u

(h)

1 THD= 186.8%RMS= 0.48 p.u

6 ~~~~DC=O p.u

. . . . . . I. I

(i) Conf. kFigure 5: Harmonic analysis for output voltage for different configurations

Left: R- load Right: RL- loadoc =400

c 800

cL=1200

(Conf. a)

(b) Conf. b

, THD = 266.9%RMS= 0.69 p.u; * DC= Op.u

(Conf. b) (Conf. c & g)

CL=400"

oc=800

CL=1200 I

(Conf. i)Alpha4

(Conf. j) (Conf. k)

cL=40

oc=80

cL=120

(Conf d & h) (Con. e) "" (Conf fi

Figure 6: Effect of varying delay angle on output voltage for different ACvoltage controllers on R-load.

V. APPLICATION CASE STUDIES:

Two different application case studies using AC voltagecontrollers will be discussed through the following sections

(e) Conf. e

82

(a) Conf. a

(c) Conf. c & g

(d) Conf. d & h

r

Io Iz H 7 3c

C- C O C o 07 0 9

ii.. -------------------------------------------------------------------------......

L------------- ------------------------------------------- -----------------d

A. Static Power Factor Improvement (FC-TCR).Recently, the AC voltage controllers have been utilized in

the field of power system quality and flexible ACtransmission FACTS [9-11]. Unlike traditional shunt reactiveelements, a fixed capacitor- thyristor- controlled reactor (FC-TCR) is able to rapidly and smoothly supply or absorbreactive power by controlling the firing delay angles ofthyristors. As shown in fig. 7 and 6, each branch has a fixedcapacitor and two anti- parallel thyristors controlled in serieswith an inductor. For such configuration, (5) can be writtenfrom [10].

I FC-TCR = VjBFC-TCRBFC-TCR = Bc -BTCR,BC = wC

BTCR (C)= Bmax 2 -2a- sin 2 ), B2T IT

practical implemented setup for both the 3 SCRs and 6 SCRscorresponding to those simulated waveforms in fig. 9 & 10 atno-load. The differences between the experimental andsimulation waveforms are due to the increase of a was donemanually unlike the simulation.

(5)1

wOLFig. 7-c shows the operating characteristics and thesusceptance (BFC-TCR ) of this type of compensator based on(5) and it can be seen that VAR (reactive power) productionas well as VAR absorption is possible by varying the delayangle of thyristors; hence the power factor changes fromleading to lagging. The firing gates of the thyristors aresynchronized with the capacitance voltage and can be variedfrom 900 to 1800.From (5) and for L= 448 mH, C= 18 [tF, =314.15 rad/sec,then:if a=90' BFC-TCR= BC - Bmax= (1.45 e-3) mho.or if a=l80 BFC-TCR= BC = (5.65e3 ) mho.Such configuration of the FC-TCR has been experimentallyconnected and simulated using Matlab. Fig. 8 shows theexperimental and the corresponding simulation waveforms ofthe FC-TCR voltage and current for different delay angles. Itcan be seen that experimental and simulation waveforms arematched and show the validity of controlling the flow ofreactive power. This configuration could be utilized toreplace the traditional bank of capacitors in manyapplications such as power factor improvement, powersystem voltage and reactive power control, voltage control ofinduction generator and performance optimization of threephase induction motor operated from a single phase supply.

B. Three Phase Induction Motor Starters (Soft Starters).Controlling the starting performance of three phase

induction motor has become one of the major concerns inindustrial fields [12 -13]. The purpose is to control thestarting voltage, current and torque as desired. Configurations(e, f) in fig. 1 were utilized to examine the startingperformance on a three phase delta connected inductionmotor of 0.75 kW, 75 V, 1.5 A, 50Hz, both experimentallyand using simulation. As seen from the results depicted in fig.9 & 10, configuration (e) produces symmetrical wave formsfor both voltage and currents, unlike configuration (f) whichproduces unsymmetrical voltage and current waveforms. Theunsymmetrical waveforms caused by the usage of only 3SCRs is the reason for the appearance of odd and evenharmonics, DC voltage and current, torque pulsations and alonger time to reach the desired speed as can be seen fromfig. 9. Heat loss in configuration (f) is higher thanconfiguration (e) due to the higher rms value of current. Fig.11 & 12 depict the experimental waveforms obtained for the

(a) Three phase (b) Per phase

VM TCR

/\di / 1YIRB

Nrduc¢don bll BFC-TCR BeB3FC-TCR =IBc\ / 3

FICTCCRConetolRiaern d

Capactistidctcrves(c) Characteristic curves

connection connection

Figure 7: Configuration and characteristics of the FC-TCR

cL= 1250 (lagging) cx= 1330 z in phase

BTrR

c= 1600 (leading)

4a 40 40

P. ----.4 0.6 007 --z 0.0 0.0 --------------.0

time(t) time(t) time(t)

cx= 1250 (lagging) c= 1330 z in phase c= 1600 (leading)

(b) Simulation Results

Figure 8: Effect of changing delay angle on the FC-TCR voltage andcurrent waveforms

CHI voltage: 20 V / div, CH2 current: 0.2 A / div, Time: 0.01 sec/div

I15.

i, F,%50

(a) Line voltage

i 1 1 -25 -1- 5(

(b) Line current

Losses= (C) Motor speed5.4 watt

(d) Average heat losses

83

(e) Developed torque

Figure 9: Simulation of three phase induction motor using 6 and 3 SCRS withalpha ramp from 0 to 220 V at 1 sec (0.5 N.m. loading)

Left: 6 SCRs Right: 3 SCRs

-:< X :1 0Xx' V,00\0\(a) Phase voltage

(bh Line current

(c) Phase current

I THD= 24]DC=O

I I I---

THD= 226.5%1.5% DC=0.05

I s- I I

(d) Harmonic AnalysisFigure 10: Simulatioii wavcioriiis ot tilrCC pIIasC IIuUCtLIol motor at a= 1300

Left: 6 SCRs Right: 3 SCRs

(a) Phase voltage and phase current

(b) Line voltage and line current

Figure 11: Experimental Waveforms of Three Phase Induction motorCHI voltage: 50 V / div, CH2 current: 0.75 A / div, Time: 0.01 sec/div

Left: 6 SCRs Right: 3 SCRs

(a) Phase voltage and line current

g1<_-_1..j_I_.

(b) Phase voltage and phase current

VII. CONCLUSION:

A comparison study for different configurations of ACvoltage controllers has been introduced through this work.Experimental and simulation waveforns are matched andvalidated for the configurations. The analysis showed thatunidirectional configurations, having a combination of diodesand thyristors, produced even and odd harmonics and alsocontained a DC offset hence they are most suitable forheating, melting and welding applications, while thebidirectional configurations are suitable for AC motors,power systems and electrical drives applications due to thewaveforns symmetry of the controlled voltages. This paperalso validates the analysis of each configuration for any valueof delay angle and control range. Using the AC voltagecontrollers in reactive power control and controlling thestarting perfornance of three phase induction motor throughFC-TCR and SOFT STARTERS respectively as twoapplication case studies ofAC regulators are demonstrated bythe aid of experimental and simulation results.

REFERENCES:

[1] K. H. Sueker, S.D. Hummel and R.D. Argent, "Power FactorCorrection and Harmonic Mitigation in a Thyristor Controlled GlassMelter," IEEE Trans. Industry Applications, Vol. 25, No. 6, pp. 972-975, November/December 1989.

[2] V. V. Sastry, M.R.Prasad and T.V. Sivakumar, " Optimal Soft Startingof Voltage- Controller Fed IM Drive Based on Voltage AcrossThyristor," IEEE Trans. Power Electronics, Vol. 12, No. 6, pp. 1041-1051, November 1997.

[3] A. Garcia- Cerrada, P. Garcia- Gonzalez, R. Collantes, T. G6mez and J.Anzola, " Comparison of Thyristor- Controlled Reactors and Voltage -Source Inverters for Compensation of Flicker Caused by ArcFurnaces," IEEE Trans. Power Delivery, Vol. 15, No. 4, pp. 1225-123 1, October 2000.

[4] A-R. A. M. Makky, F. N. Abd- Elbar and E. E. Mobarak, " SpeedControl of 3- Phase Induction Motor Using Delta- ConnectedThyristors Switch," The Ninth International Middle- East PowerSystems Conference MEPCON', pp. 827-833, December 2003.

[5] P.C. Sen, "Power Electronics", TATA McGraw-Hill, 1987, ch. 8, pp.588-689.

[6] Muhammad H. Rashid, "Power Electronics Circuits, Devices andApplications", Prentice - Hall International, Inc, 1993, ch. 6, pp. 190-222.

[7] A. Ginart, R. Esteller, A. Maduro, R. Pifiero and R. Moncada, " HighStarting Torque for AC SCR Controller," IEEE Trans. EnergyConversion, Vol. 14, No. 3, pp. 553-559, September 1999.

[8] M.S. Jamil Asghar, "Power Electronics", Prentice-Hall of India, 2004,ch.8, pp.158-170

[9] L. J. Bohmann and R. H. Lasseter, "Stability and Harmonics inThyristor Controlled Reactors," IEEE Trans. Power Delivery, Vol. 5,No. 2, pp. 1175-1181, April 1990.

[10] J. H. Chen, W. J, Lee and M. Chen, " Using a Static Var Compensatorto Balance a Distribution System," IEEE Trans. Industry Applications,Vol. 35, No. 2, pp. 298-304, March/April 1999.

[11] C. Champoo-inwai, C. Yingvivatanapong, K. Methaprayoon and W.Lee, " Reactive Compensation Techniques to Improve the Ride -

Through Capability of Wind Turbine During Disturbance," IEEETrans. Industry Applications, Vol. 41, No. 3, pp. 666-672, May/June2005.

[12] Frede Blaaberg, John K. Pedersen, Soren Rise, Hans- Henrik Hansenand Andrzej M. Trynadlowski, "Can Soft- Starters Help SaveEnergy?," IEEE Industry Applications Magazine, pp. 56-66,September/ October 1997.

[13] Adel Gastli and Mohamed Magdy Ahmed, "ANN- Based Soft Startingof Voltage- Controlled- Fed IM Drive System," IEEE Trans. EnergyConversion, pp. 1- 7, 2005.

Figure 12: Experimental waveforms of three phase induction motorcL=1300

CHI voltage: 50 V / div, CH2 current: 1 A / div, Time: 0.01 sec/divLeft: 6 SCRs Ri2ht: 3 SCRs

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