NASA TechniCal Memorandum 102375

Modeling of Power ElectronicSystems With EMTP ......

Kwa-Sur Tam .....

Virginia Polytechnic Institute and State UniversiO'

Blacksburg, Virginia

and

Narayan V. DravidLewis Research Center

Cleveland, Ohio

(NASA-TM-102375) MOOELING OF POWER

ELECTRONIC SYSTEMS WITH EMTP (NASA) 10 pCSCL 12R

N90-14060

Unclas

G3/o6 0252599

5f:_

December 1989

MODELING OF POWER ELECTRONIC SYSTEMS WITH EMTP

Kwa-Sur Tam, Member

Virginia Polytechnic Instituteand State University

Blacksburg, VA, 24061

ABSTRACT

In view of the potential impact of powerelectronics on power systems, there is need for acomputer modeling�analysis tool to perform simulationstudies on power systems with power electroniccomponents as well as to educate engineering studentsabout such systems. This paper describes thesuccessful modeling of the major power electroniccomponents of the NASA Space Station FreedomElectric Power System, EPS, with EMTP(ElectroMagnetic Transients Program) anddemonstrates that EMTP can serve as a very usefultool for teaching, design, analysis, and research in thearea of power systems with power electroniccomponents. The paper describes EMTP modeling ofpower electronic circuits and presents simulationresults.

1. INTRODUCTION

With the advent of semiconductor technology,power electronic circuits and systems will play anincreasingly significant role in electric power systems.Power electronic systems such as high voltage directcurrent (HVDC) transmission systems and static varcompensation (SVC) systems have already beeninstalled in some power systems to enhance electricpower transmission and to improve the overall systemperformance Other examples include powerconditioning units that connect alternative energysources (e.g., photovoltaic and wind energy) to utilitypower systems, motor drive systems for industrialapplications, utility interconnection to energy storagesystems (e.g., battery, fuel cells, superconductingmagnetic energy storage), etc. Besides improvementand innovative uses of existing devices and systems,there are many opportunities for developing newdevices, systems and applications.

In view of the potential impact of powerelectronics on power systems, there is need for acomputer modeling/analysis tool to perform simulationstudies of power systems with power electroniccomponents. Such a simulation tool could be used tostudy the operation of individual subsystems and theirinteractions, to investigate various system operationand design concepts, to analyze and compare differentcontrol and protection strategies, to examine theimpact of undesirable effects such as harmonicdistortions, to investigate potential problems such asresonance, etc.

The National Aeronautics and SpaceAdministration (NASA) is undertaking thedevelopment of Space Station Freedom, a mannedbase that would remain in low earth orbit for an

Narayan V. Dravid, Senior Member

National Aeronautic and Space AdministrationLewis Research Center

Cleveland, OH, 44135

indefinite length of time, for scientific, technological,and commercial purposes. NASA Lewis ResearchCenter is responsible for the design, development andfabrication of the Space Station Electric Power System(EPS). Among the technologies under investigation isa 20 kHz single phase, 440V, power management anddistribution (PMAD) system . The PMAD system isdesigned to convert power from the solar-based energysources (photovoltaic/solar dynamic) to 20 kHz fortransmission and to distribute it to various types ofcustomer loads, both dc and variable frequency ac.The initial system capacity is 75 kW. The system isplanned to grow up to 300 kW in subsequent phases.Although operated at a much higher frequency, theEPS resembles a terrestrial utility power system (60 Hz)in many respects.

NASA Lewis Research Center has built a powersystem facility for testing the hardware and software tobe developed for the EPS. Modeling capability forsimulation studies is also under development Apreliminary study has been performed to investigate thefeasibility and appropriateness of using EMTP(ElectroMagnetics Transients Program) [1] for themodeling of EPS with special emphasis on powerelectronic devices. Results of this study are describedin this paper.

The objective of this paper is to demonstratethat EMTP can be used as a tool to model powerelectronic components (including high frequencyoperation) and their interactions wire the powersystem. Engineers can use EMTP for simulation ofpower electronic systems while educators can also use itas a teaching tool for such systems. New challengesassociated with power electronics and new applicationssuch as space power systems could stimulate moreinterests among students to study power systems.

A brief overview of EMTP is provided inSection 2. The modeling of major power electroniccomponents in the EPS is described in Section 3.Applications of EMTP are discussed in Section 4.Conclusions are presented in Section 5.

2. BRIEF OVERVIEW OF EMTP

EMTP is a well-known network simulationsoftware tool for power system simulation studies, usedextensively by utility engineers. Presently, versions ofEMTP are available for a variety of computersincluding personal computers [2,3]. EMTP continuesto be updated with new features as need arises,

The theoretical basis of EMTP modeling isexplained in [1]. To model a power system, an EMTPuser assigns a name to each node of the network andspecifies how the nodes are connected. Models of avariety of power system components are available in

EMTP. Alternatively,the usermaysetupa modelofhisown device.

Control systems are modeled by using the TACS(Transient Analysis of Control Systems) program ofEMTP. During each time step, voltage, current, orother data measured from the power system are usedby the control system to determine appropriate controlactions (e.g., turning switches on/off ), which are,then, fed back to the power system for execution.Interaction between the power system and its controlsystem can be simulated accurately.

Initially, TACS was developed to model theHVDC converter control system and the resultingsimulation results were validated against field data [4].EMTP has also been used to simulate the operation ofstatic vat compensators. However, modeling of otherpower electronic circuits using EMTP is not widelyreported. The modeling of two power electroniccircuits, Which are being considered for the EPS, ispresented in this paper. One of the circuits is aphase-angle-controlled (PAC) resonant inverter thatuses thyristors and the other is apulse-width-modulated (PWM) resonant inverter thatuses transistors. Because of the increasing popularityof resonant-mode power supplies [5], these resultshave future applications. Also, the same modelingtechnique can be applied to other types of powerelectronic circuits.

3. MODELING OF EPS POWER ELECTRONIC

COMPONENTS

Although the configuration of the EPS is not yetfinal, NASA Lewis Research Center has assembled apower management and distribution (PMAD) systemhardware test bed to support the design anddevelopment of EPS where system design issues such assystem protection, control, and load management willbe addressed. A simplified schematic diagram of the

Line Resistance Inductance Capacitance(ohm) (mH) (/aF)

L1 0.0305 0.000171 0.05016L2 0.0230 0.000129 0.03784L3 0.0203 0.000114 0.03344L4 0.0728 0.000408 0.11968

Table 1. Line Data for the 20 kHz PMAD test bed

PMAD test bed is shown in Figure 1.DC power fromthe photovoltaic arrays is converted to 20 kHz ac bythe main inverter unit (MIU), which is connected tobus 1 of a 4-bus, 4-line power system. The 20 kHztransmission cable, designed to have low inductance,consists of flat foil conductors with dielectric coating.Line data for this system are given in Table 1.

Because these lines are short (less than 150feet), NASA studies indicate that a single "n'-model issufficient for line representation for most of thesimulation studies.

A major component of the PMAD system is theMIU. Two candidate circuits are currently beingconsidered for the MIU. The first is a PAC circuit

ARRAYS LI

Figure 1. A simplified schematic diagram ofthe PMAD test bed

being developed by General Dynamics while thesecond is a PWM circuit being developed by TRW.

A. Phase-angle-controlled (PAC) MIU

The basic component of this MIU is a dc/acresonant inverter. Figure 2 shows the schematicdiagram of this inverter, known as the Maphaminverter [6], wherein the basic resonant circuit isformed by the inductors (L) and the capacitor (C).However, the connected load across the capacitor alsoaffects the resonant circuit. The resonance frequency,fr, is determined by the values of L, C, and the load.The operating frequency, fo, of the output voltageacross C is determined by the gate firing frequency ofthe thyristors, provided that the ratio, fr/fo, is greaterthan unity. This ratio influences the performance ofthe circuit in terms of efficiency, harmonic distortion,and voltage regulation and is an important designparameter.

The steady-state operation of this circuit isdescribed as follows:

Mode 1 : Thyristors Q1 and Q2 are turned on andestablish a resonant circuit ; current flowsfrom the dc source to the capacitorthrough Q1 and Q2 and charges thecapacitor in the positive direction;currents in diodes D3 and D4 decrease tozero.

Mode 2: At the end of half of the resonanceperiod, current reverses direction andflows from the capacitor to the dc sourcethrough diodes DI and D2 ; thyristors QIand Q2 are turned off.

Mode 3 : Thyristors Q3 and Q4 are turned on andestablish a resonant circuit ; current flowsfrom the dc source to the-capacitor

throug.h Q3 and Q4 and charges thecapamtor in the negative direction ;currents in diodes D1 and D2 decrease tozero.

Mode 4: At the end of half of the resonanceperiod, current reverses direction andflows from the capacitor to the dc sourcethrough diodes D3 and D4 ; thyristors Q3and Q4 are turned off.

-I- Ol Ol

oc 'ff;

t T+cit'- -INPUT L L,L OUTPUT

o

_.Q4

_Q2

Figure 2. Mapham Inverter

The operation of the Mapham inverter wassimulated with EMTP and the results are shown in

Figure 3. These results are consistent with those

obtained experimentally. For this study, the operating

frequency was 20 kHz and the ratio, fr/fo, is about

1.4. The electric network portion of this circuit wasmodeled with existing EMTP features. EMTP also

offers models for thyristors and diodes while switch

characteristics such as holding current, ignition voltage,

and deionization time can be specified. Snubber

circuits were also included in the simulation. The gate

firing pulses for the thyristors were generated usingTACS.

Figure 4 shows the firing pulse generation circuitimplemented with TACS. The type-24 device is a

signal source generating a periodic sawtooth waveform.

Like the other TACS devices, this device is uniquelyidentified by the name of its output, VSIG. 1, as

defined by the user. VSIG. I is the driving signal for a

type-51 device, a relay-operated switch. The type-51device is normally open and its output, TRIG. 1 is zero.

When the driving signal VSIG. 1 is greater than or equalto a user-controlled threshold, VREF. I, TRIG. 1

changes to unity.

TRIG. 1 is fed to a pulse shaping circuit. DTRI. 1,

output of the type-53 (transport delay) device, differsfrom its input TRIG. l by a user-controlled time delayPULWID. By subtracting DTRI. 1 from TRIG. 1 a pulsewith a controlled width can be obtained. FPULSE is

the firing pulse that controls the operation of athyristor. Since SCRs respond only to positive gate

signals, the negative portion of FPULSE does not affect

the operation of the circuit and, if desired, may be

filtered away by passing FPULSE through a limiter withthe lower limit set at zero.

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200

Figure 3.

(a)

QI

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....Q3 __QI

firing pulses

....Q3

(b)

/%I

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Current through Q[ and Dt

A

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(c) Current through Q3 and D3

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(d) Output current

/% ,/'\/ 2/ X,

) Output voltage

/--. /I--,X

I1 _x ¢

,/ \ / ",\

250 _ :,',l

Time (microsecond)

Steady State operation of the MaphamInverter

VREE I

TYPE-51

DEVICE TRIG. I

TYPE-53 I

Io,,,.,

PULWID

FPULSE

Figure 4. Firing Pulse Generation Circuit

By fixing the period of the type-24 source at 50microsecond, a train of pulses at 20 kHz can begenerated. The phase angle of these pulses iscontrolled by changing the threshold, VREF. 1, whichcan be the output of a phase-angle-control (PAC)control scheme. The width of the firing pulse iscontrolled by changing the time delay PULWID,whichcan be the output of a pulse-width-modulated (PWM)control scheme.

A PAC MIU consists of two Mapham invertersconnected as shown in Figure 5 [7]. The inverters areconnected in parallel on the dc side. Capacitors areadded in series to the ac outputs to improve voltageregulation. The inverter ac outputs are then combinedthrough a transformer to provide a total output voltageof 440 V RMS.

4-

(DC) V_

_ MAPHAM __V_I _._ oVo

I -o

Figure 5. Phase-angle-controlled MIU

!

Vo

_'

VME_SLJRED[ RMS

:'tCALCU LATOR

VREFERENCEMAX

IINTEGRALCONTROLPULSE

FIRING

CIRCUIT

____J ._ PULSEFIRING

MIN - CIRCUIT

Figure 7. PAC MIU Voltage Control System modeled with TACS

MAPHAM

INVERTER t

MAPHAM

INVERTER 2

The resonance frequency, Jr, is determined bythe values of L, C, and the load. The operatingfrequency of the output voltage, fo, is determined bythe frequency at which the transistors are turned onand off, provided that the ratio, fr/fo, is greater thanunity. Thus, fr/fo is an important design parameter.

The steady-state operating cycle of this circuitconsists of the following sequence of operating modes.Mode I : Q3 is turned on and Q2 is turned off ;

current flows from the dc source to the

capacitor through D1 and Q3 andcharges the capacitor in the positivedirection.

Mode 2 : QI is turned on and Q3 is turned off ;current circulates through D J, Q1, andthe resonant circuit.

Mode 3 : Q4 is turned on and Q I is turned off ;current flows from the dc source to the

capacitor through D2 and Q4 and chargesthe capacitor in the negative direction.

Mode 4 : Q2 is turned on and Q4 is turned off ;current circulates through D2, Q2, andthe resonant circuit.

The amount of power delivered by the inverteris determined by the durations of mode 1 and mode 3in the operating cycle. These durations are determined,in turn, by the width of the base current pulses appliedto transistors Q3 and Q4. Circuit control is achievedthrough pulse-width-modulation (PWM).

The operation of PWM inverter has beensimulated with EMTP and the results are shown inFigure 10. These results are consistent with thoseobtained experimentally. The operating frequency is 20kHz and the ratio fr/fo is about 2.0. The presentversion of EMTP does not have explicit models fortransistors. However, transistor operation can bemodeled using the TACS-controlled switches (TCS).When a TACS signal supplied to a TCS becomespositive, the TCS is turned on. TCS is turned off when

the same TACS signal becomes negative. Using ascheme similar to one shown in Figure 4, pulses withcontrollable pulse width are generated at a frequencyof 20 kHz and are used to control the transistors. Therest of the circuit, including the center-tappedtransformer, can be readily modeled using availableEMTP features.

A PWM MIU model is incorporated in thesystem model shown in Figure 1. Voltage control isachieved by changing the width of the transistor basecurrent pulses. The voltage control system modeledwith TACS is shown in Figure 11. The RMS value ofthe measured voltage (at bus 1) is compared with thevoltage reference to produce a voltage error signal.This signal, processed by a proportional-integral (PI)control scheme, changes pulse width (required forcorrective control) which is fed to the pulse firingcircuit of the inverter.

The dynamic response of the PWM MIU to astep load change from 5 kW to 2 kW is shown inFigure 12. The pulse width is reduced to handle theload change and the voltage at bus 3 is regulated backto 440 V RMS within eight cycles.

Using the Fourier analysis feature of EMTP,harmonic distortions produced by power electronicscan be examined. EMTP results indicate that theharmonic distortions generated by PWM MIU varywith power output levels. Table 2 presents the voltageharmonics at the harmonic trap input (point A inFigure 9) for two different power levels. The higherharmonic contents at lower power levels could haveadverse effects on the performance of the controlsystem.

The selection of simulation time-step size isbased on the tradeoff between accuracy andcomputation time. A fixed time-step size of 0.1microsecond (0.72 electrical degree) is found to besufficient to accurately model the 20 kHz powerelectronic systems. Experience suggests that atime-step size in the order of 1 electrical degree is agood rule-of-thumb.

it,, (a) Load current at bus 3

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'"[ (b) Voltage at bus 3

600... I " "^ ? _ n. .."_; ', i i i \ . :. ; , •" ' , ' ', ; , i ' '

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(c) Voltage at bus 1

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,, : _ i . .w ;%/

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,,.[ (d) Output voltage of inverter 1l f_ A _ _, n r,

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Transistor base current pulses

.....Q4 Q3 .....Q4

160,..I

0'

(b) Resonant tank inductor current

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(c) Resonant tank capacitor voltage

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-600 ....

(d) Output voltage

/

VMEA

VRE'F r_

PROPORTIONAL

CONTROL

INTEGRAL

CONTROL

_x

PULSE-

WIDTH

PULSE

FIRING

CIRCUIT

Figure 11. PWM MIU Voltage Control System modeled with TACS

on Power Apparatus and Systems.

4

(a) Transistor base current pulses

ri!iHi!li IiIiiIf!iJIJiII!!i_.i (b) Load current at bus 38,

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/TL, 'tli "_'_t/_! ii ti' 'v _i I /i tV_ ; t,/ V7900 ii00 1300 l_,l

Time (microsecond)

Figure 12. Dynamic Response of the PWM MIU to

a step change in load

[4] D.A. Woodford, "Validation of DigitalSimulation of DC Links", IEEE Transactions

Vol.PAS- 104, No.9, September 1985,

pp.2588-259 5.

[5] F.E. Sykes, "Resonant-mode Power Supplies :

A Primer", _ Vol.26, No.5,

May 1989, pp.36-39.

[6] N. Mapham, "An SCR Inverter with GoodRegulation and Sine-wave Output", IEEETransactions on Industry and General

Applications. Vol.IGA-3, No.2, March/April

1967, pp. 176-187.

[7] "AC Power System Testbed", Final Report,NASA CR 175068; J. Mildice and R.

Sundberg, General Dynamics-Space SystemsDivision, November 1987.

[8] "Bidirectional Power Driver/Receiver", FinalTechnical Report, NASA Contractor Report180887; J.J. Biess and L.Y. lnouye, TRW

Space & Technology Group, May 1988.

[9] J. Reeve & S.P. Lane-Smith, "Integration ofReal-time Controls and Computer Programsfor Simulation of Direct Current

Transmission", 1989 IEEE/PES Summer

Meeting, Long Beach, Paper 89SM7899PWRD.

[10] R.J.Spier and M.E. Liffring, "Real-time Expert

Systems for Advanced Power Control",

Proceedings of the 23rd Intersociety EnergyConversion Engineering Conference. Denver,

1988, Paper 889084.

Kwa-Sur Tam (M) is an Assistant Professor of

Electrical Engineering at VPI & SU. He received his

Ph.D. degree from the University ofWisconsin-Madison in 1985.

Narayan V. Dravid (SM) is an Electrical Engineer atthe NASA Lewis Research Center with Space Station

Freedom Directorate. He received his Ph.D. degree in

1968 from the University of Wisconsin, Madison and is

a registered professional engineer.

Na|ional Aeronaulics andSpace Administration

1. Report No.

NASA TM-102375

4. Title and Subtitle

Modeling of Power Electronic Systems With EMTP

7. Author(s)

Kwa-Sur Tam and Narayan V. Dravid

g,

12.

Performing Organization Name and Address

National Aeronautics and Space AdministrationLewis Research Center

Cleveland, Ohio 44135-3191

Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

3. Recipient's Catalog No.

5. Report Date

December 1989

6. Performing Organization Code

8. Performing Organization Report No.

E-5109

Report Documentation Page

2. Government Accession No.

10. Work Unit No.

474-42-10

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15. Supplementary Notes

Kwa-Sur Tam, Virginia Polytechnic Institute and State University, Blacksburg, Virginia; Narayan V. Dravid,NASA Lewis Research Center.

16. Abstract

In view of the potential impact of power electronics on power systems, there is need for a computer modeling/

analysis tool to perform simulation studies on power systems with power electronic components as well as to

educate engineering students about such systems. This paper describes the successful modeling of the major power

electronic components of the NASA Space Station Freedom Electric Power System, EPS, with EMTP

(ElectroMagnetic Transients Program) and demonstrates that EMTP can serve as a very useful tool for teaching,

design, analysis, and research in the area of power systems with power electronic components. The paper describes

EMTP modeling of power electronic circuits and presents simulation results.

17. Key Words (Suggested by Author(s))

Education; Simulation; DC/AC resonant inverters; PWM;

Harmonics; Transients; Operation; Control

18. Distribution Statement

Unclassified- Unlimited

Subject Category 66

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages

Unclassified Unclassified 10

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