I 

NASA Technical Memorandum 102335 

Development and Refinement of 

Test Bed Simulations 

N.V. Dravid and D.R. Miller 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 

A. G. Patterson 

Analex Corporation 

NASA Lewis Research Center 

Cleveland, Ohio 

F.J. Gombos 

Rockwell International 

Rocketdyne Division 

Canoga Park, California 

Prepared for the 

24th Intersociety Energy Conversion Engineering Conference 

cosponsored by the IEEE, AIAA, ANS, ASME, SAE, ACS, and AIChE 

Washington, D.C., August 6-11, 1989 

{I?aSA-TH- 10 2335) DEVELOPMENT &ND BEPI NE MEIT 

OF TEST BED SIXULATIOHS [NBSA.. Lewis 

Research Center) 8 p CSCL 14B 

189-27702 

Unclas 

G3/20 0225954 
DEVELOPMENT AND REFINEMENT OF TEST BED SIMULATIONS 

N.V. Dravid and D.R. Miller 

NASA Lewis Research Center 

Cleveland, OH 

1. Abstract 

A.G. Patterson F.J. Gombos 

Analex Corporation Rockwell Corporation 

NASA Lewis Research Center 

Cleveland, OH Canoga Park, CA 

Rocketdyne Division 

Lewis Research Center of NASA (Lewis), with support 

from Rocketdyne, has been engaged in non real time computer simulation effort for the Space Station Freedom Electric Power System (EPS) [I J. EASYS. a simulation package developed by Boeing Computer Services, Inc. (BCS), 

is used as the primary tool for this activity. Early in the 

design of the EPS, two test beds have been set up at Lewis. 

The lntegrated Test Bed (ITB), that combines and upgrades these test beds, is in the planning stage. The test 

beds are designed to functionally represent many of the 

components of the EPS and their interconnections. The 

simulation effort is primarily directed towards these test 

beds. Model verification is performed using test bed data. 

This paper will present many of the component models, 

results of simulations, and results of model verification activity. 

2. Introduction 

Lewis was assigned the responsibility to direct the efforts to 

design, develop, and fabricate the electrical power system 

for the proposed Space Station Freedom. Rocketdyne Division of Rockwell In~ernational Corporation was selected 

by Lewis as the prime contractor for this effort. 11 was 

decided to set up lest beds to investigate new concepts in 

power management and distribution (PMAD), e.g., 20 

kHt distribution, to support the design process. Development of computer simulations of the test beds was undertaken as an important adjunct activity. 

While it is desirable for the simulation to precede the lest 

bed hardware design nnd installation, the two activities 

took place almost concurrently for the first two test beds. 

However, for the ITB the modeling task was completed 

nheod of the test bed fabrication. This will help in the 

evaluation of test bed performance before critical design review. 

The availability of test beds during the early modeling 

phase was critical to determine acceptable levels of model 

fidelity to yield useful results. Since many of the components from the earlier test beds would be conceptually repeated in the ITB, the task of ITB simulation is greatly 

simplified. 

This paper describes simulations of the test beds and verification of the models using experimental data from the test 

beds whenever possible. References are provided for information about test bed details and test bed simulation development. 

3. Test Bed Development 

The evolution of test beds at Lewis began with a components test bed, followed by B systems test bed, and shall 

culminate with an advanced lest bed, the ITB. Following 

is a brief description of these test beds. [2.3,4] 

The PMADIPV test bed, supplied by General Dynamics 

Corporation(GDC) in 1987, was primarily built to test 20 

kHz technology components. The test bed consists of 20 

kHz inverters , ac switches, high frequency (Litz wire) 

440V, primary distribution cable, 44OV1208V transformers, and three types of load converters to convert 208V, 

single phase, 20 kHr ac to n type to meet load requirements. The test bed is set up lo evaluate replacements for 

any of the above mentioned components. The dc source to 

the inverters can either be a power supply or the output of 

the Solar Array field nearby. 

The PMAD Systems test bed was assembled nt LeRC from 

components supplied by Rocketdyne, Westinghouse. 

TRW, and Allied Signal. The test bed has the following 

major components: Solar Array Simulator, Solar Dynamic 

Simulator, 20 kHz main inverter units (MIU), 1200 Hz to 

20 kHz frequency converter, Main Bus Switching Arsembly, MBSA, and Power Distribut!on and Control Assembly, PDCA. The PDCA and the MBSA contain power 

switching devices capable of implemenling overcurrent and 

differential protection schemes. System related issues 

such as protection against faults, end-to-end voltage regulation, load sharing among inverters, etc., are being investigated in this test bed. 

The ITB represents a two channel power distribution system containing many of the EPS functional components. 

lnitially. dc power supplies will he used as sources to the 

inverters. Solar Array input will follow when array regulators are added. The ITB will have a complete primary and 

1 
a representative secondary distribution system. The test 

bed will be used to address distribution system design issues 

such as conducted EMI, efficiency, power availability, 

etc.. Another important feature of this test bed is the hierarchical control architecture which will allow investigation 

of a variety of methods of power system control. 

4. Simulation Development 

Simulation activities for the EPS began in 1985, with a 

grant to Virginia Polytechnic Institute & State University(VPI), to generate a computer model of the PMADlPV 

test bed, and continued through Rocketdyne as part of the 

Advanced Development program. Rocketdyne developed 

computer models for many of the EPS components while 

integrating those developed by VPI. Initially, Lewis concentrated on monitoring the task order contracts and exercising the models thus generated. Later, independent 

modeling activity began at Lewis to support test bed operational issues. Presently, modeling work continues at all the 

three places. The following briefly describes some of the 

major components and subsystem models that have been 

developed. 

Developed to simulate 

the PMADlPV test bed MIU, the model consists of two 

inverters. paralleled on the dc inpui side and series connecied on the ac output side. This permits voltage control 

by controlling the phase angle between the individual ac 

output voltages. The simulation of this inverter is based 

primarily on modeling the operation of SCR switches as 

ideal switches (for simplicity). The switch operation is 

based on clock pulses at 20 kHz. This model has provided 

satisfactory information for issues such as output voltage 

harmonic distortion, and general inverter operation control. [ 51 

DW This model was developed 

to simulate the PMAD Systems test bed MIU (two inverters 

in parallel). Each inverter consists of switching elements 

arranged in a bridge configuration around a resonant circuit. The switches are power transistors, but modeled as 

ideal switches. A 20 kHz clock is used to control- the 

switch operation. Output voltage control of individual inverters is obtained by controlling the energy being supplied 

to the resonant circuit. This model also has performed satisfactorily in giving results quite similar to the hardware operation. [6] 

A transmission line model generally consists of lumped parameter elements. The simulation allows for a variety of models depending upon the fidelity requirements and simulation time constraints. On 

one extreme, the line model consists of 5 sections, connected in cascade, each section being a lumped element L 

network to represent one-fifth of the line length. The 

number 5 is a compromise between fidelity and reasonable 

computing time. Depending upon the length of the line, 

even a single section n network may suffice. On the other 

hand, when a large system with many states is simulated, 

it may be sufficient to represent the line without any propa- 

.. . 

gation time, Le., as an equivalent resistance to simulate 

the line loss. This is true when one is not interested in the 

very high frequencies associated with a transmission 

line. [ 71 

Bus Developed by 

Rocketdyne, it simulates all the functions of an intelligent 

switch with built-in sensing and operating logic. There is a 

provision to also simulate the device as a closed switch 

thereby reducing considerably the number of states. This 

model also includes representation of the Remote Power 

Controller (RPC), a device capable of inturrupting a circuit 

faster than the RBI. However, the RPC is a lossy device 

because it does not have a mechanical relay parallel to the 

solid state switch. [ 81 

Three types of load converier 

models have been developed to represent power conversion 

from 20 kHz, 208V. single-phase ac. They are: 1) load 

converter model to obtain 12OV dc; 2) load converter 

model to obtain variable frequency, variable voltage ac for 

loads such as motors; and 3) bi-directional 20 kHz, 208V 

ac l 120V dc load converter model for special applications. 

Load converter models are still evolving because or the 

changing requirements of the base line design. Some of the 

models have closed loop controls to regulate output voltagelfrequency for variable loads. [ 8,9] 

power I Init Model: Also known 

as PDCU, this represents a subsystem which acts as a mini 

substation. It consists of RBIs, RPCs, and Transformers, 

A simple controller model for the PDCU is also available. 

Another similar model is that of the Main Bus Switching 

Unit (MBSU) which is functionally similar to a switching 

station. Presently, the models of both PDCU and MBSU 

utilize the passive, ac RBllRPC models. In future, they 

will be replaced by the active, ac RBllRPC models which 

have many built-in sensing and logic functions for fault 

protection. 

.. . 

A simple model has 

been developed for the PV Source System which includes 

the solar array, battery chargeldischarge converter, array 

regulator, and controller. The time response of this system is substantially slower than that of the 20 kHz EPS 

network. Therefore, the model will mainly be used to 

study the PV Source System and its interactions with the 

MIU. It should be noted that this model is undergoing 

continued refinement to reflect latest available data. 

A model has been developed 

for the BRU-F Alternator, which has been proposed as an 

alternative to the PV source. This model is not presently in 

use but can be revived if the EPS program necessitates. [I] 

Initially, the forward Euler rule for 

numerical integration was utilized. At times, this resulied 

in unstable solutions, especially for models which include 

considerable amount of switching, e.g., inverters, RBI, 

etc.. Later, Switch-State representation was proposed as 

a better alternative allowing the use of the BCS Gear, variable time step, numerical integration method (see page 164 

of [8] and also [IZ]). Many of the models have since 

been convened lo this approach. 

.. 

2 
c 

Many other models such as R-L-C loads, 

transformers, instrumentation, dc RBI, etc. have been developed to meet the requirements of simulation 

5. Simulation Results 

After development, the models were exercised to verify 

their operation. Generally, the simulations were performed 

as functions of time. These included either cold starts or a 

steady state operation subjected to a disturbance Although 

many cases were run, only one representative case for each 

test bed is shown here as an example. More simulation 

results can be found in the references given. 

5.1 PMAD / PV Test Bed Simulation 

Figure 5. I shows a configuration of this lest bed for which 

some simulation results are given. Test bed parameters 

were used whenever available. Three, paralleled MIUs are 

connected to a 100 m cable, feeding into a 4.84 kW resistive load and a dc load converter supplying a resistive load. 

The simulation begins with zero initial conditions and the 

system is brought to a steady state. The load converter 

load is doubled to 2 kW, as a step, to create a disturbance. 

Resulting changes in the MIU output voltage. the line CUP 

rent, and the load converter response are shown. The load 

converter and system responses appear satisfactory. [ 101 

RBI 3 

TRANSMISSION LINE 

Figure 5.1: Simulation Example for PMAD 

/ PV Test Bed 

5.2 PMAD Systems Test Bed Simulation 

The PMAD Systems Test Bed model was developed to investigate the response of a distributed system to a variety of 

disturbances, e.g., varying load profiles. network topology 

changes, faults,etc.. The test bed simulation was also intended to focus on the TRW MlU and to provide subsystem 

level information such as inverter load sharing, load 

change response, fault response, etc.. 

For MIU subsystem analyses, only a small part of the test 

bed model was utilized as shown in figure 5.2. The distribution network is represented as a transmission line supplying a resistive load. Step load changes are applied one 

millisecond apart; the first step is from 4 kW to 8 kW while 

the second being its reverse. Plots of inverter output voltage and current are shown. 

This type of simulation provides information about the 

TRW inverter response to load changes which, in turn, can 

be used to design voltage control loop for optimum performance. [6] 

OUTPUT 

VOLTAOE 

"$ lo TRW INV. 

Figure 5.2: Simulation Example for PMAD 

Systems Test Bed 

5.3 ITB Simulation 

Simulation of the complete ITB system presented a challenge due lo the limitations of the computer resources. The 

large number of states associated with the ITB simulation 

require excessive amounts of execution time. Some economies were realized by using the simplest form of transmission line model (see section 4). Therefore, the ITB simulations are limited to a demonstration of steady state operation to rule out hidden programming errors. However, 

subsystem simulations for the ITB were performed with applied disturbances. Figure 5.3 shows a schematic of the 

IT9 system including the hierarchical control system and 

data network. It must be noted that the PDCU and MBSU 

blocks contain many RBIlRPC models and, thus, have 

large number of states. The figure also shows representative bus voltage plots, labeled as VI and V2, from the two 

separate, nonconnected rings. These plots show the almost identical waveforms for corresponding quantities. 

The phase angle differences are due to those between the 

respective sources. For other simulation results of the ITB, 

please refer to [a]. 

3 
300 

200 

100 

-1 00 

-200 

-300 

v1 0 

0 2E-4 4E-4 6E-4 

TIME 

BE4 0.001 

300 

200 

100 

0 

v2 -100 

-200 

-300 

0 2E-4 4E-4 6E4 8E-4 0.001 

TIME 

Figure 5.3: PMAD ITB Configuration and sample Simulation results 

6. Simulation Verification from Test Bed time honored approach. However, due to limitations of 

model fidelity, such comparisons are likely to show some 

mismatch. Nevertheless, such a comparison will he 

shown. Another method of verification is comparison of 

derived data from steady state operation of the model and 

the test bed. This is likely to yield closer comparison even 

with the limitation mentioned earlier. Some of these are 

also shown. For more information, please refer to [S,lO]. 

Data 

As noted before, the need for verification of the models 

from test bed data is paramount. These data could come 

in different forms. Comparing time function plots from the 

test bed to corresponding plots from the simulalion is the 

1 

4 
6.1 Total Harmonic Distortion (THD) 

Due to the existence of many power processing devices, 

control of generated harmonics will be a significant issue 

for the EPS. Therefore, the amount of total harmonic 

distorlion is an important specification for the design and 

performance of the EPS. Harmonic distortion from the 

MIU is, among other things, a function of the ratio of 

clock to resonant frequencies (fsn). Experiments were 

performed on the PMADlPV test bed to determine this 

function. The same was repeated using the test bed model. 

Figure 6.1 shows lhe comparison of the results. [5,10,11] 

16% 

1- 

l2% 

10% 

9 

EO% 

D 

6% 

2% 

0% 

050 035 0.60 0.65 0.70 0.75 0.80 0.85 0.90 O.% 

f sn 

Figure 6.1: Comparison of THD 

6.2 MIU input Characteristics 

A Maphnm inverter, when connected to a PV source, may 

have an unstable operating point due to the nonlinear voltage-current characteristics of each device. In order to investigate this phenomenon, input characteristics of the inverter were experimentally found from the test bed and and 

also determined by simulation. Figure 6.2 shows the comparison. [ IO] 

=1 

:l 0 

0 

impedance. Such a source must inherently exhibit voltage 

regulation, Le., the output voltage must decrease with increasing power output. Figure 6.3 shows comparison of 

the voltage regulation characteristics of a Mapham inverter 

as obtained experimenlally and from simulation. [ 101 

Figure 6.3: Inverter Voltage Regulation 

6.4 TRW MIU Start-up verification 

The start-up of a TRW MIU is somewhat peculiar because 

of the nature of the control system presently employed. 

Since the simulation is supposed to mirror the test bed the 

control system was also modeled in its present form. Comparison of the MIU start-up characteristics is shown in figure 6.4 which verifies the fidelity of the models.[6] 

INVERTER OUTPUT VOLTAGE 

/. , I I I I I 750 

500 

zy, 0 

-250 

boo 

-. 

0 5E-4 0.001 1.5E-3 0.- 

TIME 

Figure 6.4: TRW MIU Start-up 

7. Conclusions 

Figure 6.2: MlU Input Characteristics 

6.3 Inherent Voltage Regulation of Inverter 

Looking hack from the 20 kHz side. the inverter should 

appear as any other ac source with some amount of output 

Development and refinement of 20 kHz Test Bed simulation for the EPS has been described and the model verification process demonstrated. The high degree of correlation 

between the test bed data and the simulation results has 

been shown. Test bed simulations will be performed prior 

to hardware tests to uncover any hidden problems. Simulations will be substituted where hardware tests are not 

practical or convenienl. [ 51 

EASY5. as a software modeling tool, has generally performed satisfactorily. However, it has required excessive 

5 
amounts of computer time for certain simulations having a 

large number of stales. By improving upon the EASY5 

software and by selective reduced order modeling, it may 

be possible to reduce computation time to a reasonable 

level([ 121 ). Additionally, other software modeling tools 

will be evaluated and utilized to conduct specific investigations concerning the EPS. [7] 

8. Acknowledgments 

The authors wish to acknowledge the assistance provided 

by the following in supplying information for this paper. 

R. Sundberg for PMAD I PV test bed data, R. Beach for 

PMAD Systems test bed data, Dr. Ponlee Li for the initial 

model of the TRW inverter power stage, A. Peuron for the 

ac RBI model, and E. Pongracz-Bartha / J. Kidder for ITB 

simulation results. 

9. References 

1. F J. Gomhos and N.V. Dravid, "An Integrated and 

Modular Digital Modeling Approach for the Space Station 

Electrical Power System Development", NASA Technical 

Memorandum 100904. presented at IECEC-88, Denver, 

CO . 

2. R. M. Button, A. S. Brush, R. C. Sundherg, "Development and Testing of a 2OkHz Component Test Bed", to 

be presented at IECEC-89, Washington, D.C. 

3. R. F. Beach, G. L. Kimnach, T. A. Jett, L. M. 

Trash, "Evaluation of Power Control Concepts using the 

PMAD Systems Test Bed", to he presented at IECEC-89, 

Washington. D:C. 

4. 

Rocketdyne Div., Rockwell International 

PMAD Integrated Test Bed Requirements Document, 

7.* LeRC-900-100. Rev. B, Sept. 30, 1988; prepared by 

6 

5. R. C. Sundberg, A. S. Brush, R. M. Button, A. G. 

Patterson, "Distortion and Regulation Characterization of 

a Mapham Inverter", to be presented at IECEC-89, 

Washington. D.C. 

6. D. R. Miller, "Characterization of the TRW 20 kHz 

Inverter using EASYS", a preliminary information report 

under preparation at NASA LeRC 

7. N. V. Dravid, "Investigation of Transmission line 

modeling for EPS using the Electromagnetic Transients 

Program (EMTP), a preliminary information report under 

preparation at NASA LeRC 

8. Task Order 5. subtask 1 - Final Report, RIIRD89-166, 

Contract NAS 3-25078, E. Pongracz-Bartha, Rocketdyne. 

Canoga Park, CA, April 1989 

9. F. S. Tsai, F. C. Lee, "Computer Modeling and Simulation of a 20 kHz ac distribution for Space Station", Final 

Report, February 1988, VPI & SU, Blacksburg, VA 

10. A. G. Patterson, "PMAD / PV Test Bed modeling using EASYS", a preliminary information report under 

preparation at NASA LeRC 

11. R. C. Sundherg, A. S. Brush, A. G. Patterson. 

"Main Inverter Unit Distortion Test Report", NASA LeRC 

PIR No. 220, November 11, 1988 

12. A. G. Patterson, "Comparison of Power System 

Model Execution Times using Different Integration 

Schemes in EASYS", a preliminary information report under preparation at NASA LeRC 
1. Report No. 

NASA TM-102335 

2. Government Accession No. 3. Recipient’s Catalog No. 

4. Title and Subtitle 

Development and Refinement of Test Bed Simulations 

7. Author(@ 

N.V. Dravid, D.R. Miller, A.G. Patterson, and F.J. Gombos 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

5. Report Date 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-5043 

14. Sponsoring Agency Code 

~ 

9. Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

5. Supplementary Notes 

24th Intersociety Energy Conversion Engineering Conference cosponsored by the IEEE, AIAA, ANS, ASME, 

SAE, ACS, and AIChE, Washington, D.C., August 6-11, 1989. N.V. Dravid and D.R. Miller, NASA Lewis 

Research Center; A.G. Patterson, Analex Corporation, NASA Lewis Research Center; Cleveland, Ohio 44135. 

F. J. Gombos, Rockwell International, Rocketdyne Division, Canoga Park, California. 

6. Abstract 

Lewis Research Center of NASA (Lewis), with support from Rocketdyne, has been engaged in non real time 

computer simulation effort for the Space Station Freedom Electric Power System (EPS) (“An Integrated and 

Modular Digital Modeling Approach for the Space Station Electrical Power System Development, ” by F.J. 

Gombos and N.V. Dravid, NASA TM-100904, presented at IECEC-88, Denver, CO.) EASY5, a simulation 

package developed by Boeing Computer Services, Inc. (BCS), is used as the primary tool for this activity. Early 

in the design of the EPS, two test beds have been set up at Lewis. The Integrated Test Bed (ITB), that combines 

and upgrades these test beds, is in the planning stage. The test beds are designed to functionally represent many 

of the components of the EPS and their interconnections. The simulation effort is primarily directed towards these 

test beds. Model verification is performed using test bed data. 

IO. Work Unit No. 

474-42-10 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

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

Simulation 

Power system 

Computer modeling 

2. Sponsoring Agency Name and Address 

18. Distribution Statement 

Unclassified - Unlimited 

Subject Category 20 

Technical Memorandum 

9. Security Classif. (of this report) 

Unclassified 

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20. Security Classif. (of this page) 21. No of pages 22. Price’ 

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