NASA / TM--1999-209637

Testing of the Engineering Model Electrical

Power Control Unit for the Fluids and

Combustion Facility

Greg L. Kimnach and Ramon C. Lebron

Glenn Research Center, Cleveland, Ohio

David A. Fox

Hamilton Sundstrand Aerospace, Rockford, Illinois

December 1999

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NASA/TM--1999-209637

Testing of the Engineering Model Electrical

Power Control Unit for the Fluids and

Combustion Facility

Greg L. Kimnach and Ramon C. Lebron

Glenn Research Center, Cleveland, Ohio

David A. Fox

Hamilton Sundstrand Aerospace, Rockford, Illinois

Prepared for the

34th Intersociety Energy Conversion Engineering Conference

sponsored by the Society of Automotive Engineers

Vancouver, British Columbia, Canada, August 2-6, 1997

National Aeronautics and

Space Administration

Glenn Research Center

December 1999
This report contains preliminary

findings, subject to revision as

analysis proceeds.

Trade names or manufacturers' names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

Available from

National Technical Information Service

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Springfield, VA 22100

Price Code: A03
Testing of the Engineering Model Electrical Power Control Unit for the Fluids and

Combustion Facility

Greg L. Kimnach and Ramon C. Lebron

NASA Glenn Research Center

Electrical systems Development Branch

21000 Brookpark Rd. MS. 301-5

Cleveland, OH 44135

(W) 216-433-6251 and 433-6482

(Fax) 216-433-2143 and 433-8410

David A. Fox

Hamilton Sundstrand Aerospace

4747 Harrison Ave.

PO Box 7002

Rockford, IL 61125-7002

(W) 815-394-3809

(Fax) 815-226-2601

ABSTRACT

The John H. Glenn Research Center at Lewis Field

(GRC) in Cleveland, OH and the Sundstrand

Corporation in Rockford, IL have designed and

developed an Engineering Model (EM) Electrical Power

Control Unit (EPCLJ) for the Fluids Combustion Facility

(FCF) experiments to be flown on the /nternationa/

Space Station (ISS). The EPCU will be used as the

power interface to the ISS power distribution system for

the FCF's space experiments' test and telemetry

hardware. Furthermore, it is proposed to be the

common power interface for all experiments.

EM EPCU with Cover Removed

Photograph 1

The EPCU is a three kilowatt 120Vdc-to-28Vdc

converter utilizing three independent Power Converter

Units (PCUs), each rated at l kWe (36Adc @ 28Vdc)

which are paralleled and synchronized. Each converter

may be fed from one of two ISS power channels. The

28Vdc loads are connected to the EPCU output via 48

solid-state and current-limiting switches, rated at 4Adc

each. These switches may be paralleled to supply any

given load up to the 108Adc normal operational limit of

the paralleled converters. The EPCU was designed in

this manner to maximize allocated-power utilization, to

shed loads autonomously, to provide fault tolerance, and

to provide a flexible power converter and control module

to meet various ISS load demands.

Tests of the EPCU in the Power Systems Facility testbed at GRC reveal that the overall converted-power

efficiency is approximately 89%--with a nominal-input

voltage of 120Vdc and a total load in the range of 40% to

110% rated 28Vdc load. (The PCUs alone have an

efficiency of approximately 94.5%.) Furthermore, the

EM unit passed all flight-qualification level (and beyond)

vibration tests, passed ISS EMI (conducted, radiated,

and susceptibility) requirements, successfully operated

for extended periods in a thermal/vacuum chamber, was

integrated with a proto-flight experiment, and passed all

stability and functional requirements.

Due to paper length limitations, this Technical

Memorandum only overviews EPCU operation and tests;

a more detailed test report is available [1], as is a

Technical Memorandum explaining operational details

[2].

INTRODUCTION & OPERATIONAL OVERVIEW

The EPCU is an ISS power interface, which converts

120Vdc to 28Vdc, and was designed for the FCF

experiments. The rated, converted power is 3kWe @

28Vdc and is available via 48 4Adc output-channels (4

channels per connector) on the front of the unit.

Furthermore, the FPCU provides an additional 2.gkWe

of unprocessed--but protected--power at 120Vdc.

These 120Vdc feed-throughs are contained in groups of

three per connector (two connectors) on the back of the

EPCU. The EPCU's feed-through power is not

processed: these load connectors are fed by DC-to-DC

Converter Units (DDCUs) and are subject to DDCU

power quality.

Power protection is accomplished with current limiting,

solid-state switches which trip according to a v2t timing.

(The switch FET voltage is controlled to achieve the

current-limiting value of 4Adc: as the load current

increases, the voltage drop across the FET increases.

The hybrid trips more quickly as the connected load

increases beyond the current-limiting value.) These

solid-state switches, along with the control and

monitoring hardware, are called Remote Power

Controllers (RPCs) and have been packaged into a

hybrid module. These hybrid switches, which can

operate at voltage levels from 18Vdc to 150Vdc, are the

keys to the flexibility and re-configurability of the EPCU.

Because the RPC channels can operate individually or

be paralleled in groups of two, three, or four RPCs (i.e.,

channel capacities of 4Adc, 8Adc, 12Adc, or 16Adc), the

EPCU can readily feed any variety of user loads

NASA/TM-- 1999-209637 1
requiring 28Vdc simply by swapping-out external loads'

harnesses, which are connected at the front of the

EPCU (see Photograph 1). By selecting a harness,

which parallels the required number of channels, the

EPCU eliminates the need for "experiment-specific"

power interfaces. (The harnesses contain programming

pins, which cause paralleled channels to operate as if

they were one.)

One of the most significant design features of the EPCU

is its autonomous power-limiting design, which efficiently

utilizes ISS power, and ensures that the EPCU

autonomously limits the connected power drawn from

the ISS power system. This is accomplished with

Dynamic Power Sharing (DPS) and Prioritized Load

Shedding (PLS).

The EPCU has two 120Vdc input feeds which may be

connected to either of two independent ISS power

channels, and each of the three lkWe PCUs within the

EPCU can be connected to either input feed with a

3PDT relay. The maximum power allocation for each

ISS power channel is downloaded to the EPCU via its

MIL-STD-1553B interface. (The power allocation needs

to be sent only once per the load demand schedule

period and does not require that a higher level controller

monitor the EPCU.) During normal operation, the EPCU

output-paralleled PCUs (inputs are not paralleled!) share

output-power equally: as a result, the ISS power feeds

proportionally supply the load demand.

This proportional sharing of load, via the DPS controls,

continues with increasing load demand until one of the

ISS feeds reaches its allocation limit. If the load

continues to increase, the PCU(s) connected to the other

feed pick up the excess load. This adjustment continues

until that feed reaches its allocation limit, at which point

the PLS controller begins to shed loads in a prioritized

fashion (there are 16 user-selectable levels of priority).

Load shedding continues until the busses are again

within the allocation limits.

It should be noted that the DPS and PLS functions play

a more pi_votalrole during abnormal operation of the ISS

power system. These functions allow the EPCU to

continue to feed as much load (i.e., high priority level) as

possible in the event that a DDCU feed is lost, severe

DDCU voltage droop occurs, other ISS load demand

perturbations occur, an EPCU converter fails, or other

events which limit the total load capability of the EPCU.

Finally, the EM unit dimensions are approximately

17.5"W x 21.8"D x 6.6"H, and has a mass of

approximately 1081bsm. The chassis is a milled

aluminum base-plate with aluminum, vertical milledguides. The three PCUs are mounted directly on the

base-plate; whereas all other assemblies (for example,

1553B card) are wedge-locked into place in the guides.

INITIAL FUNCTIONAL TESTS

Prior to performing any higher level tests, basic

functional checkouts were performed. These tests

included functional verification of the MIL-STD-1553B

interface, RPC isolation and voltage drop

characterization, 120Vdc feeder isolation verification,

and programming pin and initial position tests.

MIL-STD-1553B INTERFACE CHECKOUT

The 1553B interface was checked to verify that the unit

received and implemented basic commands (e.g., bus

relay transfer and PCU on/off) and returned proper

status information to the software control interface.

Accuracy of the sensors and ND conversion has not yet

been quantified.

ISOLATION TESTS

Isolation tests were done on the 120Vdc input channels

and on the RPCs (120Vdc and 28Vdc). The input

channel isolation tests were done to verify that the ISS

input feeds would never be shorted together (it is an ISS

requirement that channels never be cross-tied, and this

is guaranteed by the break-before-make 3PDT relays)

and that the two ISS "hot" and "return" lines were

isolated from the EPCU chassis. These are, in fact, the

results of the tests.

Isolation of 28Vdc loads connected to the output RPCs

(called "A5"s) and 120Vdc loads connected to 120Vdc

feed-through RPCs (called "A12"s) were quantified. This

was done by turning "off" the A5 and A12 RPCs,

applying 120Vdc to the EPCU inputs, turning "on" the

converters to energize the Point of Regulation (POR) at

28Vdc, and by measuring the output voltage and current

of each RPC. The multi-meter provided the short-circuit

load for leakage current measurements. Typical A5

leakage was 0.15Vdc @141JAdc, and A12 leakage was

2.8Vdc @60pAdc.

RPC VOLTAGE DROP TESTS

The test setup had one PCU connected to one power

supply ("bus A") and two PCUs to another ("bus B").

Bus A was energized at 120Vdc, and bus B was not

energized. The voltage drop across the 28Vdc and

120Vdc RPCs were determined. The A5 channel drop

was measured by measuring the POR voltage and the

load voltage at half load (2Adc) and full load (4Adc).

The difference yields the channel drop. The channel

drop was also measured directly and compared with the

difference between the POR and load voltage values.

Furthermore, because there are 48 4Adc channels and

all hybrids were of the same batch, it was decided to

only check four 28Vdc channels. The results for the

28Vdc tests are given in Table 1.

The channel-voltage drop at 2Adc is approximately

0.19Vdc, which is an 0.4We loss. At 4Adc the drop is

approximately 0.4Vdc, which is a 1.7We loss. These

losses correspond to 0.7% and 1.5%, respectively, on a

per-channel basis.

Similarly, the 120Vdc hybrids were tested, and at 2Adc

load the drop across the channel is approximately

0.14Vdc. At 4Adc the drop is approximately 0.4Vdc.

These values, other than the J3,3 RPC, are consistent

with the 28Vdc RPCs (they are the same hybrids, after

NASA/TM-- 1999-209637 2
all). However, because the voltage drop across the FET

is current dependent, the percentage losses at the

higher input values are 0.12% and 0.34%, respectively.

8u,, I 0 I

Bus B 120,2

FRPC {Jx,n] or Ira] J22,1 322,2 322,3 J22,4

Load Current [A] 2.106 2,105 2.095 2.102

Vsense IPORI IV] 28.771 28,769 28,773 28.771

FRPC Vout IV] 28.58 28.57 28.58 28,59

Measured DropM 0.192 0,198 0.19 0.185

Calculated DropM 0.191 0,199 0,193 0.181

FRPC [Jx,n] or [m I J22,1 J22,2 J22,3 J22,4

Load Current [A] 4.141 4,139 4.139 4.138

Vsense tPORI IV] 28.77 28.769 28.769 28.769

FRPC Vout _V_ 28.35 28.33 28.36 28.4

Measured Drop M 0.42 0.436 0.408 0,371

C.alcu_ed Drop IV] 0.42 0.439 0.409 0.369

28Vdc Hybrid RPC voltage Drops

Table 1

PROGRAMMING PINS & INITIAL POSITION

The control software developed for the EM EPCU, in

conjunction with the "parallelable" loads of the

HyberLoad load bank, afforded testing of the EPCU

/nitia/ Position (IP) and parallelability of EPCU 28Vdc

channels. 120Vdc channels were similarly tested, using

dip switches on a specially designed test harness.

The control software and HyberLoad combination

eliminated the need to construct varying harnesses for

different IP and paralleled channel scenarios, because

the HyberLoads can "short" the appropriate pins within a

harness. In this manner, all channels were verified to

implement IP and paralleling properly.

EFFICIENCY of THREE PARALLELED PCUs

The efficiency of the PCUs were tested individually and

with the three units paralleled. The efficiency numbers

do not include the losses associated with the input RPC

cards; however, the minimal losses associated with

approximately 8 inches of #14 wires connecting the PCU

outputs to the POR are included.

96

95

94

_,93 /

w 91

90

89

80

100 800

Effclency of Three ParsIfe_ed Corlvetters at 120VOc In

- Core_rter efficiency only

- No other k3iles included

I

E

Y

i

i

I

1100 1600 2100 2600 3100 3800

Load IW==}

4100

Efficiency of Paralleled Converters Only

Figure 1

The efficiency is over 93% for loads in the range of

500We to 3000We, and greater than 94% in the range of

800We to 2500We. The peak efficiency of the paralleled

PCUs is approximately 94.4%. See Figure 1.

EPCU EFFICIENCY TESTS

The EPCU efficiency tests were run in two different

modes of operation:

1. The EPCU having only 28Vdc loads

connected. This mode's efficiency is referred to as the

EPCU "converted-power efficiency" and includes all

losses (e.g., A5 cards and housekeeping power

requirements).

2. The EPCU having 28Vdc and 120Vdc loads

connected. This efficiency is referred to as the "overall"

or "box" efficiency.

The converted-power efficiency tests were run by

connecting a 120Vdc power supply to each EPCU input

bus, connecting two converters to Bus A, connecting one

converter to Bus B, and measuring the input current and

voltage at the input connectors; furthermore, a special

harness was used to measure the total 28Vdc load

current and the voltage at the output of one of the load

RPCs (A5 cards). This test configuration results in a

very good approximation of the converted-power

efficiency, and is shown in Figure 2. The peak,

converted-power efficiency is approximately 89.5%.

The design requirement for EPCU efficiency is defined

for the overall efficiency, and is greater than 97% over

the "midband" range of loads.

90.00 -

85.00 -

o

80.00 - -

_ 7500- ,"

70.00 - :

05.oo-,

90.00

100 600 1100

EPCU Efficiency Cunts

..t j_ ]

i

i

"I _ lZovac _nput

1600 2100 2600 3100 3600

Load [We]

Converted Power Efficiency of EPCU

Figure 2

EPCU REGULATION, CURRENT-LIMITING, &

FOLDBACK

The POR regulation with the three PCUs paralleled and

connected to Bus B was tested at input voltages of

116Vdc, 120Vdc, and 126Vdc. The load was varied

from 0% to 100% and it was found that the EPCU

regulates the bus to within 0.17Vdc over the rated load

range.

NASA/TM-- 1999-209637 3
Voltage foldback occurs because the PWM controller

cannot maintain a long enough duty cycle to regulate the

POR voltage for a given load and input voltage. Thus,

as the load increases, the input-voltage foldback point

increases. The voltage foldback point, listed as Vin [V]

in Table 2, for various load levels was found by

decreasing the power supply voltage from 120Vdc until

fotdback occurred.

Load [A] Load [We] Vout [V] Vin [V]

12 338 27.92 101.27

53.5 1500 28.04 104.71

107 3000 27.997 106.89

Input Voltage Foldback @ Three Load Levels

Table 2

SYSTEM & EPCU STABILITY TESTS

The EPCU was tested for large and small signal stability,

as well as for stand-alone stability. Small Signal Stability

(SmSS) is determined using the Nyquist stability criterion

and Bode plots. Characterizing the response of the

EPCU input and output to load-steps tested Large Signal

Stability (LSS). A last measure of stability is the standalone stability: which is a special case of small signal

stability,

LARGE SIGNAL STABILITY

Instead of using a power supply and a Line Impedance

Simulation Network (LISN), these tests were performed

by connecting the EPCU to the ISS power system testbed located at GRC. A Rocketdyne (RKD) DDCU (rated

at 6kWe) was connected to a Type III equivalent Remote

Power Control Module (RPCM), which fed one input bus

of the EPCU via 73feet of a #4 power line. Because the

RKD DDCU is an EM unit and because the test setup

mimics an ISS channel, the response of the system (i.e.,

EPCU, RPCM and DDCU) to a load step should be more

indicative of what would occur on ISS.

LSS testing was accomplished by applying various load

steps to the EPCU and monitoring the input and output

transient responses. Of primary concern to the ISS EPS

is the input response of the EPCU, which must not

cause potential instabilities of the secondary power

distribution system which lead to, for example, nuisance

trips of RPCMs.

The worst-case test was the application of a 3kWe

@28Vdc load-step (i.e., a 0%-to-100%) to an unloaded

EPCU, having all three converters connected to one bus.

The EPCU input-current response (Figure 3)

demonstrates a time to first peak, Tp, of approximately

1.2msec., an input-current overshoot of approximately

55.7% (implies a damping ratio, _, of 0.183), and a

settling time, Ts, of approximately 3.7msec. The input

voltage (Figure 3) reaches a minimum of 119.6Vdc in

approximately 900msec (undershoot is approximately

1.65% with a _ = 0.793), and the voltage Ts is undefined

because it never exceeds 2% of its final value: That is,

the input bus remains "stiW in the presence of a large

load step placed on the EPCU. Thus, with a worst case

load application, the EPCU settles to an input current

value of 27Adc within 4msec of the load step, and--it

can be concluded--it will not introduce instability on the

input EPS.

Not shown is the EPCU output voltage response to the

0%-to-100% load step in which Vout reaches a minimum

value of approximately 25.6Vdc 2301_sec after the load

step, and it recovers to within 2% of 28Vdc

approximately 800_sec after the load step.

123.5

123

122,5

"t;_2

121.5

121

120.5

t20

11g.6

119

P\, Jli_ .............

! _r" ,,_rq --'-'rv '

46

4O

3O

E

20<

i .............................. i_ _TiT I .... 1016 i

o

0 200 400 600 800

[-- _ tvob]-- II.W_,l I

Figure 3

SMALL SIGNAL STABILITY TEST PROCEDURE &

DEFINITION

SmSS testing was done using the Venable Systems'

Impedance Measurement System (IMS). The IMS

system injects a swept-frequency small AC signal (either

voltage or current), and this small signal excitation and

the corresponding response signals are used to

determine the impedance of the system under test. For

system stability tests, the source impedance (Zsource)

seen by the EPCU input includes the DDCU output

impedance, the power line, and intermediate switchgear.

The load impedance (Zload) is defined as the input

impedance of the EPCU.

SmSS of the system is quantified in two ways. First, the

Nyquist criterion absolutely ensures stability if and only if

the contour of Zsource/Zload does not encircle the [-1, 0]

point. Alternatively, an ISS requirement for stability

imposed upon payloads is that the input impedance of

the payload (i.e., the EPCU) falls within the prescribed

magnitude and phase limits. The following sections

describe the application of these stability requirements to

the EPCU.

ISS REQUIRMENTS FOR SMALL SIGNAL STABILITY

The ISS payload stability requirements (SSP-57000)

mandate that the magnitude of the load impedance shall

not be lower than that of the source. If, however, the

magnitude of the input impedance of the toad is lower

than that of the source, then the system will be stable if

the phase of the load is bounded by the required limits.

NASA/TM-- 1999-209637 4
Again, the EPCU was connected to the RKD DDCU and

•the IMS was used for SmSS testing.

The ISS impedance limit requirements and the small

signal impedance of the EPCU, feeding a 107Adc load,

are shown in Figure 4a. This Bode plot indicates that for

frequencies below 300Hz, the magnitude of the EPCU

input impedance is below the required limit. However,

because its phase is within the required limits for

frequencies less than 300Hz, the ISS requirements are

met. Furthermore, the Nyquist plot (Figure 4b)

demonstrates that the distribution system is

unconditionally stable with the EPCU installed.

100 10oo 1oooo 100000

Froquon¢y [Hz]

Figure 4a

Nyquist Contour: (Z DDCIJ) f(Z EPCU)

................................................. ._. _...........................................................

/ 2Z_ _-_------ .

I _/_ _ ) \

'_ ( r

25 2 15 _ o5 e o5 "t---_5 2 25

: i

-2 i

................................................... •._:.S _-............................................................

Ral [_1

Figure 4b

STAND-ALONE STABILITY

Stand-alone stability was tested using the IMS with the

EPCU connected to the RKD DDCU, as discussed

above. The EPCU was tested under the following 28Vdc

loads: 12Adc, 27Adc, 54Adc, 81Adc, and 105Adc.

Opening the voltage feedback control loop of the EPCU,

injecting a swept-frequency small signal AC stimulus into

the voltage-control feedback loop, and then measuring

the EPCU output response determines the stand-alone

stability for each load condition. The loop gain (i.e.,

EPCU output response divided by the injected signal) of

the EPCU was calculated and displayed by the IMS as

both a Nyquist plot and Bode Plot.

The test results for the 105Adc load condition, using

Nyquist's criteria and Bode diagrams, indicate that the

EPCU is unconditionally stable, and that the EPCU has

a stand-alone "gain margin" (i.e., impedance magnitude)

of 15[dB_] and a minimum phase margin of 84[degrees].

The results for all aforementioned load conditions

indicate that the EPCU should not cause the ISS power

system to oscillate or become unstable.

LOAD SHARING

The EPCU is designed so that the active PCUs share

load demand equally (i.e., 33% each, if three PCUs are

active; 50% each, if two PCUs are active; 100% if only

one PCU is active). This equal power share at the

converter outputs results in unequal sharing of power by

the 120Vdc input busses, if an unequal number of

converters are connected to the input busses.

Consider the response of the EPCU to a slowly

increasing load with two PCUs on one bus and the other

PCU on the other bus: As the load demand increases,

unequal sharing at the input continues until an allocation

limit is reached; further increases in load are then met by

drawing the required power from the bus still operating

within its allocation limit. When both allocation limits are

equaled, further increases in load will result in prioritized

load shedding.

One dynamic response test of the load share control

circuit was done by loading the EPCU to l kWe,

connecting one PCU to Bus A, and connecting two

PCUs to Bus B. Bus A allocation was set to 460We and

bus B to 8190We. A 28Vdc-load step of lkWe was then

applied and the response was captured with an

oscilloscope. The scope traces are shown in Figure 5.

(The scope was set in AC-coupled mode at 100mV/div

to measure the bus voltage, and the scope was DCcoupled and set at 2.5A/div to measure each bus

current.)

NASA/TM-- 1999-209637 5
DSA 602A DIGITIZING SIGNAL ANALYZER

date_ 7-MAY-98 time: 1411]:57

Tel<

22.5A

-2,5q

Figure 5

As can be seen in the traces, the busses share power in

an approximately 2-to-1 fashion (i.e., 3.2Adc and

6.8Adc) prior to the allocation change. The introduction

of the load step causes each bus to attempt to

proportionally share the additional demand (i.e., the

slope of the current response for Bus B is twice that of

Bus A). However, because the allocation of Bus A is

460We, the DPS controller begins to limit the current

after 1.3reset. The peak Bus A current is 5.8Ado. In

steady state, Bus A is limited to 418We and Bus B picks

up the remainder of the load. Also, note that the POR

has an -64mV transient lasting approximately 500tJ.sec.

RPC TRIP CHARACTERIZATION

The hybrids used on both the 120Vdc RPC cards (i.e.,

A12 and A13) and 28Vdc RPC cards (i.e., A5) are the

same. The trip time of the hybrid is a v2t relationship:

that is, the higher the voltage drop across the hybrid's

FET, the faster (exponentially) the FET is tripped off. A

number of test conditions were run for the A5 cards to

demonstrate the tripping of the RPC.

Characterization of the RPC trip curve was

accomplished by turning the hybrid on into a short with

varying input voltages. Because the FET voltage drop is

typically 0.2Vdc under normally operating conditions, the

FET voltage drop progressively increases beyond this

value as the hybrid goes "deeper" into current limit.

Thus, by connecting a power supply directly to the input

of the hybrid and turning the hybrid on into a shortcircuit, the power supply voltage is equal to the FET

voltage--thereby simulating various overload conditions.

The trip curve is shown in Figure 6.

Hybrid RPC Trip Curve

1000 I

i 100 "_ '"_"_

1@ _"_ ..

11 ) i I t

10 100 1000 10000 101)000

Time [mliec]

-- Hybrid l_pon_

Figure 6

PRIORITIZED LOAD SHEDDING

PLS is an automatic means of ensuring that the most

critical loads remain serviced, in the event that the DPS

function cannot redistribute the load between the two

input busses such that the feeds remain within their

respective allocation limits. The load-shed control is a

slower loop than the load share loop, which ensures that

re-allocation of load (i.e., the DPS function) occurs first.

There are 16 levels of priority, including a "never shed"

priority. PLS was tested under a variety of initial

conditions (including one converter just entering currentlimit mode) and with various transients (e.g., loss of an

input bus).

Consider the PLS test for which the priority level of 16

4Adc channels was set at 16 successive levels, and the

allocation of the busses was adjusted such that the total

load on each was just equaled. (One PCU was

connected to one bus and two PCUs to the other). A

bulk load was then turned on. The total output current

was measured by using a specially designed testharness and the oscilloscope was set to trigger on the

falling edge of the output current. Because the DPS

controls could not redistribute the load without exceeding

both bus allocations, loads were shed. Figure 7 is an

oscilloscope-captured output-current transient response

of the load shedding function. (Because of insufficient

bulk load, the response indicates only 13 load-shed

steps.) Notice that the load shedding of 13 load levels

occurred within lsec and that the POR voltage does not

show any significant transient.

NASA/TM-- 1999-209637 6
k U

Figure 7

THERMAL-VACUUM TEST

Although the EPCU will be used in a pressurized

module, it was tested in a thermal-vacuum chamber.

(The rationale being that an experiment may be required

to continue in the event that depressurization occurs and

may not be sensed quickly enough to command off the

EPCU.) A total of 44 thermocouples were mounted on

and within the EPCU at points deemed critical to EPCU

operation.

The EPCU will be connected to the ISS Thermal

Management System (TMS) via two "quick disconnects"

on the back of the unit. Coolant flows through a

serpentine path within the base-plate of the EPCU. (The

Space Station thermal system coolant specifications are

40[Ibs./Hr] @ 65F and 200[Ibs./Hr] @ 115F.)

Thirty-two thermal data points were electronically

collected by a Molytec data acquisition system and sent

to a PC; the remaining 12 channels were collected with

an external data acquisition system and also sent to the

PC. Three of the data points are external temperature

measurements---most notably the faceplate

temperature, which is important because the "touch

temperature" is required to be below 114F.

Initial testing of the EPCU was done in open-air

conditions at various load levels (i.e., 50% and 100%

rated load levels) to obtain a baseline of performance.

Furthermore, these baseline tests were performed with

maximum cooling to establish the baseline data with low

risk to the unit. The tests were then repeated in vacuum

and the thermal rejection and operating temperatures

were measured.

The thermal-vacuum tests highlighted a number of

problems, which were corrected. These included the

PCB-to-card guide and the RPC-to-PCB interfaces,

which resulted in operating temperatures (in vacuum)

higher than acceptable. These interfaces were

corrected with the application of Cotherm, and the RPChybrid operating temperatures have been reduced by

30F--back to within acceptable levels.

ELECTRO-MAGNETIC INTERFERENCE TESTS

The Engineering Mode/ (EM) EPCU was tested

according to the International Space Station (ISS)

Electromagnetic Emissions and Susceptibility

Requirements (documents SSP 30237 rev C and

SSP30238 rev C). The tests were conducted in the

NASA GRC's EMI Laboratory. These tests involved

conducted emissions tests on the EPCU input and

output power terminals, conducted power line DC

transients, conducted susceptibility, radiated emissions,

and radiated susceptibility.

Additional tests were run from the ISS Pressurized

Payloads Interface Requirements Document (SSP

57000). These additional tests involved EPCU output

voltage ripple frequency spectrum, time domain input

voltage ripple measurements, large signal stability tests,

and common mode voltage and current susceptibility

tests.

The unit passed most EMI tests necessary for ISS flight

qualification. Those it did not pass were either corrected

or shall have waivers sought. EMI test procedures and

test results are fully documented in the Fluids and

Combustion Facility EPCU EM EMI Test Report.

VIBRATION TESTS

The EM EPCU was not originally intended to undergo

vibration tests, because many of its integrated circuit and

electronic components are mounted in sockets.

However, the Electrical Systems Development Branch

decided to run vibration tests with the EM unit to verify

the structural design of the unit, and to identify major

structural problems that would require a major redesign

of the EPCU. The vibration tests were run in NASA

GRC's Structural Dynamics Lab. Vibration tests

included 2.5 minutes of 20 to 2000 Hz random vibration

on the x, y, and z axes at a level of 4.4 g's RMS and 7.4

g's RMS. Also, there was a sine burst test of 30 cycles

at 20 Hz along the x-axis. This test was intended to

simulate impulse acceleration in the direction of the

flight. The levels for this sine burst test were 7 g's RMS

and 14.8 g's RMS for 1.5 seconds. Sine wave

characterization sweep tests from 20 to 500 Hz (2

octave/minute) at 0.25g's were run before and after each

vibration test in order to detect any abnormal resonance

that would indicate any damage to the unit during the

test.

NASA/TM-- 1999-209637 7
In addition, electrical checkout tests were run on the

EPCU after each vibration test to make sure that the unit

was still fully operational. The EPCU EM passed the

qualification level vibration tests required for space

shuttle launch and ISS flight. The vibration test profiles

and EPCU accelerometers data are fully documented in

the Fluids and Combustion Facility EPCU EM Vibration

Test Report.

CONCLUSIONS

The Engineering Model Electrical Power Control Unit is a

3kWe power interface designed to interface 28Vdc

Fluids/Combustion Facility Experiments with the

International Space Station's 120Vdc-distribution bus.

The EPCU incorporates Dynamic Power Sharing and

Prioritized Load Shedding to effectively utilize allocated

power and to ensure sustained power for critical loads

during abnormal operating conditions.

Initial testing of the EM EPCU was completed it1October

1998. The EPCU passed the electrical, thermal, EMI,

and thermal/vacuum tests, which identified only a few

correctable problems.

REFERENCES

[1] Test Report: "Fluid Combustion Facility

Electrical Power Control Unit Engineering Model: Test

and Verification". Kimnach, Greg & Lebron, Ramon

[GRC] and Fox, David [Hamilton Sundstrand

Aerospace]. June 1998.

[2] NASA Technical Memorandum: "Design and

Testing of a Breadboard Electrical Power Control Unit for

the Fluid Combustion Facility Experiment". Kimnach,

Greg & Lebron, Ramon [GRC] and Fox, David [Hamilton

Sundstrand Aerospace]. 1997.

CONTACT INFORMATION

Greg Kimnach, NASA Glenn RC at Lewis Field. 216-

433-6251. 21000 Brookpark Rd./MS 301-5, Cleveland,

OH. 44135.

Ramon C Lebron-Velilla, NASA Glenn RC at Lewis

Field. 216-433-6482. 21000 Brookpark Rd./MS 333-2,

Cleveland, OH. 44135.

James V. Soltis, NASA Glen RC at Lewis Field. 216-

433-5444. 21000 Brookpark Rd./MS 301-5, Cleveland,

OH. 44135.

NASA/TM-- 1999-209637 8

REPORT DOCUMENTATION PAGE FormApproved

OMB No. 0704-0188

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gathering and mainlaining me data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

December 1999 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Testing of the Engineering Model Electrical Power Control Unit

for the Fluids and Combustion Facility

6. AUTHOR(S)

Greg L. Kimnach. Ramon C. Lebron. and David A. Fox

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland. Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington. DC 20546-0001

WU-398-20-0C-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-11967

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--1999-209637

11, SUPPLEMENTARY NOTES

Prepared for the 34th Intersociety Energy Conversion Engineering Conference sponsored by the Society of Automotive

Engineers, Vancouver, British Columbia, Canada. August 2-6, 1997. Greg L. Kimnach and Ramon C. Lebron, NASA

Glenn Research Center: David A. Fox. Hamilton Sundstrand Aerospace, EO. Box 7002, Rockford. Illinois 61125-7002.

Responsible person, Greg L. Kimnach, organization code 5450, (216) 433-6251.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 20 Distribution: Nonstandard

This publication is available fiom the NASA Center for AeroSpace Information. (301) 621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The John H. Glenn Research Center at Lewis Field (GRC) in Cleveland, OH and the Sundsn'and Corporation in Rocld'ord. IL have designed and

developed an Engineering Model (EM) Electrical Power Control Unit (EPCU) for the Fluids Combustion Facility (FCF) experiments to be flown

on the bzternational Space Station !ISS). The EPCU will be used as the power interface to the ISS power distribution system for the FCF's space

experiments' test and telemet[3' hardware. Furthermore. it is proposed to be the common power interface for all experiments. The EPCU is a three

kilowatt 120Vdc-to-28Vdc convene]" utilizing three independent Power Com,erter Units (PCUs), each rated at lkWe (36Adc @ 28Vdc) which are

paralleled and synchronized. Each converter may be fed from one of two ISS power channels. The 28Vdc loads are connected to the EPCU output

via 48 solid-state and current-limiting switches, rated at 4Adc each. These switches may be paralleled to supply any given load up to the 108Arc

normal operational limit of the paralleled converters. The EPCU was designed in this manner to maximize allocated-power utilization, to shed

loads autonomously, to provide fault tolerance, and to provide a flexible power converter and control module to meet various ISS load demands.

Tests of the EPCU in the Power Systems Facility testbed at GRC reveal that the overall converted-power efficiency is approximately 89%--with

a nominal-input voltage of 120Vdc and a total load in the range of 40% to 110% rated 28Vdc load. (The PCUs alone have an efficiency of approximately 94.5%.) Furthermore. the EM unit passed all flight-qualification level (and beyond) vibration tests, passed ISS EMI (conducted, radiated.

and susceptibility) requirements, successfully operated for extended periods in a thermalhacuum chamber, was inte_ated with a proto-flight

experiment, and passed all stability and functional requirements. Due to paper length limitations, this Technical Memorandum only overviews

EPCU operation and tests: a more detailed test report is available, as is a Technical Memorandum explaining operational details.

14. SUBJECT TERMS

International Space Station; Spacecraft power supplies; Power modules;

Power converters; Power conditioning

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OF REPORT OF THIS PAGE

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OF ABSTRACT

Unclassified

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14

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