NASA/TM--1999-209638

Design and Testing of a Breadboard Electrical

Power Control Unit for the Fluid Combustion

Facility Experiment

Greg L. Kimnach and Ramon C. Lebron

Glenn Research Center, Cleveland, Ohio

December 1999

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NASA/TM--1999-209638

Design and Testing of a Breadboard Electrical

Power Control Unit for the Fluid Combustion

Facility Experiment

Greg L. Kimnach and Ramon C. Lebron

Glenn Research Center, Cleveland, Ohio

Prepared for the

32nd Intersociety Energy Conversion Engineering Conference

sponsored by the American Institute of Chemical Engineers

Honolulu, Hawaii, July 27--August 1, 1997

National Aeronautics and

Space Administration

Glenn Research Center

December 1999
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Price Code: A03
DESIGN and TESTING of a BREADBOARD ELECTRICAL POWER

CONTROL UNIT for the FLUID COMBUSTION FACILITY EXPERIMENT

Greg L. Kimnach and Ramon C. Lebron

NASA Glenn Research Center at Lewis Field

Electrical Systems Development Branch

21000 Brookpark Rd. MS. 301-5

Cleveland, OH 44135

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

(Fax) 216-433-8311 and 433-8410

ABSTRACT

The Fluid Combustion Facility (FCF) Project and the

Power Technology Division at the NASA Glenn

Research Center (GRC) at Lewis Field in Cleveland,

OH along with the Sundstrand Corporation in

Rockford, IL are jointly developing an Electrical Power

Converter Unit (EPCU) for the Fluid Combustion

Facility to be flown on the International Space Station

(ISS). The FCF facility experiment contains three

racks: A core rack, a combustion rack, and a fluids

rack.

The EPCU will be used as the power interface to the

ISS 120Vdc power distribution system by each FCF

experiment rack which requires 28Vac. The EPCU is a

modular design which contains three 120Vdc-to-28Vdc

full-bridge, power converters rated at lkWe each; bus

transferring input relays and solid-state, currentlimiting input switches; 48 current-limiting, solid-state,

output switches; and control and telemetry hardware.

The EPCU has all controls required to autonomously

share load demand between the power feeds and--if

absolutely necessary--shed loads. The EPCU, which

maximizes the usage of allocated ISS power and

minimizes loss of power to loads, can be paralleled

with other EPCUs.

This paper overviews the electrical design and

operating characteristics of the EPCU and presents

test data from the breadboard design.

INTRODUCTION

The FCF facility experiments will number between 10

and twelve per year over a 15 year period aboard the

ISS. Their long-term operational requirements, in

conjunction with their undefined requirements for the

power demands, mandates a flexible and readily reconfigurable power interface.

FCF experiments require uninterrupted power for long

periods of time, but the ISS is extremely power

constrained. Therefore, the EPCU module is

designed to maximize allocated-power usage. In

addition to accommodating power allocation

constraints the EPCU is designed to reduce

experiment development and integration by providing

a single, flexible power interface which is readily

adaptable to changing experimental power

requirements. Further discussion on this is provided

in Reference 1 [ Poljak and Soltis, 1997].

The EPCU concept achieves its goals by dynamically

sharing the load demand between the ISS channels

feeding the EPCU, by autonomously shedding loads,

and by providing limited, internal-fault tolerance. A

dynamic power controller parallels the LCUs such that

the ISS power busses equally share load as long as

channel allocations are not violated. If a channel

allocation violation occurs, then the LCU(s) fed by the

other ISS channel pick up the additional load. Load

shedding occurs only if power demand cannot be

dynamically reallocated to the another channel, if the

bus voltage droops, if an LCU is lost, or if the total load

exceeds the total allocation of the two ISS channels.

The EPCU has been bread-boarded and its operation

verified in the Power Systems Facility (PSF) located at

LeRC. The EPCU is connected to a distribution

system consisting of ISS proto-type hardware, which

conform to ISS electrical distribution system

requirements, developed by Rocketdyne (RKD). The

preliminary results for dynamic power sharing,

prioritized load shedding, and converter

synchronization of the EPCU operations are given in

this paper.

EPCU CONFIGURATION

The EPCU is a 3kWe module utilizing three

independent 120Vac-tO-28Vdc Load Converter Units

(LCUs) rated at lkWo each (see Figure 1). The LCUs

are paralleled and synchronized. Each can be fed

from one of two ISS power channels, allowing loads to

simultaneously draw power from two ISS channels via

dynamic load sharing of the input busses. Each LCU

is interfaced to an ISS power channel with three

paralleled RPCs (rated at 4,_c @ 120Vdc each) and a

NASA/TM--1999-209638 l
REAR

To RMSA

120 VDC

A

120 VDC

B

120 VDC

Cold Plate for

all Electronics

120 VDC @ X3

120VDC @ 4 A. X3

Priority Priority LoadShedding Bus

1553B

Redundant 1553B Bus

1553B

dead-facing relay. (This allows an LCU to get power

from one of two ISS channels without directly

paralleling the channels.)

Within the EPCU are analog controls to dynamically

parallel the LCUs and to shed loads under a variety of

conditions. The EPCU contains a 1553b interface

which allows EPCU telemetry to be uploaded (if

required) and to download ISS channel-power

allocations.

The EPCU faceplate has 12 power connectors (4

channels per connector) to which loads are connected

via load-configured cable harnesses. The cableharnesses can parallel 4Adc channels as required for

an experiment. This is possible because the EPCU

output consists of 48 solid-state, current-limiting

switches (referred to as F/exible Remote Power

Control/ers, FRPCs) rated at 4Adc each.

DYNAMIC POWER SHARING

To maintain load sharing between the two input

busses to the EPCU, the LCUs must be paralleled at a

common output bus and the power share among them

dynamically controlled. LCU paralleling is

accomplished with a Dynamic Power Share (DPS)

circuit which biases the converter sense voltages such

that power demand, during normal operation, is

shared equally between the two ISS power feeds to an

EPCU rack. (Normal operation is defined as the total

kW Converter'

1

3

4 28 VDC

5 48 Outputs

4 Circuits per

6 Connector

7

8

9

10

11

12

OVER TEMP

EPCU STATUS

RPC TRIP

LOAD SHED

EPCU Block Diagram

Figure 1

load connected to the EPCU is serviced without

interruption.) Then, as the allocation of a given

channel reaches its limit, the LCU(s) connected to the

other channel are forced to pick up the excess load

(this is referred to as dynamic power reallocation),

while the other channel is maintained at its allocation

limit. The dynamic response of the power share

controls is faster than that of the load shedding

controls. Therefore, the power share circuit can

reallocate load between channels before load

shedding occurs. The LCU topology and circuit used

to control the LCU power share are next detailed.

Dynamic Power Share Circuit

DPS results in EPCU loads being equally supplied by

the two ISS channels as long as an allocation is not

exceeded. This is accomplished as follows (see

Figure 2):

1) The difference between the total bus current

of each channel and the channel allocation is

calculated. During normal mode operation, the

difference amplifier output is negative.

2) Each of these "difference" signals is

weighted (one positively and one negatively) and

summed to produce an error signal. One channel

allocation being exceeded leads to a positive error

signal, while the other leads to a negative error signal.

Under normal and ideal conditions, the error signal is

zero.

NASA/TM--1999-209638 2
Bus A Current

,/J_ Difference A

Allocation A ,mk_ ,_j+_._..

I

Allocation B _ "

 erence B

Bus B Current

o ,0u 

Mult 2Out _1

jI

Summer _ T + Paralle,ing

Out ,Fb.-._V p:r ,E"oronc' ' J I Circuit rl....

" _JRef- Int Out

Ik=,,,,,=, Output Current .

Dynamic Power Share Block Diagram for Two Load Converters

Figure 2

3) This single error signal is then integrated,

inverted, and added to and subtracted from a

reference signal of 5V. With zero error, the output will

be 5Vdc and will result in control signals of 50/50 power

share.

4) These control signals are then routed to two

multipliers. The "Ref+int" is routed to those multipliers

which receive current data from an LCU which is fed

by a channel having negative weighting in step 2; the

"Ref-int" control signal is fed to the multipliers which

receive current data from an LCU which is fed by a

channel having a positive weighting. In other words,

the multiplier output is a product of the output current

of an LCU and the control signal derived from the

channel which does not feed the LCU.

5) The outputs of the multipliers are then fed to

the actual paralleling circuit, which averages the total

current supplied by the LCUs. The difference between

a multiplier's output and this average value is the "bias

voltage". Each bias voltage is added to the

corresponding LCU output voltage sense circuit.

Thus, the unit(s) which must supply more load receive

a lower sense voltage, thus forcing the LCU to slightly

increase its output voltage and thereby supply more

load current. The converse holds for the LCU(s)

which must decrease their share of load current.

It should be noted that, this paralleling scheme is

easily extended to n-LCUs because the biasing circuit

is connected in a star configuration, which results in an

average voltage (based upon the output current of

each LCU) at the common node. Also, the average

should include only the current of active LCUs and, for

this reason, the averaging bias resistor is enabled via

a relay which is controlled by the "ON/OFF" status bit

of the LCU controller. However, in the event that the

LCU does not shut off on an input undervoltage

condition--whether by design or failure--then that

LCU's zero current contribution is included in the

averaging. The result of this inclusion is a bus voltage

droop.

This bus voltage droop results from the paralleling

controller attempts to increase the "off" converter's

current share (by biasing its voltage sense lower) and

to decrease the "on" converters' current share (by

biasing the sense voltage higher). The increase in

sense voltage causes a decrease of the LCU output

voltage. Because the magnitude of current which the

paralleling controller attempts to reallocate to the "off"

converter affects the average value, the bus voltage

droop is a function of total load. The droop is

minimized by limiting the amount of bias to

approximately 0.7Vdc.

LCU Topolo.qy and Fault Tolerant Synchronization

Because the LCUs used in the FCF breadboard were

designed as stand-alone 120Vdc-to-28V_c converters,

they have individual output LC filters. These output

filters, in conjunction with non-synchronized operation,

lead to a oscillating current between the tanks. This

oscillating current adds to the ripple current (which will

be present in an independently operating LCU) and

resulted in an approximately 700mA peak-to-peak,

tank-to-tank ripple. (It must be stressed that this ripple

current is the ripple current associated with the

individual LCU outputs. The total bus current ripple

does not exhibit ripple of this magnitude.) Two

solutions are available to synchronize and to reduce

the ripple current of the converters.

NASA/TM--1999-209638 3
1) Implement a master clock.

2) Affect the PWM comparator by

superimposing a synchronizing signal on the individual

ramps.

Option (1) suffers because a signal line disruption

leads to one or more converters losing a clock. Option

(2) is preferable because, even if the synchronization

circuit fails, the worst case scenario is the converters

will operate independently and exhibit higher than

normal ripple current.

The synchronization of the converters over a small

band of frequencies was accomplished via a circuit

which produces three pulses, 120degrees out of

phase. Each of these pulses is superimposed on the

output-current ramp control signal of each LCU PWM

controller, respectively. Test data verifies--but is not

included within this paper-that the control ramps are

forced to reset with the superimposed synchronizing

signal. By synchronizing the converters, the peak-topeak ripple current has been reduced by

approximately 60%.

LOAD SHED CONTROL

A primary advantage of the EPCU is its ability to

maintain the total load connected to it within the limits

set forth by higher level ISS functions and downloaded

to it via the 1553b bus. Load sharing redistributes

power according to channel availability; however, in

the event that load cannot be serviced without

exceeding both allocations, load shedding is an

essential function. Load shedding occurs for one of

two reasons:

1) In the event that DPS cannot accommodate

the total load (due to the loss of an LCLI or too much

load) without exceeding both channel allocations, all

load in excess of either allocation must be shed in a

prioritized fashion. This is referred to as Prioritized

Load Shed (PLS) control.

2) In the event that an input channel has been

lost and the EPCU bus voltage has drooped

significantly, then the EPCU must shed all but the

most critical of loads. This is referred to as BusUndervoltage or Fail-Safe Load Shed Control.

Prioritized load-shed control is functionally

accomplished by comparing each channel's current

against a reference voltage corresponding to the

allocation (see upper two diode-or circuits in Figure

3). If either allocation is exceeded, then the allocation

comparator toggles high and the load shed-controller

integrator, corresponding to that channel, begins to

integrate down from its +15Vdc normal state. (Recall

that the load shed controller is slower than the

dynamic power share, therefore the integrator will

begin to integrate back up, if the excess load is

reallocated to the other channel and the allocation is

no longer exceeded.) These integrator outputs are

tied together in a diode-or configuration so that the

lower voltage (i.e., more overloaded channel) will

initiate tripping of load. This diode-or node is fed to

the inverting terminal of the load trip-control, "priority"

comparator of each RPC channel.

The load priority level is connected to the non-inverting

input of the "priority" comparator (Figure 3). The

priority level voltage is a dip-switch selectable value.

(There are 8 levels at 1.7V_c/priority level, and the

lowest priodty level has a 13.2Vdc voltage.) As a

result, under normal operating conditions, the load tripcontrol comparator is low, and toggles high when the

÷ - v I_

Load Shed Controller Pseudo-clrcult

Flgure 3

NASA/TM--1999-209638 4
allocation is exceeded for a sufficient amount of time.

The load shed controller comparator output is tied to

the input of an HP2200 opto-isolator, which is normally

low; the opto-isolator output is diode-or connected to

the output of the RPC controller trip gate.

Lastly, the loss of an LCU impacts a fully loaded

EPCU most drastically. In this case, the remaining

two load converters--regardless of allocation

setpoints--will result in a bus droop since the two

remaining converters will enter current limit mode

(foldback), while attempting to service 3kWa. Granted,

the load shed controls may shed some of the load;

however, the response time of the load shed controller

is much too slow for this condition.

As a result, bus-undervoltage load shed control is

implemented (see lowest block in Figure 4). When the

EPCU bus droops below 19.6Vdo, all but the most

output and have the load share control circuit having a

closed-loop response of approximately 50mseconds.

2) One "Flex" Remote Power Control (RPC)

unit having 20 channels rated at 4Adc each. The FLEX

has two 120Vdc input buses. Using a relay, an input

bus can be selected, via a 1553 command, to be

connected to an input RPC. Bus switching must be

done with the solid-state switch in the "off" mode: hot

switching cannot be accommodated. Using the FLEX,

each LCU is connected to one of the 120Vd¢ input

buses, and each LCU within the EPCU is fed by four

paralleled RPC Module channels.

3) Two Sundstrand "manual" RPCs having 14

channels, rated at 4Adc each, to service the EPCU

loads. The loads connected to the Manual RPC

Modules are purely resistive for these tests. The load

shed control circuit is tied to all 14 channels on both

RPCMs. The load shed controller has a time-to-first

FCF EPCU Breadboard

Load Shed

Controller

--4 14 Channel

I Manual

t RPCM

4

--I Trip Gate !

14 Channel

Manual

RPCM

LCU20 _ 16 _

17

. Allocations

Priodtized Load Shed Controller

Undervoltage Load Shed Co.t_o,er

i_ii_i i ii i ii ii i i_ii:ii_ii i iiili ii_i!ii !iiiii iiiiili_ii_i;i_il iiii

ntactors

' ! HP Power

:_ _ Supply

@ 120V

HP Power

i=_ Supply

120V

Breadboard & Test-bed Configuration

Figure 4

critical (i.e., initial position "on") loads are shed

instantly, thus allowing the bus to recover and the

EPCU to be operable. Again, this is fail-safe load

shedding.

TEST-BED CONFIGURATION and RESULTS

The EPCU breadboard configuration consists of the

following hardware and systems support hardware

(see Figure 4):

1) Two Sundstrand 120Vdc-tO-28Vdc LCUs

each rated at l kWe These are paralleled at the

shed and a shed-interval time of approximately 55milliseconds.

4) Two HP power supplies to emulate 160Vdc"

to'120Vdc converters. The HP supplies are tied

directly to the cold-table protective contactors which

are tied to the input Flex RPC through 3feet of 1/0

cable.

5) A total resistive load of 70Adc of which

approximately 20Adc is connected to the EPCU bus via

the Manual RPC Modules, while the remainder is tied

directly to the bus.

NASA/TM--1999-209638 5
Tests were done to verify the functionality of and the

required coordination between load sharing and load

shedding; to verify EPCU operation in the event of the

loss or droop of one ISS channels to the EPCU; to

investigate the causes of and means to mitigate busvoltage droop; and to verify the impacts of the LCU

synchronization circuit.

Dynamic Power Share Test Results

Steady-state performance data for the paralleling

circuit (refer to Figure 2) is given in Table 1. These

data demonstrate the bias voltage differences and the

resulting current share for equal and unequal

allocations for the two channels.

The data in Table 2 was gathered by setting the two

channel allocations to 3/_c for channel A and 45Adofor

channel B, respectively. (Note that reflecting these

values to the 28Vdc output of the LCUs results in

current limits of 12A and 174Adc, respectively.

Obviously, the channel B allocation cannot be

exceeded because the maximum current output for an

EPCU with three LCUs is approximately 105/_c.)

Then the load was increased and the EPCU allowed to

reach steady-state.

This steady-state data verifies that as the total load

increases the two LCUs share data equally (data

samples 1 and 2 in Table 2) until the allocation of the

channel feeding LCU21 is exceeded. Further

increases in total load are supplied by LCU20, and

LCU21 continues to supply only 12A_cof the total load.

Allocation

A=B A>B A<B

Differencel A -0.016 1.721 -0.011

Differencel B -0.013 0.119 1.653

Summer Out 0.01 1.629 -1.634

Integrator Out -0.02 -2.915 2.95

Ref + Int 5.18 2.47 7.92

Ref- Int 5,0 7.97 2.004

Multiplier 2 Out 4.26 3.511 2.639

Multiplier 3 Out 4.22 2.996 2.746

LCU20 Current 19.4 9.8 30.0

LCU21 Current 19.9 30.0 8.5

DPS Controller Data

Table 1

Total load

17.22

22.45

34.3

45.29

53.15

Dynamic

I LCU 20

8.78

11.42

22.45

33.2

41

I LCU 21

8.44

11.03

11.85

12.09

12.15

Power Sharing Load Data

Table 2

Figure 5 is the dynamic response of the EPCU to a

load step change from approximately 30A_c to 52,%c.

The allocation of one channel is set to 17,%c output

current, and the other to a maximum of 105/_¢ @

28Vdc. As seen, the two converters begin to share the

increased load equally; however, the dynamic power

share circuit limits LCU21 to 17Adc, and its share

begins to roll-off. LCU20 continues to pick-up the

remainder of the load. The steady-state share is

LCU21 at 17A_cand LCU20 at 35,%c.

T

f

!;. _...... ; |

II

Dynamic Response to Load Step

Figure 5

Prioritized Load Shed Controller Test Results

Figure 6 is a trace of a priorityload shed resulting from

the loss of input power to LCU21. As seen, upon the

loss of LC21, LCU20 picks up all of the load; however,

the channel allocation (feeding LCU20) is set to a

value which corresponds to an output current of

approximately 19A_c. As a result, the excess load

(20A_c) is shed. The load priority controller sheds six

priority levels in order to reverse the channel allocation

violation.

aSA

...... L.....

t'3,cA

1 ',i

Priodtized Load Shed: Loss of Input Power

Figure 6

NASA/TM--1999-209638 6
Also, because the bus undervoltage load shed

controller is adjusted to shed upon a bus voltage

below 19.6Vdc, it is seen that none of the load

shedding occurs due to a bus undervoltage condition.

This test scenario was selected to demonstrate that

the loss of an input channel does not necessarily

result in a severe bus droop and the resulting fail-safe

load shedding associated with it.

Bus Undervolta.qe Load Shed Test Results

In this test (Figure 7) a purely resistive load is

connected to the EPCU bus via the manual RPCs, and

the allocation of both channels is set to 3kWe (i.e.,

107Adc @ 28Vdc). Prior to the loss of input power to

LCU21, the LCUs share the load equally at 29.6Adc

and 28.4Adc. Upon turning off the input RPCs feeding

LCU21, the bus droops to 19.8Vdc and the bus

recovers due to fail-safe load shed control.

Limitin9 Bus Volta.qe Droop Test Results

Figure 8 summarizes the steady-state bus voltage

under various load conditions, after the loss of input

power to LCU21. The test parameters are

combinations of the input undervoltage shut-off status

(enabled or disabled) of LCU21 with the droop-limiting

diodes included or excluded in the paralleling circuit.

The impact of the undervoltage shut-off status is

evidently most pronounced at light load conditions in

the absence of droop-limiting diodes. The inclusion of

droop-limiting diodes yields the flattest EPCU bus

voltage and yields the best results with undervoltage

shut-off enabled. However the only case that impacts

loads interface design _nd system efficiency is

undervoltage shut-off disabled without droop-limiting

diodes, because it leads to a significant EPCU busvoltage droop at light loads. Thus, this is the only case

which should not be implemented. (The bus recovers

for all cases with loads greater than 55Adc due to failsafe load shed control.)

DSA _2A DIB[_IZING 51GNAL A_ALYZER

r_t i 1

Lt'|g'_ i j

i

....i,_iiI| _/,_ _,, _ 4_ t.,_ I

Bus Droop Limiting

Figure 7

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.=

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21

20

19

10 20 26 36.4 43.2 52.2 55.2 58

LoadCurrent(t=0-)

...... UVS Enabled;No diodes ..... E...o.-UVS Disabled.No diodo_

..... UVS Enabled;Droo_i _mitingDiodes .... UVS Disabled;_imit!r_

Bus Voltage as a Function of UMS and DLDs

Figure 8

NASA/TM--1999-209638 7
CONCLUSIONS

The FCF EPCU concept proves to be a reliable,

robust, and cost-effective power interface for the FCF

experiments. It provides an intelligent interface

through its ability to operate within tSS channel-power

allocation limits under all conditions. The dynamic

load sharing control ensures that the ISS channels are

dynamically balanced to minimize channel allocation

violations. And the load shedding schemes are

implemented such that loads are shed in priority order

only as necessary. As a result, this approach is very

suitable--and recommended--for all ISS facilities and

loads interfaces requiring 28Vat.

REFERENCES

Mark D. Poljak and James V. Soltis, NASA Lewis

Research Center; David A. Fox, Sundstrand

Aerospace. "Advanced Distribution, Switching, and

Conversion Technology for Fluids/Combustion Facility

Electric Power Control". Space Technology and

Applications International Forum, 1997. Part 1, pp.

269-275.

Design Implementation Document for the ISS EPCU.

Sundstrand Aerospace, Rockford IL. Document # DS10070Â4. 21 February 1997.

NASA/TM--1999-209638 8

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

4,

6.

December 1999

TITLE AND SUBTITLE

Design and Testing of a Breadboard Electrical Power Control Unit

for the Fluid Combustion Facility Experiment

AUTHOR(S)

Greg L. Kimnach and Ramon C. Lebron

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

Technical Memorandum

5. FUNDING NUMBERS

WU-398-20-0C-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-11968

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--1999-209638

11. SUPPLEMENTARY NOTES

Prepared for the 32nd Intersociety Energy Conversion Engineering Conference sponsored by the American Institute of

Chemical Engineers, Honolulu, Hawaii, July 27--August 1, 1997. 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 from the NASA Center for AeroSpace Information. (301) 621-0390.

12b, DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The. Fluid Combustion Facility (FCF) Project and the Power Technology Division at the NASA Gleml Research Center

(GRC) at Lewis Field in Cleveland, OH along with the Sundstrand Corporation in Rockford, IL are jointly developing an

Electrical Power Com,erter Unit (EPCU) for the Fluid Combustion Facility to be flown on the h_ternational Space

Statiolz (ISS). The FCF facility experiment contains three racks: A core rack, a combustion rack, and a fluids rack. The

EPCU will be used as the power interface to the ISS 120Vdc power distribution system by each FCF experiment rack

which requires 28Vdc. The EPCU is a modular design which contains three 120Vdc-tO-28Vdc full-bridge, power converters rated at 1kW e each: bus transferring input relays and solid-state, current-limiting input switches: 48 current-limiting,

solid-state, output switches: and control and telemetry hardware. The EPCU has all controls required to autonomously

share load demand between the power feeds and--if absolutely necessary--shed loads. The EPCU. which maximizes the

usage of allocated ISS power and minimizes loss of power to loads, can be paralleled with other EPCUs. This paper

overviews the electrical design and operating characteristics of the EPCU and presents test data from the breadboard

design.

14. SUBJECT TERMS

International Space Station; Spacecraft power supplies: Power modules;

Power converters; Power conditioning

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION

OF REPORT OF THIS PAGE

Unclassified Unclassified

NSN 7540-01-280-5500

19. SECURITY CLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

14

16. PRICE CODE

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Stci. Z39-1 8

298-102