NASA Technical Memorandum 102577

_:_........ Power Conditioning Techniques

E.D. Baumann

Lewis Research Center

Cleveland, Ohio

Prepared for the

- _ " Neutral Particle Beam Technology Symposmm i _ ' _ _:

sponsored by Strategic Defense Initiative Organization

'_ _ Monterey, California, July 18-21, 1989 .......

("JAS A-TM-1 0 9D 7 7 ) pf3w_R CQNnITInNING

TECHNI_IJt_S (NASA) 7 p CSCL O-)C

N90-20301 _-=

unclas
---- :_ ZJ

-4p
UNCLASSIFIED

POWER CONDITIONING TECHNIQUES (U)

E. D. Baumann

NASA Lewis Research Center

Cleveland, Ohio 44135

oo

o

t.o

I

(U) Power level requirements for aerospace missions continue to increase (figure 1).

Historically, aircraft have dominated their spacecraft counterparts in the need for electric

power by an order of magnitude. For example, the power systems on board the Boeing 747

and DC-10 are rated for a few hundred kilowatts. The Space Shuttle power system, on the

other hand, provides just a few tens of kilowatts. Currently, the Space Station Freedom

baseline design calls for a modest 75 kW power system; a factor of two smaller than the

jumbo airliners designed 20 years ago. Growth Station, projected for operation in 2010, will

provide 150 kW to its users. When completed, spacecraft power levels will finally approach

the size of today's aircraft power systems. Future missions, such as the National Aero-Space

Plane (NASP), a Manned Lunar Base, and a Manned Mission to Mars are each expected

to require about one megawatt. To date, the largest space type power system deployed is

aboard the Space Shuttle and is rated for only 25 kW. Figure 1 also shows that the electric

power requirements for SDIO missions, which include the Neutral Particle Beam, will

exceed one megawatt with projections to 10 MW already envisioned.

(U) State-of-the-art power electronics technology is best demonstrated by considering the

Space Station Freedom's 75 kW power system. Solar arrays provide DC power to a bank

of inverters operating at 20 kHz which, in turn, supply regulated 440 VAC to the

distribution bus. Each inverter module has a specific weight of 3.7 kg/kW and provides

12.5 kW. Since each module contains 8 active switches, each switch handles about 1600 W.

A 1984 NASA contract with Hughes resulted in a resonant converter near 2 kg/kW

supplying 25 kW at 1000 VDC from a 250-350 VDC input. It required only four D7ST

switching transistors for the active switches, each handling just over 6 kW. Scaled to one

megawatt, the Space Station design would require 640 switches and 80 modules, whereas

the Hughes design would require 160 switches and 40 modules. The major differences

between the two designs are topology and reliability. A conservative design approach is used

for the Space Station due to the nature of the mission; while the goal of the Hughes design

was to demonstrate the maximum power handling capability of state-of-the-art switches.

(U) If a 1 MW DC-to-DC converter were built using the Hughes 2 kg/kW technology, it

would weigh two metric tons, roughly the equivalent of 26, 170-1b people. Assuming launch

costs of $10,000/kg, the price of launching the converters alone into low earth orbit would

exceed $20 million.

(U) A key to reducing power system weight is to understand the Weight distribution among

the various components. In most converters, the weight is shared among the capacitors,

inductors, switching devices, control circuitry, and supporting structure. Capacitors, for

example, are used as components in filters, resonant tanks, switch snubbers, and many other

applications. Inductors are used as filters, current limiters, components in resonant tanks,

and, when mutually coupled, as transformers. Transformers provide step-up, step-down, and

UNCLASSIFIED
UNCLASSIFIED

isolation for AC voltages. Switching devices can be fully controllable (eg. BJT, FET, GTO,

and SIT), partially controllable (eg. SCR and LASS), or uncontrollable (eg. rectifiers and

flyback diodes). The switching device structure may be P-N junction, field-effect, or Schottky

Barrier which give certain operating characteristics to the device. Finally, the supporting

structure holds everything together, provides cooling, and possibly acts to furnish paths for

electrical conduction. Since circuit topology limits the choice of components and vice versa,

understanding the complex relationships between circuit operation and component selection

will result in megawatt level power converters with very light weight.

(U) In a paper study performed at the NASA Lewis Research Center on a state-of-theart 2 kg/kW, 500 kW, 20 kHz resonant converter, the weight contribution from each

component type was determined. The results show that the weight distribution in the power

circuit components is 63% capacitors, 30% inductors, and 7% switches. This seems to

indicate a need for technology development in capacitors and, to a lesser extent, inductors

to realize significant weight reductions. However, the circuit studied is a resonant circuit.

This type of topology tends to have higher capacitor weight due to the resonant capacitor.

From this example it can be determined that advances in both component technology and

circuit topology are the key to reducing the specific weight of space type power systems.

(U) Since circuit topology is one factor affecting component weight distribution in a

converter, the NASA Lewis Research Center has established programs with the University

of Wisconsin and the University of Toledo to study converter circuit topologies. The goal

of these studies is to survey the operational characteristics of a wide range of converter

topologies. Among the characteristics under consideration are: efficiency, switch utilization,

fault energy handling, component types required, and specific weight. From the survey

results, promising circuit topologies are chosen for design, build-up and characterization.

The circuits typically operate at a few kilowatts and consideration is given to potential

difficulties in scaling the units to higher power levels. The work indicates that no single

topology will meet every requirement for all space-based power systems; rather, the "right"

converter topology is application specific.

(U) Power circuit topologies under consideration range form hard-switched to resonant with

quasi-resonant types falling in between. A hard-switched converter chops DC directly into

a transformer. It may not require capacitors provided that the switches can handle the

voltage transients caused by the transformer leakage inductance. Fully controllable switches

are mandatory in a hard-switch converter if one wishes to avoid the extra weight associated

with forced commutation circuitry. Resonant converters require capacitors in their tank

circuits but provide natural commutation to the switches. Circulating current is the major

drawback in resonant topologies because the circuit Q forces the switches to handle more

power than is being supplied to the load.

(U) Switch utilization, which is defined as the converter output power divided by the total

volt-ampere product of the switches, represents how well a circuit topology uses the

capability of its switches to achieve the output power. Resonant circuits have low switch

utilization due to high circulating currents. Hard-switched converters can have high switch

utilization provided that the switching voltage transients, due to transformer leakage, can

be snubbed; but snubbering leads to the inclusion of capacitors in the circuit.

(U) Operating a converter at a fixed frequency allows filters to be tuned; thus eliminating

the need to design wide band filters and further reducing weight. Yet, tuned resonant filters

UNCLASSIFIED
UNCLASSIFIED

require inductors and capacitors which are the heaviest of all the components to choose

from. Some topologies require less output filtering but at the expense of additional input

filtering. One consideration would be to match the filtering requirements to the load and

the source.

(U) Source and load limitations impose the boundary conditions within which the power

management and distribution (PMAD) system must function. An electric power source

provides some output voltage range, output impedance, and other restrictions that must be

matched by the PMAD system to a load having a certain voltage range, ripple voltage,

and other power requirements. If power is supplied by a 100 VDC source, it is likely that

switches having very low forward voltage drop will be used in the converter. Every 1-volt

drop in a 100 VDC system results in decreasing efficiency by 1%. In full bridge

configurations for DC-to-DC conversion, two 1-volt switch drops in the chopper and two

more in the output rectifier translates to a 96% efficiency and this is before losses

contributed by other components and cables are taken into account. Another consideration

is radiation effects on the switches. Nuclear reactor powered systems, which use

thermoelectric or thermionic devices, provide high power at low voltage. Efficient

transmission of this power to the user requires converters, located near the reactor, to step

up the voltage for transmission. We have a program at the Lewis Research Center where

radiation effects on BJT's, FETs, and other switches are being studied.

(U) Finally, fault tolerance ensures that the power management and distribution system will

be able to withstand load shorts or arcs. Ideally, stored energy within the power system

should be low to protect the load from catastrophic failure by preventing burnouts. In a

neutral particle beam system this amount to only a few tens of joules.

(U) If we can develop the technology to build a 0.1 kg/kW converter operating at a

megawatt, it would weigh 0.1 metric tons, about the weight of a 220-1b man. Launch costs

would decrease dramatically from $20 million, for the 2 kg/kW converter, to $100K. The

cost savings represent an order of magnitude decrease in converter specific weight and an

order of magnitude increase in power level over state-of-the-art systems.

(U) The approach will require new circuit topologies, the development of very light weight

capacitors and inductors, and new switching devices capable of handling large currents with

low forward voltage drop. Potential circuit topologies under consideration are resonant, softswitched, square wave, voltage multiplication or some combination. The NASA Lewis

Research Center has in-house and university grant work already underway in these areas.

Light weight capacitors are being studied in-house and at the State University of New York

at Buffalo. The MOS Controlled Thyristor (MCT), a new controllable switch with low

forward voltage drop, is under development at GE.

(U) Ultra-lightweight converter technology can meet the needs of many users. The Strategic

Defense Initiative Office is planning on orbiting platforms for systems such as the Neutral

Particle Beam and Free Electron Laser systems. The Air Force and commercial aircraft

industry could replace heavy and leak prone hydraulic systems with electric control surface

actuator systems to reduce weight and minimize down time. Two weeks time is spent on the

Space Shuttle in preparation for its next launch due to checking and qualifying its hydraulic

actuator systems. NASA needs lightweight converter technology for a Manned Lunar Base,

Space Based Manufacturing, interplanetary travel, and a Manned Mission to Mars. The

Army could have large mobile power systems capable of powering electromagnetic

UNCLASSIFIED
UNCLASSIFIED

launchers. Finally, the Navy could increase the payload on board its ships by decreasing the

weight of its electric power systems.

(U) The goal of a joint development program between NASA, the Air Force, and the

Strategic Defense Initiative Office, is to build a 1 MW DC-to-DC converter with a specific

weight of 0.1 kg/kW for a total converter weight of 100 kg. The output voltage will be

100 kV DC developed from an input voltage of 500-5000 VDC. The system will be fault

tolerant and store only a few tens of joules to prevent burnouts during fault conditions. We

project that the system will be completed in a 3-5 year time span with a demonstrable

1 MW hardware as an output. NASA is currently in the procurement stage through a

NASA Research Announcement (NRA).

(U) In conclusion, a 1 MW, 0.1 kg/kW 100 kV DC-DC converter seems to be attainable

in 3 to 5 years. Circuit topology and light weight component technology are key factors in

achieving ultra-lightweight, high power converters. The goal of this work is to demonstrate

a 0.1 kg/kW converter operating at the one megawatt power level.

ACKNOWLEDGEMENTS (U)

This work is being supported by the Strategic Defense Initiative Office through the U.S. Air

Force under funding document MIPR FY1455-89-N0655, June 1989.

REFERENCES (U)

1. M. H. Kheraluwala, "High Power Density DC/DC Converter - Selection of Converter

Topology," Interim Report, NASA Grant No. NAG3-804, June 1988.

2. R. R. Robson, "25 kW Resonant dc/dc Power Converter," Final Report, NASA Contract

No. NAS3-23159, September 1983.

qlF

UNCLASSIFIED

4
UNCLASSIFIED

10,ooo, POWER LEVEL TRENDS

FOR

AEROSPACE MISSIONS

Q0

II-

,I[

3:

o

!

n.

uJ

j:

o

G.

10OO

100

10

1870

ELECTRIC

1880 1980 2000 2010

INITIAL OPERATING CONDITIONS

2020

MANNED

2030

FIGURE 1.

UNCLASSIFIED

5
Naliona[ Aeronautics and Report Documentation Page

Space Administralion

1. Report No.

NASA TM-102577

4. Title and Subtitle

Power Conditioning Techniques

7, Author(s)

E.D. Baumann

2. Government Accession No,

9. Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

3. Recipient's Catalog No.

5. Report Date

6. Performing Organization Code

8. Performing Organization Report No.

E-5408

10. Work Unit No.

506-41-3F

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14, Sponsoring Agency Code

15. Supplementary Notes

Prepared for the Neutral Particle Beam Technology Symposium sponsored by Strategic Defense Initiative

Organization, Monterey, California, July 18-21, 1989. This research is supported by the Strategic Defense

Initiative Office through the U.S. Air Force under funding document MIPR FY1455-89-NO655, June 1989.

16. Abstract

This paper discusses the technological developments required to reduce the electrical power system component

weights from the state-of-the-art 2.0 kg/kW to the range of 0.1-0.2 kg/kW.-Power level requirements and their

trends in aerospace applications are identified and presented. The projected weight and launch costs for a IMW

power converter built using state-of-the-art technology are established to illustrate the need for reliable, ultralightweight advanced power components. The key factors affecting converter weight are given and some of the

tradeoffs between component ratings and circuit topology are identified. The weight and launch costs for a IMW

converter using 0.1 kg/kW technology are presented. Finally, the objectives and goals of the Multi-Megawatt

Program at the NASA Lewis Research Center, which is funded by the SDIO through the Air Force, are given.

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

Megawatt

Power systems

Lightweight

18, Distribution Statement

Unclassified- Unlimited

Subject Category 33

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No, of pages 22, Price*

Unclassified Unclassified 7 A02

NASAFORM1626OCT86 *For sale by the National Technical Information Service, Springfield, Virginia 22161