High Voltage Space Solar Arrays

9 Pages·2002·0.7 MB·English

by  Ferguson, D. C.

IAC-024AA.6 .3.03
High Voltage Space Solar Arrays
D.C. Ferguson, G.B HiIIard, B.V. Vayner, and J.T. Galofaro
NASA Glenn
Cleveland, OH, USA
53rd International Astronautical Congress
-
The World Space Congress 2002
10-19 Oct 2002/Houston, Texas
For permission to copy or republish, contact the International Astronautical Federation
D..~M .,.;~AI;L:- 7rnir D-.;, Cr-nra
HIGH VOLTAGE SPACE SOLAR ARRAYS
D.C. Ferguson, G.B. Hillard, B.V. Vayner, and J.T.Galofaro
NASA Glenn Research Center
Cleveland, Ohio USA
dale.c .ferwson@,grc.nasa.g ov
ABSTRACT propulsion uses voltages from about 300 V
(Hall effect thrusters) to about 1000 V (ion
Recent tests performed at the NASA Glenn thrusters). For low voltage power systems,
Research Center and elsewhere have shown conversion of substantial power to high
promise in the design and construction of voltages is required for these spacecraft
high voltage (300-1000 V) solar arrays for fimctions to operate. The weight of the
space applications. Prehminary results and power conversion systems (PMAD) can be a
implications for solar array design will be substantial fraction of the total power system
discussed, with application to direct-drive weight in these cases. It is more efficient,
electric propulsion and space solar power. and can save weight, if the high voltage
functions can be directly powered fiom a
INTRODUCTION high voltage solar array, for instance.’ If the
high voltage hction is electric propulsion,
High voltage systems are used in space in we call such a system a direct-drive electric
order to save launch weight. First of all, for propulsion system. Systems have been
the same power level, higher voltages enable proposed that switch between parallel and
use of thinner wires (lighter cabling). This series combinations of different strings of
is because P = Iv, and V = IR,s o P =12R If solar cells in order to facilitate occasional
I is decreased by use of higher V, then the high voltage requirements, but to enable
wire resistance can be increased, that is, housekeeping fbnctions at a lower voltage.
thinner wires may be used, with no increase
in power loss due to cabling. In the case of Solar arrays have proven to be the major
ISS, the decision to use a 160 V primary source of reliable long-term electric power
power system was based on the decreased for both manned and unmanned orbital
cable mass possible. Of course, if one uses spacecraft. In the early days of spaceflight,
the same cable mass, higher voltages will a few satellites used energy sources other
enable higher efficiencies, as less power will than solar arrays and batteries, such as he1
be lost to resistance in the cables. For very cells or thermo-electric prime power
large power systems, the decrease in cable sources. By 1970, because of extended
mass can be substantial. mission times as well as increased power
requirements, the majority of spacecraft
Secondly, some spacecraft functions require primary power systems used solar arrays and
high voltages. For example, electric rechargeable batteries to supply the required
28 V. The choice of 28 volts for the main
Copyright Q 2002 by the American Institute of Aeronautics and
bus voltage was made to take advantage of
Astronautics, Inc. No copyright is asserted in the United States
under Title 17, U.S.Code. The U.S. Government has a royalty-Bee long-existing standards and practices within
license to exercise all rights under the copyright claimed herein for
the aircraft industry.
Government purposes. AI1 other right are reserved by the
copyright owner.
This is a preprint or reprint of a paper intended for presentation at a
conference. Because changes may be made before formal
publication, this is made available with the understanding that it will
1
not be cited or reproduced without the permission of the author.
Plasma interactions at 28 V have not been Arcs have been observed at relatively low
generally considered a degradation factor of potentials (as low as -75 V) when conductor
consequence. The only noted exceptions to surfaces are biased negative near insulator
their benign nature have occurred under surfaces in the presence of a plasma. Arc
extreme environmental conditions, rate is strongly dependent on plasma density
especiaiiy during geomagnetic substonns for and on coversiide temperature, which affects
spacecraft operating at high inclinations. the surface conductivity. It may range from
For low inclination spacecraft, those that intermittent (on a scale of minutes and
completely avoid the auroral oval, 28-volt perhaps hours or longer), to several per
systems have not been observed to arc. second. Arc currents observed in ground
tests are on the order of an ampere and may
As the power requirements for spacecraft last several microseconds. These
increased, however, high voltage solar characteristics depend on the capacitance to
arrays were baselined to minimize total mass space, increasing with increasing
and increase power production efficiency. capacitance. These arcs are usually
With the advent of 100 V systems in the late associated with solar cell array
1980’s, arcing began to be observed on a interconnects, but have also been observed
number of spacecraft solar arrays, damaging on biased conductor surfaces covered with
or disabling those spacecraft. For example, dielectric strips. They are likely to be of
see Hoeber et al, 1998. concern whenever conducting surfaces at
negative potentials with respect to plasma
abut insulating surfaces.
SOLAR ARRAY ARCING
Vayner et al (2002) and Galofaro et al
As one might expect, the initiation of a solar (2002) have shown that arcs at low voltages
array arc depends on the presence of a (- -300 V) are always associated with
strong local electric field. Frequently the contamination layers on conductors abutting
source is an exposed interconnect which, insulators and exposed to the space plasma.
depending on its location in the string, can
be at high potential.
THE SUSTAINED ARC
Most problematic are arcs that initiate at
triple points. A triple point is a point in
When the arc circuit includes the solar
space where insulator, conductor, and
arrays or other source of power, it rimy be
plasma all meet. For a solar cell operating in
possible for solar array arcs to become
LEO this is usually the solar cell
continuous (or sustained). Such continuous
interconnect, but it can also be the edge of
arcs, fed by the solar arrays, have an
the solar cell (near the substrate or the
essentially inexhaustible source of energy,
coverslide). It has been shown that arcing
and can lead to catastrophic damage. This
on solar arrays at voltages less than about
hypothesis for the loss of solar array strings
minus 1000 V is always intermediated by
on the SSLoral satellites PAS-6 and Tempo
the presence of a plasma. Identical samples
I1 was confirmed by ground-tests done by
to those that arced at -100 V in a plasma
Snyder et al (2000). Later testing on the
have been shown to withstand -1000 V bias
EOS-AM1 arrays showed that continuous
in a pure vacuum.
solar array arcs could occur in a LEO
environment at a string voltage as low as
2
minus 100 to -120 V. The scenario for the An arc that lasts long enough will locally
catastrophic loss is given in Ferguson et a1 heat the substrate and release gases. In the
(1 999), and is summarized here: case of a kapton substrate, the kapton chars,
but the char is also a good conductor, and
. First of all, an ordinary solar array arc must provides a path for the arc to continue.
get started. min. is will usuaiiy be at a tripie- Snyder et al(2000) have shown that the heat
point as described above. In the case of the generated in continuous arcs on kapton is
SSLoral arrays, the differential voltage sufficient to produce the kapton charring
between solar array and plasma could have measured after the event.
been as low as 100 V, for the SSLoral
arrays were using thin coverslides similar to
the APSA cells, which arced at voltages as
low as -75 V on orbit (PASP Plus results,
Soldi and Hastings, 1997).
When the initial arc (sometimes called the
trigger arc) is generated, it discharges only
the local capacitance, but the arc plasma
expands out from the arc site and comes in
contact with an exposed conductor at a very
different voltage. In the case of the
Array Test Sample.
SSLoral arrays, the most positive end of the
array strings was less than a millimeter away
In any event, a continuous arc can destroy a
fiom the negative end. Now the arc plasma
whole string (if the arc is between traces on
makes direct contact with the other
the same string) or adjacent strings (if the
conductor and makes for an almost dead
arc is between strings) or the entire array
short to that spot. The arc current has
power (if the arc is between combined
changed fiom one that is discharging
power traces). The possibility of losing the
capacitance to a current between two ends of
entire array power on the Deep Space-One
the solar array string.
mission caused the builders to remove a
solar panel that had already been installed to
If the current available to the arc site fiom
modifl it and its sister array to prevent
the functioning arrays is greater than a
continuous arcing. Its power traces were
certain threshold value and the voltage
only a few millimeters apart, and were
between strings is above a certain value, the
exposed htht o the plasma and to each other
arc will become continuous. In ground tests
before the modifications were made.
these arcs continued until the source of
Afterwards, insulating material prevented
power was artificially turned off. In space,
arc plasma fiom shorting out between the
presumable the arc would continue until the
power traces.
exposed conductors were melted through
and the circuit was thereby interrupted. This
NEW DATA ON VOLTAGE AND
could take seconds or minutes. Ground tests
CURRENT THRESHOLDS FOR
have shown that an arc that persists for more
SUSTAINED SOLAR ARRAY ARCING
than a few hundred microseconds will not
shut off by itself.
Of great interest to solar array designers are
techniques to prevent continuous (sustained)
3
arcing fiom occurring on space solar arrays. xenon gas. The array panels were 3x12
They may be prevented by preventing the solar cells, and the cells were 4x6 silicon
initial (the so-called "trigger") arc fiom cells on a KaptonB substrate. Adjacent
occurring, or by preventing the secondary strings were separated by 0.8 mm. The
plasma arc to adjacent strings fiom strings were biased to - 400 to 450 volts,
occurring. and the voltage difference between strings
was increased until the arcs produced
Previous work at NASA GRC has shown became sustained arcs. The lowest
that for each solar array design there is a sustained arc thresholds were obtained for
voltage threshold for the trigger arc solar cells for which the coverglass
(Ferguson, 1986), and voltage and current thickness was 300 microns (6 mils) and the
thresholds for the sustained arcs (Hoeber et coverglasses did not overhang the cell
al, 1998). Trigger arc thresholds vary fiom edges. For thinner (150 micron)
about - 100 to about -250 volts, depending on coverglasses with no overhang, the
solar array design parameters, such as the thresholds found were 60 V and 2 amps.
thickness of the solar cell coverglasses and Coverglasses of 150 micron thickness and
the amount of coverglass overhang beyond 250 micron (10 mil) overhang had
the cell edges. Sustained arc voltage thresholds of 80 V and 1.6 amp.
thresholds can be less than the trigger arc
thresholds. For instance, if two adjacent These new sustained arcing thresholds will
solar array string have a difference in cell allow array designers to prevent sustained
voltage of 60 volts, but either is operating at arcing that could damage or destroy theit
-250 volts, a trigger arc can occur which will satellite arrays in the harsh space
evolve into a sustained arc between strings. environment.
Among the techniques used to prevent
sustained arc discharges are preventing the PREVENTING TRIGGER ARCS
may voltage fiom getting above the trigger
arc threshold, lowering the voltage between Recent work by Vayner et a1 (2001, 2002)
adjacent strings to a value below its and Galofaro et al (2002) has shown that
threshold and lowering the current that can trigger arcs are initiated by contaminant
get to the trigger-arc site below its threshold. layers on conductors adjacent to insulators
If any of these values is less than the in contact with the space plasma. If
associated threshold, a sustained arc can't thoroughly baked out in a vacuum-plasma
get started. Until the present work, the chamber, samples which would otherwise
lowest known sustained arc voltage arc at less than -200 volts did not arc at
threshold was about 55 V. The present work voltages well in excess of -500 V. In
showed that sustained arcs can occur at addition, spectra of the trigger arcs showed
voltages as low as 40 V and currents as low evidence of water contamination in the
as 1 amp. hydroxyl and BaImer line spectra of the arcs.
This would suggest that a thorough bakeout
Several solar array samples were tested in a of space solar arrays would increase their
simulated plasma in a vacuum-plasma arc thresholds. However, condensable
chamber in the Plasma nteractions Facility outgassing products may make that goal
(€'IF') at NASA GRC. The plasma was unachievable on-orbit. For instance, in
produced by a Penning type source, using LEO, the cold temperatures on the night side
4
of the orbit may condense out water and voltages in a plasma environment. The
other outgassing products on the arrays. As Entech Stretched Lens Array (SLA) design
an example, arc rates on the LEO satellite uses concentrating lenses to focus sunlight
PASP Plus arrays did not change much even onto solar cell strings, and the edges of the
after a full year. Even GEO satellites solar cells are encapsulated to prevent
undergo eciipses, which may ”,, reset” the piasma contact. in separate tests at Auburn
arcing properties of the arrays. University (O’Neill et al, 2001) and
independently at NASA GRC (Ferguson et
A better technique to prevent arcing may be al, 2001), these cells did not arc at voltages
encapsulation of the arrays, such that the as high as -1 100 V even though damaged by
space plasma is excluded fkom contact with simulated micrometeoroid impacts.
conductor-insulator junctions. This was the
tactic employed on the Solar Tiles &om
Boeing Phantomworks. As discussed in
Reed et al (2001), arrays covered with a
single large coverglass to prevent plasma
contact resisted arcing even at voltages as
high as - 11 00 V in GRC testing.
Entech SLA encapsulated cell sample.
THE HAZARDS OF ENCAPSULATION
One must be carefhl with the use of
encapsulants, however, when the possibility
exists of outgassing in the presence of high
voltage components. For instance, on
SAMPIE, one of the high voltage power
Boeing Phantomworks Solar Tile.
supplies was destroyed by a Paschen
discharge that occurred on a high voltage
component where the encapsulant had
In this case, an initial tile’s coverglass
delaminated and a neutral pressure was
cracked due to entrained air when placed
enclosed with the high voltage component
under a vacuum. Even the cracked tile
(Ferguson and Hillard, 1997).
resisted arcing at voltages in excess of -750
V. A new tile produced with greater care to
When encapsulating arrays or cells, several
preventing entrained air, while still sealing
caveats must not be ignored. First, no air
the tile fkom the plasma, succeeded in
must be entrained anywhere. While this may
withstanding voltages up to -1 100 V in a
seem obvious, at least one set of
simulated space plasma without arcing.
encapsulated test arrays sent to the Glenn
Research Center had sufficient air entrained
In addition, a solar cell design encapsulating
that the coating delaminated and swelled
the edges of the solar cells has survived high
5
under vacuum. In fact, so much air was VENTED ENCAPSULANTS
entrained that the test articles under vacuum
appeared to swell up like plastic balloons. In To avoid plasma interactions, care must be
cases where only a very small amount of air taken that plasma does not enter the
is trapped, visible effects may not occur, yet encapsulant -and -.r eact with exposed
the trapped air wiil present the danger of conductors inside. I he key requirement on
Paschen breakdown under high voltage. such system is that all openings must be
smaller than the plasma Debye length,
Second, the encapsulant thickness must be which depends on the plasma density and
sufficient to withstand dielectric breakdown temperature. One can readily estimate the
at the highest array voltage. For thin-film maximum opening consistent with such a
arrays, this consideration can contribute requirement.
signiscantly to the array mass. It is
important that thin-film encapsulants be The plasma will be capable of maintaining
tested under voltage in a plasma electric fields over a distance of
environment, rather than relying solely on approximately one Debye length hD, which
published dielectric strengths.
is given by
Third, the encapsulant must not be able to
h, = (kT$4nne2)’” = 7.43 x lo2 (TJn)li2
peel away fi-om high voltage components, or
Paschen breakdown can occur due to
where T, is the electron temperature in eV, k
entrained outgassing products that may
reach sufficiently high neutral pressures. is the Boltzmann constant, n = 3.14159 ..., e
For a wide range of pressure distance is the charge of the electron, and n is the
combinations, the Paschen minima are electron density in ~ m - ~ . Placing
typically around a few hundred volts for representative values fiom International
common gases. Most outgassing products Reference Ionosphere (rru-86) simulations
have not had their Paschen curves measured. in the above equation, one hds a minimum
In the case of solar arrays, a coverglass that Debye length fiom 0.12 cm at 1100 K to
covers many cells must also make 0.17 cm at 2300 K.
allowances for escape of outgassing
products fi-om adhesives, as well. It must be Openings in an encapsulated array may have
treated for all intents and purposes as a smaller dimensions than this minimum and
vented encapsulant (see below). still be able to prohibit plasma interactions
with the experiment electronics. Larger
Fourth, the encapsulant must be able to openings may be used if covered with an
withstand other aspects of the space electrically connected conductive wire mesh
environment for its design lifetime. Atomic of spacing less than the minimum Debye
oxygen, micrometeoroids and debris, and length. To provide a reasonable margin of
W and X-ray exposure are some of the safety, a general guideline is that no opening
threats to the encapsulant. Glass stands up should exceed 0.10 cm in its largest
well to all of these environments. Some dimension. (Ferguson and Hillard, 2002).
plastics do not.
In the case of the Entech solar cells which
had been punctured by simulated
micrometeoroids, the small size of the
cracks and holes (much smaller than the
6
Debye length) prevented arcing even though orbit - From LEO to GEO,” AIM Paper
they were gas permeable by nevertheless #99-02 18, 3 7 A~er ospace Sciences Meeting
preventing plasma contact with the cell and Exhibit, Reno, NV, Jan. 11- 14.
conductors.
Ferguson, D.C., Vayner, B.V., and Galofaro,
CONCLUSIONS J.T., 2001. Private communication.
,
Significant progress has recently been made Galofaro, J., Vayner, B., Degroot, W., and
in the design and construction of space solar Ferguson, D., 2002. “The Role of Water
arrays which resist trigger arcs to -1000 V Vapor and Dissociative Recombination
and can resist sustained arcing. Ground tests Processes in Solar Array Arc Initiation,”
of the array designs allow engineers to NASA/TM-2002-211328, 40th Aerospace
confidently prevent solar array arcing on Sciences Meeting and Exhibit, Reno, NV,
orbit. Jan. 14-17.
This is of greatest importance for space Hoeber, C.F., Robertson, E.A., Katz, I.,
power systems that require high voltages, Davis, V.A., and Snyder, D.B., 1998. “Solar
such as direct-drive electric propulsion (300- array augmented electrostatic discharge in
1000 V) and the Space Solar Power project GEO,” AIAA Paper #98-1401, International
(eventually 20 kV or more). Communications Satellite Systems
Conference and Exdibit, 17th Yokohama,
@Kapton is a registered trademark of Japan, Feb.
Dupont, Inc. 23-27.
REFERENCES O’Neill, M.J., Piszczor, M.F., Eskenazi,
M.I., McDanal, A.J., George, P.J., Botke,
Ferguson, D.C., 1986. “The voltage M.M., Brandhorst, H.W., Edwards, D.L.,
threshold for arcing for solar cells in LEO: and Jaster, P.A., 2001, “The Stretched Lens
Flight and ground test results,” NASA-TM- Array (SLA): An Ultra-Light Photovoltaic
87259, 24th AIM Aerospace Sciences Concentrator,” 17* Space Photovoltaic
Meeting, Reno, NV,J an. 6-9. Research and Technology Conference, Ohio
-
Aerospace Institute, September 1 1 13.
Ferguson, D.C. and Hillard, G.B., 1997.
“Lessons for Space Power System Design Reed, B.J., Harden, D.E., Ferguson, D.C.,
fiom the SAMPIE Flight Experiment,” and Snyder, D.B., 2001. “Boeing’s High
AIAA Paper # 97-0087, 35* Aerospace Voltage Solar Tile Test Results,” 17* Space
Sciences Meeting and Exhibit, Reno, NV, Photovoltaic Research and Technology
Jan~ary6 -10. Conference, Ohio Aerospace Institute,
September 11 - 13.
Ferguson, D.C., and Hillard, G.B., 2002.
“Low Earth Orbit Spacecraft Charging Soldi, J;D., and Hastings, D.E., 1997. “Arc
Design Guidelines,” NASA SEE program, Rate Predictions and Flight Data Analysis
in publication. for the PASP Plus Experiment,” in the
Photovoltaic Array Space Power Plus
Ferguson, D.C., Snyder, D.B., Vayner, B.V. Diagnostics (PASP Plus) Experiment Final
and Galofaro J.T., 1999. “Array arcing in Report, PL-TR-97- 101 3.
7
Snyder, D.B., Ferguson, D.C., Vayner, B.V.,
and Galofaro, J.T., 2000. ‘‘New Spacecraft-
Charging Solar Array Failure Mechanism,”
6* Spacecraft Charging Technology
Coiiikrelice, MIci-‘v’3-TX-2OOOi 578, pp.
297-301.
Vayner, B., Galofaro, J., and Ferguson, D.,
2001. “Arc Inception Mechanism on a Solar
Array Immersed in a Low-Density Plasma,”
NASNTM-200 1-2 1 1070, Seventh
Spacecraft charging Technology
Conference, Noordwijk, Netherlands, Apr.
23-27.
Vayner, B., Galofaro, J., Ferguson, D., and
Degroot, W., 2002. “Electrostatic Discharge
Inception on-a High-Voltage Solar Array,”
AIAA Paper 2002-063 I, 40th Aerospace
Sciences Meeting and Exhibit, Reno, NV,
Jan. 14-17.