NO.
REV. NO.
EATM-45
i
FMEA for Solar Cell Array
PAGE
DATE
TK:ld
Dist.
EASEP Std.
-
OF
23 Jan, 1969
NO.
FMEA for Solar Cell Array
EATM-45
PAGE
DATE
1. 0
SUMMARY
REV. NO.
1
OF
23 Jan. 1969
*
A Failure Modes and Effects Analysis (FMEA) of the PSEP Solar
Cell Array is presented. The analysis covers failure modes for the solar
cells, their mounting and interconnection.
It is shown that the design incorporates a high degree of redundancy
throughout. At no point in the array is power output dependent on a single
electrical path. Therefore, no recommendations for reliability improvement
are made and the analysis has served primarily to increase confidence
in the current design.
2. 0
. DESCRIPTION OF ARRAY
Electrical power for the PSEP during lunar operations will be obtained
from a solar cell array. The solar cell panel array is composed of six
23. 75 x 13. 0 x 3/8 inch honeycomb panels. Three panels are placed
on each side of a tent configuration array. Hereafter these two sides
will be referred to as East and West. Figur·e 1 shows the general
configuration.
The solar cells used in this array are blue-sensitive, gridded,
2 x 2 ern, N/P cells • 014 inch thick. Each cell is provided with a 6 mil
rnicrosheet cover which is coated with UV and anti-reflective coatings.
The cells are mounted on the panel in seven-cell modules. The cells are
redundantly connected in parallel in each module by a metallic strip
which is attached at four points to the solder bases of each cell. Each
cell in the module is individually connected to the next adjacent module
in its series by four small connecting tabs. Fifteen modules are connected
in this fashion to form a module group within the panel.
*
This EATM is based primarily on material
contained in Spectrolab Report RR-1, Project
3644, Reliability Analysis, Bendix-ALSEP
Solar Cell Array, 7 January 1969.
15
Antenna
Passive Seismic
Experiment"
Isotope
Heater
. - Astronaut Handle
Antenna
Cable
Central Station ·~
Solar Panel
Deployment Linkage
Eas~
ALSEPAXES
z
Carrying Handle
West
Antenna Positioning
Mechanism
Antenna Mast
~
Solar
_
Outboard Solar
Panel "B"
N'"CJtx
w Pl
fl
,_. 0
,....
-.D,_.
Figure 1.
PSEP Deployed Configuration
>
!'...t OQ 1Pl (1) ~
N 1
3;(J1
*'
(J
NO.
REV. NO.
EATM-45
FMEA for Solar Cell Array
3
PAGE
15
OF
DATE 23
Jan. 1969
The solar panels are assembled by bonding a thin sheet of G-1 0
glass epoxy onto the honeycomb substrate, and the modules are bonded
on this thin sheet. This sheet of G-1 0 glass epo.xy provides electrical
insulation between the aluminum honeycomb and the solar cells. On each
honeycomb panel a series string of 4 module groups {60 modules) is assembled,
This gives a total of 420 solar cells per panel. The total number of cells for
tht complete array is 2520.
'
\
The panels are wired using redundant #24 insulated cable bonded
to 'the substrate skin, Inter -panel wiring to the center panel is imbedded in
a bonding agent in the channel between the substrate skins along the top
edge of the outboard panel and will terminate in a connector. Mating
connectors are located at the edge of the center panels to simplify panel
interconnections as shown in Figure 2. The 9 blocking diodes to prevent
reverse currents in the solar panels are located together on the backside
of the center panel substrate as shown. Three paralleled blocking diodes
are provided for each series string to provid~ high reliability. The solar array
connects to the PCU through a connector which mates with the one existing
on the present PCU.
Figure 3 shows an electrical schematic of the power system down
to the level of the module group, where each module group contains a 15 x 7
cell matrix. It should be noted from the schematic that redundant electrical
paths are provided throughout, i.e., between module groups, between diodes
and panels, and back _to the PCU connector. Thus at no point in the design is
power output dependent upon a single path.
Figure 4 shows the connections used within the module groups and
indicates the level of redundancy present, For both Figures 3 and 4 the hardware
terminology applied to. t'h.e various components and connections has been
indicated for clarity.*
3. 0
ANALYSIS AND DISCUSSION
3. 1
COMPONENT AND INTERCONNECTIONS FAILURE MODE SUMMARY
Table I is a listing of the solar cell array components. Included in this
list is a statement of each component's application, failure modes and effects,
and estimated failure rates. As noted throughout in the last column there are no
recommendations for reliability improvements.
*
Terminology Summary:
7 parallel cells :. 1 module
15 modules= 1 module group
. . . . . :'!. . ~.~.~.~!.~. . .~.::.~.~.!?. .~. . . .~ . . §~ .~~~~~~S,. . .~ . . .!. . . !?.~nel
3 panels = 1 side
6 panels = 2 sides
= 1 ~E!~Y
10
C.E.t-J"i~AL
::'>TA..TION
TO RIGI4T PA.NE.L
ID LEF"i
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--++
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Figure 2.
Diode Installation Center Panel Back Side
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-----------------------
EATM-45
Page 5 of 15
Al 1-<J
.J
23 Jan. 1969
--1
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I
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-- _j--.....~~
EATM-45
Page 6 of 15
23 Jan. 1969
Module Positive Bus
..
Solder Joints ~X
..~.;;;;;::;;;;::;;:;;:;;;;;::;;;;;;::;;;;;::;;.;;~
Solar Cell ~.,...•
-~
Module
(7 Cells
in Paral~el)
[
.
Parallel
Series
Interconnecting
T.ab~
)dule
·oup
5 Modules
series)
,(Module
f Negative Bns
Figure 4.
Module Group Electro-Mechanical Schematic
TABLE 1
SOLAR CELL ARRAY·ANALYSIS SUMMARY
Component
Application
Fail Mode
Fail Effect
Fail Rate
Remarks-
Recommendation
Solar Cell
Energy conversion
Fracture (Open)
Loss of power
f = 1. 0 x 1 0 - 8/hr
See Solar Cell Failure
Analysis
None
Solar Cell Cover
Hard radiation absorptance spectral pass
filter
Loss
Loss of protection from
radiation
Not Applicable
Slight increase in power
followed by radiation
damage degradation.
None
Fracture
Negligible
Short circuit failure
across a solar cell
Loss of power for all
solar cells parallel
to affected cell
Fracture (transverse)
Loss of power
Failure mode not
observed to date.
Fracture (longitudinal)
None
Electrical continuity
retained.
Solder tab fracture
None-redundancy
provides
Perforation
Short circuit of
solar cells to substrate
Short circuit
Maximum of 7"/o loss
on array out put for
sun angles of oo to
60° and 120° to 180°,
Open circuit
Loss of power for
affected circuit
Module bus bar
Micaply
.........
Diode
Interconnection
between adjacent
solar cells
Electrical insulation between solar
cells and panel
substrate
Blocking diode
for shadowed
panel
See Solar Cell Failure
Analysis.
f
f =
lo- 13 /hr
o. 0
f=3.5xl0
-7
/hr
Will change operating
point for remaining
cells in affected
circuit.
None
Failure mode not
observed to date.
None
At low voltage applications diode
short circuit
failures not
observed.
None
Redundancy is
provided
N'lJM
""l!J:>
>-l
l!J C1>:;:::
._.OQ
?
"'
o--
-.J
I
0
.:.
"'
.....
.....
"'\J1
TABLE 1 CONT.
Component
Application
Fail Mode
End Solder tab
Provide for connection
of module cell bus
bars and stranded
wire
Short circuit
L Solder tab
Provide for
interconnection
of module groups
·Fail Effect
Fail Rate
Remarks
Recommendation
Partial or complete
loss of power for
affected circuit
depending on position
of short
fs = 0. 0
Prevented by plotting
with insulating
materials. Short
circuit failures of
this type not
observed.
None
Open circuit
(transverse
fracture)
Complete loss of
power affected
circuit
f
/hr
Failure of end tab not
observed. Redundancy
is provided in case
of solder joint failure,
None
Open circuit
Complete loss of
power for affected
circuit
f
/hr
Failure of L tab not
observed. Redundancy
is provided i,.; c~se of
solder joint failure .•
None
Short to
adjacent
circuitry
For affected circuit
partial or complete
loss of power depending upon position
f
/hr
Insulated braided wire
plotted at points of
connection and
sharp bends.
None
Open circuit
Complete loss of
power for affected
circuit(s)
£ =1.0xl0
0
Redundancy is provided
in case of solder
joint failure,
Stress relief provided
adjacent to solder
connections.
None
Loss of mechanical
integrity for affected
solar cell region.
No loss of power
will result unless solar
cells and wiring
connections open
circuit
f =
Failures of adhesive
materials have never
been observed to
result in power
degradation for
solar cell arrays
when in deployed
position at
temperature limits.
None
0
0
(1. Oxl 0
<
· 1. OxlO
-13
-13
i
Insulated
Stranded Wire
Silastic 140
RTV-511
with primer
Circuit electrical
continuity
Bond micaply to
substrate. Bond
solar cells to
micaply.
sh
=l.Oxlo-
-8
11
/hr
o. 0
I'
N'ljt'
"'ru:x:,_
ru " .,;
?
7
c.,()Q
(X)
0
-
"""' ....
"""'
!__________________________________________________
ol>
... IJ
EATM-45
FMEA for Solar Cell Array
9
PAGE
OF
15
~a
vs;ten~ D~ion
3. 2
SOLAR CELL F AlLURE ANALYSIS
3. 2. 1
Short Circuit Failures
DATE
23 Jan. 1969
A short circuit of a solar cell results when a conducting path
exists between the upper and lower surfaces of the cell. The most
probable cause for this type of failure is a contact between the lower surface
of the cell and the lead from the module bus bar which is connected to
the top surface of the same cell. This will result in a short circuit of
the affected cell, and will also short circuit the other cells in that module.
There will be a slight variation in the operating voltage and current for
the other cells in that string resulting in a correspondingly small power
degradation. The average power degradation will be of the order of 2% for
the affected string when operating at no failure maximum power voltage.
Quality Control inspection of the inter -cell connections virtually
eliminates any possibility of such a failure under normal operating
conditions. The probability that such a failure will occur is, therefore,
assumed to be zero.
3. 2. 2
Open Circuit Failures
Each solar cell is soldered at four points on its lower surface
to a module bus bar. The series connection to the next module is
completed by four solder joints on the top surface of each cell.
Since the solder connection on each surface represents three fold
solder joint redundancy, the probability that the cell will open circuit
as a result of failure of all joints at the lower surface is negligible.
Likewise, the probability of an open circuit failure at the upper surface
is also negligible.
An open-circuit cell failure results from the fracture of 1 a cell.
The probability is extremely remote that such a failure would not be
detected during preliminary inspections or that it would occur under
normal operating conditions. An open-circuit failure may also result
from the separation of the electrode grid from the upper surface of a
cell because of thermal cycling and/ or vibration of the cell. Engineering
studies and obse~~~H9ns made to date indicate that the probability
of failure in this mode is small.
EATM-45
FMEA for Solar Cell Array
PAGE
DATE
3. 2. 3
10
I
OF
23 Jan. 1969
Power Degradation Resulting from Open Solar Cell Failures - {Worst Case
Analysis)
For the worst case, an open cell failure will result in the complete loss
of conduction area for the affected solar cell. {Comprehensive solar cell failure
studies indicate that on the average only 30o/o of the solar cell area is lost for
each fracture.) It has been shown that the probability of the loss
of more than one solar cell per module is remote. Also, the first solar cell
open circuit failure within a given circuit is ~the critical failure, subsequent
failures resulting in little if any additional degradation. Therefore, under
the worst case assumption that the first six solar cell failures will occur each
within a different panel, the resulting power degradation for the solar cell
array will be
=X D60
d
X
-6-
X
~ 6
where n 60 is the degradation for an individual solar cell panel with a single
solar cell failure.
If there are a total of X7 6 solar cell failures, the array degradation
will remain essentially constant at d = n . Using a typical 2x2 - 2 ohm-em
60
N on P solar cell characteristic curve, a composite characteristic curve
may be formulated for a 7 parallel by 60 series solar cell panel in which
there are no failures and for which there is a complete loss of a single
solar cell. The current degradation at the maximum power voltage is
determined and the circuit degradation at maximum power voltage is then
computed as
A i mpv
i.mpv
The value of
n 60 was determined to be . 057 {See Appendix A.)
Thus, on the basis of a maximum of two solar cell failures,
the power degradation would be d = 2 x • 057
019
2
6
=•
3. 3
15
ADHESIVE EVALUATION
Failure of the adhesive bond between the individual solar cells and
the module substrate will not directly result in a solar cell failure but will
leave the affected cells susceptible to a later failure resulting from vibration
fracture of a solder joint. The adhesive bond, however, is highly reliable
virtually eliminating any failure as a result of thermal cycling, vibration/
and/ or shock.
EATM-45
FMEA for Solar Cell Array
PAGE
DATE
11
FILTER EVALUATION
A clean fracture of a filter would have no appreciable effect upon
the performance of a cell. Shattering of a filter will result in a scattering
of the incident radiant energy and could, in severe cases, render the
filter opaque. In such a case, the cell affected would .still act as a conduction path and the resulting power degradation would be far less than if the
cell were to open circuit.
3.5
PANEL FAILURE EFFECTS
The probability of losing an entire panel is small because of the
series - parallel interconnection of the cells and the redundant wiring between panels, Still a certain amount of insight into system sensitivity to
major failure can be gained by examining the effects of panel failures on
.system operation.
The results of such an analysis using the minimum power expected
(minimum solar constant from EA TM 33) are given in Figures 5 through 8.
As seen from Figure 5, loss of a single panel reduces system operating
time to 60% of a full day. Loss of two panels on the same side {Figure 6)
reduces operation to 4 7% of a day, while loss of a complete side causes
little additional penalty (reduction from 47% to 42% of full day operation)
For the case of panel losses on both sides, Figure 8 shows that a
single panel lost on each side essentially stops operation since the 20%
operation time shown may or may not exist in actuality and is split in
addition. Therefore, panel losses on both sides effectively halt operation
unless actual output of the array is significantly better than in this worst
case example.
4. 0
OF
15
23 Jan. 1969
Failure of the adhesive bond betweeri the solar cell and its filter
must be included in the same category of failures. The causes of such a
failure will be the same as for adhesive failure between cell and substrate.
Loss of a filter will reduce the efficiency of the affected cell by approximately 4 percent due to increased operating temperature.
3.4
I
CONCLUSIONS
Analysis of the PSEP Solar Cell Array indicates that the design
possesses a high degree of redundancy in the interconnection of the solar
cells at the module and panel levels. In no case is power output dependent
upon the existence of a single electrical path. This statement applies both
to the .interconnecting wiring and to the solder joints used throughout the
array.
EATM-45
Page lla
23Jan. 1969
Figure 5
System Operation with One Panel Non-Operating
(Minimum Solar Constant)
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EATM-45
Page lib
23 Jan. 1969
Figure 6
System Operation with Two Panels of One Side Non-Operating
(Minimum Solar Constant)
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EATM-45
Page llc
23 Jan. 1969
I
Figure 7
System Operation with Three Panels of One Side Non-Operating
(Minimum Solar Constant}
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EATM-45
Page lld
23 Jan. 1969
I
Figure 8
System Operation with One Panel on Each Side Non-Operating
(Minimum Solar Constant}
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EATM-45
FMEA for Solar Cell Array
Aerospace
Systems Divi~
PAGE
DATE
12
I
OF
15
23 Jan, 1969
Redundancy in the blocking diodes should provide a high degree
of reliability throughout the lunar day as shadowing of the East and
West sides changes.
An evaluation of the solar cell failure modes, both short and
open circuit types, indicates that the probability of a single cell failing
is negligible. Moreover, loss of a single cell results in only a 5. 7%
loss of power output in the affected panel. Since the array is designed
with a 3 volt margin at its lowest daytime output point {sun at 60 and
120 degrees), a loss of this magnitude is not serious.
Thus, the overall conclusion of the analysis must be that the
design is inherently reliable. There are no recommendations for improvement.
HO.
REV. HO.
EATM-45
FMEA for Solar Cell Array
PAGE
~pace
~~D~
DATE
13
OF
15
23 Jan. 1969
APPENDIX A
OPEN CIRCUIT F AlLURE DEGRADATION ANALYSIS
INTRODUCTION
Included within this appendix is a description of methods and data
used in evaluation of the power degradation resulting from a single open
circuit solar cell failure within a panel of the solar cell array.
The panel consists of 60 modules in series, each with 7 solar
cells in parallel for a total of 420 solar cells per panel. Only open circuit
solar cell failures are considered, since the probability of a short circuit
failure across a solar cell is negligible for the fabrication techniques
employed. For purposes here only total loss of both generation and
conduction for the failed cell is considered. Composite characteristic
curves are constructed for a panel in which such a failure is assumed.
From this curve the percent power degradation resulting from the assumed
failure can be calculated. Since only total solar cell failures are
considered, the amount of loss per failure so calculated is substantially
greater than that which would normally be anticipated.
The following procedure is used in the construction of the
composite characteristic curves. Values of current and cor_responding
voltages for 46 points were taken from a characteristic curve for a
single 2x2 - 2 ohm-em N/P solar cell, These values are given in
Table A-1.
NO FAILURES
Since the voltage drop for all cells in any module will be equal,
so also will be the currents through each individual cell, if the cells are
assumed to have identical cp.aracteristics. Therefore, the total current
through any module will be equal to the product of the individual cell
current at a given voltage and the number of cells per module. Likewise,
all modules being in series will have identical currents, and the total
voltage drop for a module group will be the product of the voltage drop
per module and the number of modules per group. Therefore, to form
composite characteristic curves for a circuit with no failures, it is
necessary only to multiply the values of the current given in Table A-1,
by the number of cells per module, and the voltages by the number of modules.
HO.
REV. HO.
EATM-45
FMEA for Solar Cell Array
14
PAGE
DATE
23 Jan. 1969
TABLE A-1
TYPICAL CHARACTERISTIC CURVE COORDINATES
N/P - 2x2 em 2 ohm-em Solar Cell
VOLTAGE
(Volts)
-20.0127
-16.0127
-12.0127
-8.0127
-4.0127
-0.0127
0.0372
0.0872
0.1372
0.1872
0.2040
0.2200
0.2372
0.2540
0.2700
0.2872
0.3010
0.3122
0.3250
0.3372
0.3500
0.3622
0.3750
0.3872
0.3974
0.4072
0.4173
0.4272
CURRENT
(Amps)
.1456
. 1414
.1390
. 1370
.1352
.1340
.1340
.1340
• 1340
.1340
. 1340
• 1340
.1340
• 1338
• 1338
. 1338
.1338
.1334
. 1332
.1328
.1324
.1316
. 1308
.1298
.1286
.1272
.1252
.1232
VOLTAGE
(Volts)
0.4374
0.4472
0.4550
0.4622
0.4711
0.4772
0.4828
0.4872
0.4920
0.4947
0.4976
0.5002
0.5082
o. 5152
0.5202
0.5262
0.5297
0.5362
15
OF
CURRENT
(Amps)
.1204
• 1166
.1136
. 1100
.1040
• 0990
. 0956
• 0900
. 0860
. 0830
. 0804
• 0774
• 0670
• 0560
. 0490
. 0380
• 0300
. 0120
NO.
FMEA for Solar Cell Array
REV. MO.
EATM-45
15
PAGE
DATE
OF
23 Jan. 1969
Using these data, an I-V curve can be plotted for the 60 x 7 solar cell
panel. For this circuit, the maximum power can. be computed and the
maximum power voltage may be determined.
OPEN CIRCUIT FAILURE
/
Consider again a panel of 60 solar cell modules in series, 7 solar
cells in parallel per module. In such a circuit, which has a single open
total cell failure, there will be a total of 59 modules, which have 7 cells
ea'.ch in parallel, in series with a single module which has 6 cells in
pa'rallel. In the actual case where partial cell losses will occur, there
will be on the average about 6-2/3 solar cells remaining in the failed
module. Since all modules are in series, the current through each will
be equal, but the current through each cell will not. The effect of the loss
. of a conducting path {an open circuit cell) will be to produce a current
limiting effect on the affected panel.
1
Composite characteristic curves are formed for such a panel in
the following manner: A composite characteristic curve is constructed
for. a "short string" of 7 parallel by 59 series cells. Also, a characteristic
curve for one open circuit failure is then formed by adding together the
values of voltage for equal values of current.
ANALYSIS OF DATA
For the configuration examined using the individual solar cell
characteristic curve shown in Table A-1, the maximum power voltage
was evaluated as 25. 98 volts with corresponding current of . 854 amperes
for the no failure case. For the circuit in which a single open circuit
failure is assumed, the current at maximum power voltage {no failure)
is evaluated as . 805 amperes. The power degradation equals
(3 impv
i
15
mpv
which results from the single open solar cell failure is thus evaluated
as . 057 per solar cell failure per panel.