TK:ld

Dist. EASEP Std.

FMEA for Solar Cell Array

NO.

EATM-45

i

REV. NO.

PAGE - OF

DATE 23 Jan, 1969

NO. REV. NO.

FMEA for Solar Cell Array EATM-45

PAGE 1

OF 15

DATE 23 Jan. 1969

* 1. 0 SUMMARY

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.

West ~

Antenna

Isotope Heater

Antenna Cable

Solar

Figure 1.

Passive Seismic Experiment"

.- Astronaut Handle

Central Station ·~

Solar Panel Deployment Linkage

Eas~

ALSEPAXES

z

Carrying Handle

Antenna Positioning Mechanism

Antenna Mast

_ Outboard Solar Panel "B"

PSEP Deployed Configuration

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FMEA for Solar Cell Array

NO.

EATM-45

REV. NO.

3 15 PAGE 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

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EATM-45 Page 6 of 15 23 Jan. 1969

Solder Joints ~X .. ~.;;;;;::;;;;::;;:;;:;;;;;::;;;;;;::;;;;;::;;.;;~ Solar Cell ~.,... •

Parallel

Series Interconnecting T.ab~

,(Module

f Negative Bns

Figure 4. Module Group Electro-Mechanical Schematic

Component

Solar Cell

Solar Cell Cover

Module bus bar

Micaply

.........

Diode

Application

Energy conversion

Hard radiation absorp­tance spectral pass filter

Interconnection between adjacent solar cells

Electrical insul­ation between solar cells and panel substrate

Blocking diode for shadowed panel

TABLE 1

SOLAR CELL ARRAY·ANALYSIS SUMMARY

Fail Mode

Fracture (Open)

Loss

Fracture

Short circuit failure across a solar cell

Fracture (transverse)

Fracture (longitudinal)

Solder tab fracture

Perforation

Short circuit

Open circuit

Fail Effect

Loss of power

Loss of protection from radiation

Negligible

Loss of power for all solar cells parallel to affected cell

Loss of power

None

None-redundancy provides

Short circuit of solar cells to substrate

Maximum of 7"/o loss on array out put for sun angles of oo to 60° and 120° to 180°,

Loss of power for affected circuit

Fail Rate

f = 1. 0 x 1 0 - 8/hr

Not Applicable

f lo-13 /hr

f = o. 0

-7 f=3.5xl0 /hr

Remarks-

See Solar Cell Failure Analysis

Slight increase in power followed by radiation damage degradation.

See Solar Cell Failure Analysis.

Will change operating point for remaining cells in affected circuit.

Failure mode not observed to date.

Electrical continuity retained.

Failure mode not observed to date.

At low voltage app­lications diode short circuit failures not observed.

Redundancy is provided

Recommendation

None

None

None

None

None

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Component

End Solder tab

L Solder tab

Insulated Stranded Wire

Silastic 140 RTV-511 with primer

Application

Provide for connection of module cell bus bars and stranded wire

Provide for interconnection of module groups

Circuit electrical continuity

Bond micaply to substrate. Bond solar cells to micaply.

i

Fail Mode

Short circuit

Open circuit (transverse fracture)

Open circuit

Short to adjacent circuitry

Open circuit

TABLE 1 CONT.

·Fail Effect

Partial or complete loss of power for affected circuit depending on position of short

Complete loss of power affected circuit

Complete loss of power for affected circuit

For affected circuit partial or complete loss of power depend­ing upon position

Complete loss of power for affected circuit(s)

Loss of mechanical integrity for affected solar cell region. No loss of power will result unless solar cells and wiring connections open circuit

Fail Rate

fs = 0. 0

-13 f

0(1. Oxl 0 /hr

< -13 f0

· 1. OxlO /hr

f =l.Oxlo-11

/hr sh

-8 £0=1.0xl0 /hr

f = o. 0

! _________________________________________________ _

Remarks

Prevented by plotting with insulating materials. Short circuit failures of this type not observed.

Failure of end tab not observed. Redundancy is provided in case of solder joint failure,

Failure of L tab not observed. Redundancy is provided i,.; c~se of solder joint failure .•

Insulated braided wire plotted at points of connection and sharp bends.

Redundancy is provided in case of solder joint failure, Stress relief provided adjacent to solder connections.

Failures of adhesive materials have never been observed to result in power degradation for solar cell arrays when in deployed position at temperature limits. I'

Recommendation

None

None

None

None

None

None

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EATM-45 FMEA for Solar Cell Array

9 15 PAGE OF

~a vs;ten~ D~ion

DATE 23 Jan. 1969

3. 2 SOLAR CELL F AlLURE ANALYSIS

3. 2. 1 Short Circuit Failures

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 of1 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 I FMEA for Solar Cell Array

PAGE 10 15

OF

DATE 23 Jan. 1969

3. 2. 3 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

d =X D60 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

60. Using a typical 2x2 - 2 ohm-em

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 = 2 x • 057 = • 019

6

3. 3 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.

FMEA for Solar Cell Array EATM-45 I PAGE 11 OF 15

DATE 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 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 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.

0

Figure 5

System Operation with One Panel Non-Operating (Minimum Solar Constant)

£A 51

EATM-45 Page lla 23Jan. 1969

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EATM-45 Page lib 23 Jan. 1969

System Operation with Two Panels of One Side Non-Operating (Minimum Solar Constant)

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EATM-45 Page llc 23 Jan. 1969

System Operation with Three Panels of One Side Non-Operating

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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 I FMEA for Solar Cell Array

PAGE 12 OF 15 Aerospace Systems Divi~

DATE 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

15 ~pace ~~D~

PAGE 13 OF

DATE 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 15 PAGE OF

DATE 23 Jan. 1969

TABLE A-1

TYPICAL CHARACTERISTIC CURVE COORDINATES N/P - 2x2 em 2 ohm-em Solar Cell

VOLTAGE CURRENT VOLTAGE CURRENT (Volts) (Amps) (Volts) (Amps)

-20.0127 .1456 0.4374 .1204 -16.0127 . 1414 0.4472 • 1166 -12.0127 .1390 0.4550 .1136

-8.0127 . 1370 0.4622 . 1100 -4.0127 .1352 0.4711 .1040 -0.0127 .1340 0.4772 • 0990 0.0372 .1340 0.4828 . 0956 0.0872 .1340 0.4872 • 0900 0.1372 • 1340 0.4920 . 0860 0.1872 .1340 0.4947 . 0830 0.2040 . 1340 0.4976 . 0804 0.2200 • 1340 0.5002 • 0774 0.2372 .1340 0.5082 • 0670 0.2540 • 1338 o. 5152 • 0560 0.2700 • 1338 0.5202 . 0490 0.2872 . 1338 0.5262 . 0380 0.3010 .1338 0.5297 • 0300 0.3122 .1334 0.5362 . 0120 0.3250 . 1332 0.3372 .1328 0.3500 .1324 0.3622 .1316 0.3750 . 1308 0.3872 .1298 0.3974 .1286 0.4072 .1272 0.4173 .1252 0.4272 .1232

NO. REV. MO.

FMEA for Solar Cell Array EATM-45

15 15 PAGE OF

DATE 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'1.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.

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 mpv

which results from the single open solar cell failure is thus evaluated as . 057 per solar cell failure per panel.