IEEE - Aerospace and Electronic Systems - May 2021 - 53

Bai et al.
current degradation of the SA for 18 months is expected to
be around 0.3 A in the orbital space environment of TTS5, which is much smaller than the actual current drop.
Moreover, the on-orbit output current drop caused by the
irradiation of charged particles is a slow changing process,
which is not consistent with the failure phenomenon.
Therefore, F2-1 can be excluded.
When the potential difference between the SA strings
is higher than the threshold voltage, the secondary discharge is easy to occur between the adjacent two SA
strings. The parallel gap of the TTS-5 SA is increased to 2
mm to prevent secondary discharge. It has been verified
by the ground test that when the parallel gap is 1.7 mm,
the threshold voltage of secondary discharge is 145 V.
However, the maximum potential difference between the
adjacent strings is 81 V, which is much lower than the
threshold voltage of secondary discharge. There is no possibility of damage for solar cell due to electrostatic discharge (F2-2).
The impact of space micrometeoroids and space debris
on the performance of SAs has received widespread attention [6]. If the TTS-5 solar wing is impacted by micrometeoroids or space debris on orbit, the output performance
of the solar cell will be affected to varying degrees
depending on the size of the space micrometeoroid or
debris. When the size is appropriate, it will cause solar
cell string damage to the output current of SA. However,
there is a phenomenon that the solar cell strings on different panels are open at the same time. Therefore, the probability of different parts of the SA circuit being hit by
micrometeoroids or space debris at the same time is
extremely small and F2-3 can be excluded.
As one of the TTS-5 tasks is to image the ground targets, the SA will carry out directional operations to the
sun when necessary. If the solar wing fails to orient to the
sun, the output current of the two SSAs of the same wing
will drop by roughly the same magnitude. However, the
telemetry data show that the X wing inner panel has a current drop but the outer panel does not, which does not
coincide with the failure phenomenon and F2-4 cannot
happen.
The telemetry of the output current of the SA circuit is
carried out by the power controller, which communicates
with the satellite mission management unit via the CAN
bus. By telemetry in January 2018, the TTS-5 SA circuit
current is 28 A, its load current is 9.7 A and the charging
current is about 17.9 A. Three different current telemetry
values meet the equation of " SA Circuit Current - Load
Current % Charging Current. " Similarly in June 2019, the
TTS-5 SA circuit current is 23 A, its load current is 8.8A,
and the charging current is about 14 A. They also meet the
equation of " SA Circuit Current - Load Current % Charging Current. " Since the SA circuit current and charging
current use independent current sensors and sampling circuits, and the relationship between the two has always
MAY 2021

Figure 5.
(a) Schematic diagram of interconnect and (b) cross-sectional
schematic diagram of silver interconnector indicated by the four
circles in (a).

been matched in the one and a half years since launch, it
can rule out the possibility of abnormal current telemetry
of the SA circuit (F2-5).

SA CIRCUIT OPEN
Similar to the above analysis that excludes F1-1 and
F2-1, the effect of the open-circuit of power connector
solder joints and the disconnection of inner-panel or
interpanel power connectors on the output current of
the SA will also be circuit level, that is, at least about
6.5 A of output current is loss, which is not consistent
with the failure phenomenon. Therefore, F3-1 and F3-2
can be excluded.
The isolation diodes used in TTS-5 SSA have been
used in dozens of similar satellites and have rich flight
experience. They are all used in accordance with MILSTD-975M [24], Appendix A. By reviewing the prelaunch
process assembly data and physical photos, the probability
of open-circuits caused by some diodes in multiple SSAs
or multiple strings of SAs is very small. Therefore, the
open-circuit fault of the body or solder joint of these
diodes (F3-3) can be completely excluded.
The adjacent two solar cell of the TTS-5 SA is connected by four interconnectors with being soldered and
each interconnector has three or four solder pins, as being
shown in Figure 5(a). The connector is made of pure silver
with a design thickness of 20 mm. The cross-section of the
interconnector is shown in Figure 5(b). The replacement
of solar cells during the production process may cause
damage to the interconnector. Hence, a re-examination
was carried out for the replacement process of solar cell.
The results of the re-examination are shown in Table 4.
From the table, we can see that there exists some continuous replacement cases of adjacent solar cells in different
time during the SA circuit manufacturing process and the
open circuit string number of a certain panel on orbit has a
certain corresponding relationship with the continuous
replacement number of adjacent solar cells. The panel
without continuous replacement of adjacent solar cells
was not found to have an open-circuit string on orbit.
Through the analysis of the solar cells replacement processes (see section " Damage Mechanism of the Interconnector During Solar Cell Replacement Process " for

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