The requirements of power supply capacity of spacecraft increase as human continue to explore the space environment. Future spacecraft’s solar array is required for high power, long life and small attitude control pressure. For instance, the power of communications satellite is usually more than 10kW, and the life of the high power communications satellite is over 10 years. The bus voltage of the satellite is typically higher than 100V to reduce the transmission loss. Therefore, high voltage solar array will be an important development direction in the future [1-3]. Furthermore, the solar array arcing will happen when the voltage of the solar array string exceeds 90V, and resulting in
partial or total failure of the solar array [4-5]. For example, the power of Laura four communications satellites was reduced by more than 20% due to solar array arcing in 1997. There was failure in ADEOS-II satellite because of the salary array electrostatic discharge (ESD)[6-7]. So far, there is little study on threshold voltage of the ignition discharge and secondary discharge of LEO solar array at home and abroad. The determination of the ignition discharge threshold voltage and secondary discharge threshold voltage play an important role in the selection of the bus voltage.
The plasma density of charged particles in LEO (Low Earth Orbit) is 4~6 orders of magnitude higher than the plasma density of charged particles in GEO (Geosynchronous orbit). The solar arrays will interact with high pressure plasma in the LEO region and cause electrostatic discharge phenomenon. In generally, we believed that the electrostatic discharge will last when the threshold voltage exceeds 75V. Consequently, the primary and the secondary discharge voltage of the high-voltage solar array need to be determined.
In this paper, a simulated LEO plasma-vacuum environment was established, and the rigid substrate triple junction gallium arsenide solar cell specimens were used to experiment the occurrence of the primary discharge and secondary discharge under conditions of LEO. In particular, the threshold voltages of primary and secondary discharge are determined under the conditions of LEO orbit to provide ESD protection reference for solar array of LEO orbit spacecraft.
II. EXPERIMENTAL SETUP
A. Experimental systems
The schematic of the experimental system is shown in Fig. 1. Chamber geometry was 1.5 m in length and 1.3 m in diameter. The pressure inside of the chamber was approximately 1.0 ×10-3 Pa during the experiment. A plasma source (ECR) was mounted in the chamber, and the plasma concentration is approximately 5×1011m-3~ 5×1012m-3. This plasma source can irradiate the plasma energy up to 1.5 eV. The plasma concentration of the chamber was measured by electrostatic probe. The test coupon is set in the center of the chamber. For acquisition of the discharge image, we used the image intensifier ( Nikon Eclipe Lv150) to analyze the weak emission of discharge.
Supported by National Natural Science Foundation of China (No.51407008)
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2015 3rd International Conference on Electric Power Equipment - Switching Technology (ICEPE-ST) Oct. 25-28, 2015 Busan, Korea
probe 4-the test coupon 5- camera 6- observation window
7-plasma source
P
N P
N
R
Vb
CH1
Cext
N
P
CH3
CH2
CH4
Fig.1 The schematic of the experimental system.
The test circuits of primary discharge and the secondary discharge are shown in Fig.2 and Fig.3, respectively. The external circuit used in the primary discharge experiments is shown in Fig.2. We measured the arc current by current probes; and we measured the coupon potential by using a high voltage probe. Vb is used to simulate spacecraft potential. The arc energy was supplied by capacitance Cext. We changed the value of capacitance in order to change the arc energy. The primary arc energy is supplied by this capacitance. In the primary arc, the charge stored in Cext flows from arc site to the chamber wall through the primary arc plasma. This current is called a blow off current and measured by CH4.
The external circuit used in the secondary discharge experiments is shown in Fig.3. DC power sources of V1 and V2 simulate the operation of the solar array generating electric power. These power supplies are needed to follow the phenomenon of several microseconds like the primary arc. C1 to C3 correspond to the capacitance of one series circuit in which about 50 cells are connected on a substrate. Vb and Cext imitate the potential and capacitance of the satellite respectively. Variable resistance RL simulates the load of the satellite. We measured the blow off current with a current probe, CH3.
Fig. 2 The primary discharge test circuit.
P N
P N
RRL
Rb
V2
Vb
Rext
Lext
C1
C2C3V1
Cext
CH3
CH2CH1
CH4
Fig.3 The secondary discharge test circuit
Fig.4 Schematic and the picture of the primary discharge test coupon
B. Test coupons
The test coupons were constituted of the primary discharge coupons and secondary discharge coupons. There is one group of primary discharge test coupons, and there are six groups of the second discharge test coupons. The test coupons will be described in detail as follow.
The characteristics of primary arc, such as discharge current peak, duration, total charge, energy, are affected by the size of the solar array paddle mounted on the satellite. In order to test properly solar array strength, the primary arcs on the solar array must be simulated. The usage of a large solar array coupon is difficult and not realistic in ground test systems. For this problem, the coupon test as shown in Fig.4 is used to predict the discharge phenomena on much larger solar array of
realistic size. The size of the test coupon is 184mm × 247mm;
it has 6 strings of 3 cells connected in series. TJ (GaAs) solar
cells are used. The size of the solar sell is 40.4mm×30.3mm.
The test coupon shown in Fig.5 was used to verify the occurrence of secondary discharge, which is affected by voltage and current between adjacent strings. It is confirmed whether secondary discharge occurs or not for the given condition of string voltage and current. The gap between strings is also an important parameter in addition to the string
voltage and current. The size of the test coupon is 142mm ×
340mm, and it has 9 strings of 2 cells connected in series. TJ (GaAs) solar cells are used. The size of the solar sell is 40.4mm
×30.3mm. There are six groups of the test coupons as shown
in table 1.
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Fig.5 Schematic and the picture of the secondary discharge test coupon
TABLE I. THE COMPOSITION OF THE SECONDARY DISCHARGE TEST
COUPONS
Number Composition
Coupons
number
Solar sell
type
Form Gap between the
strings
1 TG1, TG2 TJ (GaAs) 9s2p 1mm (no protection)
2 TG3, TG4
3 TG5, TG6 TJ (GaAs) 9s2p 2mm (gap)
4 TG7, TG8
5 TG9, TG10 TJ (GaAs) 9s2p 1.5mm (gap)
III. RESULTS
A. Primary discharge results
The primary discharge procedures are as follow: 1) Discharges are allowed to occur with fixed Vb until the cell degrades. 2) If degradation of the cell is confirmed, Vb is increased. The discharge phenomenon between cells and inter-cell strings is emphasis observation in the paper. In the experiment, it is considered the primary discharge occur as peak value of the solar cell is not less than 0.5 A and the duration of the current pulse continues less than 5 us.
The initial voltage was -60 V and waited for 1.5 hours. The test ends when the primary discharge occurs. If the primary discharge did not occur, we would increase the ground bias -5 V and wait for 1.5 hours. When the ground bias voltage reaches -95 V, there was primary discharge, as shown in Fig. 6 (CH4).
Fig.6 The waveform of the primary discharge
B. Secondary discharge results
Secondary discharge is generated by a negative bias and the string current. We applied the negative potential and simulated plasma environment to cause primary discharge. In the case of a given string currents, the threshold voltage of secondary discharge will be find by gradually increasing the voltage between the solar array string.
a) The Gap between string is 1 mm
In the experiment, the secondary discharge was happened when the voltage between the strings was higher than 130 V, as shown in Fig. 7.
The location of the secondary discharge was measured by the Nikon Eclipes Lv150, and the location of the second discharge gap was 1.079 mm. There are obvious traces of discharge in the secondary discharge position. The voltage between the discharge positions of the string is 120 V. The traces of discharge do not occur in other locations.
b) The Gap between strings is 1.5 mm
As shown in Fig. 8, the secondary discharge occurred when the voltage between the strings was higher than 143.3 V in the experiment.
Fig.7 The waveform of the secondary discharge at 1mm gap.
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Fig. 8 The waveform of the secondary discharge at 1.5 mm gap.
After the experiment, we checked the appearance of the secondary discharge coupons, and there were no damaged in the solar array cell and the cover glass before and after the experiment.
c) The Gap between strings is 2 mm
As shown in Fig. 9, the secondary discharge occurred when the voltage between the strings was higher than 145 V in the experiment. The location of the second discharge gap was 1.748 mm which was measured by the Nikon Eclipes Lv150.
IV. DISCUSSION
In this paper, the electrostatic discharge experiments were studied by primary discharge coupon and secondary discharge coupon. The primary coupon 6s3p was designed to target the discharge possible circumstances. Furthermore, since the secondary discharge easily occurs between the cell strings, the secondary discharge test coupon was designed as 9s2p. Experimental studies have shown that the threshold voltage of the primary discharge is 95 V, and the voltage of the secondary discharge increased with the gap between the strings. Most of the discharges occurred at cell edges and inter connectors. In
the situation of no protective, the threshold voltage of the secondary is 130 V, and the gap between strings is 1.079 mm. In the situation of wide gap (1.5 mm), the threshold voltage of the secondary discharge is 143.3 V, and the actual gap of the wide gap measured by camera is 1.536 mm. In addition, the threshold voltage of the secondary discharge is 145 V in the situation of wide gap (2 mm), and the actual gap of the wide gap measured by camera is 1.748 mm. The threshold voltage of the secondary discharge occurs significantly increased as the gap between the cells parallel strings increases. In a word, the experiments demonstrate the effectiveness of the wide gap protective measures.
V. CONCLUSION
In this paper, experimental methods and protective measures of high-power LEO solar array electrostatic discharge were discussed and experimental verification. The following conclusion as follow:
a) The electrostatic discharge experiment system and
the experiment circuit were confirmed in the paper. The
primary test coupons and the secondary test coupons were
designed.
b) The threshold voltage of the secondary discharge
occurs significantly increased as the gap between the cells
parallel strings increases.
c) The experiments demonstrate the effectiveness of the
wide gap protective measures.
REFERENCES
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[3] Gaillot L., Fille M.L., Levy L. Secondary Arcs on Solar Array:Test Results of EMAGS 2[C]. 9th Spacecraft Charging Technology Conference, Japan Aerospace Exploration Agency (JAXA) , Tsukuba, Japan, 2005, pp. 345–352.
[4] Jongeward G.A., Katz I., Mandell M.J., Parks D.E. The role of unneutralized surface ions in negative potential arcing [J]. IEEE Transactions on Nuclear Science. Vol. NS-32, pp. 4087-409, 1985.
[5] Hastings E D， Weyl G，Kaufman D. The threshold voltage for arcing on negatively biased solar array [J]. Journal of Spacecraft and Rockets, 1990, 27(5)：539~544.
[6] Cho M., Kim J.-H., Hosoda S., et al. Electrostatic Discharge Ground Test of a Polar Orbit Satellite Solar Panel[J]. IEEE Transactions on Plasma Science, Vol. 34, No. 5, 2006.
[7] Li Kai, Wang Li, Qin Xiaogang, et al. The study on high-voltage solar cell array charging and discharging effects in GEO[J]. Spacecraft Environment Engineering, 2008, 25(2): 125-131
Fig. 9 The waveform of the secondary discharge at 2 mm gap.
