Secondary arcs are described as non-sustained when the arc lasts only during the primary discharge and are described as self-sustained when the arc lasts longer than the primary discharge. A self-sustained arc is described as temporary if it stops even though power is still available, or permanent if it does not stop while external power is still available.
Secondary arcs, fed from power sources on the spacecraft, are an important additional damage mechanism since the power source can become permanently short-circuited. Systems affected include solar arrays and associated power lines, power slip rings of solar array driving mechanisms, electrical propulsion power units and motors, and high-voltage power supplies of travelling wave tubes.
• Requirements for solar arrays are presented in clause 7.2, and they apply to any photovoltaic generator including substrates, coverglasses and interconnexions. The purpose of these design requirements is to avoid self-sustained secondary arc occurrence and its effect on solar panels and to prevent degradation from primary ESD’s. These are in addition to the provisions relating to surface charging in clause 6.
Specific requirement on testing of solar arrays are presented in clause 7.2.3. In general testing is always performed for solar arrays because the surface charging requirements are not fulfilled (typically at GEO). In addition to the surface charging testing described in clause 6.6, the testing requirements presented in 7.2.3, associated with controlling sustained arcs, apply to solar arrays.
Tests have been performed which proved that no sustained arcs can be established for the design of Silicon and GaAs solar array coupon investigated in EMAGS 2 study [39] when the maximum voltage-current couple available between two adjacent cells on the panel, separated with 0,9 mm as nominal value, is below the voltage and current thresholds given in the first two columns of Table 7‑1.
The voltage is the maximum voltage between two adjacent cells, taking into account possible over-voltage. The current is the maximum current that can flow through a conductive part of the array (usually the current of a single string if each is protected by a diode). The cell separation is the nominal cell separation and assumes a 20 % variation about this value.
• Requirements for solar array drive motors are presented in clause 7.3, and apply to the electric motors, gearing, control electronics and drive systems used to maintain solar array orientation.
Table 7‑1: Tested voltage-current combinations
|
Voltage |
Current |
Comments |
|
70 V |
0,6 A |
No self sustained secondary arcing observed |
|
50 V |
1,5 A |
No self sustained secondary arcing observed |
|
30 V |
2 A |
No self sustained secondary arcing observed |
|
10 V |
No requirements |
Voltage is too low to allow any arcing between non pure electrodes |