Managing space station solar-array electrical hazards for sequential shunt unit replacement

The U.S. solar-array strings on the International Space Station are connected to a sequential shunt unit. The sequential shunt unit's job is to shunt excess current from the solar array, such that just enough current is provided downstream to maintain the bus voltage while meeting the power load demand. Should a unit fail, its removal and replacement would normally be done with the solar arrays retracted to reduce the voltages and currents at the sequential shunt unit to safe levels. However, an alternate approach was desired to avoid the inherent risks associated with array retraction and redeployment. This approach allowed the replacement to be conducted via astronaut "space walk" or extra vehicular activity with the solar array still deployed. Removing a sequential shunt unit with the solar array in sunlight could result in substantial hardware damage and/or safety risk to the astronaut due to the voltages that may be present. Replacing the sequential shunt unit during eclipse would seem optimal, except that the maximum eclipse period is only 36 min, which is insufficient time. To guide the assessment and ameliorate hazards, the athours analyzed array string current and voltage capability during the various operating conditions using System Power Analysis for Capability Evaluation (SPACE), an electrical power system modeling code. This paper discusses six sequential shunt unit remove and replacement options and the associated analysis to develop a workable sequential shunt unit replacement procedure via extravehicular activity.