ISS Random Power Failure Hardware Protection

Primary source of electric power for the International Space Station (ISS) is the photovoltaic (PV) module. At assembly complete stage, the ISS will be served by four PV modules. Each module contains two independent primary power channels such that one failure will result in loss of only one-half of the power generation capability. During the early stages of assembly, the ISS is served by only one PV module. Flight 12A brings one more module to the ISS and so on until the fourth module is installed during Flight 15A. Power channel components are arranged in orbit replaceable units (ORU) and repairs are accomplished by replacing the failed ORU. While a power channel is operating, all of its ORUs are maintained within their allowable temperature ranges by a balance between the heat generated and heat removed. Heat removal is accomplished either passively or by an active thermal control system (TCS), called photovoltaic thermal control system (PV TCS). The PV module P6 has been operating flawlessly since December 2000 with 100% power availability, as of this writing. This far exceeds the design goal and expectations. However, since a random failure may occur at anytime, the operators must be prepared to handle this situation. An ORU failure may result in dormancy of one or more ORUs; in the worst case, dormancy of all ORUs within one power channel and shutdown of the PV TCS for that power channel. Dormant ORUs may exceed their allowable temperature ranges and that may result in failure of additional ORUs. It is very important to ensure that all dormant, but otherwise operable ORUs remain within their allowable temperature ranges to prevent failure propagation. This describes how the dormant ORUs in the failed power channel will be thermally protected until replacement and restoration has been completed. These procedures use the following processes: (a) backfeed of primary power from another operating power channel, (b) supply of dc control power from the operating channel on the same PV module, and (c) perform thermal control based on the monitored parameters. These procedures have been thoroughly coordinated by all affected technical teams and approved by all required boards.