—during normal operation from the USSS (to the PM only which is either in the integral—or the attached mode). In order to exhaust HERMES's fuel cells, power might also flow from serviced COLUMBUS elements to HERMES, thus, bi-directional flow of power must be an essential property of the corresponding power interface unit. The USSS dedicated power interface unit has to convert US AC power to regulated 150 V DC power. This can be achieved either by applying conventional methods of phase controlled rectification or modern techniques of resonant conversion which avoid bulky 400 Hz transformers. • The Power System Internal Data Processing (PINDAP) Assembly (Block IV) finally performed overall control and management of the above described building blocks. A central data link to the on-board Data Management System is provided for interactions between the EPS and other subsystems. Common to all described building blocks is a high degree of modular design (on equipment—and on bus level), such that the EPS can cope with the ORU concept and that it can grow in an orderly manner from IOC to AOC. In order to satisfy the needs of the individual COLUMBUS elements, the described building blocks are combined in a varying fashion: • Blocks II and IV and parts of block III for the PM. • Blocks I and IV; block II up to level I (level II is on the payload carrier) and parts of block III for PF and RM. In many cases the physical interface between COLUMBUS elements goes right through the described building blocks. For the man-tended free-flier, for example, parts of the power distribution assembly are on the RM; the main portion of this assembly, however, is on the PM. Hence, power system engineers have to design across the element interfaces which underlines the overall system engineering-related aspect being part of the EPS design activities. 3. Impacts of Low Earth Orbit Environment on EPS The operation of COLUMBUS elements in the gruelling low earth orbit (LEO) environment causes a lot of headaches to power system designers. The preferred operational orbits have the following characteristics: (a) 28.5° inclination, 400 km altitude with a sun phase of 0.95 hr and a shadow phase of 0.6 hr. (b) Polar orbits (98° inclination) with altitudes ranging from 400 to 800 km and sun and shadow phases as under (a). The short revolution time leads to a high number of sun/eclipse-cycles (approximately 5600 per year) which primarily stress solar arrays and batteries. Modern solar arrays are very light configurations and represent, therefore, a low thermal mass. This has the effect that the temperature extremes will change from + 80°C to — 100°C during one cycle. For example, a 5 years mission will accumulate 28000 thermal cycles which may cause material fatigue and mechanical stresses in compounds of materials which are not properly matched. Batteries are affected in a similar way. The high number of charge/discharge cycles has a limiting effect on battery life time and requires a low depth of discharge (DOD). This in combination with the relative long eclipse phase (approximately 63% of sun
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