Finally some important interface requirements are imposed to the EPS. During launch and deployment sequences, for servicing and operation, COLUMBUS elements have interfaces to: • Orbiter, • ARIANE, • HERMES, • US Space Station (USSS). In view of the ‘integral' and the ‘attached mode' of the Pressurized Module, the power interface to the USSS is of particular importance. A major goal of all COLUMBUS-related design activities is to have as far as possible a compatible user interface with the USSS. A decisive question in this context is whether AC or DC shall be distributed on COLUMBUS elements. Present NASA activities concentrate very much on the AC option (440 V; 400 Hz; 3 ohms), whereas in Europe DC systems are favoured. This is primarily for the following reasons: • Regarding present state of the art of European technology, DC sources (solar arrays/batteries) are available for COLUMBUS. Conversion of the DC power to e.g. 400 Hz AC power would require large static inverters. Consequence: additional complexity, additional mass and increased costs. • Modern technolgies favour DC interfaces. DC/DC conversion can be performed with minimum mass and power losses when high frequency operated power FETs are used. • Induction motors will be replaced by brushless DC motors which are of smaller physical size and mass and provide better dynamic properties. For these reasons the experts from ESTeC, AEG and other involved companies decided that the ‘working horse' for all COLUMBUS elements will be a DC system. 2.2 Power System Reference Configuration The results of section 2.1 have been synthesized into an EPS reference configuration which is shown in Fig. 1. This reference configuration consists of four major building blocks: • The Primary Power Assembly (Block I) where the power is generated, stored and conditioned. Because three independent regulated mainbuses are provided, the EPS can be classified as fail-operational/fail-safe. In order to handle the high power levels within certain mass and volume constraints, the operational voltage is increased to 150 V de. • The Power Distribution Assembly (Block II) which basically comprises two distribution levels and the power and signal harness. Each PDU on Level I splits one mainbus in a series of subsystems and payload dedicated subbuses. The Modular and Standardized Distribution Units on Level II offer versatile standard interfaces to the individual users. On both distribution Levels comprehensive monitoring, switching and protection functions are implemented. • The Power Interface Units (Block III) which feed power to COLUMBUS elements: —during launch and deployment sequences from Orbiter/ARIANE, —during servicing periods from USSS/HERMES;
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