4. POWER DISTRIBUTION SYSTEM The Power Distribution System (PDS) controls, conditions, and transmits power from the conversion system to the MPTS. It also provides energy storage and handles fault detection and isolation. The major mass components of the PDS are: the feeders which carry power from the individual power conversion units, such as a single generator, to the main bus; the main bus, which carries power from the conversion section of the SPS to the rotary joint; the electrical rotary joint, which carries power across the joint between the sun-facing and the earth-facing parts of the SPS; the voltage conversion units which transform the bus potential to the various potentials required by electrical subsystems; switchgear which allows individual power sources and sinks to be separated from the rest of the PDS; and energy storage facilities for powering essential subsystems during occultations and shutdowns. The details of the PDS are dependent on the final SPS configuration; however, some concepts for the major subsystems are discussed below. 4.1 MAIN BUS Four options were considered for the main power buses. These were to use sheet aluminum conductors, refrigerated conductors, superconducting buses, or microwave transmission. The refrigerated conductor option utilizes the decreased electrical resistance of most solid conductors at low temperature, resulting in less lost power and a less massive power conversion system. The superconducting option utilizes the fact that electrical resistance vanishes in a few materials at temperatures of 20 K or less, so no power is lost to resistance heating. The microwave transmission option replaces massive conductors with massless beams of radiant energy, but inserts an additional conversion process with its losses and mass. The sheet aluminum option requires essentially no non-lunar material, and resistance losses can be made arbitrarily small by increasing the width or thickness of the bus. Both of the cooled systems require a great deal of Earth material as coolant and, in the superconducting case, as conductor. They also add to the system complexity and absorb some parasitic power in the cooling system. The microwave system is limited by poor efficiency and requires some non-lunar material as coolant. Thus, the sheet aluminum option was selected because it requires the least non-lunar material and is the least complex. 4.2 SWITCHGEAR The reference SPS(2) uses liquid metal plasma valve switchgear. Two other options have been identified. The first is to use mechanical switches with solid conductors. The other is to use semiconductor switches. Mechanical switches require little non-lunar material, but they are relatively massive because of the need for large contact areas and cannot respond very quickly. Semiconductor switches (silicon controlled rectifiers
RkJQdWJsaXNoZXIy MTU5NjU0Mg==