6. Power Distribution COLUMBUS elements will see during their operational life a great variety of payloads and missions. Effective distribution of on-board electrical power in order to provide both reliable/safe load supply and flexibility with respect to payload accommodation is, therefore, of primary importance. The particular functions of the distribution assembly may be summarized as follows: • activation/deactivation of subsystems; • switching of power to experiments according to predefined time lining; • connection of buses and loads to active alternate or back-up modes; • isolation of single point failures to prevent failure propagation. In order to perform these tasks, the distribution equipment consists primarily of switchgear and the associated monitoring and control hardware. Switchgear may respond to crew initiated signals, computer generated commands, or operate automatically when predetermined overload conditions exist. Several types of load control switches are available but they all can be classified as electromechanical relays or solid state power controllers (SSPCs). An immediate consequence of the increased operation voltage of 150V DC is that mechanical relays can hardly by used for COLUMBUS. This is due to the arcing problem. An arc always will form between metallic contacts as they separate, provided that the initial current exceeds about one ampere and there is sufficient voltage (^50V) to establish the ionized plasma or to initiate transfer of conducting metal vapour. There are several methods available for arc interruption. However, the corresponding auxiliary equipment is so bulky and heavy that it is not suitable for space applications. Resulting thereof the application of mechanical relays for COLUMBUS is limited to the special case of switching under zero load conditions only (in order to provide galvanic isolation for safety reasons). The availability of new technologies as power FETs resulted in the development of so called solid state power controllers (SSPCs) where a semiconductor performs current interruption and thus, eliminates the arcing problem. Such a solution provides the following advantages: • increased reliability and cycle life; • immunity to shock and vibration effects; • the FET inherent fast response time allows rapid failure isolation; • easy parallel operation due to inherent current control loops. The main disadvantage is that the ‘on-resistance' of presently available devices is approximately ten times higher than that of a mechanical relay contact which causes additional losses and deteriorates the power quality at the user interface. Consideration is to be given furthermore, to the radiation sensitivity of power FETs which means for example that a negative gate voltage is needed in order to bring an n-channel device really in the off-condition. SSPCs cover both, the switching and protection task and hence, represent the indispensable main elements of medium voltage distribution systems. Regarding the fact that back-up solutions are not available (as for example NiCd batteries for energy storage), switching and protection is considered as the most critical area of the COLUMBUS EPS.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==