for terrestrial power systems. The main intent is to define the correspondence between the functions of a power management system (PMS) and the functions of an EMS. The third section outlines the basic structure of the major functions for a spacecraft power management system. All major functions are described in sufficient detail to compare the proposed PMS with an energy management system. The fourth section proposes a unique hardware implementation for a power management system. The fifth section gives a status report on the software development effort at Auburn University. Operating Modes There are four modes defined for terrestrial power systems: normal (secure), alert, restorative and emergency [1]. The normal secure mode implies that all schedules are dispatched as planned, that all equipment is in service or available for service and that all equipment is operating within all appropriate limits. The normal insecure mode implies that the next unplanned event is expected to cause system degradation. The amount of degradation depends on the severity of the outage and may range from equipment limit violations to major outages. The restorative mode implies that the system has experienced an equipment outage and that the power system has been reconfigured. A degradation of service is implied and action to repair the damaged equipment is needed. The emergency mode implies that equipment is overloaded, that nonessential load has been curtailed and that the system could quickly become inoperative. All of the above modes are appropriate for spacecraft power systems. Additionally, we would add another mode: inoperative. The inoperative mode implies total degradation of the power system and the use of a backup system for critical services. The initial operational mode of the power system is the inoperative state. We assume that manual intervention is required to move the system from the inoperative mode to the normal secure mode. The fundamental difference is that the transition time between modes for a spacecraft power system is expected to be faster than for a terrestrial power system. A terrestrial power system is spread over a vast area, with many generators and many motors connected to the power grid at any point in time. The inertia of these machines provides a buffer between the response of the loads and electrical system and the response of the power sources. The small size of a spacecraft power system and the small number of machines is not expected to provide a significant buffer for the control system to respond. It is also assumed that the power sources will not be able to respond quickly if they are of the thermal cycle type. If they are of the direct conversion type, it is expected that they will respond too quickly. Instead, a damping system will have to be included to maintain generation at a fixed level and to balance the load demands as the loads are switched between buses and between experiments. The basic assumptions of the spacecraft power system include: that the generation is to be maintained at a constant loading level, that a damping system is included and that a dissipation system is included. The dissipation system will convert the unusable electrical power to heat when the generation exceeds the load for a significant period of time. The expected damping system is a battery and converter system to store and to generate electrical power. The dissipation system is expected to be a thermal radiator directed to deep space.
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