Square wave AC power generation and distribution of high power spacecraft

In spacecrafts, where a reasonable part of all electrical loads require individual DC voltage conditioning, the introduction of a central AC line in the power system in addition to its DC main bus is assumed to bring benefits in the overall voltage conditioning equipment. These benefits are less circuit effort and mass as well as higher flexibility and overall conversion efficiency. The above assumption could be especially valid in systems with high power capability feeding a large number of different payload equipments. Considerations on this subject have been made in several contracts of the EUROPEAN SPACE AGENCY (ESA). Their aim was to define a basic AC system approach and to study the problems and interfaces of the different functions involved in the selected system. These functions are the DC/AC generation, the AC distribution and switching, the AC/DC voltage processing. For each function several breadboard developments were performed. The type of AC voltage selected for the system, is a 20 kHz square wave with a 100 V amplitude generated from a 50 V regulated DC bus. Description of two breadboard developments is given which contain all typical functions of the AC system approach and which are representative for the high power range. One development presented is a 2 KW modular DC/ AC square wave inverter, which was designed to guarantee full performance in case of any single component failure. The modular approach applies three paralleled bridge inverter modules to handle the full output power. A fourth identical module is used in standby to replace any of the three operated modules in a case of a failure. The clearing of a failure is performed automatically by a redundant three out of four logic within less than 0,1 ms. The second development presented is a 600 W AC/DC power processing unit providing typical outputs at voltages needed to power a TWT. It generates a 600 V regulated anode supply, a 2 A regulated heater current source and a regulated 5 kV/120 mA- cathode/collector supply subdivided in five collector/ collector outputs of 1 kV each. The design results achieved in both units prove the applicability and flexibility of square wave AC distribution in the high power range.