Stability Testing and Analysis of a PMAD DC Test Bed for the Space Station Freedom
ABSTRACT The Power Management and Distribution (PMAD) DC Test Bed at the NASA Lewis Research Center is introduced. Its usefulness to the Space Station Freedom Electrical Power System (EPS) development and design are discussed in context of verifying system stability. Stability criteria developed by Middlebrook and Cuk are discussed as they apply to constant power DC to DC converters exhibiting negative input impedance at low frequencies. The utility-type Secondary Subsystem is presented and each component is described. The instrumentation used to measure input and output impedance under load is defined. Test results obtained from input and output impedance measurements of test bed components are presented. It is shown that the PMAD DC Test Bed Secondary Subsystem meets the Middlebrook stability criterion for certain loading conditions. INTRODUCTION The Space Station Freedom (SSF) Electrical Power System (EPS) is a multi-kilowatt DC power system. The eventual goal is to provide 56kW (75kW peak) of power through six (6) 9.3kW (12.5kW peak) channels. Each channel is comprised of a source subsystem, a 16OVdc (nominal) primary distribution system, and a 120Vdc secondary distribution system. The source subsystem generates and regulates the 16OVdc primary power. The primary distribution power is converted to 120Vdc by DC to DC Converters (DDCUs). The secondary distribution system then distributes the 120Vdc power to the SSF users. A Power Management and Distribution (PMAD) DC Test Bed has been developed at the NASA Lewis Research Center to provide early component and systems level data to support the development of flight hardware and EPS design. The PMAD DC Test Bed consists of breadboard hardware, configured as a single EPS channel, from power sources to user loads[l]. The Space Station Freedom Electrical Power System is unique in many ways. It will be the first space-based power system that will not have clearly defined loading conditions. Over the life of the power system hundreds
The development of test beds to support the definition and evolution of the Space Station Freedom power system
Since the beginning of the Space Station Freedom Program (SSFP), the NASA Lewis Research Center (LeRC) and the Rocketdyne Division of Rockwell International have had extensive efforts underway to develop testbeds to support the definition of the detailed electrical power system design. Because of the extensive redirections that have taken place in the Space Station Freedom Program in the past several years, the test bed effort was forced to accommodate a large number of changes. A short history of these program changes and their impact on the LeRC test beds is presented to understand how the current test bed configuration has evolved. The current test objectives and the development approach for the current DC test bed are discussed. A description of the test bed configuration, along with its power and controller hardware and its software components, is presented. Next, the uses of the test bed during the mature design and verification phase of SSFP are examined. Finally, the uses of the test bed in the operation and evolution of the SSF are addressed.
A new method of near field analysis
The problem of the accurate determination of the total electromagnetic field produced by a current distribution on a circular disc type aperture is examined. For comparison of the near and Fresnel region solutions, an exact Maxwellian integral is used as the starting point. Aperture and space coordinates are separated by the use of Bessel function addition theorems, allowing the field to be expressed as a sum of terms containing radial and angular integrals. The radial integrals involve only the disc (aperture) distribution and coordinates while the angular integrals involve only the elevation angle. The distance from origin appears implicitly in the series coefficients. These integrals are suitable for high speed computations and have been evaluated for a number of cases. The results are compared with the small angle and general Fresnel formula values for various observation angles, distances from the aperture, and aperture distributions.