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
Overview and Evolution of the LeRC PMAD DC Test Bed
Since the beginning of the Space Station Freedom Program (SSFP), the Lewis Research Center (LeRC) has been developed electrical power system test beds to support the overall design effort. Through this time, the SSFP has changed the design baseline numerous times, however, the test bed effort has endeavored to track these changes. Beginning in August 1989 with the baseline and an all DC system, a test bed was developed to support the design baseline. The LeRC power measurement and distribution (PMAD) DC test bed and the changes in the restructure are described. The changes includeed the size reduction of primary power channel and various power processing elements. A substantial reduction was also made in the amount of flight software with the subsequent migration of these functions to ground control centers. The impact of these changes on the design of the power hardware, the controller algorithms, the control software, and a description of their current status is presented. An overview of the testing using the test bed is described, which includes investigation of stability and source impedance, primary and secondary fault protection, and performance of a rotary utility transfer device. Finally, information is presented on the evolution of the test bed to support the verification and operational phases of the SSFP in light of these restructure scrubs.
Intensification of Bubble Disintegration and Dispersion by Mechanical Stirring in Gas Injection Refining
Water model experiments were performed for establishing highly efficient gas injection refining processes. Mechanical stirring was applied to disintegrate the injected bubbles and to disperse them widely in the bath. The bubble disintegration and dispersion were investigated by changing rotation mode (direction of rotation), rotation speed, blade size of the impeller and gas flow rate. Forward rotation of the impeller induced a stable tangential flow and could not disperse bubbles in the bath due to formation of a vortex around the impeller shaft. The tangential flow could be suppressed by forward–interrupt rotation, which could reduce the vortex formation to some degree. However, forward–interrupt rotation could not disperse the bubbles widely in the bath. Forward–reverse rotation could prevent the vortex formation completely and create a strong shear stress field, which intensified the bubble disintegration and dispersion in the bath. Higher impeller rotation speed and larger blade length in forward–reverse rotation could enhance the bubble disintegration and make the dispersed bubbles smaller. The bubble dispersion zone became wider with larger blade length. The bubble size tended to be larger at higher gas flow rates. However, its dependence on the gas flow rate became smaller at higher impeller rotation speed.