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As direct-current space power systems continue to grow in size,
switching power converters are playing an ever larger role in
power conditioning and control. When a large direct-current system
that uses power converters of this type is being designed, special
attention must be placed on the electrical stability of the system
and of the individual loads on the system. In the design of the
electric power system of the International Space Station (ISS),
NASA and its contractor team led by the Boeing Defense & Space
Group placed a great deal of emphasis on designing for system
and load stability. To achieve this goal, the team expended considerable
effort deriving a clear concept on defining system stability in
both a general sense and specifically with respect to the space
station.
The ISS power system presents numerous challenges with respect
to system stability--such as high power, complex sources, and
undefined loads. These were further complicated by source and
load components being designed in parallel by three major subcontractors
(the Boeing Company, Rocketdyne Division/Rockwell International,
and McDonnell Douglas Corporation) with interfaces to both sources
and loads being designed in different countries (Russia, Japan,
Canada, Europe, and others). These issues, coupled with the program
goal of limiting costs, have proven to be a significant challenge
to the project.
As a result, the program derived an impedance specification approach
for system stability. This approach is based on the significant
relationship between source and load impedances and the effect
of this relationship on system stability. The impedance specification
approach is limited in its applicability by the theoretical and
practical limits on component designs as presented by each system
segment. Therefore, the overall approach to system stability implemented
by the ISS program consists of specific hardware requirements
coupled with extensive system analysis and hardware testing. The
requirements for hardware integrators are that the system phase
and gain margins must be 30° and 30 dB, respectively. In addition,
wherever practical hardware elements will be tested together,
end-to-end stability and functionality must be ensured. Following
this approach, the ISS program plans to begin construction of
the world's largest orbiting power system in 1997.
The impedance specification approach for system stability was
accomplished as a result of cooperative work of the International
Space Station program team, which consists of the NASA Lewis Research
Center, the Boeing Company, and Rocketdyne Division/Rockwell International.
In addition, major contributions were provided by the Virginia
Polytechnic Institute and State University working under a grant
to NASA Lewis.
Previous articleLast updated May 1, 1997
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