Development and application of general circuit theory to support capacitive coupling.
Date
2012-09-13
Authors
Mahomed, Yusuf.
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Abstract
In textbook literature, the phenomenon of mutual inductance has been described in
rigorous detail from the magnetic field-level behaviour all the way to equivalent circuit
models, and these are valid for circuits consisting of any number of coils where there
may be magnetic flux linkage. Unlike mutual inductance, the description of multi-body
systems which exhibit electric flux coupling has not been carried through from a field
level to equivalent circuit models in the same way. Most circuit models used to
describe capacitive coupling are therefore different and cannot be easily compared.
In this dissertation, a general circuit model describing capacitive coupling is developed
from field-level theory. This model is based on a four-body physical structure, and
forms a restricted dual to the well-known two-body inductive coupling circuit model.
A quantity representing coupling capacitance was defined and given the symbol S, and
this quantity is the dual to the mutual inductance term commonly referred to by the
symbol M in textbook literature. An in-depth analysis is documented into the coupling
capacitance term S, showing that it is possible to obtain a system which exhibits
positive, negative or zero coupling. Experimental verification was done for systems
exhibiting zero coupling, 100 % coupling and arbitrary coupling. For all cases, the
experimental results had very good agreement to the values predicted using the
capacitive coupling circuit model.
The circuit properties of the capacitive coupling model hold in the same way as it does
for inductive coupling, as expected of a dual model. In general, interconnections of
capacitive coupled networks can also be made as long as specific conditions are met.
A concise discussion into different possible practical applications of the circuit model
is then provided, together with circuit diagrams. This is followed by a detailed
discussion into a condition-monitoring application for inductive (power) transformers.
It is shown theoretically and experimentally that the capacitive coupling circuit model
can be used in condition-monitoring of power transformers to detect mechanical
movements of coils.
The dissertation is concluded with a discussion on possible future work.