The subject of this thesis is to study the mechanical integrity and performance of insulation
materials used in power transformers under short circuit conditions. There are a number of
methods to calculate short circuit forces in the literature. These methods were developed to
determine the magnetic properties related to the short circuit condition and cannot be used
in the open circuit condition due to the assumption of infinitely permeable core. To this end,
a new solution strategy is introduced which is able to calculate magnetic properties of power
transformers both in open circuit and in short circuit conditions. A solution was derived
utilizing transform techniques and multiple connected permeable regions can now be
solved.
Mechanical failure modes of transformer winding are presented and new solution methods
are introduced for some failure modes. Dynamic representation of the transformer winding
is achieved by treating the winding turn by turn. Particular attention is given to stress
calculations of Continuously Transposed Cable (CTC) and resin bonded CTC.
Digital Image Correlation (DIC) technique maps the strain distribution on the test specimen
and the strain distribution can be extracted at any cross section of interest. 3‐D DIC
technique is used to determine the response of transformerboard material under tensional
and bending loads and material properties are determined. It is also shown that the DIC
method provides much more accurate results compared to strain gauges due to its
manufacturing technique. DIC results are used to determine material properties related to
both tensional and bending type loading conditions. Resin laminate wood is also compared
to transformerboard.
Viscoelastic properties of transformerboard are examined. Because a short circuit event is
dynamic in nature, storage and loss modulus of transformerboard are determined as a
function of temperature and frequency under cyclic loading conditions. Conventional creep
test setups cannot be used for transformerboard. A test setup is designed to measure creep
curves of transformerboard. The test setup measures the creep curves both in oil and in air
and the displacements are recorded automatically. Tests are performed at a wide range of
temperatures encountered in real service conditions of transformerboard material. The developed method can be introduced in PC codes to determine the magnetic properties
related to the magnetic field. Also the method can be used to determine turn to turn or disc
to disc mutual inductances of a transformer. Electromagnetic forces calculated with the
introduced method are also more accurate than the methods developed earlier. Methods to
calculate mechanical stresses acting on transformer components are also reviewed and new
solution techniques for some failure modes are developed. Mechanical performance of
insulation materials under tension and bending type loadings are measured and monitored
using DIC technique. Finally, time dependent material properties of the transformerboard
under constant and cyclic loading conditions are determined.
Both analytical and experimental techniques are utilized to determine the material
properties of the transformer components and their behaviour under different loading
conditions. New solution techniques are developed and the material behaviour of the
insulation materials under different loading conditions is determined. With the data
obtained and solutions developed, mechanical stress calculations of the power transformer
components can be made more accurately.