Geomagnetically induced currents (GIC) could saturate a transformer core. Two
significant effects are the abnormal stray flux in transformers and extremely large
harmonic contents in excitation currents, which can lead to serious equipment damage
and power system misoperation. Such incidents have occurred during the March 1989
K-9 solar magnetic disturbance.
This dissertation starts with a systematically study of transformer magnetization under
GIC. It reviews both dc and ac saturation patterns of five transformer core designs.
Magnetic fields along various traverses for dc excitation are presented. Impedance
matrix entries of a single phase transformer are compared for normal and dc operations.
New observations have been formed based on the simulation results. The study helps to
reveal the fundamental transformer magnetization mechanism under GIC in order to
assess potential stray flux heating possibilities of geologically vulnerable transformer
units.
Based on the finite element analysis, an improved method of modeling transformer
excitation under dc bias using equivalent magnetic circuit is developed. There are two
unique points in this approach: first, information of 3D finite element magnetic flux
distribution analysis is used to construct and verify the circuit model; second, the effect
of the transformer tank: is included The model is capable of simulating transformer excitation currents under different levels of dc bias with good accuracy. As a
consequence, the complete variations of excitation current harmonics with respect to an
extended range of dc bias are revealed. The sensitivity of transformer winding
impedances and core loss on the excitation characteristics are examined. The saturated
transformer under no-load and various loading conditions is simulated. A laboratory test
is performed on a small scale transformer and compared with the model results.
Excitation harmonics generated from dc biased three phase transformer banks with
different types of equivalent loads are also simulated The effect of both unbalanced dc
excitations and unbalanced loads are investigated The results of this study contribute in
understanding transformers as harmonic sources and the impact on power systems during
a geomagnetic disturbance.
next to an author's name indicates that all files or directories associated with their ETD are accessible from the Virginia Tech campus network only.
![[VT]](http://scholar.lib.vt.edu/images/ETD-db/restricted.gif)
