Development of Magnetic Models to
Assess Transformers’ Susceptibility to
Geomagnetic Disturbances
A thesis submitted to The University of Manchester for the degree of
Doctor of Philosophy
in the Faculty of Science and Engineering
2018
Yufan Ni
School of Electrical and Electronic Engineering
Abstract
During the peak years of solar activity, the magnetic field held by the solar wind has an
impact on the Earth’s magnetic field and induce an electric field on the Earth’s surface. The
Geomagnetically Induced Current (GIC) is generated between two neutral points of
transformers. The GIC can do severe harm to a power system, including to its transformers.
The worst GIC event caused a power system blackout for several hours in Quebec in 1989.
The research aims to build a representative model of core saturation and carry out simulation
studies to understand the performance of transformer cores in the high flux density region.
This in turn helps to identify the design features that need to be taken into account when
assessing the capability of a transformer to withstand over-excitation.
ATP is a kind of user-maintained software so it allows self-developed code to be added into
the software package. The results simulated by the existing ATP models are inaccurate
compared to the measured results. In addition, the existing models cannot provide flux
distribution results, so it is difficult to understand the process of how the core is pushed into
the deep saturation region by DC offset.
A new model is developed to include the equivalent electric and magnetic circuit
representations, taking flux leakage, in particular, into consideration. The flux leakage paths
are composed of the oil gaps and tank in series. This model is validated by the consistency
shown between the measured and simulated HV winding currents of a 5-limb transformer.
The peaks of magnetising currents are identified with the peaks of magnetic flux which
saturate the core.
The model can identify the design features, such as the core structure, dimension of flux
leakage paths and winding impedance that need to be taken into account when assessing the
capability of a transformer to withstand over-excitation. A 3-limb model and a 5-limb core
model are built to assess the susceptibility to GIC for different core types in high flux density
region. The delta winding plays a role in holding the 3rd harmonics and unbalanced current
generated by core saturation, and in delaying the core saturation. Lastly, Transformers are
simulated under realistic GIC waveforms, for situations with and without load.
The new model is expected to be coded into ATP to conduct a GIC study for a power system.
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