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.