POWER TRANSFORMER OVERHEATING DUE TO GEOMAGNETICALLY INDUCED CURRENTS

This thesis concerns power transformer overheating due to (near d-c) geomagnetically-induced currents (GIC). In order to understand how the magnetic field behaves when an iron core is in deep saturation under GIC conditions, the field of a deeply saturated two-dimensional soft-iron ring is examined in detail using the well-known finite-element Newton-Raphson numerical technique. The governing non-linear vector-potential field equation used in the field calculations is derived from Maxwell's equations. The analysis adopts higher order isoparametric elements for improvement in accuracy of the results. The nonlinearity of the iron characteristic is fully accounted for in this study. This is accomplished by using the cubic spline method for modeling the entire reluctivity curve of the iron. A special feature is also added whereby the field due to the magnetized iron itself can be examined in detail.; The deep saturation condition, the nonlinearity of the iron core under ac-plus-dc excitation conditions, and the limited applicability of magnetic circuit theory to an iron circuit under deep saturation conditions make quantitative analysis of the problem almost impossible. In view of this difficulty, experimental investigations were emphasized. These include Hall-effect flux probe measurements, true power and true rms exciting current measurements, and temperature measurements. Experimental data are analyzed in conjunction with knowledge gained from the study of the field behavior of a deeply saturated iron ring.; A device based on the sensing of a unique symptom of power transformer behavior under GIC conditions has also been designed and tested on the Manitoba Hydro System. The demonstration of the slow rate-of-rise of sufficiently high 2nd-harmonic current is also included.