AbstractsEngineering

In a power transformer, the electrical steel core serves as a low reluctance path for the main magnetic flux linking primary and secondary windings. It is also one of the most costly components, whose properties are vital to design an efficient and reliable transformer. Normally, power transformers are predominantly operated within the linear portion of the core steel’s magnetisation curve with the maximum flux density limited at a certain value in the knee area. Nowadays, more technical challenges from the core saturation are raised, which are caused by the geomagnetically induced currents or the normal operation of Quadrature Boosters. The substantial power losses generated at such high flux densities can lead to the core overheating and consequential thermal degradation of the surrounding insulation, and even transformer failure.The characteristics of transformer core in deep saturation, however, are not readily available from measurements, and neither are the current IEC standards applicable above 1.8 Tesla for the measurement of magnetic properties of electrical steels owing to measurement difficulties, such as magnetic flux waveform stabilization. The simulation studies often need to extrapolate the steel’s magnetisation curve to high flux densities, which brings uncertainties to the results. In addition, the industry has often adopted a conservative transformer core design due to the insufficient knowledge of core loss and temperature rise under the extreme scenario.In order to fill the knowledge gap of electrical steels and transformer cores at high flux densities, this thesis uses an improved single strip test bench developed at Wolfson Centre for Magnetics to measure the magnetic properties of modern grain- oriented electrical steels up to 2.0 T under AC magnetisation up to 400 Hz. Based on the latest measurement results, a new single explicit expression is proposed to approximate and predict the AC magnetisation curve accurately over a wide range up to 2.0 T. A simple and accurate power loss separation algorithm is also proposed to identify the percentages of hysteresis loss, eddy current loss and anomalous loss, and predict the power loss at high flux densities.The finite element computational method based on Maxwell’s equations together with the measured magnetic properties of electrical steels up to 2.0 T leads to a more accurate predication for the distribution of magnetic flux and core losses in the power transformer core at high flux densities. The effects of core joint types, overlapping techniques, air gaps on the magnetic flux distribution are investigated in both 2D and 3D core corner joints. The distributions of the main flux, the leakage flux and the power loss in the core and the clamping structures are also obtained.