Electromagnetic field calculation and power loss evaluation in power transformers - Analysis of Leakage Fields and Stray Losses under Special Operation Conditions

Abstract

The PhD research work presented in this thesis deals with electromagnetic field calculation and loss evaluation in power transformers under special operation conditions. The special loading conditions under investigation include over-excitation, heavy loading, inductive loading and dc-bias introduced either by geomagnetically induced current (GIC) or by power converter operations. In a power transformer, the leakage flux enters the steel laminations of the iron core in different directions. The leakage flux can, depending on the orientation, add eddy current or hysteresis losses to the loss caused by the main flux. To study the principles of the influence of the leakage flux on the losses in transformer cores, an instrument that enables a loss measurement under the main flux superimposed with the transverse flux or normal flux was developed. The system was modelled using finite elements to interpret the physical phenomena. The results revealed that the loading conditions (heavy loading or inductive loading) have a significant impact on the local eddy current loss and on the overall hysteresis loss in the core. The identified additional losses indicate that under inductive loading, conventional no-load tests can underestimate the core losses considerably. Dc magnetisation due to GIC or HVDC converter operation may cause core saturation and result in a serious decline in the transformer performance as well as the power system stability. To distinguish dc-bias caused by GIC and converters, the concept of common mode and differential mode was introduced. An experimental investigation was conducted on a threephase three-limb transformer to study the power loss and reactive power consumption. The test revealed a significant difference in stray loss and winding loss between the two modes in threephase transformers. Finally, a solution to mitigate the undesired effects associated with dc-bias of differential mode was discussed. Computation of iron losses in transformers requires significant numerical efforts, particularly under magnetic saturation when the magnetic nonlinearity needs to be considered. In this PhD work, a Fourier-based effective permeability is proposed to calculate the magnetic flux density for the core loss in transformers under saturation. This includes pre-processing of the nonlinear material. A permeability frequency spectrum is obtained from Fourier analysis, where the fundamental part is used as the magnetisation definition and the harmonic components are used for loss calculation. The proposed Fourier permeability is more efficient than the time-domain method and yields significantly higher accuracy in stray loss calculation under heavy saturation than energy-based permeability.