Modelling, computer simulation and constituting an algorithm for harmonic analyses of three phase transformers

ABSTRACT Harmonic phenomena in three phase transformers are quite important. It is because under certain conditions, the exciting current harmonics may cause inadvertent operation of the protective equipment and may also cause interface with communication circuits. The study is to discuss the harmonics in three phase transformers. For the harmonic analysis of power systems, it is essential to have all compenents represented in the harmonic frequency domain. This study shows how nonlinear elements can be directly represented in the harmonic domain by using a convolution equivalent to time domain multiplications. The procedure is then applieed to power transformers. The three-phase models with the specially connected transformers offered in this study distinctly represent the feeding system by means of the physical three-phase circuirt and the symetrical-compenent equivalent circuit. The three- phase model could be joined to three-phase load flow program to exactly analyse the system imbalance in the future. Transformer models for harmonic studies based on rated parameters are presented. Results from simulations and measurements on transformers are compared. Sensitivity studies are carried out to show how to total harmonic distortion of current and voltage behaves as parameters of the change. A variety of operating conditions simulated on a computer were used to test the algorithm. The results of the tests show that the algorithm performs well. Measurements of transient inrush currents in three-phase transformers offer important data for power system operation and protection. Because three-phase transformers are more widely used in industrial applications than single-phase transformers, the inrush current of the for three-phase transformer is worthy of investigation This study proposes a simple method, extended from that for three-phase transformer inrush currents. Harmonic analysis of the inrush currents is carried out. Various structures of three-phase transformers, including winding connections and loading conditions, are discussed. The simulation results are compared with experimental results. The proposed method is simple and effective for transformer banks and five-limb transformer. The harmonic analysis of inrush currents is useful for protective relay design. This study outlines an improved lecture or experiment that quantitatively analyses the wave forms of various transformer connectings under steady-state no-load conditions while taking into account the nonideal voltage supply. The experimental data obtained with the aid of a harmonic analyser are then compared to the quantitative predictions and the discrepancies are discussed. New iterative procedures using fourier transform algorithms are proposed for the harmonic studies of three-phase transformer banks with various connections under steady-state operating conditions produce different harmonic distortions, which are deliberately analysed and compared in this thesis. Good convergence is achieved for each iterative procedure of the corresponding transformer connection and the resulting current waveform and its spectrum are presented. An application program is advanced using Microsoft Visual C++ for harmonic analyses in this thesis. Simulation results proves the program's accuracy. Keywords: Harmonic analysis, three-phase transformers, saturation xiv