Transformers are subjected to voltages and currents of various waveforms while in service or during insulation tests. They could be system voltages, ferroresonance, and harmonics at low frequencies, lightning or switching impulses at high frequencies, and corona/partial discharges at ultra-high frequencies (a brief explanation is given at the end of the chapter). It is of great importance to understand the frequency characteristics of transformer windings, so that technical problems such as impulse distribution, resonance, and partial discharge attenuation can be more readily solved. The frequency characteristics of a transformer winding depend on its layout, core structure, and insulation materials.
Analysis Methods Of Winding Frequency Charateristics
The frequency characteristics of a transformer winding may be analysed from its terminal transfer function. Sinusoidal low voltages with different frequencies are applied at one end of the winding, and the response is measured at the other end. If the winding can be accessed at several points, the sinusoidal voltage distribution along the winding can also be analysed. Although the detailed equivalent circuit of a winding consists of distributed inductance, capacitance and resistance, within a certain frequency range it may perform approximately as a transmission line or a capacitive ladder network. These frequency ranges are important in impulse voltage distribution analysis and partial discharge location.
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