The Transient Response and Frequency Characteristics of Power Transformers having Non-uniform Winding Insulation

Abstract

Abstract

This paper presents a Laplace-domain distributed-parameter technique for analyzing the transient and frequency response of power transformers having non-uniform winding insulation. Stronger winding insulation near the windings source side terminals is usually applied in order to mitigate the concentration of the transient voltage stresses close to these locations. This will result in a location-dependent inter-turn capacitance distribution. The suggested procedure for analyzing such windings is based on the cascade connection of several equivalent two-ports. The s-domain expressions for the corresponding surge impedances and propagation constants for the different sections will therefore be location-dependent. Analytical closed-form expressions for the complex input impedance as well as the voltage and current distributions along the different winding sections could then be obtained. The windings series and parallel resonance frequencies and the corresponding impedance values can then be easily found. Applying numerical Laplace inversion, the windings response to any applied voltage or current stimuli is also computed. Furthermore, the presented method can identify the locations of excessive voltage or current stress concentrations along the winding and the way they are affected by the non-uniformity of the capacitance distribution. The result of detailed parameter study has been presented in order to demonstrate the effect of both the non-uniform capacitance distribution and of the transformers neutral treatment on the windings frequency and transient responses. In particular, the visualization of the modified locations and magnitudes of the overstresses as well as the changes in their resonance frequencies are also discussed. The effect of the windings copper and insulation losses and of the source internal resistance on the computation accuracy is also addressed.

Keywords: Power transformer, equivalent circuit, power system transients, simulation, transformer winding, distributed parameters, voltage distribution,
high-frequency model, non-uniform insulation