Date of Award

2019

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering (PhD)

Administrative Home Department

Department of Electrical and Computer Engineering

Advisor 1

Bruce A. Mork

Advisor 2

Jeffrey B. Burl

Committee Member 1

Sumit Paudyal

Committee Member 2

Laura E. Brown

Abstract

A power transformer is a very important and expensive asset for a utility. As for any other power equipment, the transformer is subject to internal faults. The extent of transformer damage depends on the fault duration. Therefore, the protective relays should provide for sensitivity, selectivity, and speed of operation to minimize the effects of damaging conditions. High sensitivity of the relay allows it to identify internal faults quickly. However, the relay should not jeopardize the normal operation of the transformer. To determine desirable relay settings, a transformer model for simulation of internal faults is required.

Transformer models, shown in the literature, require detailed design data for simulation of internal faults. Such information is rarely available outside the transformer manufacturer. This dissertation proposes development and implementation of a model for simulation of internal faults in three-phase three-winding autotransformers. The model is based on typical test report data and limited core window design information.

The first objective for developing the model is to identify the components of the transformer leakage field and their interaction during internal faults.

The second objective is to develop the Cylindrical and Segmented approaches for segmentation of a faulted coil. The main attraction of the Cylindrical approach is that the sub-coils of the faulted coil form individual coils of the same height as the healthy coils and the faulted transformer remains an N-coil transformer. Therefore, calculation of binary reactances is carried out by applying the well-known energy method and formulation of the leakage admittance matrix is performed by applying Dommel’s method. This allows an easy implementation of the internal fault model in the ATP software.

The third objective is to incorporate the magnetic core circuit in the transformer model. The role of the core circuit is to provide the interphase coupling and increase the transformer zero-sequence inductance compared to the air core. The interphase coupling allows simulation of internal faults during unbalanced operation and core saturation. The core circuit and its attachment method are adopted from the Hybrid model of ATPDraw.

The research showed that the Cylindrical approach is not as reliable as expected. However, it is a good start to acquiring sufficient experience before shifting the focus towards the Segmented approach. The author does not recommend continuing development of the Cylindrical approach for internal faults on the series and delta coils. The suggestion is to proceed with development and implementation of the Segmented approach using the guidance of this dissertation. The other important finding is the necessity to re-derive the adopted core circuit in the faulted phase. The adopted core circuit corresponds to a healthy three-legged transformer and, as per the results, is not suitable for simulation of internal faults.

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