Technical Papers and Presentations

Power Transformers are the most important piece of power system elements and protecting this device from failures, especially from the internal faults has been a major challenge. The internal faults such as Turn-to-Turn (T2T) and Turn-to-Ground (T2G) has been attributed to most of the transformer failures. These faults typically evolve from a small winding fault to a full-blown large magnitude fault current showdown. Conventionally to study the transformer behavior under faulted condition, an EMTP environment is approached. The leakage inductances for transformers are typically estimated using simplified approximation from traditional formula-based approaches but in the case of either T2T or T2G condition, the estimation becomes more tedious using analytical approaches. Fig. 1 presents a simple case of inductance estimated using different analytical approximates for a varying breadth for an air core coil. The problem becomes more complex with the inclusion of core effects, non-standard winding configuration and dimensions.

In this work, COMSOL Multiphysics^® simulation software is used for parameter estimation of leakage reactance for segmented coils of the power transformer for T2T and T2G faults. The 2D axisymmetric model of a three winding autotransformer is modeled in COMSOL Multiphysics^® and validated against the values estimated from other means for healthy condition. This binary SC reactance for coils within the same group and with other coils are estimated using the AC/DC Module of the COMSOL Multiphysics^® software. The location of the faulted coil, configuration and number of turns are defined under Parameters node (Pi), Global definitions which plays a very vital role in automating the complex procedure. The Parametric Sweep in the Study node is also performed in continuity for studying the sensitivity of the coil inductance with respect to fault position on certain coil and for varying number of faulted turns contribution.

The empirical curves generated from Derived Values are used to produce the inverse inductance matrix using Matlab scripts. This output library files are then exported to ATPDraw template of Hybrid transformer and used for studies in the EMTP environment such as energization, fault disturbance and also cases for relay testing. The finite element model built for healthy and faulted (T2T or T2G) is will be coupled with the electric circuit to simulate the comprehensive electric connection to validate the model in the FEM environment and to investigate the developed model. The main objective of the work is to demonstrate that using COMSOL Multiphysics^®, FEM based method can be implemented to build Hybrid transformer models with internal faults and overcome some of the serious limitations observed from the analytical approaches.