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Design and Development of a Transformer Restricted Earth Fault Protection Immune to Current Transformer Saturation

Anani, Esmaeil | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55470 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Hajipour, Ehsan
  7. Abstract:
  8. One of the shortcomings of differential logic in power transformers is its inability to detect small percentage faults at the end of the winding. The occurrence of a phase-to-ground short circuit in the last few percent of the winding near the neutral causes a very large fault current to pass through the neutral and the faulted coils, while the changes in the current observed at the transformer terminal will be very small. To improve the performance of the power transformer protection system, a restricted earth fault (REF) protection is normally used in parallel with the differential protection. To implement REF protection, several solutions have been proposed in scientific and industrial references, which can be categorized into two categories: protection based on the amplitude of zero sequence currents and protection based on the angle of zero sequence currents. In modern industrial relays, the second category is more common. Until now, the comparison of different REF protection methods has been only theoretical and based on fault steady state studies (symmetric components). In the implementation of transient tests, references have focused on the comparison of these algorithms in a scattered manner, and a focused and comprehensive study was felt missing. With the introduction of the IEC 60255-187-1 standard, which proposed all the transient test requirements for REF protection algorithms, a platform was provided for different REF algorithms to be properly tested. In this thesis, at first, the REF protection algorithms mentioned in reliable industrial references are introduced. Then, theoretically, it is shown that Siemens 7UT 613 relay algorithm performs better than other algorithms with a small difference. After that, the transient behavior of this algorithm is evaluated by using IEC 60255-187-1 standard tests. Also, by examining the behavior of this algorithm against current transformer saturation, it is shown that this algorithm is sensitive to current transformer saturation, and as a result, this thesis proposes a practical and appropriate solution to improve the behavior of REF algorithms against current transformer saturation. It is shown by simulations of the standard that the proposed method improves the performance of REF protection. In order to validate the proposed method, in this thesis, three-dimensional finite element modeling has been used to model the percentage faults. Finally, the proposed method has been implemented on an industrial relay and its operation time and accuracy have been reported.
  9. Keywords:
  10. Transformer Protection ; Protection Relays ; Current Transformer Saturation ; Current Transformer ; Restricted Earth Fault (REF) ; IEC 60255-187-1 Standard

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