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Short Circuit Force Evaluation in 3D Core Distribution Transformers

Moradnouri, Ahmad | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 47522 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Vakilian, Mehdi
  7. Abstract:
  8. The work starts with comparison of 3D-wound core transformers against the conventional transformers using the published research results. The past works on transformer short circuit force calculation are reviewed. An algorithm is developed to design 3D-wound core distribution transformers. To determine the maximum short circuit forces, currents in different type of short circuits have been calculated. The worst case in different core configurations and different type of winding connections is determined. Different analytical methods are investigated for transformer short circuit force calculation. Two-dimensional and three-dimensioal finite element methods (using Comsol software) employed to solve the related electromagnetic field problem and to calculate the short circuit forces in 3D-wound core transformers. It is showed that the analytical methods do not determine the distribution of forces in transformer windings accurately, due to neglecting the effect of other windings and yokes. The axial forces in single phase short circuit faults are higher than the forces in three-phase short circuit faults (assuming the same magnitude of short cicuit current) due to effect of the other phase windings currents in the latter case which reduces the applied force, while the radial forces in three-phase short circuit at similar conditions are higher than the single phase short circuit faults radial forces. These results show that besides the magnitude of short circuit current, the type of short circuit (due to the distribution of leakage fluxes) plays role in the magnitude of the short circuit forces. Since the two-dimensional finite element method has a limited accuracy in field computation, the three-dimensional finite element simulations should be carried out to accurately determine the distribution of short circuit forces in a transformer windings. The reported transformers mechanical failures and the withstand capability of used materials in a winding construction is investigated. Multiobjective optimal design of 3D-wound core and core type transformers using genetic algorithm are carried out to optimize the design, simultaneously, in terms of the short circuit forces, the mechanical stresses and the total owning cost as the objective functions. Parreto fronts for 3D-wound core transformers are lower than these curves for classic core type transformers. A 3D-wound core transformer has lower total owning costs under equal force magnitude, while under an equal value of the total owning costs, these transformers have lower forces. Additionally, using the developed algorithm, new designs (by application of amorphous metals as ferromagnetic material of the 3D-wound core) are developed. It is shown that in these designs on average 66 percent reduction in the transformer no-load losses is achieved. And, despite of an increase in the transformer manufacturing cost, the transformer total owning cost is reduced
  9. Keywords:
  10. Short Circuit Current ; Genetic Algorithm ; Finite Element Method ; Optimal Design ; Transformers ; Three Dimentional Wound Core Transformer ; Short Circuit Forces ; Transformer Failures

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