Hyun-Mo Ahn

Experimental Verification and Finite Element Analysis of Short-Circuit Electromagnetic Force for Dry-Type Transformer

The short-circuit force induces critical mechanical stress on a transformer. This paper deals with experimental verification and finite element analysis (FEA) for short-circuit force prediction of a 50 kVA dry-type transformer. We modeled high voltage (HV) winding into 20 sections and low voltage (LV) winding into 22 sections as similar as those windings of a model transformer. With this modeling technique, we could calculate electromagnetic forces acting on each section of the windings of a dry-type transformer under short-circuit condition. The magnetic vector potentials, magnetic flux densities, and electromagnetic forces due to short-circuit current are solved by FEA. The electromagnetic forces consisting of radial and axial directions depend both on short-circuit current and leakage flux density. These results were used as input source of sequential finite element method (FEM) to predict the resultant mechanical forces considering the structural characteristics such as stress distributions or deformations of windings, accurately. The obtained resultant mechanical forces in HV winding are compared with those of the experimental ones.

Finite-Element Analysis of Short-Circuit Electromagnetic Force in Power Transformer

Transient electromagnetic forces in radial and axial directions induce critical mechanical stress on windings and transformers. In this paper, short-circuit electromagnetic forces that are exerted on transformer windings are investigated. A 3-D transformer model is considered to calculate the transient electromagnetic forces. The magnetic vector potential, magnetic flux density, and electromagnetic forces due to the short-circuit transient currents applied to the power transformer are analyzed by a coupled electromechanical finite-element method. The results obtained are compared with the analytical results and show good agreement. The numerical modeling technique dealt with in this paper is expected to be useful in the design of power transformers.

An efficient investigation of coupled electromagnetic-thermal-fluid numerical model for temperature rise prediction of power transformer

This paper deals with coupled electromagnetic-thermal-fluid analysis for temperature prediction of a power transformer. Electric power losses are calculated from finite element method (FEM), and are used as input source of thermal-fluid analysis based on computational fluid dynamics (CFD). In order to accurately investigate the temperature distribution in a power transformer, the thermal problem should be coupled with the electromagnetic problem. The coupling method proposed in this paper is compared with the experimental values for verifying the validity of the analysis. The predicted temperatures show good agreements with the experimental values.

Finite element analysis of short circuit electromagnetic force in power transformer

The electromagnetic forces acting upon the transformer windings are calculated by numerical analysis in this paper. The electromagnetic forces consisting of radial and axial directions induce the injurious mechanical stress to windings as well as transformer itself. 3-Dimensional power transformer model by finite element analysis is considered to calculate the electromagnetic forces. The magnetic vector potential, magnetic flux density and electromagnetic forces of the power transformer are calculated numerically, and the results are compared with the analytical results.

Multi-physics Analysis for Temperature Rise Prediction of Power Transformer

In this paper, a method for multi-physics analysis of the temperature-dependent properties of an oil-immersed transformer is discussed. To couple thermal fields with electromagnetic and fluid fields, an algorithm employing a user defined function (UDF) is proposed. Using electromagnetic analysis, electric power loss dependent on temperature rise is calculated; these are used as input data for multi-physics analysis in order to predict the temperature rise. A heat transfer coefficient is applied only at the outermost boundary between transformer and the atmosphere in order to reduce the analysis region. To verify the validity of the proposed method, the predicted temperature rises in high-voltage (HV) and low-voltage (LV) windings and radiators were compared with the experimental values.

Computation of transient electromagnetic force on power transformer windings by inrush current

In general, a lot of computing time is needed to analyze the transient characteristics of a power transformer with complex geometry by field-circuit coupling method. This paper proposed a modified 2-D model and an efficient field-circuit coupling technique of a power transformer in order to reduce the computing time. To analyze the transient electromagnetic force acting on each disk of transformer windings, we made 2-D axisymmetric model. First, the transient current equation considering the residual flux was solved to get the inrush current. Next, the transient electromagnetic force due to the inrush current is obtained by applying finite element method (F.E.M.) to the modified 2-D model. The proposed method will be useful for the structure design of the power transformers.

Optimal Design of Permanent Magnetic Actuator for Permanent Magnet Reduction and Dynamic Characteristic Improvement Using Response Surface Methodology

Permanent magnetic actuators (P.M.A.s) are widely used to drive medium-voltage-class vacuum circuit breakers (V.C.B.s). In this paper, a method for design optimization of a P.M.A. for V.C.B.s is discussed. An optimal design process employing the response surface method (R.S.M.) is proposed. In order to calculate electromagnetic and mechanical dynamic characteristics, an initial P.M.A. model is subjected to numerical analysis using finite element analysis (F.E.A.), which is validated by comparing the calculated dynamic characteristics of the initial P.M.A. model with no-load test results. Using tables of mixed orthogonal arrays and the R.S.M., the initial P.M.A. model is optimized to minimize the weight of the permanent magnet (P.M.) and to improve the dynamic characteristics. Finally, the dynamic characteristics of the optimally designed P.M.A. are compared to those of the initially designed P.M.A.

Numerical investigation for stray loss analysis of power transformer

The accurate computation of stray loss is needed to find ways of lowering the losses. In this paper, the stray losses induced in the flitch plates, frames, and tank walls are investigated. A 3-D transformer model is considered to calculate the stray losses. In order to estimate the stray loss on a power transformer, the stray flux, eddy current, and etc. are analyzed by 3-D finite element method (FEM).

A Study of Coupled Electromagnetic-Thermal Field Analysis for Temperature Rise Prediction of Power Transformer

This paper deals with coupled electromagnetic-thermal field analysis for thermal fluid analysis of oil immersed power transformer. Electric power losses are calculated from electromagnetic field analysis and are used as input source of thermal field analysis based on computational fluid dynamics(CFD). Particularly, In order to accurately predict the temperature rise in oil immersed power transformer, the thermal problem should be coupled with the electromagnetic problem. Moreover, to reduce analysis region, the heat transfer coefficient is applied to boundary surface of the power transformer model. The coupling method results are compared with the experimental values for verifying the validity of the analysis. The predicted temperature rises show good agreements with the experimental values.

Short Circuit Electromagnetic Force Prediction by Coupled Electromagnetic-Mechanical Field Analysis of Dry-Type Transformer

This paper deals with the coupled electromagnetic-mechanical field analysis for short-circuit electromagnetic force of the dry-type transformer. The short-circuit currents are calculated using external circuit in accordance with short-circuit test equipment. According to short-circuit current, the generated magnetic leakage flux density in dry-type transformer model is calculated by finite element method. The radially-directed electromagnetic forces in windings are calculated using electromagnetic field analysis and then axially-directed electromagnetic forces in windings are calculated using electromagnetic-mechanical field analysis. The calculated axially-directed electromagnetic forces in high voltage winding are compared to those of measured ones and showed good agreement with experimental results.