Yeon-Ho Oh

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.

Calculation of Temperature Rise in Gas Insulated Busbar by Coupled Magneto-Thermal-Fluid Analysis

This paper presents the coupled analysis method to calculate the temperature rise in a gas insulated busbar (GIB). Harmonic eddy current analysis is carried out and the power losses are calculated in the conductor and enclosure tank. Two methods are presented to analyze the temperature distribution in the conductor and tank. One is to solve the thermal conduction problem with the equivalent natural convection coefficient and is applied to a single phase GIB. The other is to employ the computational fluid dynamics (CFD) tool which directly solves the thermal-fluid equations and is applied to a three-phase GIB. The accuracy of both methods is verified by the comparison of the measured and calculated temperature in a single phase and three-phase GIB.

Prediction of temperature rise in gas insulated busbar using multi-physics analysis

This paper describes the coupled magneto-thermal analysis technique to calculate the temperature rise in the gas insulated switchgear (GIS) bus bar. Harmonic eddy current analysis is carried out and the power losses are calculated in the conductor and shield tank. Two methods are presented to analyze the temperature distribution on the conductor and tank surface. The one is to solve the thermal conduction problem with the equivalent natural convection coefficient and applied to the analysis of single phase GIS system. The other is to employ the computational fluid dynamics technique which directly solves the thermal-fluid equations inside the tank. The method is applied to the analysis of a three phase GIS problem. The accuracy of the developed technique is verified through the comparison of the measured and simulated temperature rise in a single phase and three phase GIS system.

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.

Investigation of the Supercritical Fluids as an Insulating Medium for High Speed Switching

The paper investigates the insulation properties of the supercritical CO₂ (SCCO₂) fluid as an insulating medium for electrical apparatuses. The insulating material is crucial for electrical apparatuses and SF6 gas has been widely used for high power electrical apparatuses. There have been many research efforts to develop substituents for SF6 gas because of high global warming potential. We obtained above 350 kV/mm insulation strength with 12.0 MPa SCCO₂. The positive and negative IEC standard pulses are applied between two 10 mm diameter spherical electrodes. The insulation strength of SCCO₂ is at least 2.5 times higher than that of CO₂ gas at 6.0MPa. The insulation strength of SCCO₂ fluid is about 10 times higher than that of SF6 at 0.5MPa which is the ordinary operating pressure of electrical switchgears. Using the result, we expect that the time for switching and dielectric recovery could be reduced using SCCO₂ fluid as an insulating medium.

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.

Thermal Recovery Characteristics of a CO 2 Mixture Gas Circuit Breaker

Interruption tests were conducted using the same circuit breaker for an initial pressure of SF6 0.5 MPa (gauge pressure) and CO 2 mixture 1.0 MPa, 0.8 MPa, and 0.6 MPa. The pressure-rises in the compression and thermal expansion chambers were measured for verifying the computational results using a simplified synthetic test facility. Further, the possibility of the CO 2 mixture substituting SF 6 gas was confirmed. Moreover, in view of the thermal recovery capability, it has also been confirmed that the pressure of the CO 2 mixture can be reduced almost to the same value as that of the SF 6 gas by optimizing the design parameters of the interrupter.