Ki-Dong Song

Analysis of Dielectric Breakdown of Hot SF 6 Gas in a Gas Circuit Breaker

This paper presents the analysis of the dielectric characteristics of a hot SF6 gas in a gas circuit breaker. Hot gas flow is analyzed using the FVFLIC method considering the moving boundary, material properties of real SF6 gas, and arc plasma. In the arc model, the re-absorption of the emitted radiation is approximated with the boundary source layer where the re-absorbed radiation energy is in- put as an energy source term in the energy conservation equation. The breakdown criterion of a hot gas is predicted using the critical electric field as a function of temperature and pressure. To validate the simulation method, breakdown voltage for a 145kV 40kA circuit breaker was measured for various conditions. Consistent results between the simulation and experiment were confirmed.

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.

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.

Prediction of Insulation Capability for Ground Fault to Consider Asymmetry in SF6 Circuit Breaker

Currently, most high-voltage gas circuit breakers (CBs) include asymmetrical geometries in the shield, the tank, the hot-gas exhaust, and the connection parts for bushings. For this reason, a 3-dimensional (3-D) analysis of the insulation capability is necessary, rather than a 2-D analysis. However, a 3-D analysis has difficulties due to the computational time and complex modeling. This paper presents a 3-D analysis considering the asymmetry in high-voltage gas CBs and a technique to reduce the calculation time. In the proposed technique, the arc plasma requiring the most computational time is first calculated by a 2-D analysis. Then, the results such as pressure, temperature, and velocity are input as a source for the 3-D analysis. This technique is applied to a 145kV self-blast-type CB and the analysis result exhibits good agreement with the experimental result.

Modified Finite Element Method to Consider the Singularity of the Electric Field Using a Singularity Function

In the insulation design of an electric power apparatus, the triple junction which is a joining point of conductor, insulator, and gas should be considered carefully. At this point, three different materials are connected and the electric field intensity is not smooth. Local concentration of the electric field at this point can result in the surface flashover. Therefore, the accurate analysis of the electric field is necessary to design the apparatus with a triple junction. The conventional electric field analysis method such as the FEM (finite element method) can not take into account the triple junction problem accurately. In this paper, the singularity function is introduced and combined with the FEM to solve the triple junction problem. A discrete stress intensity factor is employed to prove the convergence of the proposed method. Several numerical tests show that the method can give an accurate solution around the triple junction and is very promising in solving the problem with the geometric singularity.