J.L. He

Calculation of the potential distribution of high voltage metal oxide arrester by using an improved semi-analytic finite element method

This paper proposes a semi-analytic finite element method to treat the open boundary axisymmetric electrostatic field problems. The method is applied to computing the potential distribution of the high voltage porcelain metal oxide arrester (MOA). The numerical example shows a good agreement between the calculated and measured results.

Numerical Green's function of a point current source in horizontal multilayer soils by utilizing the vector matrix pencil technique

Numerical Greens function approach is proposed to calculate Greens function of a point current source in an arbitrary horizontal multilayer soil. The coefficients of Greens function are straightforwardly sampled in an iterative way in terms of the simultaneous equation system satisfying the pertinent boundary value problem, then the closed-form expression of Greens function for multilayer soil can be given by the vector matrix pencil technology. The approach proposed could be practically applied in grounding problems in an arbitrarily layered soil without needing to derive the analytical expression of Greens function.

Two-stage algorithm for inverting structure parameters of the horizontal multilayer soil

The multilayer soil structure model is the fundament to design grounding systems. A fast algorithm is presented to invert the structure parameters of the horizontal multilayer soil. The procedure is divided into two independent stages. First, Fredholm equation of the first kind with respect to the apparent resistivity is solved by using the technology of decay spectrum, which can reduce the computation time greatly. Second, the structure parameter of soil is determined by the generalized Newton-Kantorovich method, which is more robust and less noise sensitive because of using the generalized inverse algorithm to solve the nonlinear equation system. The numerical results show the validities and main features of the proposed approach. The numerical results by using the proposed method are in a good agreement with ones from geophysical exploration.

Operating temperature margin and heat load in PF superconducting coils of KSTAR

The code SAITOKPF has been developed for the design and analysis of the poloidal field (PF) coil system of KSTAR (Korean Superconducting Tokamak Advanced Research) device. The plasma operation is studied by consideration of flux conservation. An equivalent circuit model is used to simulate the magnetic coupling among the plasma current, superconducting coils, the tokamak supporting structure and the cryostat. Due to the high changing rates in the operating currents of the PF coils, the magnetic coupling can generate high AC losses including the hysteresis loss, the eddy current loss, and the coupling loss. The helium is forced-flowed through the cable in conduit conductor (CICC) to remove the losses and to keep the temperature rise in the superconducting cable lower than its current sharing temperature. The thermal coupling between pancakes, layers, and cooling channels in PF coils is simulated by a quasithree-dimensional thermo-hydraulic model. In this paper, the physical model, the numerical method and the structure of the simulation code are introduced. A nominal operating scenario of the KSTAR device is used to simulate these phenomena.

Magnetic field analysis of iron-core reactor coils by the finite-volume method

An accurate evaluation of the magnetic field of the iron-core reactor coils by the finite-volume method (FVM) is presented. The linear and quadratic interpolation scheme of FVM is developed. The numerical results are compared with those obtained by the finite-element method. Good agreement is obtained. The main advantage of the approach proposed is that the formation of coefficient matrix is very straightforward without needing the complicated mathematical derivation.

Influence of current diffusion in superconducting magnet fabricated by high stabilizer aluminum on quench propagation

The development of conductors with an aluminum super-stabilizer was proposed for some applications of accelerator detector magnets for high-energy physics and superconducting magnet storage energy devices. The conductors are designed with the cryogenics stabilization by using a large amount of low resistance aluminum stabilizer. However, it has induced to some new problems. One of these is the effect of the current redistribution between superconducting strands and stabilizer on the quench propagation and stability. In this study, we assume a superconducting magnet to be immersed in liquid helium. A quasi-three-dimensional model is proposed to study thermal diffusion in superconducting magnet and current diffusion is based on the solution of Maxwells equation. The uniform temperature distribution in the cross-sectional area of conductor is taken into account due to the high thermal conduction of stabilizer and small size compared with the length. An adaptive mesh scheme COLSYS is employed to capture the solution of temperature and current in the front of normal position. The computational model includes nonlinear thermal and electrical characteristics in superconducting strands and aluminum stabilizer and heat transfer of helium. The influence of current re-distribution between stabilizer and superconducting strands on the quench is studied.

Moving Mesh Application for Thermal-Hydraulic Analysis in Cable-In-Conduit-Conductors of KSTAR Superconducting Magnet

In order to study the thermal-hydraulic behavior of the cable-in-conduit-conductor (CICC), a numerical model has been developed. In the model, the high heat transfer approximation between superconducting strands and supercritical helium is adopted. The strong coupling of heat transfer at the front of normal zone generates a contact discontinuity in temperature and density. In order to obtain the converged numerical solutions, a moving mesh method is used to capture the contact discontinuity in the short front region of the normal zone. The coupled equation is solved using the finite element method with the artificial viscosity term. Details of the numerical implementation are discussed and the validation of the code is performed for comparison of the results with thse of GANDALF and QSAIT.

Simulation on helium expansion and quench in CICC for KSTAR with moving mesh methods

A numerical code, MFEM1D, has been developed to study the helium expansion and quench propagation in CICC. Since the heat induced flow gives cause to a high heat transfer coefficient between supercritical helium and the strands, the temperature difference between the strands and helium is assumed to be very small. The strong coupling of heat transfer at the front of the normal zone generates a contact discontinuity in temperature and density. In order to obtain convergence of numerical solutions, a moving mesh method is used to capture the contact discontinuity in the short front region of the normal zone. The coupled equation is solved by the moving mesh finite element method with artificial viscosity. Details of the numerical implementation are discussed and the quench study for the KSTAR coils is performed.

Creating multivariable response surfaces by the two-stage adaptive sampling techniques and its application in the optimization of grounding grids

A fast and efficient adaptive sampling algorithm to obtain the response surface of multivariable objective function is presented. The algorithm is divided into two stages. First, starting with a low-rank interpolation model, the technique systematically increases the rank by optimally choosing new support points in the areas of the highest error until the required accuracy is achieved. Second, the verification between the fitted result and the exact one is carried out. The optimization of grounding grids is performed to show the effectiveness of the proposed algorithm

Analysis of the toroidal HVDC grounding systems in horizontal multilayer soils

A Fredholm integral equation of the first kind has been developed for analyzing the performance of the toroidal high-voltage direct current (HVDC) grounding system buried in the horizontal multilayer soil. The method of moment is used to solve the equations. The formulation of potential in the earth due to a circular current is proposed in closed form by means of conception of the complex resistivity and depth, which leads to the saving of the computational time greatly. The numerical results show the good agreements with those in other papers and the measured data