CST – Computer Simulation Technology

Electrostatic Simulation of a 24 kV SF6 Gas Insulated Ring-Main Unit Load Break Switch

This article is concerned with the features required for a 3D electrostatic computation, using CST EM STUDIO® (CST EMS) of a power engineering application for which a medium voltage SF6 main ring unit load break switch manufactured by ABB Distribution, Skien, Norway [1] has been taken.

A load break switch is used to interrupt the flow of current in an electrical circuit under load conditions. Electrostatic simulations are used to indicate the locations of critical spots where discharges may occur. This aids the determination of, for example, optimal distances between fixed and rotary components. For such simulations, efficient workflow and accuracy are essential requirements.

For this analysis, simplification of the geometry defeats the purpose of the simulation. Therefore, seamless CAD support is a critical part of the simulation workflow. This entails the ability to import a wide range of CAD formats and perform automatic healing of problematic components....

The CAD model of the load break switch used in this article was provided with the permission and courtesy of ABB, Baden-Dättwil, Switzerland.



Figure 1: Imported Pro/Engineer Geometry of the load-break switch

Figure 1 shows the load break switch imported directly from Pro/Engineer and with automatic healing applied. A Pro/E assembly file consists of all parts correctly positioned and this was imported for this project.

The imported model needs to be set up with the appropriate material properties, excitations and boundary conditions. In an electrostatic simulation, the components can be handled either as Perfect Electric Conductors (PEC) or as dielectrics.



Figure 2: Calculated potential distribution for instantaneous voltages of 19.6 kV on the middle phase and -9.8 kV on the outer phases

Fixed potentials are applied to each conductor phase corresponding to the instant potentials arising during a current interruption. In this instance, 19.6 kV on middle phase and -9.8 kV on outer phases. The calculated potentials for all regions are shown in figure 2. The definition of metallic and dielectric components can be seen in the potential plot.



Figure 3: Mesh with applied curvature ratio and isotropic mesh refinement settings

The accuracy of the simulation is strongly influenced by the mesh, which must approximate the geometry as accurately as possible, and the solver order. A standard tetrahedral mesh has been applied in this case under the condition that rounded components are well approximated by the mesh. A good mesh approximation for rounded components may be generated with the aid of the curvature refinement global mesh feature where the quality of the mesh on all rounded components can be set. This can be combined with another feature, anisotropic curvature refinement, which ensures that the mesh on cylindrical surfaces will be refined only along the circumference.

As an example, figure 3 shows the application of the above features for the generation of an appropriate mesh for a single phase assembly.



Figure 4: Electric field distribution with mesh view

The aforementioned mesh settings were made to ensure that a good mesh is created but it nevertheless helps to improve the result with automatic mesh refinement.

The electric field distribution obtained from the simulation, superimposed on the generated mesh, is shown in figure 4.



Figure 5: Electric field lines for spark-over evaluation

Important in the simulation process is the computation of electric field lines which can be used with additional processing to predict the spark-over voltage or streamer criterion [2] [3].

To calculate the streamlines, seed points are required and this is facilitated in CST EMS by selecting a series of faces on the component(s) of interest. This is demonstrated in figure 5 where all of the faces on one of the switch blades were selected.

The field value is interactively available via the mouse but more useful is the ability to export the data to a text file for use in external spark-over evaluation tools. The streamline visualization and export features are fully integrated in the CST EMS environment.

This article has demonstrated some but not all of the functionality available in CST EMS. Automatic post-processing such as the extraction of field values, capacitance matrices etc. as well as fully-integrated parameterisation and opitimisation modules can also be applied.

References

[1] http://www05.abb.com/global/scot/scot235.nsf/veritydisplay/134c08f362080c33c12577ba00420459/$file/safering_24kv_e.pdf

[2] A. Blaszczyk, H. Boehme, A. Pedersen, M. Piemontesi: “Simulation based spark-over prediction in the medium voltage range”, ISH Bangalore 2001

[3] http://scee2012.ethz.ch/abstracts_new/SCEE12_Abstract_67_talk_Sterz.pdf

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