Leading manufacturers of Transformer On-Load Tap Changers (OLTC) ensure quality and secure functionality of their products by a high amount of physical tests. However, these tests are not performed in the environment of a transformer, i.e. not in a built-in situation and without connected tap-leads. The system as a whole (transformer/OLTC/tap-leads) is finally tested in the transformer plant.
The effect of the transformer and the tap-leads on the dielectric field strength may be investigated with the application of the CST EM STUDIO® (CST EMS) electrostatic field solver.
Details, results and findings of the simulation work carried out on a typical OLTC are given in  and . The functionality of CST EMS and workflow required for dielectric breakdown strength estimation is presented in articles  and ....
For such analyses, simplification of the geometry defeats the purpose of simulation. Therefore, seamless CAD support is a critical part of the simulation workflow. This entails the need to import complex geometries in a wide range of CAD formats and perform automatic healing of problematic components. Figure 1 shows the geometry of the OLTC.
For accurate electrostatic field simulation, the approximation of rounded features plays an important role. To achieve this, curved elements of 2nd order basis functions are applied. CST EMS solvers apply the Multi-Grid approach for the efficient simulation of large models. Robust tetrahedral meshing algorithms are used to generate large meshes such as those shown in figure 1 which results in a system of 22 million unknowns.
Figure 2 shows the electrostatic scalar potential in the OLTC. Other results such as the electric field strength are also automatically available, some of which are found in references  and .
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. Field lines generated by this process for the OLTC are shown in figure 3.
This data can be exported in ASCII format for external dielectric breakdown voltage estimation.
The application of the aforementioned functionality and results are given in  and .
 M. Wiesmüller, B. Glaser, F. Fuchs, and O. Sterz: "Dielectric Breakdown Simulations of an OLTC in a Transformer", COMPEL, Issue #4, Vol 33, July 2014