Jens Seifert

Three dimensional FEM electrical field calculations for EHV composite insulator strings

Numerical simulations play a key role in various fields of engineering as approach to verify and optimize functional properties for complex configurations which cannot be studied analytically, starting from the early stages of design even before construction of the first prototype. Numerical methods can be adopted to calculate electric field magnitude on the components of high voltage composite insulating strings: resulting data can be subsequently used to verify and if necessary to optimize the design of corona rings and field grading hardware, which constitutes a key feature especially for insulator sets operating at the highest voltage levels. Three dimensional Finite Element Method (3D-FEM) is a particularly suitable tool for such purposes since both symmetrical (mirror- or rotational-) as well as non-symmetrical geometries can be taken in account in the field calculation. Overhead Transmission Line (OHTL) composite insulator sets operating within and above EHV system voltages (>345kV) are always equipped with field grading hardware in order to reduce peak field stresses along the insulator string. String configuration (V-string, single or double suspension and tension sets etc.) geometry of conductor bundle, presence of metal structures in the vicinity like i.e. the lattice tower and cross arms as well as distance of insulator sets from the ground may have remarkable influence on the actual field magnitudes encountered under operating conditions. When all these details of geometry are considered, resulting geometry is typically neither mirror-nor rotational symmetric, thus complete study can only be performed by 3D-analysis. Beside an accurate knowledge of electric field magnitude around the insulator string, accurate information about critical field level is necessary in order to perform design evaluation properly. Stabile partial discharge on polymeric insulating materials (Silicone, EPDM, etc.) may generate long term degradation of material properties and consequent reduction of reliability in service. The simulation procedure in its main steps including modeling, meshing, numerical solving and post-processing is illustrated in the paper and some of the critical features are reported and discussed. Since the number of Degrees of freedom (Dof) growths quickly with model complexity, advanced modeling and simulation techniques for electrostatic or electro-quasistatic field distributions are necessary to perform bulk calculations in a reasonable amount of time. In this regard, a full 420 kV case study is presented.

Investigation of composite insulators with microvaristor filled silicone rubber components

This paper reports possible applications of microvaristor filled silicone rubber at composite insulators. Different insulator variations fitted with microvaristor filled sheds or shanks have been investigated. The insulators are dimensioned for Um = 170 kV and Um = 420 kV. Different variations of the 420 kV type were tested for their a.c. behaviour under artificial rain. In these tests the insulators with microvaristor filled silicone rubber components showed interesting properties. The suitability of microvaristor filled silicone rubber for application both in the sheds and in the sheath are discussed based on these positive results. The applicability for outdoor applications is investigated by an “inclined plane test” (material test) as well as by a “weather ageing test” under salt fog (material and design test).

Dimension Reduction for the Design Optimization of Large Scale High Voltage Devices Using Co-Kriging Surrogate Modeling

In high voltage (HV) technology, the electrical field distribution can be improved using different field grading methods. These can be categorized into two groups: 1) capacitive or geometrical field grading and 2) resistive field grading. To obtain the optimized field grading effects, some geometrical or material parameters of the HV devices should be optimized. However, these devices are often nonrotationally symmetric and simulation is computationally very time consuming. In this paper, a multilevel surrogate method using co-Kriging methodology is proposed to optimize such large-scale 3-D HV devices. To compute the electrical field distribution of these HV devices, a finite-element method simulator can be run at different levels of complexity, i.e., by reducing the 3-D model into a 2-D model under certain additional assumptions. The co-Kriging method combines expensive runs of highly complex 3-D simulations with relatively inexpensive dimension reduced 2-D simulations. This approach is shown to allow for a faster optimization of a large-scale nonrotationally symmetric problem, while preserving a sufficiently high level of accuracy.

Investigation of electrical field grading of bushings with microvaristor filled epoxy resin components

High electric field intensities along the composite insulator surface of high voltage bushings, in particular around the area of the grounded electrode, can result in electrical partial discharges on the silicone rubber surface. The microvaristor filled epoxy resin components are applied to bushings in order to reduce the electric field intensity locally. Using this nonlinear component makes it possible to reduce the diameter of bushings, while still having a reasonable electric field distribution. This new compact design is analyzed using numerical simulations. The corresponding prototypes are experimentally tested in the high voltage laboratory.

Effects of microvaristor material on the occurrence of partial discharges upon insulators in rain test

Finite element method (FEM) simulations of high voltage composite insulators including nonlinear field grading materials in rain test are presented. These silicone polymer materials are filled with ZnO microvaristors, which feature semi-conductive field-dependent material properties. The effects of adding layers to a composite insulator made of semi-conductive ZnO microvaristor material on the voltage resistance is analyzed using numerical simulations. The resulting homogenization of the electric field is compared for various ZnO layer setups in rain test.

Electric field grading using insulators with microvaristor filled silicon rubber

The lifetime of insulators for Ultra High Voltage Overhead Transmission Lines (UHV OHTL) for voltages of 765 kV or 1100 kV is currently restricted to limited years due to unknown effects regarding the onset of streamer discharge. To overcome this limit special insulators are developed that incorporate a microvaristor layer thus restricting the peak field strength to 0.5 kV/mm (according to the switching point to be determined by the microvaristor microstructure). In this paper the current status including most recent FEM modelling and HV laboratory test results is presented. This resulted in the development of a prototypical design to be tested in a forthcoming field test.