Stefan Kempen

Testing of nano-insulation materials: Some ideas, some experiences

Today it seems to be beneficial to investigate how nanoparticles can serve to improve the electrical properties of insulation materials and how this affects their behavior during processing and other physical or chemical characteristics. However; a crucial factor is the choice of the filling material in combination with the carrying matrix as well as the filling technology. Hence it is useful to develop a testing arrangement that allows one to perform a significant number of tests on different materials and enables a high voltage laboratory to investigate the strength under electrical stress levels that generate partial discharges. Therefore; a testing device in accordance to IEC 60343 has been modified to use a very common and easy-to-produce plate shape for specimens and to increase throughput. Also this device allows to detect gases or to change the electrical field geometry. This paper reports on experiences with different contents of nano sized silica particles in epoxy resin matrices. This is the basis for the development of new insulation systems to be used for a more effective design of high voltage devices like rotating machines.

Calculation of electric field distribution and temperature profile of end corona protection systems on large rotating machines by use of finite element model

Future developments of end corona protection (ECP) systems require calculation of the electrical field strength along the insulating surface and the temperature profile with satisfactory accuracy. This report presents an approach for the exact calculation of relevant physical parameters for a typical ECP configuration based on the finite element method, this approach accounting for the large electrical nonlinearity as well as the weaker thermal nonlinearity of the ECP material. The peculiarities of the implemented calculation algorithm for modeling the electrically and thermally coupled system are presented and the function of the model is verified based on practical tests which confirm the extremely good correlation between the calculated and measured results.

Computer aided design of an end corona protection system for accelerated voltage endurance testing at increased line frequency

Due to highly nonlinear material characteristics the design of end corona protection systems (ecp-system) is a time consuming process. In order to accelerate this process a finite element model is developed. The model takes the nonlinear electrical and thermal coupled material properties into account. Furthermore it is able to calculate the electric and thermal behaviour of a painted or taped ecp-system. In this paper a special model is used to design a 500 Hz ecp-system. The paper quantifies why it is not possible to apply the ecp-system that was designed for power frequency also at a ten times higher frequency. Such a system is needed to accelerate the determination of the voltage endurance characteristics enabling the qualifying process of new or modified stator groundwall insulation of large turbine generators. In the first step the electrical and thermal behaviour of the insulation system with an existing ecp-configuration (50 Hz and rated voltage of 27 kV) is recalculated for an increased frequency of 500 Hz and 33 kV. In the next step an optimized layout is calculated with a new numerical algorithm, which is implemented in the finite element calculation and being efficient with calculation time. The newly developed design is verified by a test setup operating at 500 Hz and the electrical field strength distribution and temperature profile is measured.

Parameter studies on surface partial discharge inception of polluted, tangential electrically stressed boundary surfaces

Compact stator end windings of large turbine generators are stressed by tangentially stressed boundary surfaces. To achieve an optimal design, the electrical field strength has to be below the partial discharge (PD) inception field strength. In regular operation — especially for air cooled machines — several environmental impacts could affect the PD inception field strength inside stator end winding configurations, especially the influence of dust, dirt and ambient moisture. Therefore investigations are necessary to classify contamination influences in regard to its potential to cause surface corona.