Friedhelm Pohlmann

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.

Comparison of transient time-domain and harmonic quasi-static solution of electrical and thermal coupled numerical stress grading calculations for large rotating machines

In terms of future development of stress grading systems for large rotating machines it is required to calculate the electric surface field strength distribution as well as the temperature profile along the stress grading surface with sufficient accuracy. Due to high nonlinear material characteristics the design and optimization of such systems are a very time consuming process. In order to accelerate this process a finite element model with implemented numerical optimization algorithm was developed in the past. The model takes the nonlinear electrical and thermal coupled material properties into account. In terms of a short design time an optimization is needed to design a typical multi-layer stress grading configuration for large rotating machines, i.e. turbo-generator with a rated voltage higher than 16.5 kV and an output higher than 400 MVA, a harmonic quasistatic approximation is performed to solve the FEM-model. The advantage of this procedure is a decrease of the calculation time by a factor of about 25-100 compared to a full transient time-domain solution. Caused by the nonlinear specific resistive behaviour of the stress grading material it is usually necessary to perform a time-domain solution to consider the development of harmonic content. In case of a quasi-static harmonic calculation with an applied sinusoidal voltage this effect is neglected. This paper shows the results on a real stator bar geometry with stress grading systems by means of two different numerical models based on FEM. The first model allows calculating the time-domain behavior of the stress grading system and takes the harmonic content caused by the stress grading material into account (transient model). Introduction of electrical and thermal time constants is presented as well as the electrical potential and field strength distribution along the insulation surface over time. Finally electrical and thermal results in case of a steady state condition are compared qualitatively as well as quantitatively with results obtained by a quasi-static harmonic calculation.

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.

Investigations of modified nonlinear electrical materials for end corona protection in large rotating machines

Materials with nonlinear dielectric behavior are applied in rotating electrical machines for a defined control of the electrical field strength in the insulation system of stator end windings. To achieve better control of the electrical parameters a new filler material has been developed. Due to platelet shapes and coating with oxide layers the electrical behavior indicates a minimized spreading. The electrical resistance could be adjusted by different doping conditions of the oxide layers. Nevertheless, operational and environmental influences are not sufficiently investigated for operational application. In this paper the combined electrical, thermal and chemical properties under direct voltage stresses are worked out.

Analysis of resistance characteristics of multilayered field grading material structures on rotating machines

The proper resistance range for field grading constructions, such as the Outer Corona Protection (OCP) and the End Corona Protection (ECP), on rotating machines appears to be an essential parameter for correct performance of the entire insulation system. The insulation system of high voltage stator coils or bars usually is assembled using tape material with a certain overlap. Furthermore field grading systems with highly complex structure, such as e.g. the so called “Double-OCP-System”, were established to handle thermo-mechanical stresses occurring on large GVPI turbine generators. The complexity as well as the variety of designs makes it difficult to define the targeted system resistance value. Furthermore electrical properties measured on tape material cannot be simply used to describe the behavior of entire system. The approach in this work is to understand how the targeted resistance of complex field grading structures can be approximated based on properties of tape material. Or in reverse, how the tape material must be designed to achieve the targeted overall resistance on a global structure.

Ageing of corona protection material on rotating machines

The outer corona protection (OCP) is a field grading part of an insulation system, which is located in the stator slot area and the step iron area of an electrical machine. Recent publications show increasing numbers of ageing phenomena regarding the OCP, unrelated to a specific manufacturer. Although no direct correlation could be determined between the phenomenon and essential damage of machine insulation, it is necessary to establish a basic understanding of its main cause and the contributing parameters in order to estimate possible consequences for the machine operation and future designs.

Examination of anisotropic material characteristics in Outer Corona Protection (OCP) systems in large rotating machines

In large rotating machines the insulation system is extended by a field grading systems to provide an optimal electrical field distribution in the main insulation. In this study the anisotropic electrical material properties of the Outer Corona Protection (OCP) field grading system are analyzed. By modeling real OCP test samples and subsequent numeric calculation the systems field properties are characterized. Due to the consideration of complex spatial substructures, like the overlapping tape layers in the examined field grading system, the local field distributions will lead to an optimization of OCP resistivity configurations.