Christian Staubach

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.

Particle swarm based simplex optimization implemented in a nonlinear, multiple-coupled finite-element-model for stress grading in generator end windings

Due to highly nonlinear material characteristics in combination with electrical-thermal coupled partial differential equations and the complex geometry the design of stress grading systems for large rotating machines is a difficult and 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 behavior of a painted or taped stress grading system. The goal of this work is to present strategies to determine optimal stress grading-configurations for a minimization of the electrical as well as the combined electrical-thermal stress caused by the potential grading. Therefore, several numerical, global bounded optimization algorithms are implemented in the finite-element-model and analyzed regarding efficiency and effectiveness. As a result a self developed partial swarm based simplex optimization algorithm (PSBSO), is introduced which obtains the best result for this special optimization problem. This hybrid-algorithm combines the positive features of particle swarm optimization (PSO) and globalized bounded nelder-mead algorithm (GBNM).

Analysis of localized dissipation factor measurements on the insulation system of mechanically aged generator stator bars

Integration of decentralised renewable energy sources lead to an increasingly fluctuating load profile of power engineering equipment. In particular, on large rotating machines this changing load profiles continuously stress the insulation in form of mechanical and thermo-mechanical forces. Eventually, a mechanical overstressing can ultimately result in an electrical breakdown of the insulation. To evaluate the condition of electrical insulation systems the knowledge of predominant aging mechanisms is of particular importance. This paper presents accelerated multi-factor aging tests with simultaneous mechanical and thermal loads on generator stator bars. Localized dissipation factor measurements as well as partial discharge detection are performed between aging cycles. An interrelationship between mechanical stress and the results of experimental examinations is indicated and analyzed. It can be shown, that mechanical stress can lead to cracking of the insulation, which is verified by the use of computer tomography.

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.

Development of a finite-element-model for transient thermal analysis of thermal cycle tests on stator bars for large rotating machines

This paper presents a three dimensional, full parameterized numerical model based on finite-element-method (FEM) to determine transient temperature distributions on real stator bar geometries. The model allows calculating the resulting timedomain thermal gradient across the insulation for internal (current) as well as external heating (oven) and therefore the comparison with measurements provided by thermal cycling tests (IEEE 1310-1990 / IEC 60034-18.34). FEM-results for simplified models of stator bar geometry are compared with the analytical results of the describing partial differential equations and advanced thermal lumped models. A detailed geometry of an exemplary generator stator bar (20 kV, 450 MVA) is modeled. This model takes several temperature depending material parameters into account. Furthermore the resulting convection, depending on cooling gas flow velocity, dynamic viscosity and temperature distribution on the stator surface, is calculated within this transient model. Finally several measurements are performed on two test-setups of generator stator bars for both situations, an internal current heating in the copper bar and external heating in an oven. The measurement and calculation results are evaluated and compared. As a result a very good correlation is obtained between theoretical and experimental investigations, which show the capability of the new developed numerical calculation model.

Electrical and thermal analysis of the stress grading system of a large hydro turbine

This paper gives an overview about comprehensive investigations and analysis on the stress grading system of a 20 kV rated voltage hydro bar. This includes relevant electrical and thermal calculation results of 3 different FEM-models to assess the efficiency of the stress grading system. Further investigations show sufficient correlation of calculation results with measurements conducted in the laboratory.

Ageing behaviour of the insulation system used in rotating machines

The integration of decentralized renewable energy sources leads to a changed load profile of electrical power engineering utilities such as generators, transformers etc. A decreased application of thermal power plants as primary supply of electrical energy results in progressive energy transition and rising load dynamics. Frequent start-up processes etc. lead to rapid changes in the operation conditions of large turbine generators and increasing thermo-mechanical and mechanical stress of the involved devices. The electrical insulation system of high-voltage engineering components is a common reason for destructive failure. To improve the reliability of electrical energy supply and to estimate the expected lifetime of power engineering devices, especially in consideration of increasing stress the complex nature of multi-stress insulation ageing has to be investigated. [1]

Detection of faults in rotor-windings of turbogenerators

This paper should give an overview of different measurement methods for detecting faults in rotor-windings of turbogenerators and explains their principal functionality in detail. Advantages and disadvantages of each individual measurement are summarized and compared. In addition theoretical calculation models to evaluate the specific methods are illustrated while at the same time the accuracy of this theoretical models are validated with the help of practical testing.