Frank Jenau

Demanding response time requirements on coherent receivers due to fast polarization rotations caused by lightning events.

Lightning events can cause fast polarization rotations and phase changes in optical transmission fibers due to strong electrical currents and magnetic fields. Whereas these are unlikely to affect legacy transmission systems with direct detection, different mechanisms have to be considered in systems with local oscillator based coherent receivers and digital signal processing. A theoretical analysis reveals that lightning events can result in polarization rotations with speeds as fast as a few hundred kRad/s. We discuss possible mechanisms how such lightning events can affect coherent receivers with digital signal processing. In experimental investigations with a high current pulse generator and transponder prototypes, we observed post FEC errors after polarization rotation events which can be expected from lightning strikes.

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.

Measurement and statistical analysis of Partial Discharges at DC voltage

Partial Discharge (PD) detection and diagnostics is an established tool to test the condition of the insulation in HV-facilities. One basic question while performing PD measurements is the one concerning the type and location of the defect. Under AC stress there are several instruments to interpret the measured PD, most important the phase angle histogram. At DC voltages this approach is inapplicable and therefore new parameters to analyze PD under DC stress have to be identified. In this paper it is shown, that a statistical analysis of pulse shape parameters can give information about the type of defect and thus could be used for DC-PD diagnostics.

Time domain analysis of partial discharges at DC voltage in air and insulation oil

An important tool for the estimation of the insulations condition in electrical devices is the partial discharge (PD) measurement. Under AC stress, the PD phase distribution can be used to classify the type and location of different defects. At DC voltage this approach is inapplicable. Therefore, other parameters for the classification of different PD-sources have to be identified. Here the pulse shape can provide information about the source of a measured PD. To prevent distortions of the PD-pulses by the measuring circuit a high frequency (HF) optimized setup is used to record the pulse shape. The pulse shape of corona in air and corona in mineral insulation oil is measured, analyzed and discussed.

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).

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.

Comparative investigation on pulse shape parameters of partial discharges in air under AC and DC voltage stress

In order to identify the type and the location of fault, partial discharge diagnosis is a valuable method. Until now, a majority of the evaluation criteria used for partial discharge diagnosis is based on investigations performed at AC voltage. The increasing importance of DC transmission systems requires additional research concerning the behaviour of partial discharges at DC voltage. Especially with regard to the different dominating physical processes and the associated influence on the appearance of partial discharges, a comparison of the behaviour under AC and DC voltage stress is necessary. By focussing on the pulse shape, two typical types of fault in air are emulated and stressed at AC and DC voltage under comparable test conditions. The pulse shapes occurring under negative and positive voltage stress are captured with a broadband measuring circuit. Characterizing parameters such as the rise time, the pulse width and the fall time are analysed, compared to each other and discussed.

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.

Ion current studies using a small corona cage

To increase the transmittable electrical power without building more transmission lines, a conversion from an AC circuit into a DC circuit seems reasonable. Within the resulting hybrid transmission line system, coupling phenomena have to be considered. This paper concentrates on the measurements concerning the coupling effect induced by the HVDC system, influencing the HVAC systems: the ohmic coupling. In order to investigate this coupling phenomenon under laboratory conditions, a small scale measurement setup is designed to observe the magnitude and the dependencies of the underlying effects. In this paper, the small scale test setup is presented and evaluated concerning its capability. Different conductors are tested and the influence of wind on the ohmic coupling is analyzed.