Petr Martínek

Mesh statistics for robust curvature estimation

While it is usually not difficult to compute principal curvatures of a smooth surface of sufficient differentiability, it is a rather difficult task when only a polygonal approximation of the surface is available, because of the inherent ambiguity of such representation. A number of different approaches has been proposed in the past that tackle this problem using various techniques. Most papers tend to focus on a particular method, while an comprehensive comparison of the different approaches is usually missing.

Partial Discharge Potential Free Test Methods

During the last few decades, partial discharge measurement and analysis have been considered one of the most important test methods for evaluating the quality of electrical machines and insulating systems. Test methods based on potential free capacitive and/or inductive couplers have taken place as well as methods based on the galvanic circuit with measuring impedance. These potential free couplers detect the electromagnetic field that is generated by partial discharge activity. The partial discharge detection, with using of the capacitive coupler, was the main reason to perform the measurements. The conclusions of the test are presented in this paper. Furthermore, the analysis of partial discharge of several physical models was performed because of the couplers calibration and sensitivity evaluation. The different methods and apparatuses were used for partial discharge detection with the aim of the accuracy and sensitivity of the potential free method determination. The diagnostic equipment consists of a wideband analog apparatus, a digital apparatus, couplers and measuring impedance. The measurement was done according to IEC 270 standard. The capacitive couplers have been used as well for performing the potential free test method measurement.

Partial discharges of thermally and electrically aged insulation

The working life of electrical machines is primary affected by the insulation system quality. Diagnostics methods help to understand the momentary state of insulation and to avoid the possible damage or electrical breakdown of machines through the repair or insulation rewinding. Partial discharges testing belongs to one of the high applicable test method of insulating materials within electrical machines. The described experiment consists of laboratory thermal and electrical aging of main wall insulation and consequently testing of partial discharges. The test material consists of resin rich mica composites based on glass fabric and epoxy resin. The flat specimens of insulation were cured and thermally and electrically aged and the partial discharges were measured accordingly. The thermal aging was performed at four different temperatures (170, 175, 180, 186°C) at different times. The electrical aging was completed at two different voltage levels (10, 12 kV) at different times. The flat specimens were tested at special electrode setup. The conductive rubber electrodes were used to avoid the gliding discharges on the surface of specimen. The trends of partial discharge main parameters (ignition voltage, extinguish voltage, pulse count, average discharge current, peak charge level) are studied and described in dependence on exposure time, temperature and applied voltage.

Aspects of statistical treatment of electrical breakdown behaviour in gas-insulated apparatus

The knowledge of stochastic breakdown behaviour of environmental friendly gas mixtures become more and more important with respect to the required replacement of SF6 - insulated apparatus. To obtain the necessary withstand values of breakdown voltage at the variety of desired parameters a statistical treatment of the measured data is required. The paper deals with the estimation of these values typical for such type of gas- insulated systems e.g. the approximation by a theoretical distribution function and necessary conditions. Problems at estimation will be discussed as non-conventional statistical treatment including some recommendations for their overcoming.

Partial discharge measurement at high voltage test circuit having static frequency converters

For testing of high voltage (h.v.) components under so-called unconventional frequency test conditions as h.v.-test source very often static frequency converters (SFC) were applied. A typical example for that is the testing of power transformers at higher frequency. In the past this test was provided by standardized motor-generator sets, but more and more alternative power sources having SFC-units are used. In that case the performing of sensitive diagnostic test procedure like partial discharge (pd) measurement seems to be impossible caused by vast electromagnetic disturbances which are generated by the switching part of the SFC. On the other hand the performing of such sensitive diagnostic measurement is obligatory. For overcoming of these problems two possibilities will be described in the paper. At first the EMI pulses which are coupled into the test circuit must be suppressed. Unfortunately, because of the mostly unknown “current-way” of these disturbances, the optimal solution of suppression must be found by a “half-empirical” way, taking into account the well-known EMC requirements. In addition to that the right choice of the commonly used filter located after the SFC-component is very important and should be designed following the performed EMC measures. For an exemplary h.v. test circuit the appropriate measures to minimize the EMI disturbances are described, at which a pd-sensitivity of ~ 30 pC could be obtained at the full range operation of SFC. For further increasing of pd-sensitivity additional measuring methods were investigated. For a desirable measuring sensitivity of ~ 10 pC the suppression of EMI signals, reached by additional measures should have at least a value of ~ 10 (20 dB), what could be obtained either by digital or by analogous methods. Using all the described measures for h.v. test circuits having SFC a pd measuring sensitivity in the range of few pC seems to be reachable.

Erosion-Inspired Simulation of Aging for Deformation-Based Head Modeling

Abstract Simulation of age progression of 3D head models is an open problem in the field of computer graphics. Existing methods usually require a large set of training data, which may not be available. In this paper, a method for aging simulation of models created by deformation-based modeling is proposed that requires no training data. A user defines the position of wrinkles by selecting the position of endpoints of the desired wrinkles and the wrinkles are then generated using an erosion-inspired approach. The method can be used to simulate aging of any head model, however, if used for models created by deformations of a base model, the erosion factors can be calculated only for the base model and applied to the derived models. The results show that the approach is capable of creating visually plausible aged models.

Curvature-Based Feature Detection for Head Modeling

Abstract In the field of 3D head modeling and animation, feature points are often needed to mark important regions of the face and can be used to animate or deform the input model. However, an automatic detection of features remains a challenging task. This paper presents a novel approach to feature detection based on curvature and its derived descriptors, such as shape index, curvedness and Willmore energy. Four important feature regions are detected using the proposed approach - eyes, nose, mouth, ears. For each region, feature points are detected. Results show that the feature points are detected with sufficient accuracy for further use.

Dielectric behaviour of gaseous insulation with synthetic air at AC stress

The paper deals with basic investigation of AC breakdown behaviour of gaseous insulation with synthetic air at different level of field homogeneity and gas pressure. The measurement was performed at different gas pressure ranging from 0.1 to 0.25 MPa and gap distances from 1 to 9 cm, using an electrode test-set “tip-bowl”. The test parameters were chosen in such a way that different field types could be realized. At small distances the electrical field is weakly inhomogeneous and the breakdown values correspond to the discharge inception level. The measured values show a reasonable agreement to those values based on the calculation of the streamer criterion by using a relevant approximation formula for the effective ionisation coefficient. For larger distances the electrical field becomes more inhomogeneous and the calculated streamer inception voltage corresponds to the partial discharge (PD) inception voltage only. The calculated inception voltage values fit the measured values over the applied gap distance and gas pressure quite sufficiently. The best fit to the measured results was reached by slightly changing the parameters of the approximation formula depending on the gas pressure value. Additional PD measurement shows different PD activity dependent on gas pressure and gap distance.

Partial discharges in high-voltage insulation of turbogenerators

This paper is aimed at the development of one of the main turbogenerator diagnostic methods — partial discharge galvanic measuring on high-voltage stator winding insulation. For this purpose the stator slot model with inserted stator bars which, in maximal possible measure, respect the real machine electrode configuration were constructed. Artificial defects were created on stator bar samples similar to those which may occur during the technological production process or the machine operation. Samples with a large scale of types of defects like delamination from copper stack, interlayer delamination, stress grading tape on the slot exit, defects on external corona protection in the slot part and samples with undamaged insulation were analyzed in detail. Partial discharge patterns of these defects show considerable dissimilarities. Individual measurements were analyzed and compared in term of phase distribution of measured discharge levels and count of discharge occurrence. Obtained results were confronted with measurements on individual winding phases of turbo generator and the applicability of the model measurements for practical utilization was considered.

Partial discharges of solid insulation in different insulating fluids

Partial discharge measurement and subsequent analyses is one of important and capable test of electro-technological diagnostics. This test evaluates the properties of electrical insulation system of electrical machines and devices. There are a lot of partial discharge test methods based on physical or chemical detection of partial discharge. Described experiment consists of laboratory testing of partial discharges activity of insulating material samples in different insulating fluids. Flat specimens of insulation were prepared, aged and the partial discharges were measured accordingly. The trends of partial discharge main parameters (ignition voltage, extinguish voltage, pulse count, average discharge current, peak charge level) are studied and described in dependence on exposure time, temperature and applied voltage.