Search

The acoustic emission signals produced by discharges of a point-plane electrode on the surface (surface discharge) of different oil-impregnated aramid pressboards are detected by a 150 kHz acoustic emission sensor mounted on the outside of the experimental tank sample. Numbers, maximum amplitudes and average amplitudes of acoustic bursts are measured via a digital signal processor device and evaluated. The effect of transition mechanism from spark to pseudoglow discharge on acoustic emission signals at different voltages is examined. The tracking patterns on the surface of four different aramid pressboard insulations are compared together by using Matlab image processing program. By experimental results, tracking patterns on the surface of the bulk impregnated pressboard insulation have a significant effect on the acoustic emission signals. Acoustic emission signals amplitudes decrease dramatically on damaged pressboards and cause so called silent PDs. [ABSTRACT FROM AUTHOR]

This paper proposes an explanation for the aging of solid insulating materials exposed to high frequency voltage impulses based on thermal stimulating processes triggered by losses generated in insulating materials due to damped oscillations of molecular dipoles in high frequency electrical fields. For this purpose, total dissipation of insulating materials is calculated based on frequency domain description of applied voltage and frequency dependent dielectric properties of insulating materials. To validate the proposed theory, different experimental investigations are carried out on thin oil impregnated paper exposed to high voltage square wave shape pulses with different pulse durations, rise times and repetition rates. The trends obtained in experimental studies can be satisfactorily explained using the proposed theory. [ABSTRACT FROM AUTHOR]

Accelerated aging tests have been extensively used on motor and generator insulation systems to simulate in a short time period the deterioration mechanisms occurring during normal operation of many years, to identify insulation system designs with longer life time and to support the qualification of a new system. The contribution presents results from extensive investigations on the behavior of two VPI machine test insulation systems (reference and candidate) with thermal class F (155 °C), which were subjected to three thermal overstresses in repeated cycles. After each thermal cycle the insulation system is subjected to mechanical stress, moisture, and voltage. In order to check the condition of the insulation systems in a non-destructive manner several properties of insulation like dissipation factor, insulation resistance and partial discharge quantities were measured after each thermal cycle. After that the end of life of the insulation systems were checked with ac voltage and impulse voltage. The results support the qualification by comparative evaluation and they indicate, how the changes in electrical and dielectric properties of insulation take place during the accelerated aging test. [ABSTRACT FROM PUBLISHER]

Partial discharge (PD) detection in the ultra-high frequency (UHF) range is state of the art. One of the most critical points in this detection method is adopting proper and suitable antenna design. In this paper, a new design of UHF antennas is introduced which is suitable to be used on the transformers in the HV laboratories of the manufacturers. To ensure the applicability of the new antenna, experimental UHF PD measurements are carried out on a transformer tank model as well as on the real power transformers. Installing several UHF antennas on the transformer tank to perform defect localization is also an important issue. There are rigorous limitations and lack of the places to install the antennas. This problem is almost solved by using the mentioned new antenna design. The proper positioning of the antennas is the other critical point in the UHF method. Placing the antennas in improper positions will damage the localization accuracy. In this paper, an automatic algorithm is introduced to determine the best installation positions for the antennas to enhance the localization accuracy. The efficacy of this algorithm is investigated by using the results of experimental measurements on power transformers. [ABSTRACT FROM AUTHOR]

Dielectric spectroscopy in the wide frequency range of 1 mHz to I kHz has been used to study the polarization and conduction phenomena in insulating liquids. The dielectric spectroscopy of mineral oil and ester oil at different conditions, i.e. temperatures and electric fields, has been measured. It came out that at low frequencies, space charge polarization changes the permittivity of insulating liquid and at very low frequencies, the consideration of a RC parallel model for the dielectric behavior of insulating liquids is not adequate. The results indicate that the permittivity of insulating liquid increases with aging duration and therefore it can be used as a diagnosis parameter for insulating liquids. The calculation of the activation energy of the conduction process shows that consideration of single activation energy for compensation of temperature effect on dielectric loss for dielectric spectroscopy in the wide frequency range is not perfect. [ABSTRACT FROM AUTHOR]

A significant method to characterize the electrical properties of materials and to evaluate the condition of the insulating systems is the dielectric response study which can be done in the time or frequency domain. The dielectric response measuring results can be affected by different parameters like measuring voltage, temperature, water content and aging condition of insulating system. In this work numerous measurements are presented, thus to understand, how the dielectric response measurement results at the insulating system of high voltage rotating machines can be affected by these parameters in the time and frequency domain. Because of the importance of the dielectric response application as a non-destructive diagnostic method the possibilities and limitations of this method to reflect the deterioration processes due to thermal and electrical stress were also investigated. In addition the possibility to transfer the measuring data from the time to frequency domain was investigated and the calculated and measured data were compared. For the test object actual stator bars were used that were manufactured with epoxy VPI technology. [ABSTRACT FROM AUTHOR]

The present paper reports the results about a study of mechanical, thermal, dynamic mechanical and electrical properties of housing (weather shed) materials for outdoor polymeric insulators. Silicone rubber, ethylene–propylene–diene monomer (EPDM) and alloys of silicon–EPDM are known polymers for use as housing in high voltage insulators. The result of dynamical mechanical measurement shows that the storage modulus of blends enhances with increase EPDM in formulation. It can be seen from the result of TGA measurement that initial thermal degradation of silicone rubber improves by the effect of EPDM in blends. The blends of silicone–EPDM show good breakdown voltage strength compared to silicone rubber. Surface and volume resistance of silicone rubber improve by EPDM content. The mechanical properties of EPDM such as strength, modulus and elongation at break improve by silicone. [Copyright &y& Elsevier]

Investigates the humidity uptake of some transformer liquids and its influence on the electrical breakdown. Evolution of the breakdown voltage in terms of the insulating liquid humidity content; Description of the evolution of insulating liquid humidity content in terms of surrounding relative air humidity; Correlation between the surrounding air humidity, temperature and humidity uptake rate.

To diagnose electrical and mechanical failures on the active part of power transformers efficiently, different state-of-the-art testing methods are recommended to be performed since transformers are complex devices. Measurement results can be equivalent electrical parameters relating to the windings (resistances, leakage/zero-sequence inductances and capacitances), core magnetic property (via induced voltages and exciting currents) or terminal frequency responses (standard FRA tests). Consequently, it is not easy for normal users to implement all necessary measurements, which require much time and training skills due to use of different specialized testing devices. To help users for fast and efficient diagnostics, the paper presents a new method in determining most electrical parameters of the transformer active part based on only the drivingpoint impedance tests performed by means of a vector-network analyzer. More importantly, the method can be applied to give a FRA interpretation based on physical electrical parameters, which is currently requested from relevant standards. The physical FRA interpretation is found to be useful in calculating immeasurable series capacitances of windings in transformer bulk, which is still problematic until now. Investigation on a large distribution transformer shows that the new method is simple, less time consuming but efficient. [ABSTRACT FROM PUBLISHER]

Results of Frequency Domain Spectroscopy measurements are known to be largely influenced by environmental conditions, such as the temperature. Because field measurements, last hours after de-energizing the transformer, the ambient temperature may affect the results. Especially in cold regions of the world, extreme care are required to interpret the results when performing tests at relatively low surrounding temperatures. A better understanding and analysis of the dielectric test results are therefore only possible with a clear understanding of the physical behavior of the insulation system in response to the ambient conditions. In this contribution, the dielectric behavior of a composite oil paper insulation system has been explained from the properties of Debye basic model. A series of experiments have been performed under controlled laboratory conditions with preset moisture content inside the insulation. The equivalent circuit parameters of a laboratory made oil paper condenser bushing model were obtained using a non-linear optimization procedure. Since the dielectric parameters values are geometry dependent, poles, calculated from resistances and capacitances, were used as they are independent of the geometry. It was shown that the poles can be regarded as parameters able to be used for insulation condition assessment. [ABSTRACT FROM AUTHOR]

For reliable operation of power transformers, the condition of the insulation system is essential. This paper reports on a detailed study of the effect of ageing, temperature and moisture on frequency and time domain spectroscopic measurements carried out on oil-impregnated pressboard samples as well as on a distribution transformer under controlled laboratory conditions. Because field measurements are generally performed after dc-energizing the transformer, extreme care is required in interpreting the results due to inherent temperature instabilities. To avoid large thermal variations that may affect the results, a customized adiabatic room was built around the transformer for measurements above the ambient. Capacitance ratio and direct current conductivity deduced from the spectroscopic measurements, helped to interpret the data. Because, low frequency measurements techniques are time consuming, alternative to a transfer of time domain data into frequency domain data was investigated. [ABSTRACT FROM AUTHOR]

In this paper, premature damage to some static inverter loading resistor modules is analyzed. Furthermore, investigations were carried out to determine the origins and causes of this serious damage occurring on the resistor modules in service. Investigations were preformed on the insulating liquid samples as well as new resistor modules. From the results obtained, it seems that the origin of the damage is from a long-standing, free-burning arc between two resistor windings, following mechanical breakdown. [ABSTRACT FROM AUTHOR]

In order to remove the present IEC 60060-1 and IEEE Standard 4 ambiguities in the evaluation of the test voltage when oscillations and/or an overshoot appear superimposed in a lightning impulse, at the end of the 1990s, a European Project within the Standard, Measurement and Testing (SMT) program was developed by KEMA (Netherlands), Schering Institute (Germany), LCOE (Spain), and NGC (United Kingdom) in coordination with the CIGRE Working Group 33.03. The conclusion of this European Project is a new evaluation method called test voltage function K(f) derived on the breakdown behavior of different dielectric media, that removes the standard ambiguity to determine the test voltage of an impulse with oscillations and/or an overshoot. This paper presents the experimental results of the project and deals in depth with the definition of K(f). [ABSTRACT FROM AUTHOR]

Presents the results of investigations into the retrofilling conditions of a Transclene filled transformer. Explanation for the accelerated aging process of the electrical properties of insulating liquids; Discussion on breakdown voltage of the hydrochloric acid and ester liquid mixture; Effects of the addition of Transclene to mineral oil or ester liquid.