W. Lick

Dissolved gas analysis investigations on ester liquids after breakdown

To offer more personal safety and a better environmental behavior of electrical power supply, the usage of alternative insulation fluids is demanded in large power transformers. The minimizing of fire loads is another significant reason for these new liquids. A number of companies are offering such alternative insulation fluids which are already used in distribution transformers but there is still little experience with those liquids in large power transformers. One reason for this is missing maintenance information like limit values of the different DGA detectable gasses. For mineral oil there are many different interpretation methods for DGA (Dissolved Gas Analysis) results and also many known patterns for specific reasons of the increase of one or more gas values. This investigation shows which gas values change significantly caused by breakdown in an oil/cellulose insulation system resp. an ester/cellulose system. A test setup was created and experiments were done with mineral oil, a natural ester and a synthetic ester. A DGA analysis was carried out before and after several breakdowns in the liquids. With the results the gasses which indicate an oil/cellulose breakdown could be identified.

Comparison of alternative insulating fluids

In times when personal safety and the security of electrical power supply become more and more important, the usage of alternative insulation fluids is demanded in large power transformers too. The minimizing of fire loads is another significant reason for new transformers. Several companies offer such alternative insulation liquids and they are already used in distribution transformers. On the other hand there is only few experience with those fluids in large power transformers. Alternative insulation liquids should be compared with mineral oil at equal conditions and voltage levels of several hundred kV AC. Comparison methods and test situations must be found. In this paper it will be shown how the new insulation liquids can be compared with conventional mineral oil and among each other.

New test method for dielectric breakdown voltage of insulating oils

The dielectric breakdown voltage of an insulating liquid is of importance as a measure of the liquidpsilas ability to withstand electric stress. The parameters of the standard test methods from different countries are not similar. In the different standards there are used five or six single breakdowns in a glass or a plastic vessel with about 0.5 litter test oil content. The voltage raising up velocities are between 0.5 and 3 kV/s. After filling the test oil in the test cell until the start of the first breakdown test there is a pause between 3 to 10 minutes and between each breakdown there are breaks of 1 or 2 minutes. Also the form of electrodes are different, sphere formed (according VDE) or flat formed (ASTM). With these single breakdown values the mean value, which height is a parameter for the oil quality, and the coefficient of variation were calculated. A weak point of the standard dielectric breakdown tests is that it is possible to reach values for the coefficient of variation up to 30 % and higher. Also the total quantity of test oil needed (1 liter for flushing and testing) should be reduced.

Experiences in testing of AC and DC bushings

Bushings are very important parts of the electrical power supply. They are used as transformer bushings to allow the electrical connection from the transformer winding to external equipment like busbar or overhead lines or as wall bushings for bringing “high voltage” through a wall. The design of bushings usually consists of an insulating body, for instance oil impregnated paper with concentrically inserted layers for electrical field grading or a gas chamber, a housing, usually porcelain or polymeric, and the flange. For testing of bushings it is necessary to simulate service conditions. This means oil vessels for transformer bushings and a special structure for wall bushings to simulate installation conditions are necessary. The performed electrical tests should verify that the bushing withstand the electrical stress in service. Challenges for testing bushings in the UHV range are the big dimensions of the test objects, avoiding any kind of external influences to the measuring results and the lack of relevant standards for the UHV range. This paper summarizes the experiences in testing AC and DC bushings up to 800 kV system voltages. Problems during the preparation process or during the tests which can occur and there possible solution will be shown.

Behaviour of alternative insulating liquids at cold temperatures

In times when personal safety and the security of electrical power supply become more and more important, the usage of alternative insulation fluids is demanded in large power transformers. The minimizing of fire loads is another significant reason for new liquids. Different companies are offering such alternative insulation liquids which are already used in distribution transformers. On the other hand there is little experience with those fluids in large power transformers. For the confirmation of the usability of new insulating liquids comparison methods must be tested to find the differences between the alternative insulating fluids and commonly used mineral oil. There are different methods for characterising insulating materials, like AC-Breakdown tests, partial discharge tests, lightning and switching impulse voltage tests, etcetera. Also the electrical behaviour of alternative insulation liquids at low temperatures is very important. This paper shows the results of investigations with alternative fluids at low temperatures.

Improvement of the standard dielectric breakdown test for insulating liquids

The dielectric breakdown voltage of an insulating liquid is of importance as a measure of the liquids ability to withstand electric stress. The parameters of the standard test methods from different countries are not similar. In the different standards there are used five or six single breakdowns in a glass or a plastic vessel with about 0.5 lifter test oil content. The voltage raising up velocities are between 0.5 and 3 kV/s. After filling the test oil in the test cell until the start of the first breakdown test there is a pause between 3 to 10 minutes and between each breakdown there are breaks of 1 or 2 minutes. Also the form of electrodes are different, sphere formed (according VDE) or flat formed (ASTM). With these single breakdown values the mean value, which height is a parameter for the oil quality, and the coefficient of variation were calculated. A weak point of the standard dielectric breakdown tests is that it is possible to reach values for the coefficient of variation up to 30 % and higher. In this paper it will be shown where the problems are and some ways are given how to minimize the coefficient of variation

Prüfmöglichkeiten an der Versuchsanstalt für Hochspannungstechnik Graz GmbH

ZusammenfassungDieser Beitrag beschreibt die Prüfmöglichkeiten der Versuchsanstalt für Hochspannungstechnik Graz GmbH an der Technischen Universität Graz. Dazu werden die einzelnen Laboratorien, welche die Versuchsanstalt bilden, vorgestellt. Natürlich wird ein wesentlicher Teil des Berichtes sich mit der „Großen Halle“ (Nikola Tesla Labor) der Versuchsanstalt beschäftigen. Die Einrichtungen der Labors werden beschrieben, versehen mit den wichtigsten technischen Daten. Auch soll aufgezeigt werden, welche Prüfungen und Untersuchungen durchgeführt werden können und für welche Prüfungen die Versuchsanstalt akkreditiert ist. Eine Auswahl an durchgeführten Prüfungen soll die Palette der Prüftätigkeit darstellen. Abschließend sollen die geplanten Neuanschaffungen für die Versuchsanstalt angeführt werden, die notwendig sind, um auch in der Zukunft konkurrenzfähig zu bleiben.AbstractThis paper describes the test possibilities of the Test Institution of High Voltage Engineering Graz Ltd. at Graz University of Technology. The individual laboratories which form the Test Institution, are presented. An essential part of the paper will deal with the ”Large Hall” (Nikola Tesla Laboratory) of the Test Institution. The facilities of the laboratories are described, provided with the most important technical data. It is also pointed out which tests and examinations can be carried out and for which tests the Test Institution is accredited. A variety of performed tests shall illustrate the range of test activities. Finally, the planned new acquisitions for the Test Institution will be given that are necessary in order to remain competitive in the future.

Partial discharge behaviour of an alternative insulating liquid compared to mineral oil

In times when personal safety and the security of electrical power supply become more and more important, the usage of alternative insulation fluids is demanded in large power transformers. The minimizing of fire loads is another significant reason for new liquids. Different companies are offering such alternative insulation liquids which are already used in distribution transformers. On the other hand there is little experience with those fluids in large power transformers. For the confirmation of the usability of new insulating liquids comparison methods must be tested to find the differences between the alternative insulating fluids and commonly used mineral oil. There are different methods for characterising insulating materials, like AC-Breakdown tests, partial discharge tests, lightning and switching impulse voltage tests, etcetera. Partial discharge behaviour is a distinguishing characteristic of an insulating medium. This paper shows the partial discharge behaviour of alternative insulation liquids such as synthetic and natural esters, compared to commonly used mineral oil. To generate the PD patterns, voltages up to 30kV and inhomogeneous test arrangements were used. A model of an oil/board insulation system was created by using sheets of pressboard.

Improvement of the test method for dielectric breakdown voltage of insulating oils

The dielectric breakdown voltage of an insulating liquid is important as a measure of the liquids ability to withstand electric stress. The parameters of the standard test methods from different countries are not similar. In the different standards there are used five or six single breakdowns in a glass or a plastic vessel with about 0.5 litter test oil content. The voltage raising up velocities are between 0.5 and 3 kV/s. After filling the test oil in the test cell until the start of the first breakdown test there is a pause between 3 to 10 minutes and between each breakdown there are breaks of 1 or 2 minutes. Also the form of electrodes are different, sphere formed (according VDE) or flat formed (ASTM). The mean value of these single breakdowns, which is a parameter for the oil quality, and the coefficient of variation were calculated. A weak point of the standard dielectric breakdown tests is that it is possible to reach values for the coefficient of variation up to 30 % and higher. In this paper it will be shown where the problems are and some ways are given how to minimize the coefficient of variation.

Improvement of the test method for insulating oils

Standard dielectric breakdown tests for insulating oils consist of five or six single breakdowns in a glass or a plastic vessel with about 0.5 litter test oil content. For each breakdown the voltage is raised up linear from zero to the breakdown value usually by 0,5 ... 3 kV/s. After filling the test cell with the test oil there is a pause between 3 to 10 minutes until the start of the first breakdown test Between each breakdown there are breaks about 1... 2 minutes. Optional, a stirrer produces an oil circulation in the test oil. The form of electrodes are different, according VDE (sphere formed) or ASTM (flat formed). With these single breakdown values the mean value, a parameter for the oil quality, and the coefficient of variation were calculated. A weak point of the standard dielectric breakdown tests is that it is possible to reach values for the coefficient of variation up to 30 % and higher. Further repeated tests lead frequently to contradictory results. So the standard oil test is very common but the results are sometimes not satisfied. Therefore we have performed breakdown tests with changing test parameters like pause times, different voltage raising up velocities and with and without stirring. Further we have tried to use additional methods for filling the test cell with test oil or better cleaning of the electrodes after each breakdown. And last but not least we have also created a new test cell. This paper will show the results of our efforts