Hans Michael Muhr

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.

An algorithm for the three-pole controlled auto-reclosing of shunt compensated transmission lines with an optimization for the second and third pole

This article presents an algorithm for auto-reclosing of shunt compensated transmission lines. The algorithm uses signal analysis methods to estimate the future voltage shape over the circuit-breaker. Based on this voltage shape switching instants are estimated in the zero crossings. The minimization of the probability of a prearcing is reduced be adding time shift parameters. These parameters take the limited rise of strength recovery of the circuit breaker and the mutual influence of the coupling of the poles by a three-pole auto-reclosing into consideration. The advantage of the descript method is significant reduction of the switching overvoltage.ZusammenfassungDieser Artikel beschreibt einen Algorithmus für die kontrollierte Wiedereinschaltung auf parallel-kompensierten Übertragungsleitungen. Der Algorithmus verwendet ein Signalanalyseverfahren zur Berechnung des zukünftigen Verlaufs der Spannung über dem Leistungsschalter. Mit Hilfe des berechneten Spannungsverlaufs ist eine Bestimmung von Nulldurchgängen als Einschaltzeitpunkte möglich. Die Wahrscheinlichkeit eines Vorüberschlages wird durch Addition eines Verschiebezeitparameters minimiert. Dieser Parameter berücksichtigt den Verlauf der Einschaltkennlinie des Leistungsschalters und die Beeinflussung der einzeinen Leiter untereinander bei einer dreipoligen automatischen Wiedereinschaltung. Der Vorteil des beschriebenen Verfahrens ist eine bedeutende Reduzierung der Schaltüberspannung.

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.

Umfassende Zustandsbewertung von Generatoren

SummaryTechnical diagnostics present a main part in generator maintenance, however, for condition evaluation besides aspects like equipment properties and rate of utilization the results of measurements are to be considered, too. Special emphasis is put on visual inspections which are mostly implemented during revision processes. For a comprehensive condition evaluation different technical and optical investigations have to be applied and the results have to be analyzed. In this way the aim of localization and repair of early failures can be achieved efficiently, so that a reliable operation during the whole life time period can be ensured.ZusammenfassungDie technische Diagnostik stellt einen wesentlichen Teil der Generatorinstandhaltung dar, jedoch sind für die Zustandsbewertung neben den Resultaten dieser messtechnischen Untersuchungen auch Aspekte wie Betriebsmitteleigenschaften und Maschinenauslastung zu beachten. In den Vordergrund ist auch die visuelle Inspektion zu stellen, die zumeist im Rahmen der Revision durchgeführt wird. Für eine umfassende Zustandsbewertung sind daher unterschiedlichste technische und optische Untersuchungen durchzuführen und deren Ergebnisse zu analysieren. Das Ziel zur Lokalisierung und frühzeitigen Behebung von Schwachstellen kann somit effizient erreicht werden, so dass ein zuverlässiger Betrieb während der gesamten Lebensdauer gewährleistet werden kann.

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

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.