B. Wieser

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.

Partial discharge behavior of environmentally friendly and hardly inflammable ester liquids compared to mineral oil for power transformers

Conventional insulation systems of power transformers consist of mineral oil, cellulose paper, and pressboard. Due to the low biodegradability level of mineral oil and its susceptibility to fire, there has been an increase in the use of environmentally-friendly fluids such as synthetic esters and natural esters in recent years. To improve personal safety and to reduce the environmental impact of electrical power supply, alternative insulation fluids are demanded for large power transformers. These liquids are already used in the area of distribution transformers, but in case of large power transformers insignificant experience exists so far with alternative insulating fluids. For characterization of the insulating material and optimization of electrical design different electrical test methods (e.g. breakdown voltage, partial discharge) are used. The partial discharge behavior is a distinguishing characteristic of an insulating medium. In conclusion, it can be said that this paper shows the partial discharge behavior of alternative insulation liquids compared to commonly used mineral oil based on various investigations of an inhomogeneous test setup to generate reproducible partial discharges.

Static electrification of different solid-liquid couples used in transformers for insulation

Since the middle of the 1970s the static electrification phenomenon was made responsible for several outages of forced cooled, high voltage power transformers. The forced oil flow results in electrostatic charging between the liquid and the solid insulation. Most of the research done till now in this field concerned the electrostatic charging behavior of mineral oil - cellulose insulation systems, conventionally applied in power transformers. The aim of the research activity at the Institute of High Voltage Engineering and System Management is the investigation of not so well studied solid - liquid couples. In this paper first investigations by the institute concerning static electrification of aramid in combination with different insulating liquids are shown.

Partial Discharge characteristics of mineral oil using needle — Plane and needle — Sphere electrode configuration base on pulse current measurement

The aim of this paper is to describe the study of the Partial Discharge (PD) characteristics of mineral oil obtained from Partial Discharge Inception Voltage(PDIV) testing using the needle - plane and needle - sphere electrode configurations. This paper represents the experimental investigation of the PD characteristics of the mineral oil, Nynas 4000x, with the water content not more than 10 ppm at the PDIV level using various tip radii of needle - plane and needle - sphere electrode arrangements. The tungsten needle electrodes with the tip radius of 10μm, 20μm, and 40μm respectively were used as the high voltage electrode while the brass plane electrode with a diameter of 50 mm was connected to ground via the shunt resistor of 50 ohm. The gap distance of the electrode system was set up at 50 mm. The test experiment, conventional PD measurement, was set up according to IEC 60270. The test procedure was performed in accordance with IEC 61294 under room temperature. PD activities at the PDIV and 1.1 PDIV test level were recorded. Additionally, PD currents were also detected and recorded by a high quality oscilloscope. Furthermore, the brass plane electrode with 75 mm diameter and the sphere electrodes with the diameter of 50.8 mm and 76.2 mm respectively were used also as the grounded electrode tested with the mentioned needles as high voltage electrode. The average PDIV value, the phase - resolved PD characteristics of mineral oil and PD pulse currents of the tested electrode systems are analyzed and presented in this paper. From the test results, we will gain the knowledge of PD characteristics of mineral oil testing by needle - plane and needle - sphere electrode system and we also found that the PD pulse current measurement can be used as an alternative indicator of the mineral oil degradation.

Study of electrode for measuring dielectric properties of oil immersed material

Dielectric properties including relative permittivity and dc resistivity has been used widely as a design parameter of insulation system. In measuring dielectric properties of oil immersed material (OIM), we have to make some adaptations from IEC 60093 and IEC 60250 i.e. the detail of live electrode curvature corner shape and the effect of electrodes weight. The effect of the electrode curvature shape on the electric field stress distribution should be reduced as small as possible when measuring the dielectric properties of the pressboard. Meanwhile the weight of electrode plays an important role to reduce the effect of non-flatness surface of the pressboard. This paper focused on electric field analysis distribution for three electrode configurations and pressure simulation of different electrodes weight. Then the simulation results were deepened with the laboratory experiment, permittivity, partial discharges inception voltage (PDIV) and partial discharge (PD) pattern respectively. In this experiment, the dielectric interface configurations consists of oil immersed pressboard operated under oil bath in a test vessel. For PDIV measurement, the pressboard samples were placed on a grounded plane under cylindrical live electrode with three types of corner shape. The live electrodes used were sharp-cornered electrode and rounded-cornered with radius 3 mm and 5 mm. Meanwhile for the permittivity measurements, a guard electrode was used to prevent edge capacitance. The solid dielectric were oil immersed transformer board type B 3.1A with 2 mm and 4 mm thickness operated under mineral oil, Nynas Nytro 4000x. The PD experiment was set up according to IEC 60270, the PDIV test procedure was performed in accordance with IEC 61294 and the permittivity and tan δ measurements performed in accordance with IEC 60250. We concluded that, eventhough round-cornered electrode yields more uniform electric field at the pressboard side, it also yields steep electric field enhancement at the oil side. This nature of rounded-corner electrode makes it easier to generate oil discharge in the wedge (small oil gap) when compared to sharp-cornered electrode. Meanwhile, the D/R (ratio of the sample thickness to corner radius of electrode) parameter does not seem to have any real important meaning. It was also found that electrode with pressure higher than 1.9 N/cm2 give enough pressure to prevent a thin oil layer that present on surface. These results showed the effect of electrode corner shape on electric field enhancement and pressure of electrode at the contact point of dielectric interface. Furthermore, the results could be used as a consideration on which electrode is suitable for measuring dielectric properties of pressboard immersed in oil in compliment with IEC 60093 and IEC 60250. We concluded that, eventhough round-cornered electrode yields more uniform electric field at the pressboard side, it also yields steep electric field enhancement at the oil side. This nature of rounded-corner electrode makes it easier to generate oil discharge in the wedge (small oil gap) when compared to sharpcornered electrode. Meanwhile, the D/R (ratio of the sample thickness to corner radius of electrode) parameter does not seem to have any real important meaning. It was also found that electrode with pressure higher than 1.9 N/cm2 give enough pressure to prevent a thin oil layer that present on surface. These results showed the effect of electrode corner shape on electric field enhancement and pressure of electrode at the contact point of dielectric interface. Furthermore, the results could be used as a consideration on which electrode is suitable for measuring dielectric properties of pressboard immersed in oil in compliment with IEC 60093 and IEC 60250.

The study of the arcing phenomena of the mineral oil using the rod — Plane electrode configuration

The aim of this paper is to describe the study of the arcing phenomena of mineral oil using a rod - plane electrode configuration. This paper represents the experimental investigation of the arcing phenomena of the mineral oil, Nynas 4000x, with the water content not more than 10 ppm under various tip diameters of the rod - plane electrode arrangements. The tungsten rod electrodes with the tip diameter of 1mm, and 2mm with the curvature of 0.2 mm respectively were used as the high voltage electrode, while the brass plane electrode of 75 mm diameter was used as the grounded electrode. The gap distance of the electrode system was set up at 0.3 mm and 0.8 mm respectively. The electric field pattern of these electrode systems was calculated using Finite Element Program. The test experiment was modified from IEC 60156 and performed under room temperature. The arcing current, the arcing voltage and the partial discharge pulse current before the arc occurred including the arcing current pulse signal were investigated. The test results show that the 1 mm diameter rod - plane electrode yielded the highest electric field and the lowest arcing voltage at 9.9 kV including the maximum arcing current density of 54.24 A/cm2. This electrode configuration gave higher a bit the arcing voltage than that of the 2 mm diameter rod - plane electrode. In addition, the PD pulse signals before arcing occurrence and the arcing pulse current signals were relatively similar in shape. Furthermore, the significant degradation of the mineral oil was clearly observed after testing.

Dielectric properties measurements of oil immersed pressboard

Dielectric properties including relative permittivity, dielectric dissipation factor and dc resistivity has been used widely as a design parameter of insulation system. In measuring dielectric properties of oil immersed pressboard (OIP), we have to make some adaptations from IEC 60093 and IEC 60250 e.g. effect of electrodes weight. This paper is focused on study of permittivity and d.c. conductivity measurement of oil immersed pressboard (OIP). In this experiment, the dielectric interface configuration consisted of oil immersed pressboard operated under oil bath in a vacuum tight test vessel. The pressboard samples were placed on a grounded plane under live electrode. Rounded-cornered electrode with radius 3mm was used as live electrode. For this experiment, the solid and liquid dielectric were oil immersed transformerboard type B 3.1A with 2 mm thickness and operated under mineral oil bath, Nynas Nytro 4000x. The pressboard and mineral oil used in the experiment were in dry condition with moisture level <; 0.5% and <;5 ppm respectively. The test vessel, electrode system and test procedure for OIP were set up according to IEC 60093 and IEC 60250. We concluded that, first, relative permitivitty and dielectric dissipation factor of oil immersed pressboard are highly influenced by temperature. The change of dielectric dissipation factor is significant only for temperature ≥ 90°C. Second, the relative permitivitty and dielectric dissipation factor of pressboard do not depend on the electric field stress for range 0.125 kV/mm-5 kV/mm. Third, the calculation of effective surface area A for calculating relative permittivity and d.c. conductivity is not significant for B factor >; 0.83. Fourth, it was also found that electrode with pressure higher than 1.9 N/cm2 give enough pressure to prevent a thin oil layer that present on surface. Furthermore, the results could be used as a consideration on measuring the dielectric properties of oil immersed pressboard in compliment with IEC 60093 and IEC 60250.

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.