Thomas Judendorfer

Conductivity measurements of electrical insulating oils

Mineral oils play - in combination with cellulose (paper and pressboard), a vital part in the insulation system of large transformers and bushings. The dielectric stress within such insulation systems during high voltage direct current (HVDC) network operation is defined by a mixed stress that consists of AC, DC and transient components. Exemplary, the valve winding of converter transformers is to be named here.

Review of Partial Discharge Monitoring techniques used in High Voltage Equipment

To ensure economic operation of high voltage equipment, reliable condition evaluation concepts are essential. They enable long-term inspection strategies with regard to a maximized service life of each component. Condition based maintenance (CBM) is therefore desirable and it is already used more and more in practice. Monitoring and diagnostic systems are on the way to get important tools for modern high voltage units or electrical equipment. With the help of modern monitoring techniques, it is possible to record and process significant parameters which is helpful for diagnostics. The quantity and magnitude of partial discharges (PD) are major quality characteristics in the assessment of high voltage equipment. For the PD measurements, different physical effects can be utilized, such as electrical, optical, acoustic, chemical and high-frequency effects. Therefore, in addition to the conventional PD measurements (according to IEC 60270), depending on the electrical equipment, a multiplicity of different alternative methods are currently being used. At the same time a distinction between on-and off-line measurements is possible. For new high voltage equipment, the direct integration of sensors for PD detection during production is practical as this enables the possibility of on-line measurements and furthermore a permanent operation monitoring. This paper deals with different possibilities for on- and off-line PD measurements in variety of electrical equipment. Moreover, new methods to measure and detect PD and the influence of interferences at on-site measurements are discussed.

Challenges to machine windings used in electrical generators in wave and tidal power plants

The majority of electrical generators in wave and tidal power plants are face harsh environmental conditions. Particularly the seals between the moving shaft and the stationary generator as these are mechanically stressed under pressure of the sea water. Flooding the electrical generator and the air gap respectively is a possible solution that would overcome seal failure. With this proposal, the strains on the seals can be reduced as they would have to seal non-moving parts against the sea water only. The lifetime of seals could be prolonged with this measure. On the other hand, this will also result in changes to the electrical behavior of the windings and the electrical machine respectively. This is the subject of this paper. Mainly, these are changes to the winding capacitances. They result in a different transient behavior, for example when the machine is operated with switched inverters. The performance of six different insulation materials for the submersion in sea water are studied and the results are compared to the standard operation in air. Furthermore, with the changed sealing methods the thermal characteristics are altered, which will be also analyzed within this work.

Voltage upgrading - An insulation coordination challenge

When voltage upgrading, lightning-, switching- ice-and pollution performance of the insulators and airgaps in the overhead line play a very important role. The dimensioning procedure is different and technically more challenging from the one used for new lines. Amongst others, the margins available in voltage upgrading are much smaller and therefore the engineering needs to be done more precise. To be able to perform the required engineering for voltage upgrading Statnett has performed and is busy performing a number of R&D projects. Different types of insulators have been studied, as well as pollution and lightning performance of the most used glass insulator. The precise gapfactors have been determined theoretically and practically in full scale. Besides that, tools for the determination of the exact air clearances have been taken into use. A tool for determination of the total line performance with correct gapfactors and insulator performance has been and is being developed.

Voltage withstand tests on a full scale overhead line tower model for line upgrading

In times of rising electrical energy demands, higher transmission capacities are getting more and more important; in Europe as well as all over the world. The construction of new overhead lines is not only cost and time intensive, but it is also difficult to get permits. For that reason the trend is towards upgrading the existing grid to higher operating voltage levels, which raises issues in terms of insulation coordination. Especially the dimensioning of required clearances at the overhead line towers is an important point to ensure a safe electricity transmission. These clearances are influenced by the lengthened insulator string as well as environmental factors. In a cooperative research project between Statnett SF1) and Graz University of Technology a full scale model of a part of an affected overhead line tower has been designed and constructed. Using this model, impulse voltage tests have been performed for several gaps to investigate the gap factor and the withstand behavior. Furthermore the given gap factors in the European standard EN 50341 [1] have been evaluated and compared with the experimental values within the project. This paper will present the development of the model and a summary of the results of the full scale tests as well as some background on the required clearances

Assessment of space charge behavior of oil-cellulose insulation systems by means of the PEA method

Despite recent advancements in material technology, oil-cellulose insulation systems in form of so called Transformerboard combined with paper and mineral oil are still one of the backbones of high voltage technology. This insulation system is widely used in the high voltage industry, especially in high voltage transformers for AC and HVDC applications, e.g. converter transformers. The space charge behaviour of Transformerboard has rarely been investigated, causing the presumption oil-cellulose insulation systems would not collect significant amounts of space charges. This exploratory work investigated oil-cellulose insulation in new condition and at different aging stages. Main objective of this series of measurements was to investigate the influence of isothermal aging on the charge behaviour of Transformerboard. To achieve this, a number of oil-impregnated samples were measured with the pulsed electro-acoustic (PEA) method. The non-destructive PEA method has the advantage of enabling multiple measurements on the same sample. The surface of the Transformerboard samples has been grinded to eliminate possible surface effects here. These samples had a thickness of 1 mm and they have been impregnated with mineral insulating oil. Space charge profiles were recorded during polarisation and depolarisation at 20°C and 60°C. New samples were recorded first and compared to samples, which were aged for 11 weeks at 135°C. A preliminary explanation for the charge mechanisms involved is given in the analysis.

Ausgewählte technologische Entwicklungen in der Hochspannungstechnik

ZusammenfassungDie Wende in der Energiepolitik stellt die Hochspannungstechnik als Schlüsseltechnologie für eine nachhaltige, ausfallsichere, umweltfreundlichere und effiziente Energieversorgung vor neue Herausforderungen. Dieser Artikel beleuchtet ausgewählte technologische Entwicklungen der Hochspannungstechnik. Besondere Bedeutung kommt dabei der Entwicklung in der Isolierstofftechnik, z. B. neue, umweltfreundliche Gase oder Gasgemische, biologische oder synthetische Isolierflüssigkeiten sowie neuen Systemen von Feststoffisolierungen zu. Einfluss auf diese Entwicklungen nimmt auch die Nanotechnologie im Bereich der Nanodielectrics bzw. der Nanofluids. Zu diesen Entwicklungen kommen auch die neuen Möglichkeiten in der Messtechnik und der Diagnostik, die wesentlich für die Qualität der Systeme sind, wie etwa die Teilentladungsdiagnostik oder die Messung der elektrischen Leitfähigkeit der verschiedenen Isolierstoffe.AbstractHigh-voltage engineering faces new challenges due to arising changes in energy policy. As a key technology, it needs to provide a sustainable, reliable, environmentally friendly as well as an efficient supply of electrical energy. This paper highlights selected technological developments in high-voltage engineering. New, environmental friendly gases and gas mixtures, biological or synthetic insulating liquids as well as new solid insulation systems are of particular importance. Additionally, nanotechnology has a strong impact on these developments within the area of nanodielectrics and nanofluids. Furthermore, there are new approaches in measurement techniques and diagnostics which are essential for equipment quality, for example partial discharge diagnosis or the determination of electrical conductivities of various insulation materials.