Author: Maik Koch and Michael Krueger – Omicron Electronics Austria
Application at Onsite Measurements
Here the example of a heavily aged transformer dedicated for scrapping should be considered. Such a case delivers a good opportunity to compare different approaches to assess moisture content. Mineral oil type Shell Diala 6KX having a neutralization number of 0,49 mg KOH/g oil from 1954 filled the transformer. This high value together with an oil conductivity of 900 pS/m indicates a strong influence of conductive aging products and therefore a progressed aging state. Paper samples were taken after measuring the dielectric properties (polarization and depolarization currents, frequency domain spectroscopy) and oil sampling (moisture content in ppm, moisture saturation in %). Figure 9 depicts the measurement data of the dielectric measurements. The fast decaying currents in time domain and the high dissipation factor in frequency domain indicate a highly conductive insulation.
Figure 10 (left) compares the moisture results of the different measurement and analysis approaches. Karl Fischer titration at paper samples came to 2,6 % moisture by weight (KFT). The analysis results of the dielectric response methods differ from each other: Two algorithms had no compensation for conductive aging products and determined 3,8 and 4 % moisture by weight (FDS, PDC). The new analysis software having a compensation for conductive aging products indicates 2,9 % moisture relative to weight (Dira).
In an oil sample the moisture saturation was measured directly onsite and also the moisture content in ppm by Karl Fischer titration in a laboratory. Via an moisture sorption isotherm 11 the relative saturation reading lead to 2,5 % moisture in cellulose (RS), which well agrees with the paper samples and the dielectric response analysis compensating for conductive aging products. An equilibrium chart based on moisture content in oil in ppm  determined a too high moisture content of 6,0 % (PPM), proving that equilibrium diagrams not adopted to aging state and moisture adsorption capacity cannot reliably evaluate moisture in power transformers.To conclude, the findings at this very aged transformer show, that a compensation for aging products is necessary both for the measurement based on moisture equilibrium and them based on dielectric properties.
Assessment of Moisture Results
IEC 60422 categorizes moisture saturations of more than 6 % as “moderately wet”, which is equivalent to a moisture content of approximately 2,2 %. In this area the water molecules become more and more active, causing the dangerous effects of water. At this level, maintenance actions such as drying should be considered taking into account the importance and future operation of the transformer. Figure 10 (right) shows the relationship between moisture content and moisture saturation which helps to assess the analyzed results.
This paper discusses approaches to measure moisture in power transformers using dielectric response methods and equilibrium diagrams.
- Dielectric diagnostic methods deduce moisture in the solid insulation from dielectric properties like polarisation depolarisation currents and dissipation factor frequency.
- A new instrument “Dirana” combines time domain (PDC) and frequency domain (FDS) measurements and thus substantially shortens the measurement
- A new software was developed which bases on a new data pool, measured at new and aged pressboard with various moisture contents and oil
- The new analysis algorithm compares measurements from a transformer to modelled dielectric responses, obtained from the so called XY-model. The new software features weighting of low frequency data and an extended XY-model.
- The analysis algorithm compensates for the influence of conductive aging by-products.
- The new software was successfully utilized for onsite measurements in comparison to other measurement and analysis
- The conventional application of equilibrium diagrams to derive moisture in cellulose (%) from moisture in oil (ppm) is effected by substantial
- To exclude the interference due to oil aging the moisture in oil relative to saturation level (%) is more appropriated instead of moisture in oil in
- One step forward constitutes the use of moisture relative to saturation in oil and This measure is easy, continually and accurate measurable.
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Maik Koch is a product manager at OMICRON Austria. Before that he worked as a research assistant at the University of Stuttgart in Germany where he graduated as a Doctor of Philosophy. His fields of research were ageing and moisture determination in oil paper insulated power transformers using chemical and dielectric analysis methods. He received a diploma for electrical engineering at the Technical University of Cottbus in Germany. He has written about twenty scientific papers and collaborates in Cigrè working groups.
Michael Krueger is head of the OMICRON competence center primary testing. He studied electrical engineering at the University of Aachen (RWTH) and the University of Kaiserslautern (Germany) and graduated in 1976 (Dipl.-Ing.). In 1990 he received the Dr. techn. from the University of Vienna. Michael Krueger has 30 years experience in high voltage engineering and insulation diagnosis. He is member of VDE, Cigré and IEEE.