Dielectric Response Diagnoses for Transformer Windings
The dryness of the oil-paper insulation systems in power transformers is a key factor in both their short and long term reliability since moisture has deleterious effects on dielectric integrity and insulation ageing rates. Traditionally, moisture in oil measurements have been used to estimate the dryness of transformers, but this has not been a particularly reliable approach because of possible sampling and analysis errors and because moisture distributes unequally between cellulose and oil, the greater part residing within the solid insulation, with the water content in the oil being very dependent on temperature and also oil condition, and therefore not simply correlated to solid insulation dryness. The moisture content in the solid cellulose insulation is therefore the key parameter.
Recent attention has been directed to methods of determining moisture content by measuring the effects of the moisture on electrical properties of the insulation. Rather than traditional measurements of power frequency dissipation factor, the variation of various dielectric parameters in the time and frequency domains have been studied in an attempt to isolate the effects of moisture. All the dielectric response methods used today (RVM, PDC and FDS) reflect the same fundamental polarisation and conduction phenomena in the insulation system as a composite of two different dielectric media. The insulating liquid (oil) with ionic conduction is mixed with a less conducting impregnated solid (pressboard or paper), having different characteristic dielectric responses, reflecting not only properties of each material but also the way they are combined.
The lecture presents basic principles and guidelines on how meaningful dielectric response measurements on power transformers should be performed. Robust and reliable instruments have been developed. Recent evaluation of the proposed measurement methodologies and interpretations schemes by CIGRE confirmed that the dielectric response measurements, in both time and frequency domain, provide very useful information for equipment users.
About Keynote Speaker
Stanislaw Gubanski is a Professor at Chalmers University of Technology, Göteborg, head of High Voltage Engineering Division since 1996.
His research activities concentrate on introducing polymeric materials into electric insulation systems and on measurement techniques for diagnosing insulation condition. The goal is to develop reliable, cost efficient, and environmentally friendly components of future transmission and distribution electric power networks based on high voltage direct current technology, HVDC. Solutions for components of smart grids are also in focus. Stanislaw Gubanski is a Fellow of IEEE and a distinguished member of CIGRE. 1997 – 2002 co-ordinator of the Swedish national research program on “High performance outdoor electrical insulation – ELIS”, financedby the Foundation for Strategic Research (SSF)
2005 – 2008 research leader in High Voltage Valley in Ludvika , a joint facility for Chalmers, Uppsala University, KTH and ABB for education of PhD students in near-contact to industry.