05_scopy_cromo - 586 C. T. Dervos et al.: Dielectric...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
C. T. Dervos et al.: Dielectric Spectroscopy and Gas Chromatography Methods Applied on High-Voltage Transformer Oils 1070-9878/06/$20.00 © 2006 IEEE 586 Dielectric Spectroscopy and Gas Chromatography Methods Applied on High-Voltage Transformer Oils C. T. Dervos, C. D. Paraskevas, P. D. Skafidas School of Electrical and Computer Eng., National Technical University of Athens Zografou Campus, Athens 15780, Greece and N. Stefanou Public Power Corporation, Transmission System Dept., 70 Ag. Annis str., Aegaleo 12241, Greece Abstract This paper points out the advantages offered by the temperature dependent dielectric spectroscopy in frequency domain, as an additional diagnostic technique for insulation testing of power transformers for life-time prediction. Complex permittivity measurements have been carried out as a function of operating frequency and oil temperature for a wide selection of high voltage transformers and the gaseous byproduct concentrations were also monitored. The work correlates dielectrometry data to the gas chromatography ones. Index Terms— Dielectric spectroscopy, oil insulation, insulation testing, permittivity. 1 INTRODUCTION P OWER transformer operational reliability and life time expectancy depend on insulation ageing [1]. This is mainly related to the synergy effects [2] induced by the high electric field and the thermal stress on the kraft paper [3,4], together with the electrochemical decomposition of the oil insulation [5,6]. The majority of power transformers contain mineral oils for insulation and cooling purposes [7]. Mineral oils are fabricated by refining a fraction of the hydrocarbons collected during the distillation of petroleum crude stock. Mineral oils consist of hydrocarbon liquids of a complex nature such as paraffin (40 to 60%), naphthene (30 to 50%), aromatic (5 to 20%) and olefin (roughly 1%). Their chemical structure is given in relevant literature [8]. To improve their insulation capabilities additives may also be employed. Mineral oils enhanced with stabilizers, antioxidants (such as 2,6-di-tert- butyl-p-cresol) and very low PCB concentrations, are currently in use [9,10]. Due to ecological considerations, the ester liquids (consisting of pentaerythritol tetraester and different fatty acids) [8], the silicone fluids (poly-dimethyl siloxanes) [9] and the vegetable oils fabricated from sunflower, canola/rapeseed, and soyabean oil seeds [11] have also been proposed as alternative insulating liquids for transformer applications. During oil degradation process gases will evolve [12]. The gassing of insulating oils is considered to be a potential threat to the operational safety of power transformers, since the gases evolve when the hydrocarbon chains breakdown, Manuscript received on 15 September 2005, in final form 21 November 2005. leaving behind large free radicals in the liquid phase [13]. The
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

Page1 / 7

05_scopy_cromo - 586 C. T. Dervos et al.: Dielectric...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online