Corrosive sulphur in large transformers: impact, quantification and detection

Abstract

Several power apparatus use some type of oil insulation system. Transmission level transformers, one of the most expensive components in power networks, use mineral oil as both electrical insulation and cooling. Due to the financial risk posed, if this equipment fails unexpectedly, several quality and condition monitoring techniques are used to test the insulation system. Nevertheless, in 1990 approximately 50 % of the mineral oils used in transformers was contaminated with corrosive sulphur species [1], which passed undetected by the standard corrosion tests at the time, resulting in transformers failures worldwide. CIGRE estimates that between 2000 and 2009 there were approximately 100 related failures, not considering small distribution transformers too small for any investigation to be carried out.

The work presented investigates how CuxS faults develop, examines possible techniques to monitor the corrosion process and the failure scenarios caused by sulphur corrosion. A sample production method capable of generating 90 mm square Kraft paper samples with homogeneous CuxS deposition, with a less than 1 % variation, and a reproducibility of 3:5% is detailed. Empirical data demonstrate how the generation of CuxS deposits to DBDS concentration, temperature, ageing atmosphere, paper-copper interfaces and location of deposits. Based on the empirical data obtained, a copper sulphide deposition model using multiple parallel CuxS formation mechanisms was developed.

The development of condition monitoring techniques and laboratory-scale validation experiments have been completed successfully, with the use of both field and laboratory samples. It has been shown that x-ray fluorescence (XRF) is technically capable of quantifying very accurately sulphur and copper in oil, with a limit of quantization at 4.05 and 1.95 ppm respectively. In addition, XRF measurements demonstrated that through historical sulphur trends, sulphur depletion rates can be calculated to extrapolate if there is an onset sulphur corrosion. Frequency dielectric spectroscopic analysis of CuxS contaminated Kraft paper demonstrated empirically that CuxS deposits have resonance frequency, at approximately 1 mHz, and that the magnitude of the resonance relates to the quantity of CuxS. Furthermore, laboratory samples accurately replicate field samples containing CuxS deposits and other impurities in terms of dielectric data analysis.

The effect of sulphur corrosion and possible failure scenarios are investigated. The electrical breakdown experiments demonstrate that the CuxS deposits reduced the electrical breakdown strength of each Kraft paper layer by 79.5 %. By applying DP life expectancy models to empirical data it is demonstrated that the corrosion process reduces 25 % of the transformer life expectancy at normal working conditions. The excess dielectric loss, generated from a CuxS contaminated insulation paper system, has increased by a factor of approximately 27.5. The thermal conductivity also increased 75%. It is demonstrated the generation of a hotspot is very likely, which then generates a snowball effect of generating further CuxS deposits and further increasing the local temperature.

More information

Identifiers

ORCID for P.L. Lewin: orcid.org/0000-0002-3299-2556

Catalogue record

Export record