New corrosive sulphur test

Corrosive Sulphur Analysis with SEM / EDS

Caused by Dibenzyl Disulfide (DBDS) corrosive sulphur has received significant attention in the HV transformer industry over the last decade.

The underlying reason for concerns started in 2000 when several large and relatively new transformers unexpectedly failed.

Subsequent investigations found fine deposits and corrosion on some of the windings and fine deposits embedded in some of the paper insulation. These deposits were identified as copper sulphide, which is an electrically conductive compound. Its presence in the paper insulation formed a conductive path, lowering the dielectric strength of the paper, therefore resulting in failure.

This copper sulphide was created when corrosive sulphur compounds in the oil reacted with the copper conductors.

• When a transformer operates at high temperatures, unstable sulphur compounds in the oil breakdown to corrosive compounds.
• The corrosive sulphur compounds attack the copper conductor, causing the formation of copper sulphide, some of which then dissolves in the transformer’s oil.
• Copper sulphide has an attraction for the paper insulation and collects on and in the paper.
• The copper sulphide damages the insulating properties of the insulation paper by forming a conductive path that results in failure.

About the test: Our analysis is not just the standard ASTM D 1275B test but also a second analysis using a scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) to confirm the result. We believe that this confirmation is important before any expensive treatment or replacement is carried out.

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