The criticality of “Corrosive sulfur from sulfur combustion by-products – C3” is caused by oil regeneration treatments that involve the re-activation of fuller’s earth (and other particulate adsorbents) by a combustion process. This uncontrolled thermo-oxidation process (> 370°C) degrades the sulfur present in the oil, producing three distinct criticalities:
A. contamination of regenerated oil by the formation of highly corrosive by-products (H2S-hydrogen sulphide, mercaptans, elemental sulfur, etc.)
B. corrosion of copper and silver parts with copper sulfide and silver sulfide formation inside the transformer impregnated with regenerated oil (e. g. contacts of the on-load tap-changer)
C. emissions of CO2 and contaminants such as H2S and PCDD-Dioxins and PCDF-Furans into the environment in the event of contamination by PCB-Polychlorinated biphenyls and other chlorinated and persistent organic pollutants (POPs)
| Cause of criticality “Corrosive sulfur from sulfur combustion by-products – C3” | When it can occur (life cycle phases) |
| Lack of requisites for purchase of oils (new or recycled) | Requisites and purchase |
| Defciencies in quality control for individual lots or individual supplies of insulating oil | Acceptance of insulating oils |
| Deficiencies in analytical procedures for the verification of corrosive sulfur compounds | Oil acceptance, factory test, installation and pre-energisation, operation, old age, post-mortem |
| Cross-contamination through use of oil, plants, tanks or containers contaminated by corrosive sulfur compounds (for toppings up, impregnations, fillings or treatments) |
Factory test, installation and pre-energisation, operation, old age, post-mortem (oil recycling) |
To understand the damageing effects of oil regeneration treatments that reactivate fuller’s earth by combustion, it is necessary to know more about how reactivation takes place.
Further reading
Oil regeneration treatments are performed using different techniques and system solutions. Some of these “regeneration” operations make the oil flow through columns containing fuller’s earth (or other adsorbent particulate matter). The oil passes through the earth at a temperature of 60-80°C
Fuller’s earth is unable to decontaminate PCB, DBDS or other corrosive sulfur compounds
When the fuller’s earth becomes saturated, it can be replaced (with production of waste to be disposed of) or reactivated by combustion. The C3 criticality is generated precisely by this phase.
To reactivate the earth, the oil flow in the column is interrupted and oil drainage is continued
N.B. A significant aliquot of oil after drainage remains impregnated in fuller’s earth cavities.
The next step is combustion. In detail, the following occurs in this phase:
A. heating of one end of the column up to the ignition temperature (about 350-400°C);
B. injection, at the opposite end of the column, of the combustive agent (oxygen of air) under pressure;
C. the real combustion of oil impregnated in fuller’s earth until complete exhaustion of the fuel (oil).
During combustion, the flame front (700-800°C) moves progressively from the trigger point towards the opposite side of the column. At the end of combustion, input of the combustive agent is stopped and the column and the particulate media inside are cooled
The duration of reactivation is about 12-18 hours
Real example
A column with a volume of 200 litres can contain about 150 kg of fuller’s earth (dry); fuller’s earth can retain oil up to 50% of its weight. Consequently, in spite of the drainage of oil, 75 kg of oil still remain trapped in the fuller’s earth to be reactivated. Assuming a total sulfur concentration of 10,000 mg/kg, this means that there are 750,000 mg of sulfur (that is, 750 g) in the oil aliquot!
In conclusion, in order to reactivate the fuller’s earth, 75 kg of oil will in fact be burned with 750 g of sulfur, generating highly corrosive by-products in the transformer oil mass and generating dangerous emissions into the environment.
Failure mechanisms
Contamination of corrosive sulfur compounds in regenerated oil creates an uncontrolled phenomenon of cross-contamination on the transformer pool with high probability of failure due to the formation of copper sulfide and silver sulfide (e. g. on-load tap changers or switches) . Copper sulfide increases as the temperature rises, reaching its peak in the presence of localised hot spots. The result is the formation of deposits and macroparticles that can circulate dangerously in the oil causing partial discharges and power arcs.
However, copper sulfide can also be formed from windings which are also made of copper. In this case there is a progressive migration of copper sulfide from the conductors on the windings to the layers of paper around them. Copper sulfide crystals push against the layers of paper, gradually arriving at the outermost layer until causing the loss of its insulating properties. Partial discharges and power arcs can also be generated in this case up to catastrophic failure.
Corrosion can increase if the starting oil contains significant concentrations of chlorinated organic compounds (e. g. PCB, trichlorobenzene) which, when subjected to thermal degradation, tend to form highly toxic by-products (PCDD-dioxins, PCDF-furans) as well as other compounds with free chlorine or hydrochloric acid (HCl).
