Over the last couple of years, transformers in various locations worldwide have been failing due corrosive sulphur deposits on the copper winding insulation.
Since the copper contains some impurities (no matter how small), the sulphur in the oil may react with these and this product (Copper Sulphide) will then float in the oil and attach itself to the paper insulation, reducing its insulation ability.
Silvered contacts have also, been known to be effected by corrosive sulphur. Flaking on the silver can occur and such flakes can become dislodged during mechanical movement, these may then reduce insulation gaps.
Sulphur is a natural element contained in crude oil, however, the sulphur contained is made up of various types, some of which are bad but some of which are beneficial and improve oil stability.
Although this beneficial sulphur is contained as a very small percentage, it can never be zero.
Even though refiners select only certain crude oils to form the basis of transformer oil, unrefined oil contains these elements:
- Elemental Sulphur
During the refining process, most of these elements are removed, however, remaining sulphur will be in the order of 0,05% – 0,6%. Elemental Sulphur is accepted to be the most corrosive form of Sulphur.
Repeating the Hydro and Acid refining process will remove more sulphur; unfortunately it will remove natural inhibitors, which we want in the oil, also.
Sulphur is also contained in materials used in the transformer construction and manufacturing process.
Although this may be in smaller quantities, they cannot be excluded. Examples of these are found in:
- Some glue in the paper wrapping
- Some types of Gaskets
- Catalyst and hardener products
- Copper itself
- Certain paints.
Consideration should also be taken to ensure that the equipment and hoses used during both the manufacturing process and future (oil) processing, does not introduce sulphur.
Certain transformer operating parameters have been proven to increase the risks introduced, and accelerate the formation of Corrosive Sulphur.
Oil Acidity will increase the risks created.
Oil Temperature also has been cited as a contributing factor. Failures being more evident when a transformer average operating temperature is elevated.
Transformer “hot spots” further accelerate the problem.
Certain Designs, which limit oxygen and utilise unvarnished copper warrant priority rating.
Transformers with closed conservator, which are heavily loaded or operate at high temperature, have a high risk factor when containing corrosive oils.
SOLUTIONS CONCIDERED FOR EXISTING TRANSFORMERS
In the long term, the possibilities to use new non-intrusive monitoring methods is being developed in order to more easily identify transformers at risk.
Direct inspection of the activate transformer internals is neither, practical or recommended unless access is given during major transformer refurbishment.
In the past, no doubt copper sulphide was seldom identified. Deposits on paper, often mixed with sludge and blackening of copper were, were usually considered to be caused by high temperatures. Deposits of copper sulphide usually only evident after degreasing and careful de-winding of the paper has been performed.
Future failures should be fully recorded and all available data recorded in order fully reveal cause and assist with future design.
New Oil Specifications will now allow oils that may cause copper sulphide formation to be identified.
However, to minimize such apparent problem with existing transformers, much attention has been given to a technical solution, which involves adding a passivating agent to the oil. Being easily soluble in oil a final concentration in the region of 100ppm appears to provide the required protection.
This solution has been adopted by many utilities throughout the world and no adverse effects on paper, copper or other materials, used during transformer manufacture, has been recorded.
Before the passivator is added, it is recommended that Oil Regeneration be carried out on the transformer, in order to ensure that all oil parameters are within specified limits for new oil. This procedure indicates that once the Passivator is added and circulated within the transformer, the passivator adheres to the copper, and blocks the reaction of other molecules with the conducting surface.
Further more, it is expected that the passivator so installed, will last the whole equipment lifetime, without requiring to be topped up.
In addition, tests advise that the passivator will not affect future oil analysis results.