When should the Corrosive Sulfur Test be performed? - Part 2

In Part 1 of this series, we defined corrosive sulfur, briefly reviewed the history of the issue, and described how corrosive sulfur can cause a transformer to fail. In Part 2, we will discuss when the Corrosive Sulfur test should be performed.

In thinking about when the Corrosive Sulfur test should be performed, it helps to consider the operating conditions of the transformers that have actually failed due to corrosive sulfur. Transformers that have failed due to corrosive sulfur have been manufactured since about the year 2000, and have also had at least some of the following characteristics:

1. They often were operated with top oil temperatures being at least 70°C for at least six months of the year. This criterion is the most important in choosing which units to test for corrosive sulfur. Units running this warm tend to be GSUs (generator step-ups) and/or units in the tropics and in warmer parts of the world.
2. They often were operated under high load and/or experienced high electrical stress.
3. They often were sealed units with low levels of dissolved oxygen in the oil. Higher levels of oxygen seem to play a role in protecting the copper windings from attack by corrosive sulfur.
4. They had unvarnished copper coils. Varnish would tend to shield coils from attack by corrosive sulfur.
5. They contained oil without metal passivator. Metal passivators are oil additives that function to react with the metals in the transformer in such a way as to protect them from corrosive attack (forming a "protective shield").

Therefore, as a general guideline, if a mineral oil-filled transformer was manufactured or retrofilled since about the year 2000, and if it spends at least six months of the year with its top oil at a minimum of 70°C, then we recommend that the oil be tested for corrosive sulfur.

In addition to those transformers that fulfill the preceding guideline, there are a few other cases where Corrosive Sulfur testing could also be warranted:

Some transformers may not completely meet the temperature guideline, but may meet one or more of the other characteristics of transformers that have failed due to corrosive sulfur. For example, furnace transformers will not always fulfill the rule regarding a minimum 70°C top oil temperature for at least six months of the year. However, due to their critical nature and the high electrical stresses they experience, it is often wise to test furnace transformers for corrosive sulfur, assuming they were manufactured or retrofilled since about the year 2000.

Some insurance companies recommend Corrosive Sulfur testing for a main transformer that runs an industrial facility. It may also be desirable to test new oil for filling or topping-off a transformer. ASTM D3487, Standard Specification for Mineral Insulating Oil Used in Electrical Apparatus, specifies multiple tests for new oil, among them the Corrosive Sulfur test (ASTM D1275).

In Part 3, we will describe how the Corrosive Sulfur test is performed, the possible results, and our recommendation if the test result is Corrosive.