Last week, we talked about how mineral insulating oil ages in the LTC. As oil ages in a load tap changer, it polymerizes and forms a resinous, varnish-like film over the contacts and mechanism. This week, we are going to talk about conditions that cause that filming to proceed faster. It is very important to understand these conditions since accelerated filming may lead to potentially destructive conditions prior to the end of the normal preventative maintenance interval. If filming is well advanced, arcing may be sustained too long on the contact causing overheating and premature wear. In extreme cases, this may lead to failure. Similarly, filming of the mechanism will cause the device to work harder during normal changing of the taps under load. These conditions can cause an increase in operating temperature of the device. More importantly, in extreme cases, filming can become so advanced that it may lead to binding and subsequent mechanical failure of the LTC.
Although oxidation is not generally the most critical aging mechanism of the oil in an LTC, oxidized oil does form film at a more rapid rate than clean, unoxidized oil. Since there is less concern of damage to solid insulation by oil oxidation products in an LTC, the industry has tended to pay less attention to oxidative aging and most owners use guideline values for acid and interfacial tension that are much less restrictive when evaluating oil in an LTC compared to those they use for oil in transformers. Our experience, as we began testing for particles and filming compounds in LTCs several years ago, indicates that a better strategy is to use the same acceptable, questionable, and unacceptable ranges for values that we use for in-service oil from transformers. Recommendations from these values and ranges however are interpreted differently than they are for transformer oil. In the case of LTCs, replacing or reclaiming the oil is recommended when the values for acid or IFT become unacceptable (acid number 0.10 mg KOH/g, IFT 28 mN/m).
Just as higher moisture content causes oxidative aging of transformer oil to proceed more rapidly in transformers, elevated moisture content in LTC oil will cause faster filming of the contacts and mechanism. This process is not as clearly defined or documented with LTCs as the effects of higher moisture on the aging of oil in transformers. However, our experience from running particles and filming compounds analysis on oil from load tap changers, and reviews conducted during preventative maintenance of such devices, confirm such a correlation between moisture and filming. Furthermore, high moisture leads to other difficulties in an LTC related to dielectric strength of the oil and moisture tracking within the device. Unacceptable moisture levels indicate that there is an unacceptable risk of tracking and even dielectric failure of the oil. The values that we use to define unacceptable moisture also correspond to the levels where we expect to encounter runaway filming in the device. Moisture content 60 ppm or greater in an LTC is classified as unacceptable according to our oil classification system.
We mentioned last week that film gets “rubbed off” during the normal operation of the LTC. Movement of the contacts across each other, arcing at the contact surface, and arcing in the oil also produce other types of metallic and non-metallic particles. These particles are incorporated into new film. If the oil has an unusually high number of particles, or if the particles themselves are unusually large, the new film forms more quickly. Incorporating the existing, suspended particles into film as it forms in an LTC can be of particular concern if there are large numbers of metallic particles or carbon particles. These typically are much more abrasive that the other non-metallic particles generally found in load tap changers. When film forms on the contact surfaces and incorporates metal and carbon particles, normal operation of the device can erode those contacts at an accelerated rate. Similarly, film incorporating such particles is more mechanically resistive on moving parts of the mechanism, having a further detrimental effect on the efficient operation of the LTC.
Incompatible compounds in the insulating oil also may greatly accelerate filming in an LTC. One frequent source of such incompatible is the use of solvents as cleaners during LTC maintenance and inspection to clean the contacts and mechanism. Only hot mineral oil dielectric fluid should be used for this application. Any other material such as brake cleaner, spot remover, chlorinated or aromatic solvents, and paint or lacquer thinner may make film removal easier for the current project. In the future, though, the use of these materials will greatly increase the rate at which the film forms. Further, films from incompatible materials may frequently be more difficult to remove. So, the short term gain from using such cleaners and solvents is far outweighed by the more frequent need to clean the LTC and the greater difficulty in cleaning the device in the future. If an LTC exhibits an unusually severe filming tendency, there are trouble-shooting oil tests that may be performed to help identify the cause. Our experience is that frequently inappropriate materials used in cleaning account for a large number of these types of problems.
Testing the oil in load tap changers provides valuable information concerning whether the unit may have conditions that can create filming at a rate that may compromise the operation of the unit. These oil tests also indicate other conditions that may require preventative maintenance before the “normal” or planned scheduled maintenance. Next week, we will talk about how liquid screen tests, moisture content, and dissolved gas analysis are used to monitor conditions between preventative maintenance inspections and what abnormal results for these tests may indicate.
