When oil maintenance is needed to arrest oxidation of the oil and aging of the solid insulation, the service recommendation is most often for hot oil cleaning. Hot oil cleaning involves the use of a mobile oil processing unit (sometimes referred to as a VOP or an oil reclamation “rig”) with a filtering clay such as fuller’s earth to process the oil in electrical equipment. The oil is reclaimed in the VOP in a process utilizing heat and vacuum and sub-micronic filtration- then returned to the equipment being cleaned.
There are two keys to this process. The first is that the oil is thoroughly cleaned of oxidation products such as acids, alcohols, peroxides, aldehydes, and both dissolved and suspended sludges. This requires the use of an appropriate adsorbent material in the processing rig. Fuller’s earth and activated alumina are two examples of adsorbent materials that provide proper reclaiming of the oil. Traditionally, this spent adsorbent was removed from the rig for disposal. Another current, more environmentally responsible approach is to use a reclaiming technology that reactivates the adsorbent so that it can continue to be used and recycled.
The second and very important key to this process is returning heated, clean oil to the equipment being hot oil cleaned at the proper flow rates, oil quality, and temperature. If the oil is heated to its aniline point, typically 65 to 80 oC, the hot oil will act as a solvent for its own oxidation decay products. This means that it will dissolve sludge that formed from the oxidation of the oil, and the hot oil will also remove oil oxidation products from the solid insulation. Complete cleaning of the solid insulation involves recirculating the oil from the equipment through the processing rig, and back to the equipment several times. The number of recirculations or “passes” needed depends on how oxidized the oil has become and on how long the equipment has been allowed to operate with this degree of contamination prior to the start of the project. Care in the prescribed treatment should also be taken to consider other properties such as but not limited to the PCB content and oxidation inhibitor content of the oil.
Hot oil cleaning also involves the oil passing through a vacuum chamber and submicron filter. Some of the reclaiming technologies, particularly some of those that do not reactivate the adsorbent, result in an increase in moisture content of the reclaimed oil as it passes through this stage of the process. In this vacuum step of the process, moisture is removed so that the overall moisture level of the equipment and the solid insulation do not increase as a result of the oil maintenance. A typical goal for processing is voltage dependent but normally requires the oil to always stay under 15 ppm as it leaves the purifier. Responsible equipment operators have instrumentation or test equipment on site to monitor key parameters.
We have experienced considerable confusion among transformer owners and service providers concerning terminology used for oil maintenance processes. The use of the term “reclaiming” should be used to describe the process of effecting a chemical change in the oil to remove contamination and oxidation or aging products by use of Fuller’s earth or some other appropriate adsorbent material. That definition comes from IEEE Standard 637, Guide for the Reclamation of Insulating Oil and Criteria for Its Use. Although some use the term “filtering” for this process, that is actually incorrect. Filtering and vacuum purification of oil are both methods used to “recondition” the oil, as defined in the same standard. Reconditioning is the removal of moisture, gases, and/or solid materials by mechanical means.
Overseas, particular in areas where IEC standards are followed, reclaiming is frequently called “regeneration”.
Once the sometimes confusing terminology is reconciled, hot oil cleaning of a transformer using a suitable reclaiming technology to process the oil is an effective maintenance tool for life extension. In next week’s article, we will carry this discussion further by relating this maintenance activity to what we know about the aging process of oil and the transformer’s solid insulation to indicate why this is the case.