Transformer Oil Maintenance
by Judie White
Perhaps, the easiest way to explain the reasoning for proper transformer oil maintenance is to compare a transformer to a car. A car requires oil for lubrication while a transformer requires oil for insulation and cooling. Just as a car requires regular checks of both oil quality and oil level, a transformer requires the same proactive treatment. Oil in each instance degrades over time resulting in the potential for failure or costly repair bills. Regular maintenance and upgrading of the oil will add years to the life of both your car and your transformer.
Transformer oil is a mineral based oil. It is commonly used in transformers because of its chemical properties and dielectric strength. Minimal breakdown of the oil occurs under normal operating conditions.
The insulation and cooling properties of the transformer are effected by the quality of the oil. The quality of the oil is reduced by oxidization and contamination. The results of each of these can be summarized briefly as follows:
1. Contamination commonly found in oil can be water and particulate. The presence of either contaminant will cause reduced insulation quality of the oil.
2. Oxidization is the acid that forms in the oil when it reacts to oxygen. This acid will form sludge which will settle on the windings of the transformer reducing the heat dissipation from the transformer. The heat transfer from the windings to the oil is limited thus causing the windings to run hotter. Sludge formation on the windings has a snowball effect on the transformer with more sludge creating more heat, creating more sludge etc. The high acid content together with the excessive temperatures will cause the deterioration of the transformer insulation to be accelerated and if left untreated the transformer will fail.
The first part of a preventative maintenance program requires establishing when remedial action is necessary. With transformer oil this is done through testing of the oil. Development of an ongoing maintenance program will help prevent costly failures. Initial testing will establish a base line and annual testing will plot any changes happening internally in the transformer. The following 5 part tests are a minimal requirement of a yearly maintenance program.
1. Dielectric breakdown: Dielectric strength is a measure of the voltage which the oil will conduct. Many contaminants conduct electricity better than oil therefore lowering the dielectric breakdown voltage.
2. Neutralization/Acid Number: Oxidation occurs in the oil causing the buildup of acid which will lead to the formation of sludge. The test indicates the level of acid present in the oil.
3. Interfacial tension: This test points to the presence of polar compounds, which indicate oxidation contaminates or deterioration from the transformer materials i.e. paint, varnish, paper.
4. Colour: Indicates the quality, aging, and the presence of contaminants.
5. Water: Is measured in parts per million. The presence of water will decrease the dielectric breakdown voltage.
Another useful tool in a maintenance program is a Dissolved Gas Analysis (DGA). By analyzing the gasses present in the oil it is possible to determine if there is a fault such as arcing, corona or overheated connections.
From the testing results, it can be determined whether remedial action needs to be taken. Predetermined limits must be set based on voltage class and KVA rating of the equipment. Any analysis showing test results lower than the parameters set indicate further investigation. A downward trend in any of the testing values warrants further testing and an evaluation of these results. Equipment should not be condemned by one test but by a series of tests.
If remedial action is required to the transformer oil, a recent PCB analysis is required in addition to the preceding testing. If the PCB level of the oil is less than 2 ppm, in most cases on-site reclamation is recommended based on the customer’s preference. If the oil is over 2 ppm but less than 50 ppm, it can be shipped to a recycling facility and the transformer can be retrofilled with either new oil or recycled oil. In all cases, any results over the 50 ppm limit require special handling.
If testing indicates a problem with the oil quality, the owner must determine if the cost of reclamation of the oil is warranted. Factors influencing this decision would include the replacement cost of the transformer or expenses associated with a failure of the equipment at a critical time.
After the decision has been made to upgrade the oil, the owner will need to evaluate if the oil is to be replaced or treated. Onsite reclamation includes fuller’s earth processing and degasification. It is possible to restore oil to new oil specifications with a combination of these treatments. Any transformer oil can be brought back to new oil specifications, although some contamination levels may make it more economically viable to replace the oil. Antioxidant may be added at this time if an inhibited oil is required.
A regular maintenance program allows the transformer oil to be upgraded before sludge created by oxidization occurs. This results in a cost saving to the transformer owner. Once sludge occurs, the internal parts of the transformer require flushing with hot oil to remove the sediment. If not rinsed down, the core and coil assembly will hold the acids which will leach back into the oil over time. This will cause the re-deterioration of the oil to be accelerated. If reclamation is done in the early stages of the acid build up (approx. 0.1) before sludging occurs, the oil will retain its properties longer under normal operating conditions.
Reclamation of oil with a high acid content consists of fuller’s earth treatment and degasification. Fuller’s earth treatment will remove both acid and particulate in the oil. This process corrects the acid number as well as the colour. Degasification removes gasses and water. Transformer oil will hold water in suspension. The amount of water it will hold depends on the temperature of the oil. For example, if the oil temperature is 20C the maximum amount of water that can stay in suspension is approx. 53 ppm. It is likely that if oil is at its saturation point there will be free water at the bottom of the transformer. The ASTM limit for water is 30 ppm but the temperature must be considered. Due to the decreased dielectric strength with high water content it is important to complete a degasification on the oil. If the oil is especially high in water a hot oil dry out should be considered. Although it is more costly than just a degasification, it also removes any water that may be in the core and coil assembly.This is completed by circulating the oil through the degasification process and the transformer to heat the transformer. Then if the transformer is capable of vacuum the oil will be removed and will be held under vacuum for a determined number of hours. The oil is then degasified back in to the transformer.
If the decision has been made to replace the oil, either reclaimed or new oil can be used as retrofill material. If the tank is capable of a vacuum, the transformer should be filled under vacuum, based on manufacturer’s recommendations. If the transformer cannot withstand a vacuum, the oil should be degasified into the transformer and circulated through the degasifier three times the volume of the transformer to help remove any moisture present in the insulation.
New oil often requires further degasification to remove air and moisture added during transportation and handling procedures. This further degasification will increase the life expectancy of the oil in the transformer.
Mineral insulating oil is a valuable resource that can be recycled many times and returned to its original condition. The benefits of a regular maintenance program for your transformer include both economical and environmental factors. A failure of a transformer can result in significant environmental clean up costs.
In conclusion, a good preventative maintenance program for your transformers is money well spent. You will have peace of mind that comes from knowing that you have done due diligence in maintaining the continuing operation of your transformers.
Zhongneng Transformer Oil Treatment Plant can help you in any transformer oil contamination problems to purify your transformer oil and offer best maintenance for your oil.