Transformer Oil Maintenance from luis huang's blog

Transformer oil is a mineral based oil that is commonly used in transformers for its chemical properties and dielectric strength. This oil in your transformer acts as an insulator and a cooling agent. Over time, the oil will degrade resulting in the potential for faults and costly repairs. With a proper preventative maintenance program, you can avoid any costly down time and expensive repairs.

The quality of your transformer oil will effect its insulation and cooling properties. Under normal operating conditions, a minimal breakdown of oil quality will occur from oxidization and contamination. These are summarized as follows:

Oxidization is acid that forms in the oil when it comes in contact with oxygen. The acid will form sludge which settles on the windings of the transformer resulting in reduced heat dissipation. The windings will run hotter thereby creating more sludge which in turn will create even more heat. The high acid content and increased temperatures will accelerate the deterioration of the insulating qualities of the oil and if left untreated will cause the transformer to fail.

Contamination commonly found in transformer oil includes water and particulate. The presence of either of these contaminants will reduce the insulating qualities of your transformer oil.

Testing

Testing your transformer oil should be part of your yearly preventative maintenance program. Testing the oil will help to determine when corrective measures are required. Initial testing will establish a base line for comparison and annual testing will plot any changes internally in your transformer.

The following 5 part test is a minimum requirement of an annual maintenance program:

Dielectric breakdown: Dielectric strength is a measure of voltage the oil will insulate against. Many contaminants conduct electricity better than oil therefore lowering the dielectric breakdown.

Neutralization/Acid Number: This test will measure the level of sludge causing acid present in the oil.

Interfacial tension: This test identifies the presence of polar compounds. These would indicate oxidation contaminates or deterioration from the transformer materials. i.e. paint, varnish, paper.

Colour: The colour of the oil indicates quality, aging and the presence of contaminants.

Water content: This tests for the amount of water present in the oil in parts per million. The presence of water in your oil will decrease the dielectric strength.

The Dissolved Gas Analysis (DGA) test is another useful tool as part of your maintenance program. Examining the gasses present in the oil can help determine if there are faults in the transformer including arcing, corona or overheated connections.

The results of the tests performed will help determine when further action is required. Predetermined limits for these tests must be set based on the voltage class and KVA of your transformer. Any tests performed showing results outside of the predetermined parameters indicate further investigation is required. A downward trend of your test results over time also warrants further testing and an evaluation of the results.

If remedial action is required for your transformer oil, a recent PCB analysis is required in addition to the preceding testing. If the result of the PCB testing is less than 2 ppm, in most cases an onsite reclamation of your oil can be performed. If the oil is over 2 ppm but less than 50 ppm, the oil can be shipped to a recycling facility and your transformer can be retrofilled with new or recycled oil. Any PCB analysis with results over 50 ppm will require special handling.

Remediation Treatment

If the quality of your oil has declined below acceptable levels a decision has to be made to replace or reclaim the existing oil. Often, the rapid degradation of the oil in your transformer indicates that remedial treatment is required for your transformer itself as well as the oil.

Reclamation of your existing oil can be performed onsite with limited down time. It is possible to restore your oil to new oil specifications with a combination of treatments including fuller’s earth and degasification. If some contaminant levels are significantly high, it may be more economically practical to replace the oil rather than reclaim it.

A proactive approach should be taken if your transformer oil has a high acid content. Any sludge formed by the acid has to be rinsed out of the transformer with hot oil to remove the sediment. There is a cost saving to you if the oil is reclaimed in the early stages of the acid build up, before the sludging occurs, as the oil will retain its quality longer under normal operating conditions.

Reclamation of oil with a high acid content includes fuller’s earth treatment to remove acid and particulate, and degasification to remove gasses and water. This process will also correct the acid number and the colour.

Transformer oil can hold water particles in suspension depending on the temperature of the oil. If the oil is at its saturation point, there is likely to be free water at the bottom of the transformer. The dielectric strength of the oil is decreased with water present in the oil and a degasification of the oil is recommended. If the water content is particularly high, a hot oil dry out should be considered. Although more costly than a degasification, this will also remove any water that may be in the core and coil assembly.

If you have decided to replace the oil in the transformer, either new or recycled oil can be used. If the transformer tank is capable of vacuum, it should be filled under vacuum based on manufacturer’s recommendations. If the tank cannot withstand a vacuum, the oil should be degasified into the transformer and circulated through the degasifier three times the volume of the transformer. This will help remove any moisture present in the insulation of the transformer.

New oil often requires further degasification to remove air and moisture added during the transportation and handling process. This will increase the life expectancy of the oil in the transformer.

Environmental Concerns

Mineral insulating oil is a valuable resource that can be recycled many times and returned to its original condition. Using new quality recycled oil or reclaiming your existing oil saves the depletion of a non-renewable resource and can be much more cost effective than replacing with new oil.

A preventative maintenance program for your transformer has both economic and environmental benefits. A failure of a transformer can result in significant environmental clean up costs and considerable replacement or repair costs.