Written by Harvey Ursaki, September 21, 2019
All power transformers are filled with an insulating/cooling oil. The oil is a petroleum-based insulating oil refined specifically to meet the desired electrical and chemical properties of the transformer. The quality of the oil in a transformer plays an important role in maintaining the longevity and performance of the transformer, extending the life of the transformer for many years.
Periodic testing of a transformer’s insulating oil is the most important part of extending the life of the transformer. For example, testing can detect sludge build up within the transformer thus allowing the removal of the sludge before it can find its way into the windings and other interior surfaces of the transformer. Eliminating sludge prolongs transformer life.
Another advantage of oil testing is the prevention of unscheduled outages. If problems are detected early enough, corrective action can be scheduled when disruption of electrical service will be minimal.
Finally because transformer oil breaks down in a predictable fashion, periodic testing will prove helpful in determining any trends. This allows comparisons between normal and abnormal rates of deterioration. Although one set of test data will indicate the presence of contaminants, it will not enable accurate analysis of any trends that are developing.
‘Types of Insulating Oils A. Mineral Oil: These are Petroleum products and are mainly following two types:
B. Synthetic Oil: These are generally the chemical products e.g. Silicon Oil. while having fire retardant properties, they are used in fire prone area. Lower heat dissipation capacity and high moisture absorbing capacity make these less desirable in normal operating condiyions. This type of oil is cost prohibative compared to the mineral oils.
Oil TestingTo measure the quality of insulating oil and establish a benchmark for the degree of deterioration, several tests are used. Samples of the fluid can be drawn while the transformer is in normal operation through drain valves or sampling ports. The following list describes some of the most common laboratory tests.
Several key gases are attributed to certain fault conditions that generated them. Since normal operation causes the formation of certain gases, simply determining the presence of gases within the oil should not cause alarm. What is important is the rate and amount of gases generated. As in other tests, gas analysis should be conducted on a regular basis to indicate trends or changes in results.
Thermal faults within the transformer are detected by the presence of by-products produced by the faults. For example, the solid insulation is commonly constructed of cellulose material. The solid insulation breaks down naturally but the rate increases as the temperature of the insulation increases.
Another example is when an electrical fault occurs it breaks the chemical bonds of the insulating oil. Once the bonds are broken these elements quickly reform into the fault gases.
The energies and rates at which the gases are formed are different for each of the gases which allows the gas data to be examined to determine the kind of faulting activity taking place within the transfomer.
When gassing occurs in transformers there are several gases that are created. Of all the types of gases created during the transformer operation only the information from nine gases are usually examined. The nine gases examined are:
Atmospheric Gases: Nitrogen and Oxygen.Carbon Oxides: Carbon Monoxide and Carbon Dioxide. Overheating windings typically lead to thermal decomposition of the cellulose insulation. In this case the results show high concentrations of carbon oxides (monoxide and dioxide). In extreme cases methane and ethylene are detected at higher levels. Hydrocarbons: Acetylene, Ethylene, Methane and Ethane. Oil overheating results in breakdown of liquid by heat and formation of methane, ethane and ethylene.Hydrogen. Corona is a partial discharge and detected by elevated hydrogen.Acetylene: Arcing produces large amounts of hydrogen or acetylene or minor quantities of ethylene and methane. Arcing is the most severe condition in a transformer and indicated by even the lowest levels of acetylene.
Oil Filtration Large Power Transformers need to be maintained to ensure safe operation and long life. One way to achieve this is a good quality and clean insulating oil. However, transformer oil tends to degrade over time and steps need to be taken to ensure the oil is a its peak performance. Under normal operating conditions the oil is exposed to undesirable materials like gases, acids, metal dust, moisture etc., following a well planned maintenance schedule, shows when it\s is time to take action to filtered and purified the oil.
To do this, high quality equipment for treating oil and following recommended transformer oil filtration techniques must be used, thus sustaining a healthy, safe operation of the transformer.
Research shows that most of oil related failures and breakdowns are caused by contaminated oil. Preventive oil maintenance is therefore an important factor to ensure optimum equipment reliability.
Insulating oil filtration process:
To further understand how the transformer oil is tested, below is the 3 step process.
A successful insulating oil filtration procedure can be achieved by using quality transformer oil filtration equipment. This will extend the efficiency and reliability of the transformer throughout it’s life-cycle!
The quality, testing and maintaining of the insulating oil in a transformer plays an important role in the longevity and performance of the transformer, extending the life of the transformer for many years.
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Mr. Harvey Ursaki is the Director of Programming and Operations for Electrical Design Management Software Ltd. He has over 37 years experience in the electrical industry.
Experienced in thermal and hydro generation, radial distribution substations, multi-breaker, ring bus transmission terminals. Along with many years in the oil and gas industry, he has a well-rounded knowledge of the electrical consulting industry.
Prior to forming EDM, Mr. Ursaki was Director of CLA Utility Services Ltd. an electrical consulting service, specializing in developing electrical engineering standards, serving clients in Canada, USA and in the Caribbean.
He also served as a Supervisor of Transmission Engineering for a privately- owned utility in southern British Columbia, Canada.
He now brings his years of experience to EDM company, developing an electronic standards toolbox for Engineers, Technologists and Electricians worldwide.