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:

• NAPTHANIC OIL : Naphtha oil is more easily oxidized than Paraffin oil but oxidation product i.e. sludge in the naphtha oil is more soluble than Paraffin oil. Thus sludge of naphtha based oil is not precipitated in bottom of the transformer. Hence it does not obstruct convection circulation of the oil, means it does not disturb the transformer cooling system.
• PARAFFINIC OIL : Oxidation rate of Paraffin oil is lower than that of Naphtha oil but the oxidation product or sludge is insoluble and precipitated at bottom of the tank and obstruct the transformer cooling system. It has high pour point due to the wax content.

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 Testing
To 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.

• Dielectric Breakdown is the minimum voltage at which electrical flash-over occurs in an oil. It\s a measure of the ability of an oil to withstand electrical stress at power frequencies without failure. A low value for the dielectric breakdown voltage generally indicates the presence of contaminants, such as water, dirt, or other conducting particles in the oil.
• Neutralization Number of an oil is a measure of the amount of acidic or alkaline materials present. As an oil ages in service, the acidity and, therefore, the neutralization number increases. A used oil having a high neutralization number indicates the oil is either oxidized or contaminated with materials such as varnish, paint, or other foreign matter. A negative neutralization number results from an alkaline contaminant in the oil.
• Interfacial Tension of an oil is the force required to rupture the oil film existing at an oil-water interface. When certain contaminants such as soaps, paints, varnishes, and oxidation products are present in the oil, the film strength of the oil is weakened; thus less force is required to rupture the oil film. For oils in service, a decreasing value indicates the accumulation of contaminants, oxidation products, or both. It\’s a precursor of the presence of objectionable oxidation products that may attack the insulation and interfere with the cooling of the transformer windings.
• Specific Gravity of an oil is the ratio of the weights of equal volumes of oil and water. A high specific gravity indicates the oil\’s ability to suspend water. In extremely cold climates, specific gravity can be used to determine whether ice, resulting from freezing of water in oil-filled apparatus, will float on the oil. Such a condition possibly may result in flashover of conductors extending above the oil level.
• Water Content measures the concentration of water contained within the oil. A low water content is necessary to obtain and maintain acceptable electrical strength and low dielectric losses in insulation systems.
• Color compares the actual color of the oil to an established spectrum of colors. Expressed numerically from 0 to 5, a high color number indicates contamination caused by carbon or the deterioration of either insulation material or the oil.
• Visual Examination an oil sample is visually examined by passing a beam of light through it to determine transparency and identify foreign matters. Poor transparency, cloudiness, or the observation of particles indicate contamination, such as moisture, sludge, or other foreign matter.
• Power Factor indicates the dielectric loss of an oil. A high power factor is an indication of the presence of contamination or deterioration products such as moisture, carbon, or other conducting matter, metal soaps, and products of oxidation.
• Flash Point is the minimum temperature at which heated oil gives off sufficient vapor to form a flammable mixture with air. It is an indicator of the volatility of the oil.
• Pour Point the lowest temperature at which the oil will flow. A low pour point is important, particularly in cold climates, to ensure that the oil will circulate and serve its purpose as an insulating and cooling medium.
• Corrosive Sulfur detects the presence of objectionable quantities of elemental and thermally unstable sulfur-bearing compounds in an oil. When present, these compounds can cause corrosion of certain transformer metals, such as copper and silver.
• Viscosity is the resistance of oil to flow under specified conditions and is the principal factor in the convection flow of oil in an electrical device. It influences heat transfer and, consequently, the temperature rise in apparatus.
• Dissolved Gas Analysis All liquid-filled transformers generate gases during normal operation. When a transformer begins to function abnormally, the rate of gas production increases. Analyzing these gases and their rate of production is another valuable laboratory tool for evaluating the condition of an operating transformer.The most accurate method of analyzing dissolved gas in a transformer is using gas chromatography. The gases dissolved in the oil are extracted from a sample and analyzed by a gas chromatography. This method identifies the individual gases present and also the quantitative amounts.

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.

Oil Analysis

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.

• 1. Raising oil temperature. First step is to raise oil temperature up to 65°C to give latent heat which separates moisture and gases. Heating the oil will make it easier to filter because of the decrease in oil viscosity.
• 2. Removal of dirt and impurities Second step is to remove sludge and dirt from the insulating oil. There are 2 ways to eliminate dirt in the transformer oil.
  • By paper filter (candles): Insulating oil filtration by filter candles can be classified by using classical edge type paper filter or depth type paper filter. However, new advancements were made in which transformer oil filtration machines use filter cartridges instead of edge type paper filters.
  • By centrifuging action: Another method for separating dirt from oil is through centrifuging. With this process, you can save recurring cost of changing filters.
• 3 Dehydration and Degasification of Insulating Oil.
  ‘This is the stage of dehumidification of insulating oil and removal of gasses. This process is completed in the degassing chamber (part of the oil filtration equipment).’+ ‘The most efficient method of industrial oil degassing is vacuum processing, which removes air and water solved in the oil. This can be achieved by distributing the oil into a thin layer over special surfaces (spiral rings) in vacuum chambers. Under vacuum, an equilibrium between the content of moisture and air (solved gases) in the liquid and gaseous phase is achieved. The equilibrium depends on the temperature and the residual pressure. The lower that pressure, the faster and more efficiently are water and gas are removed.
  

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.