Regular sampling and testing of insulation oil taken from the transformer is a valuable technique in the preventative maintenance program. The transformer can be used longer if a proactive approach undertaken based on the transformer oil’s condition. Hence, transformer oil must be periodically tested to ensure its basic electrical properties. These tests can be divided into:
The IEC standard method for this test is IEC 247. This involves measuring the power loss through a thin film of liquid test. Water, contamination, and the decay products of oil oxidation tend to increase the power factor of oil. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)
Dielectric Breakdown Strength
The dielectric breakdown voltage is a measure of the ability of the oil to withstand electric stress. Dry and clean oil showed the inherent high breakdown voltage. Free water and solid particles, especially the latter in combination with high levels of dissolved water, tend to migrate to areas of high electric stress and dramatically reduce the breakdown voltage. The measurement of breakdown voltage, therefore, serves primarily to indicate the presence of contaminants such as water or conducting particles. A low breakdown voltage can be indicating that one or more of these are present. However, a high breakdown voltage does not necessarily indicate the absence of all contaminants. This test was conducted in accordance with IEC 156. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)
Moisture
The purpose of dielectric tests are conducted is to ensure the monitoring moisture can be done directly. IEC 733 is a well established and can measure the moisture down to the low part of the million levels. While the acceptable values have been set by the voltage class for moisture, these are somewhat misleading. A truer picture of moisture in the transformer must be taken into account so that percentage saturation of the oil by moisture and percentage moisture by dry weight of the solid insulation can be calculated. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)
Neutralization Number (Acidity)
This value, measured by IEC standard method IEC 1125A reported as mg KOH / g sample, reports the relative amount of oil oxidation products, especially acids, alcohol and soap. As oil continues to oxidize, the acid increased gradually, generally over the years. Running the acid number regularly provides guidance as to how far oxidation of the oil has proceeded. The acceptable limit by the test is usually used as general guidelines to determine when the oil should be replaced or reclaimed. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)
Interfacial Tension
The test methods for interfacial tension (IFT), IEC 6295, measuring the strength in mN/m from the interface that will form between service aged oil and distilled water. Because the decay products of oil oxidation are oil and water soluble, their presence would tend to weaken the interface and reduce the interfacial tension value. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)
Colour/Visual
Field inspection of liquid insulation (IEC 296) includes examination for the presence of cloudy or sediment and the general appearance as well as a colour inspection. As oil ages, it will be darken gradually. Very dark oil or oil that changes drastically over a short period of time may indicate a problem. Any cloudiness or sediment indicates the presence of free water or particles that may be harmful to continued the equipment operation. Taken alone, without considering the past history or other test parameters, the colour is not very important to diagnose transformer problems. If the oil has an acrid or unusual odor, consideration should be given to carrying out further tests. (A Guide To Transformer Oil Analysis, by I.A.R. GRAY)
Sludge/Sediment
The IEC 296 test distinguishes between the sediment and sludge. Sediment is an insoluble substance present in the oil. Sediment may consist of insoluble oxidation or degradation products of solid or liquid materials, solid products such as carbon or metallic oxide and fibres or other foreign matter. Sludge is polymerized oxidation products of solid and liquid insulating material. Sludge is soluble in oil up to a certain limit. At sludge levels above this, the sludge comes out of the solution contributing an additional component to the sediment. The presence of sludge and sediment can change the electrical properties of the oil and prevent the exchange of heat, so encouraging damage to the insulating material. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)
Inhibitor Content
Inhibited oil deteriorates more slowly than uninhibited oil so long as active oxidation inhibitor is present. However, after the oxidation inhibitor is consumed, the oil can be oxidized at a higher level. Determination of oxidation inhibitor remaining in the in-service transformer oil is based on IEC 666. (A Guide to Transformer Oil Analysis, by I.A.R. GRAY)
Dissolved Gas Analysis
The purpose and functions of the DGA is to provide an indication as to whether there may be an active or incipient transformer fault affecting the operation and continued health of the equipment. DGA is used to detect and measure nine of dissolved gases which are Hydrogen, Oxygen, Nitrogen, Methane, Carbon Monoxide, Carbon Dioxide, Ethan, Ethylene, and Acetylene. (A Guide To Transformer Oil Analysis, by I.A.R. GRAY)
Dissolved Metals Analysis
Analysis of dissolved metals can be used in further identifying the location of transformer faults discovered by dissolved gas analysis. For example, the dissolved metal analysis indicating the presences of conductor metals may indicate a fault is occurring in the winding or at a connection while the presence of iron indicates involvement of the core steel. (A Guide To Transformer Oil Analysis, by I.A.R. GRAY)
Furanic Compounds
When paper breaks down, the cellulose chains are broken and glucose molecules (which serve as the “building blocks” of the cellulose) are chemically changed. Each of the glucose monomer molecules that are removed from the polymer chain becomes one of a series of related compounds called “furans” or “furanic compounds”. Because these furanic compounds are partially soluble in oil, they are present in both the oil and the paper. Measuring the concentration of the oil can tell us a little more about the paper. The standard method typically tests for five compounds that are normally only present in the oil as a result of the paper breaking down. (A Guide To Transformer Oil Analysis, by I.A.R. GRAY)
