THE PAIN: Aging Of Transformer Oil And
Transformer Oil serves as a cooling medium and an insulating material. Oil, together with the Kraft Insulating Paper and thick cardboard spacers protect the transformer and allow the transformer to perform even under severe electrical surges, vibration, shorts and other stressful events. There are a number of different aging processes that affect the oil and the paper, but the paper is the most important part of the transformer that must be protected. If the paper fails, the transformer fails
Transformer oil consists mostly of hydrocarbons (more than 99%), a mix that is similar to conventional diesel. The oil's minor components however, which constitute less than 1% of the oil, have the greatest impact on the oil's operating properties and performance capabilities The Oxidation Process is the most common cause of oil decay. It is a highly complicated process because oil oxidation and paper degradation are connected with each other. The paper however, depends heavily on the oil to control and keep the level of damaging contaminates circulating into the paper down to a minimum. Transformer oils can be divided into two categories: inhibited transformer oil and uninhibited oils. inhibited transformer oil contain synthetic anti-oxidants, while uninhibited oils rely on natural sulfur anti-oxidants that are usually present in less refined oils. Inhibited oils normally have a longer life cycle, but inhibitor content still needs to be monitored throughout the oil's service life to keep the inhibitor additives at a sufficient level. This requires implementation of monitoring programs.
Transformer suffered catastrophic failure due to damaged insulating paper as the result of neglecting timely preventive maintenance
Water, acids, and sludge, produced during the oxidation process, are the most harmful compounds for your transformer's insulation system. Sludge is a highly polar mixture of contaminates that do not dissolve in oil and instead, settle in the transformer tank and find their way into the components of the insulation system forming deposits. These undesirable sludge deposits prevent the normal oil flow within the transformer and can cause the transformer to overheat even with low outside air temperatures. Acids are formed and grow in strength and begin to attack and degrade the copper parts of the transformer windings. The copper windings then begin to experience corrosion which leads to subsequent transformer failure. Additionally, the acids become aggressive towards the insulating paper and as the acid number in the oil rises, the paper degradation process begins to accelerate also leading to transformer failure.
The Service Life of the transformer insulating paper is highly influenced by water and acid content in the oil and the paper itself. The Insulating Paper contains the majority of the water and moisture found in a transformer. The amount of water and moisture found in the oil is relatively small (especially if the oil is purified regularly), compared to what is contributed to the degradation of the insulating paper by the water content of the paper itself. Water content in the oil can be easily measured with the use of moisture sensors, but measuring water and moisture content in the paper is much more difficult to determine directly.
Insulating paper consists of Cellulose, Hemi-cellulose and Lignin as its main components. It is the degradation of the "Cellulose" however, that has the most significant impact on the paper's strength and its ability to protect the transformer. Changes in the paper’s physical properties are the main causes of concern, as they directly affect the paper’s ability to withstand short-circuits and vibration stresses. Excessive water content also negatively influences the dielectric strength of the oil, but the oil's water content can be reduced by application of timely preventive maintenance procedures and the oil can be restored. On the other hand negative changes in the paper’s physical properties are impossible to reverse. The paper degradation process can be stopped with the GlobeCore Process, but any damage done to the paper before stopping the process will remain present.
The IEEE has determined that a transformer is at, or is near, the end of its service life when the insulating paper has lost 75% of its tensile strength or "DP" value. Beyond this point, the transformer may not be able to reliably withstand the next overload surge or short circuit. Since the life of the transformer is directly related to the life of the insulating paper, it is important to protect the paper by removing the water, moisture and other undesirable contaminates from the insulating oil and the insulating paper. This action must be taken before the paper becomes damaged and unable to perform its job. There are number of maintenance procedures to address the problems discussed above. They include, but are not limited to oil changes, oil purification, and oil regeneration in conjunction with oil monitoring and oil analysis programs. Decisions should always to be made on a case-by-case basis and should use as much information as possible, including any data from DGA oil tests and continuous gas-in-oil monitoring.