Most substation transformer failures, excluding acts of man or nature, can often be traced to poor maintenance of the transformer or its related components. Catastrophic failure of a substation transformer can be prevented by accurately testing the condition of the oil on a yearly basis and by having a monthly visual inspection program in place. Most of the checks on a substation transformer are relatively simple: checks for oil leaks, oil level, temperature, the operation of the on-load tap-changer, etc. However, if the oil paper insulating system of the transformer is breaking down, it can only be detected by oil analysis. By testing the physical and chemical properties of transformer oil, a number of problem conditions can be identified before they lead to transformer failure.
The reason transformer oil care and filtering is such an important part of substation transformer maintenance is because of the makeup of most transformer insulation systems. Oil and cellulose paper insulation systems are organic compounds containing hydrogen, and gradual oxidation of these compounds allows for the formation of moisture. The byproducts of the process produce an increase in moisture levels and create an environment that overheats the transformer and further degrades the paper insulation, which in turn produces more byproducts. This cycle continues until the transformer fails. The two major contaminants that cause paper insulation and oil degradation are the presence of oxygen and/or water in the oil.
Transformer oil is organic and therefore oxidizes and decays; creating oxidation by-products which destroy the paper insulation. The aging of transformer oil is caused by the oxidation process of the oil; therefore, controlling the access of oxygen to the transformer oil is critical. Oxygen is introduced through the venting pipe of the transformer expansion reservoir present on older transformers if the silica gel filters are not properly maintained. In order to control the oxygen that may be absorbed by transformer oil, several other methods are also used:
• Incorporating a sealed tank with a positive pressure inert gas layer maintained above the oil by means of a permanently connected tank of nitrogen gas.
• A tank completely filled with oil but connected to a raised tank or oil conservator, which maintains a positive oil pressure in the main tank and provides a place for expansion and contraction of the oil.
• A conservator tank with a divided expansion tank with two sections and a flexible diaphragm conservator tank.
• A conservator tank with a bladder within the conservator tank as a variation of the flexible diaphragm.
While these systems are effective in protecting the oil from outside oxygen and limiting access of oxygen to the paper, they do nothing to deal with the water present in insulating paper.
The paper used as insulating material is cellulose and naturally contains a certain amount of oxygen...