E+C August 2018

TRANSFORMERS + SUBSTATIONS

Major factors influencing transformer oil degradation

Corné Dames, Transformer Tech Repair

Since oxygen has two free electrons, any in- gress of air in oil makes it more susceptible to an auto-oxidation process. Free radicals can easily break the hydrocarbon chains in oil and produce peroxides. Under heat, the peroxides decay to produce more free radicals, and thus the oxidation process compounds as a chain reaction process. Oxidation inhibitors could be added to slow down the oxidation process in the oil. However, if the sulphur content of the oil is high, oxidation effects on the unit might be more severe as more corrosive by-products form, with adverse effect on the unit. Some oils have natural inhibition proper- ties, depending on the source of the oil. Contamination The presence of water in oil, either in dissolved or suspended form, affects the dielectric properties of oil adversely. Thermal degradation of cellulose and oil will produce water internally, through a process of hydrolysis of the hydrocarbon chains. This can adversely increase if the unit operates at high temperatures. Measuring water content in oil is only a fraction of the total amount of water in the transformer, as the major portion of the water is absorbed by the Kraft paper of the transformer core. If the transformer core heats up, the equilib- rium state between the core and the oil favours movement from the core into the oil. When the unit cools down the opposite is true. Undertaking moisture analysis on a unit that is not operational is of very little value as it is impossi- Major factors influencing the degrada- tion of transformer oil include excessive heating, oxidation, contamination, par- tial discharges, and related by-products.

Thermal degradation Excessive rise of operating temperature is one of the most severe factors influencing the degrada- tion of transformer oil, and it is estimated that an increase of 8 – 10°C in temperature approximately doubles the rate of oil degradation.Therefore, units with higher temperature variations show greater degradation than units under constant load. Tem- perature changes in the transformer cause the oil to push air from the transformer when heating, and suck in air when cooling down, if the unit is free breathing. Chemical reactions in the hydrocar- bon oil take place at a much more rapid rate under high temperatures than in units with cooler oper- ating temperatures. As the oil decomposes, several by-products are formed, which further accelerate the ageing of the oil and solid insulation. As a result, the oil degra- dation rate increases with the formation of degra- dation products as this actually fuels the process. Oxidation Oxidation of oil causes a breakdown of bonds, followed by further chemical reaction. The main oxidation products of oil include water, carbon di- oxide, carbon monoxide, acids, and sludge. In ad- dition, the presence of dissolved metals such as iron, copper, etc, act as a catalyst to accelerate the oil degradation process. Bus bars, leads and wind- ings in the transformer’s tank act as catalysts for oxidation to take place even quicker than would be the case if they were not present.

Take Note!

Typical diagnostic tests used to determine transformer deg- radation include: Testing of new and old oil. Dissolved gas-in-oil anal- ysis (DGA). 1 2

14 Electricity + Control

AUGUST 2018