Moisture management of transformers - Why? and How?
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About the Event
Power transformers represent 60% of the asset value in a high voltage substation, making them among the most expensive assets in electricity transmission and distribution. Knowing the condition of power transformers is key to ensuring reliable grid operation, improving risk assessment, and implementing more effective maintenance strategies. Moisture is one of the dominant factors that limit the lifetime of a power transformer. For effective transformer lifetime management, the moisture content of the insulation system, cellulose, and oil should be kept at a low level. After initial factory drying, the moisture content in solid insulation is typically between 0.5 and 1% depending on requirements, and starts to increase gradually over the operating lifetime of the transformer. The main moisture sources are residual moisture in thick structural cellulose components, ingress from the atmosphere, and small amounts of moisture generated as a byproduct of cellulose decomposition.
In a power transformer, the majority of water is found in the solid paper insulation. Higher water content leads to accelerated deterioration of the paper, reducing its degree of polymerization (DP). Moisture and heat provide an optimal environment for various compounds in the oil – such as acids and metal ions – to react with the cellulose molecules and break them down. This is important as when the water content in the paper doubles, it can cut the remaining life of the transformer in half. Poor oil quality and high moisture levels can lead to low dielectric strength and eventual system failure. It has been reported that when relative moisture saturation goes above 20%, the relative saturation ‘RS’ dielectric strength of oil starts to decrease rapidly. When the temperature rises, high water content in the insulation, combined with dissolved gases in the oil, may cause bubble formation if the total pressure of gases and water vapor exceeds the oil pressure of the transformer. If bubbles form or travel into energized parts of the transformer like the windings, there is a severe risk of system failure due to sudden local collapse of the dielectric-medium. In a wet transformer, exceptional high-loading events can drive excess moisture from the paper into the oil, creating relative moisture saturation of 100% RS and the formation of free water. The same may happen if a heavily loaded wet transformer cools down quickly. Free water may cause corrosion and rust particles in oil circulation pipes and radiators. In the worst case, free water can form around active parts, resulting in discharge that damages the transformer.