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“Green” transformers, which use eco-friendly biodegradable vegetable oil insulation, need moisture equilibrium charts for their monitoring and diagnosis. In-depth experiments have been carried out to produce and validate such charts. This is an important step toward longer service life with effective condition monitoring.
Green transformers, a sustainable and environment-friendly range of power transformers (PTs), are gaining in popularity. Demand is steadily growing. They are used as simple distribution transformers or sophisticated high voltage power transformers. They owe their development to the introduction of innovative technologies such as biodegradable insulating liquids combined with a hermetically sealed design, as well as noise reduction, etc.
Green Energy Transformer
Natural esters, or “vegetable oils,” offer several advantages over mineral oils: they are biologically degradable and thus do not affect groundwater. They are less flammable and they have a protective effect on transformer insulation, notably because they are less sensitive to humidity (higher water solubility), which means that they can absorb more water. Power transformer insulation consists of a composite system made of cellulosic materials (mainly paper and pressboard) impregnated with insulating liquid (mainly mineral or ester oils). Since the cellulose insulation is more hygroscopic than the oil, it contains most of the water, which can constitute several percentages by weight of the total mass. “Excessive moisture in cellulosic materials can affect the life of transformers by significantly reducing both dielectric and mechanical strengths,” explains Christophe Perrier, R&D Project Manager at GE Grid Solutions in Villeurbanne, France. “This is why the in-service evaluation of moisture in cellulosic insulation is a very important aspect of the condition assessment of power transformers—whether using mineral or vegetable oil.” Since it is not possible to remove cellulose samples from the active windings of a transformer in operation, indirect measurements were developed by assessment of the oil humidity and the use of experimentally derived moisture equilibrium charts. “Knowledge of the moisture content distribution within the various materials (oil, paper, pressboard), which are dependent on diffusion and absorption-desorption processes, is valuable in insulation diagnosis and manufacturing dry-out processes,” says Perrier.
Fig. 1 – Perrier-Lukic equilibrium curves for paper + pressboard together in mineral oil (MO) and vegetable oil (VO)
Fig. 2 –Perrier-Lukic equilibrium curves for paper and pressboard in mineral oil.
Fig. 3 –Perrier-Lukic equilibrium curves for paper and pressboard in natural ester oil
Nonetheless, the establishment of these complete moisture equilibrium charts for natural ester fluid insulation represents a significant contribution to the development and application of green transformers, “starting with the possibility of checking and ensuring manufacturing quality at the power transformer manufacturing stage. Above all, they provide power utility operators with an efficient tool to monitor and control transformer insulation and increase its lifetime,” Perrier concludes.
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