DRYING OF POWER TRANSFORMERS IN THE FIELD, APPLYING THE LFHTECHNOLOGY IN COMBINATION WITH OIL RECLAMATION

摘要

Lifetime extension of power transformers has become very important as the bulk of the installed transformers in the western world is now coming into the final life phase. The main point of concern when it comes to life expectation is the condition of the insulation system that is based on organic products: Mineral oil and paper. They degrade over time and finally lose the capability to withstand the stresses a transformer might see in the daily life (short circuits, energizing, vibration…). Main factors for the degradation processes of the organic materials are: Temperature, moisture, oxygen and acids. When a transformer leaves the factory, it is dry and almost free of acids. This will change over the time as water and acids are degradation products and will accumulate in the transformer. As a result, the degradation speed of paper and oil will increase and the “evil circle” with the self-accelerated degradation will start. Removing moisture and acids from the transformer can slow down the ageing process and thus extend the lifetime of the insulation system. Preferably these removal processes will be executed on site, as it involves substantial amounts of time and money to move a large power transformer to the next transformer workshop equipped with a vapour phase drying plant. Onsite drying and online oil reclamation are two processes that will extend the remaining lifetime of the insulation system on site. Onsite drying of power transformers is a process that has been developed in the past years. There is a wide range of possibilities known in the market. A new possibility is offered with the so-called Low Frequency Heating (LFH) system. With this technology a much better drying effect can be achieved, in much shorter time, than with other known processes. In addition to the efficient removal of water, substantial amounts of acids can be removed with the combination of heat and vacuum. The latest development is the combination of the LFH process with other known processes like online oil reclamation. With such a combination, both the oil and the paper ageing can be delayed dramatically. Thus extending the technical lifetime of the transformer.