The service life of transformers is normally estimated by manufacturers at 30–50 years and basically depends on the insulation condition, which is affected by degradation processes. One of the most serious threats to cellulose insulation is a high water content. It may result in cellulose depolymerization, which deteriorates the mechanical strength of the insulation and makes it more susceptible to degradation due to short circuits or other stresses.
Moreover, a high level of water in the paper insulation induces a “bubble effect,” which sharply decreases the dielectric strength and increases the tank pressure, as well as leads to operation shutdown or failure of a transformer. Therefore, drying of power transformers has a significant effect on the service life of both new and already operating equipment.
Challenges of Transformer Insulation Drying
Each of the existing insulation drying methods has its advantages and disadvantages, but the problem of complete warmup and moisture removal from the deepest layers of the windings insulation is common to most of them.
GlobeCore Hot Oil Spraying Technology
In order to solve this problem, GlobeCore professionals have developed a hot oil spraying technology, which is implemented by means of the CMM-RM hot oil sprayer with a throughput of 30 m³/h.
Transformer drying with the use of the CMM-RM hot oil sprayer begins with almost complete draining of oil from the transformer tank. GlobeCore recommends that not more than 10% of the initial volume be allowed to remain in the tank. Next, the hot oil sprayer is installed above the transformer’s active part in combination with a manifold. The manifold consists of attachment flanges and pipes joined by welding and is used to attach the sprayers to the transformer tank.
After that, the oil from the transformer tank bottom is fed through a filter into the heater, where it is heated and sprayed onto the transformer windings under a pressure of 2.5 bar, thereby ensuring that the windings are flushed and evenly warmed up. As a result, moisture begins to exude even from the deepest layers of insulation.
Drying Cycle and Vacuuming
Upon completion of the windings warmup stage, oil feed is stopped, and transformer tank vacuuming begins and lasts a few hours. Transformer oil heating and circulation, as well as transformer tank vacuuming, are performed by a GlobeCore CMM plant equipped with a heater, transfer pumps, and a two-stage vacuum system capable of developing a high vacuum. This cycle — heating with hot oil and vacuuming — is repeated several times until the moisture content of the solid insulation is not more than 0.5% by mass for new transformers and not more than 1.5% by mass for transformers in operation.
The tanks of some older transformers may not withstand a full vacuum. In this case, it is recommended to either reinforce the tank before vacuuming, or to extract the windings from the transformer and dry them in a US vacuum oven, and then insert them back into the transformer.
Benefits of Hot Oil Spraying and Vacuuming
Timely drying by hot oil spraying and vacuuming increases the dielectric strength and extends the service life of transformer’s solid insulation by at least 10 years, and more often than not — by a longer period.