Oil Treatment

It is generally accepted that when the water concentration exceeds 2.5%, the transformer is classified as "wet" and requires oil treatment. In essence, the lifespan of a transformer means the lifespan of its insulation. The theoretical service life of transformers is over 70 years, but it may be shortened under normal operation, considering both the load and the protection of solid and liquid insulation.

Drying the solid insulation is achieved through heating and dehydrating the oil in a vacuum drying unit and returning it to the transformer, where it becomes saturated with moisture once again. This process continues until the required moisture criteria are met. Although this process is slow, it is economically effective due to low implementation costs and the ability to perform oil treatment while the transformer is still in operation.

Consequences of Transformer Oil Contamination

The mineral oils directly involved in the operation of large transformers can accumulate a variety of contaminants over time. These contaminants can manifest themselves as unwanted chemical compounds and mechanical impurities in oil. Such contamination can lead to unsteady operation, or malfunctioning of a transformer. In order to prevent these adverse effects, transformer oils undergo deep purification, which can be done by different methods.

What is Transformer Oil Treatment?

During preparation, the transformer oil is subjected to several stages of treatment,which include heating, dehydration, filtration, degassing (for transformers with film-type oil protection), and nitrogen treatment (for transformers with nitrogen protection).

Oil treatment for large transformers is typically carried out in two stages:

1. Before filling the transformer with oil.
2. Directly within the transformer tank after filling it with oil.

The most common activity during preparation of transformer oil is its dehydration. The desired quality of oil (with a mass moisture content of not more than 0.001%) is achieved using either an adsorption or thermal vacuum method.

Mechanical impurities adversely affect both oil characteristics and overall transformer performance. This effect is further exacerbated by the presence of water in the oil. During installation, mechanical impurities may originate from poorly cleaned oil storage tanks and oil transfer piping. Contamination inside the transformer can also result from poor industrial hygiene in assembly workshops, insufficient cleaning of the transformer tank’s active parts and other components after assembly, as well as improper handling during transportation, storage, and installation.

According to the standards adopted in many countries, the content of mechanical impurities in treated oil shall not exceed 5 grams per ton of oil, with particle sizes not greater than 5 microns. Mechanical impurities are mainly removed from the oil with the use of filters. Malfunctioning of filters may cause mechanical impurities to be released, which can subsequently contaminate the purified oil.

Types of Transformer Oil Treatment: Filtration, Dehydration, and Degassing

As is well known, transformer oil in service contains dissolved water that develops during the aging of both the oil and the insulation, as well as moisture that enters the oil from the surrounding environment. Dissolved water is the most dangerous impurity, because even small amounts can significantly reduce the dielectric strength of transformer oil. Oil stability is also affected by the presence of contaminants and sludge, including those resulting from the aging of solid insulation, such as the windings varnish constituents and degradation products. In addition, the chemical composition of transformer oil, including the acids caused by oxidation, asphaltenes, and other compounds, has a direct impact on its properties.

Purification of waste oils is carried out with the use of various technological methods, based on physical, physicochemical, and chemical processes. Typically, combined methods of purification are applied to ensure the high quality of reclaimed oils. Waste oil purification involves the removal of mechanical impurities and water from the oil through physical methods.

FLD D Transformer Oil Treatment Units

The FLD D vacuum oil treatment unit is a machine that ensures heating, dehydration, degassing, and filtration of transformer oil. When the oil is heated and enters a vessel with a very high vacuum level, the water contained in the oil gradually evaporates and is removed by a vacuum pump. As the oil passes through the filtration system, impurities are captured by a filtering element equipped with an ultrafine mesh. As a result, the treated oil exhibits improved parameters, including reduced moisture and gas content, smaller particle sizes, and enhanced overall quality.

Product range of FLD D transformer oil treatment units:

• The units manufactured by Fluidex can process oil at a rate of 5 to 53 gpm.
• Filtration is performed with the use of filters that capture particles larger than five microns. 
• The weight of this kind of machines is up to 5,180 lbs; the machine is mounted on a skid, which allows it to be transported to hardly accessible locations.