Dry power transformers


       Power transformers are one of the most important elements of electrical networks and electrical installations. Earlier, up to the last third of the last century, power oil transformers were exclusively used in power grids. However, over the past 40 years, instead of them, dry power transformers have become more and more widely used, which is one of the modern fire and environmentally friendly types of transformers in which the magnetic system and windings are not immersed in transformer oil, silicon-based fluid (LEO) or any another liquid dielectric.

   In this way, dry transformers significantly differ from traditional designs of fire and environmentally hazardous oil transformers, which, in order to eliminate the risk of ignition of oil and paper-oil insulation, must be placed in specially equipped rooms with fireproof outer walls, ceilings and floors, as well as oil receivers in the form of pits. to drain the oil, or place them in separate chambers with an outlet to the outside. In addition, oil-immersed transformers, in contrast to dry-type transformers, require constant maintenance, which leads to additional operating costs.

Dry power transformers in comparison with oil transformers have a number of advantages, the main of which are given in Table 1 [1-6]. Although low-power single-phase dry transformers were used in radio engineering, automation, signaling, communication, etc. back in the first half of the last century, the technology for the production of power single- and, in particular, three-phase dry transformers, designed to convert electricity in power grids and electrical installations, was developed much later - in the last third of the last century. This is clearly shown by the curves of voltage levels and power of distribution dry-type transformers with cast resin, achieved by Siemens for the period from 1966 to 1 997, shown in Fig. 1, where it is indicated:
  1 - rated power, MVA;

2 - operating voltage, kV;                                                                                                              

3 - rated test voltage of a lightning impulse, kV.

As these curves show, over a thirty-year period of time, the power of dry transformers was increased 40 times (from 0.5 to 20 MVA), the operating voltage - 2 times (from 18 to 36 kV), the test voltage - 2.5 times (from 80 to 200 kV), which made it possible to establish the serial production of various designs of dry transformers, which, according to the international standard IEC-726, have the classification given in Table 2 [1].

   Note that the use of transformers in electrical installations makes it possible to generate electricity at the same voltage level, and to use a higher voltage to minimize losses for its transmission. At the same time, the transmission of electricity over long distances from the place of its production to the place of consumption requires the use of at least 5-, 6-fold transformation in modern power grids, carried out by using step-up and step-down transformers. Therefore, due to the need to distribute energy in different radial directions between many consumers, it is required to install a significantly larger number of individual transformers compared to the number of generators. In this case, the total power of the transformers in the network at each next stage of transformation with a lower voltage in order to more freely maneuver the electric power is usually chosen to be greater than the power of the previous stage of a higher voltage. Therefore, the total capacity of all transformers installed in power grids currently exceeds the total capacity of generators not by 5-6, but by 7-8 times. In this regard, the most important tasks are: improving the quality of power transformers, using advanced technology for their production, saving materials in their manufacture and achieving the lowest possible energy losses during their operation in the network.

Although the efficiency of the overwhelming majority of modern transformers is 98 ... 99% and more, however, due to the need for multiple energy transformation and installation in this regard in transformer networks with a total power 7-8 times higher than the generator power, the total energy losses in the entire fleet of transformers are big enough. So, in the mid-50s of the last century, they amounted to about 6% of all energy generated by power plants, and in subsequent years, when XX losses were reduced to 50%, and short-circuit losses - by 20 ... 25%, total losses in the transformer fleet decreased slightly. An even greater reduction in these losses can be achieved due to the widespread use of dry power transformers with low XX and short-circuit losses. The field of application of dry power transformers, due to their numerous advantages indicated in Table 1, is quite extensive, despite the fact that they better consumer properties in comparison with oil transformers, such as increased reliability, safety, ease of use, etc., cost 2.5-3 times more than oil transformers. These transformers are widely used in power distribution systems in residential, public, administrative and residential buildings, as well as in a number of other facilities, which are subject to increased requirements in terms of fire safety and explosion protection, environmental friendliness and low noise level. Such facilities with an increased level of safety for people, equipment and the environment include hospitals, hotels, banks, office centers, high-rise buildings, the subway, ground electric transport, etc. In addition, dry power transformers made by special orders are also used in special conditions operation, including for marine, arctic or tropical climates, for areas with increased seismic activity, etc.

      Design features of dry transformers manufactured using vacuum technology

 Currently, the overwhelming majority of foreign and domestic firms produce dry transformers using one of the following technologies: vacuum or vacuumless (roving) [2-6].

Let us first characterize the essence of the vacuum technology for the production of dry power transformers.

In the production of dry transformers using vacuum technology, the finished transformer windings are filled in vacuum with an epoxy compound with quartz filler, the preparation process of which also takes place in a vacuum. Transformers with windings made in this way are called CAST RESIN TRANSFORMERS, or CAST RESIN for short.


   The advantage of this technology is that it allows you to exclude various impurities from the composition of the insulation, as well as gas microcavities, which sharply deteriorate the dielectric strength of the insulation in relation to partial discharges, the action of which causes rapid aging of the insulation and reduces its service life. As a result of vacuum treatment, the transformer winding receives a strong, closed on all sides, epoxy shell 5 ... 20 mm thick, which gives the winding the necessary rigidity and protects it from moisture and the effects of aggressive compounds.

A general view of a dry power transformer of the CAST RESIN type, manufactured using vacuum technology, is shown in the photo at the beginning of the article, and the view of the most important structural elements of this transformer is shown in Fig. 2, where it is indicated:

1 - a three-rod magnetic conductor, consisting of three columns, made of magnetic steel with an optimal grain structure;

2, 3 - LV and HV windings, respectively, made of aluminum tape

4 - LV inputs, which can have either a normal arrangement - on the top on the opposite side in relation to the HV inputs, or a special arrangement - at the bottom;

5 - HV bushings with jumpers for matching the HV winding with the mains voltage, which are located on the side of the LV bushings; switching of jumpers is carried out when the transformer is not excited;

6 - elastic support pads designed to reduce the noise level of the transformer;

7 - support frame with adjustable rollers for moving the transformer in the longitudinal and transverse directions;
    8 - insulation, which is a mixture of epoxy resin and quartz filler, which does not require additional maintenance.

The main technical characteristics of transformers manufactured using the ABB vacuum technology are shown in Table 3 [2,3].

Let us note the most significant features of the windings and magnetic core of dry power transformers, manufactured using the vacuum technology of casting windings [2-4].

A distinctive design feature of the heat-treated HV winding of dry power transformers with cast resin is that it is manufactured using automatic winding and consists of a set of coils made of aluminum foil. The insulation between the turns is carried out with a polyester film. Each coil is fiberglass-reinforced, deep-dried and then vacuumed with Class F epoxy mixed with quartz and aluminum trihydroxide. This technology for manufacturing the HV winding provides a low voltage level between adjacent conductors. The insignificant potential difference between adjacent turns of the winding made it possible to abandon the use of interlayer insulation in cast resin transformers and thereby reduce the size of the coils and ensure high quality cast insulation covering all conductors.

The LV winding of dry power transformers manufactured using vacuum technology is also made of aluminum foil insulated with a class F dielectric film. the integrity of the transformer in case of temperature deformations and emergency short-circuit currents, many times higher than the rated operating current of the transformer, but also an order of magnitude reduce eddy current losses in the winding compared to losses in conventional windings.

The magnetic circuit, which is one of the most important elements of the transformer, is made of a magnetic plate with an oriented grain structure, which is protected from specific energy losses and has a high magnetic permeability.



        In addition, the components of the magnetic circuit during its manufacture are located at an angle of 45 °, with overlapping joints according to the so-called "Step Lap" technology, which leads to a decrease in losses and current XX, as well as the noise level of the transformer.

     Dry power transformers manufactured using non-vacuum technology

  In addition to the manufacture of dry power transformers using the vacuum technology of casting windings, dry transformers are also created using a different technology. So, in the late 70s of the last century, ASEA - LEPPER (the present name of ABB) developed a vacuum-free technology for the production of dry power transformers [5, 6]. According to this technology, the HV winding of a dry power transformer is manufactured by alternately winding a layer of winding and interlayer insulation consisting of roving saturated with epoxy compound without filler, and the winding is done "wet" at atmospheric pressure. Transformers with windings made according to this technology were named "REZIBLOK", reflecting the fact that such windings have the form of a monolithic block, reinforced with fiberglass impregnated with an epoxy compound, and therefore, after subsequent joint heat treatment, they are able to withstand significant short-circuit forces. The mechanical strength of the winding of such a transformer eliminates the risk of cracks in the windings and guarantees a long service life of this type of transformer.

The main advantages of RES / BLOC transformers include the following:

• low losses XX and short-circuit;

• low level of partial discharges;                                                                                                                                                                              

• high dynamic strength of the windings,

• linear distribution of atmospheric overvoltage;

• an efficient free cooling system (thanks to the built-in cooling channels), which allows them to be operated in normal operation when the windings are heated up to 140 ° C;

• low noise;

• exceptionally high explosion and fire safety.
  A general view of a RESIBLOC dry power transformer manufactured by ABB, manufactured using vacuum-free technology, is shown in Fig. 3, and its main technical characteristics are given in Table 4 [5].

RESIBLOC transformers have undergone long-term tests at an ambient temperature of -60 ° C, which have proven that this type of transformer exceeds the requirements of existing standards in terms of its characteristics.

A comparative assessment of the main advantages of dry (epoxy) type CAST RESIN transformers manufactured using vacuum technology and RESIBLOC type transformers manufactured using vacuumless technology shows that both types of transformers are practically equivalent in terms of fire safety; moisture and chemical resistance; environmental safety [4]. At the same time, RESIBLOC transformers are capable of surpassing CAST RESIN transformers in terms of mechanical strength, dynamic resistance to short-circuit forces, and resistance to high and low temperatures.

                   K.Yu. Gurd, Kiev

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