What are dry transformers? Technical characteristics and scope

When using electricity, you need to change the voltage of one level to another. Dry transformers (otherwise - air-cooled) perform this function so safely and effectively that they are widely used for indoor installation in public and residential buildings, where other types of these devices are considered too risky.

Types of transformers: liquid and dry

In principle, there are two different types of such a device: with liquid insulation and cooling (liquid type) and with air cooling or air-gas mixture (dry type).

For transformers of the first type, the cooling medium can be ordinary mineral oil. Other substances, such as refractory hydrocarbons and organosilicon fluids, are also used. Such transformers have a core and windings immersed in a tank with a liquid medium, which serves both as an insulator and as a cooler.

The most common power dry transformers have windings filled with epoxy resin, which serves as an insulator. It protects conductors from dust and atmospheric corrosion. However, since molds for casting coils are only used with fixed dimensions, there are fewer possibilities for changing the design of such devices. In the range usually used in the power supply of small industrial enterprises, as well as public and residential buildings, dry transformers completely duplicate a number of capacities of their liquid analogs.

Main settings

The most important moment in the operation of the devices under consideration is to ensure the temperature regime of the windings. To help in the selection or purchase of a dry type device for power supply to various facilities, let us consider a few basic operational parameters:

1. Power, kVA.
2. Rated voltage of the primary and secondary windings.
3. The heat transfer of the insulation system is the sum of the maximum ambient temperature + the average temperature rise in the windings + the difference between the average rise in temperature in the windings and the highest in them.
4. Core and coils - possible damage to the core or accumulation of stratifications (copper or aluminum conductors) is of particular importance.

There are various design types of transformers, determined primarily by the methods used to isolate their windings. Among them are known: vacuum impregnation, encapsulation and cast coil. Let's consider each of them separately.

Insulation by vacuum impregnation (VPI)

This technology creates a varnish coating of conductors by alternating the cycles of pressure and vacuum. The VPI process uses polyester resins. It provides conductors with better varnish coverage than with conventional immersion. Coils coated with it, then placed in the oven, where the baking takes place. They are much more resistant to the appearance of corona discharges. What does such a transformer look like? The photo is located below.

Vacuum encapsulation isolation (VPE)

This method usually exceeds the VPI process. Several dives are added during the manufacturing process to encapsulate the coil, after which their coating is baked in the oven. These transformers have better protection against the effects of aggressive and humid environments than their VPI counterparts. What does such a transformer look like? The photo is presented below.

Encapsulation (sealing)

Encapsulated transformers are conventional devices with windings coated with silicon-containing compounds or epoxy resin and completely enclosed in a heavy-duty casing. The production process fills the windings with a dense epoxy resin with high dielectric strength, protecting the transformer from any influencing media.

Cast coils (in molded packed epoxy resin)

These devices contain coils encapsulated in epoxy resin during the formation process. They are completely filled with resin under the action of vacuum.

Each of the winding insulation methods is especially suitable for specific media. It is very important to understand where it is best to use the appropriate types of devices. For example, dry cast-insulated transformers cost about 50% more than VPE or VPI products. Thus, the choice of a particular type of device can significantly affect the overall cost of the project.

Recommendations for selection

If it is necessary to increase the resistance to corona discharges (i.e., electrical discharges caused by a field strength exceeding the dielectric strength of insulation), when an increased mechanical strength of the windings is not required, a VPI type of transformer should be used.

Use them with cast coils, when additional strength and protection is required, for example, in aggressive environments, such as chemical processing plants, building materials plants, and also for outdoor installation. Aggressive environments include substances that can adversely affect the windings of other dry transformers, including salts, dust, corrosive gases, moisture and metal particles.

In addition, cast-insulated windings have improved ability to withstand short-term and repeated overloads, typical for many production processes.

The engineer often has to choose between a cast-insulated device or its VPI / VPE-type for use in critical environments and aggressive environments. The first type, as a rule, is considered the best. Some manufacturers, however, indicate that molded resin insulation limits the life of the transformer. The coefficient of thermal expansion of epoxy resin is lower than that of copper conductors. Cyclic expansion and contraction with heating and cooling of the coils can eventually cause cracking of the resin. It is also noted that a VPI-type transformer can better cope with such processes and therefore serves longer. In the end, the final choice is the energy engineer.

Liquid type against dry

Liquid-filled transformers, as a rule, have a higher efficiency than dry ones, therefore they have a longer service life. In addition, the liquid is a more efficient medium for cooling local heat regions in the windings. Plus, liquid-filled devices have better overload capacity.

So, a dry 1000 kVA transformer at half load has a loss level of about 8 kW, and at a full power of about 16 kW. At the same time, the same "thousand", but liquid, has about half the waste. Oil "dvuhtysyachnik" at half load carries losses of 8 kW, and at full - 16 kW. Its dry analog is characterized by a cost of 13 and 26.5 kW, respectively. This means that dry transformers hold a dubious primacy in terms of losses. Their price is higher than that of liquid ones.

Due to more intensive cooling of windings, liquid devices have smaller dimensions (depth and width) than dry ones of similar power. This can affect the necessary area of transformer substations (especially embedded ones), and therefore, the cost of the entire facility. Thus, a typical dry 1000 kVA transformer has a depth of 1.6 m and a width of 2.44 m. At the same time, a similar oil at a near depth has a width of about 1.5 m. But this type has, however, a number of drawbacks.

For example, fire protection is more important for liquid transformers when using a cooling medium that is capable of ignition. True, dry transformers can also catch fire. An improperly operated liquid-type device may even explode.

Depending on the operating conditions for liquid-filled products, it may be necessary to install a sump to collect the coolant for possible leaks.

Probably, when choosing transformers, the transition section from the unique preference of dry type to liquid one is between 500 kVA and 2.5 MVA, the first type being preferably used up to the lower limit of the range, and the second one is higher than it.

An important factor in the choice of type is the place of installation of the transformer, for example, inside an office building or outside, and also servicing industrial loads.

Dry transformers with a capacity of more than 5 MVA are quite affordable, but many of them are liquid-filled. For outdoor installations, this type is also predominant.

A few words about ventilation

When the transformer is equipped with a fan blower, the load can be substantially raised. Thus, for cast windings, this function can raise the long-term permissible load by 50% above the rated load. For VPE or VPI types, the power increase in this case can be up to 33%.

For example, the power of a standard transformer of 3000 kVA with a cast winding when equipped with a blower blower increases to 4500 kVA (by 50%). At the same time, a VPE- or VPI-type power of 2500 kVA with a fan will raise it to 3.333 kVA (by 33%).

However, one must always take into account that the presence of a blower fan reduces the overall reliability of the system. If the ventilator fails when working with a blower under the load above the rated one, then there is a real risk of a severe accident, due to which it is possible to lose the entire transformer.

And what about the Russian market?

It should be noted that in recent years a stable tendency has been formed in Russia to repeat the experience of Europe, where up to 90% of all newly installed transformers are of the dry type. Accordingly, the market reacts. Today in Russia there are offers of such devices from two groups of manufacturers. The first of these can be Russian, Italian, Chinese and Korean brands. Basically, constructive analogs of well-known Russian brands are proposed: TSZ, TSL, TSGL. How much is such a dry transformer? The price of a typical "thousandaire" varies from 900 thousand to 1 million rubles.

The second group includes German and French manufacturers. They offer brands of DTTH, GDNN, GDHN. What is the cost of such an imported transformer? The price of the same "thousand" will be from 1.5 to 2 million rubles.