When using electricity, it is necessary to change the voltage of one level to another. Dry transformers (otherwise - air-cooled) perform this function so safely and efficiently that they are widely used for indoor installation in public and residential buildings, where other types of these devices are considered too risky.
Transformer types: liquid and dry
Basically, there are two different types of such a device: with liquid insulation and cooling (liquid type) and with cooling with air or air-gas mixture (dry type).
For transformers of the first type, the cooling medium may be ordinary mineral oil. Other substances are also used, such as flammable hydrocarbons and organosilicon liquids. Such transformers have a core and windings immersed in a tank with a liquid medium, which serves as an insulator and cooler.
The most common dry power transformers have windings drenched with epoxy that serves as an insulator. It protects conductors from dust and atmospheric corrosion. However, since the molds for casting coils are used only with fixed sizes, the possibilities for changing the design of such devices are less. In the range commonly 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 counterparts.
main parameters
The most crucial moment in the operation of the devices in question is the temperature regime of the windings. To help in the selection or purchase of a dry type apparatus for powering various objects, we consider several key operational parameters:
- Power, kVA.
- The rated voltage of the primary and secondary windings.
- 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 temperature rise in the windings and the highest rate in them.
- Core and coils - possible damage to the core or the accumulation of bundles (copper or aluminum conductors) are of particular importance.
There are various structural types of transformers, determined primarily by the applied methods of insulation of their windings. Among them are known: vacuum impregnation, encapsulation and cast coil. Let's consider each of them separately.
Vacuum Impregnation Insulation (VPI)
This technology creates a varnish coating of conductors by alternating pressure and vacuum cycles. The VPI process uses polyester resins. It provides conductors with a better lacquer coating than with conventional immersion. The coils coated with it are then placed in the oven where the baking takes place. They are much more resistant to corona discharges. What does a transformer look like? A photo of him is posted below.
Vacuum Encapsulation Insulation (VPE)
This method is usually superior to 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 aggressive and humid environments than their VPI counterparts. What does a transformer look like? A photo of him is presented below.
Encapsulation (sealing)
Encapsulated transformers are conventional devices with windings coated with silicon-containing compounds or epoxy and completely enclosed in a heavy casing. The production process fills the windings with a dense epoxy resin with high dielectric strength, protecting the transformer from any exposure environments.
Cast Coils (molded packed epoxy)
These devices comprise coils encapsulated in epoxy during formation. They are completely filled with resin under vacuum.
Each of the winding insulation methods is specifically suited to specific environments. It is very important to understand where it is better to use the appropriate types of devices. For example, cast cast dry 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.
Selection recommendations
If you need increased resistance to corona discharges (i.e., electric discharges caused by field strengths exceeding the dielectric strength of the insulation), when increased mechanical strength of the windings is not required, use a VPI-type transformer.
Use them with cast coils when extra strength and protection are required, for example, in aggressive environments such as chemical process plants, building materials plants, and also for outdoor installations. 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 windings have improved ability to withstand the short-term and repeated overloads that are characteristic of many manufacturing processes.
An engineer often has to make a choice between a device with cast insulation or its VPI / VPE type for use in critical conditions and aggressive environments. The first type is generally considered the best. Some manufacturers, however, indicate that cast resin insulation limits the life of the transformer. The coefficient of thermal expansion of epoxy is lower than that of copper conductors. Cyclic expansion and contraction with heating and cooling coils can ultimately cause cracking of the resin. It is also noted that a VPI-type transformer can better cope with such processes and therefore lasts longer. In the end, the final choice rests with the power engineer.
Liquid versus dry
Liquid-filled transformers, as a rule, have higher efficiency than dry ones, therefore they have a longer service life. In addition, liquid is a more efficient medium for cooling local areas of high temperature in windings. Plus, liquid-filled devices have better overload capacity.
So, a dry transformer 1000 KVA at half load has a loss level of about 8 kW, and at full - about 16 kW. At the same time, the same "thousandth", but liquid, has approximately half the waste. The oil "two-thousandth" at half load carries a loss of 8 kW, and at full - 16 kW. Its dry counterpart is characterized by costs of 13 and 26.5 kW, respectively. This means that it is dry transformers that hold the dubious primacy in terms of loss. Their price is higher than that of liquid ones.
Due to the more intensive cooling of the windings, liquid devices have smaller dimensions (depth and width) than dry ones of a similar power. This can affect the required area of transformer substations (especially built-in ones), and therefore the cost of the entire facility. So, a typical 1000 KVA dry transformer has a depth of 1.6 m and a width of 2.44 m. Moreover, a similar oil transformer with a similar depth has a width of about 1.5 m. But this type, however, has a number of drawbacks.
For example, fire protection is more important for liquid transformers when using a coolant that can ignite. 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, a drip tray may be required to collect the cooler in case of possible leaks.
Possibly, when choosing transformers, the section of the transition from the unequivocal preference of the dry type to the liquid type is between 500 kVA and 2.5 MVA, the first type being preferably used to the lower end of the range, and the second above it.
An important factor in choosing the type is the installation location of the transformer, for example, inside an office building or outside, as well as serving industrial loads.
Dry transformers with a capacity of more than 5 MVA are quite affordable, but many of them are liquid-filled. For outdoor installation, this type is also predominant.
A few words about ventilation
When equipping a transformer with a blower fan, the load can be significantly increased. So, for cast windings, such a function can raise the long-term permissible load by 50% above the nominal. For VPE or VPI types, the power gain in this case can be up to 33%.
For example, the power of a standard transformer of 3000 kVA with molded winding when equipped with a blower fan increases to 4500 kVA (by 50%). At the same time, the VPE or VPI type with a capacity of 2500 kVA in the presence of a fan will raise it to 3.333 kVA (by 33%).
However, you should always consider that having a blower fan reduces the overall reliability of the system. If the fan fails when working with blowing under a load above the rated one, then there is a real risk of a severe accident, due to which the entire transformer can be lost.
And what about the Russian market?
It is worth noting that in recent years in Russia there has been a steady tendency to repeat the experience of Europe, where up to 90% of all newly installed transformers are of the dry type. The market reacts accordingly. Today in the Russian Federation there are offers of such devices from two groups of manufacturers. The first of them can be attributed to Russian, Italian, Chinese and Korean brands. Basically, structural analogues of well-known Russian brands are offered: TSZ, TSL, TSGL. How much does such a dry transformer cost? The price of a typical "thousandth" varies from 900 thousand to 1 million rubles.
The second group includes German and French manufacturers. They offer brands of the DTTH, GDNN, GDHN series. What would such an imported transformer cost? The price of the same "thousandth" will be from 1.5 to 2 million rubles.