When using electricity, it is necessary to change the voltage from one level to another. Dry-type transformers (aka air-cooled) perform this function so safely and efficiently that they are widely used for indoor installations in public and residential buildings where other types of these devices are considered too risky.
Types of transformers: liquid and dry
Basically, there are two different types of such a device: liquid insulated and cooled (liquid type) and air or air-gas mixture cooled (dry type).
For transformers of the first type, the cooling medium can be ordinary mineral oil. Other substances are also used, such as flame retardant hydrocarbons and silicone fluids. Such transformers have a core and windings immersed in a tank of liquid medium, which serves as both an insulator and a coolant.
The most common power drytransformers have windings filled with epoxy resin, which serves as an insulator. It protects conductors from dust and atmospheric corrosion. However, since coil casting molds are used only with fixed dimensions, there is less room for change in the design of such devices. In the range commonly used in the power supply of small industrial enterprises, as well as public and residential buildings, dry-type transformers completely duplicate the range of capacities of their liquid counterparts.
Main parameters
The most crucial moment in the operation of the devices in question is to ensure the temperature regime of the windings. To assist in the selection or purchase of a dry-type apparatus for the power supply of various objects, we will consider several basic operational parameters:
- Power, kVA.
- Rated primary and secondary voltage.
- The heat dissipation of an 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 temperature in them.
- Core and coils - possible damage to the core or accumulation of delaminations (copper or aluminum conductors) are of particular concern.
There are various structural 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.
Vacuum impregnation (VPI) insulation
This technology creates a lacquer finish on conductors by alternating pressure and vacuum cycles. The VPI process uses polyester resins. It provides the conductors with a better lacquer finish than conventional dipping. Coils coated with it are then placed in an oven where baking takes place. They are much more resistant to corona discharges. What does such a transformer look like? His photo is posted below.
Vacuum Encapsulation (VPE)
This method usually outperforms the VPI process. Several dips are added during the manufacturing process to encapsulate the coil, after which their coating is baked in an oven. These transformers have better protection against aggressive and damp environments than their VPI counterparts. What does such a transformer look like? His photo is presented below.
Encapsulation (sealing)
Encapsulated transformers are conventional devices with windings coated with silicon compounds or epoxy resin and completely enclosed in a heavy casing. The manufacturing process fills the windings with a dense, high dielectric strength epoxy resin, protecting the transformer from all environments.
Casted coils (in molded densified epoxy)
These devices contain coils encapsulated in epoxy resin during the molding process. They are completely filled with resin under the action ofvacuum.
Each winding insulation method is specially suited for specific environments. It is very important to understand where it is best to use the appropriate device types. For example, dry cast resin transformers cost about 50% more than VPE or VPI products. Thus, the choice of a specific type of device can significantly affect the overall cost of the project.
Recommendations for selection
Where increased resistance to corona discharges (i.e. electrical discharges caused by a field strength exceeding the dielectric strength of the insulation) is required, when increased mechanical strength of the windings is not required, a VPI-type transformer should be used.
Use these with cast coils when extra strength and protection is required, such as in harsh environments such as chemical process plants, building materials plants, and outdoor installations. Aggressive environments include substances that can adversely affect the windings of other dry-type transformers, including s alts, dust, corrosive gases, moisture and metal particles.
Furthermore, cast resin windings have improved ability to withstand transient and repeated overloads common in many manufacturing processes.
An engineer often has to make a choice between cast resin or VPI/VPE type for critical applications and harsh environments. The first type is generally considered the best. Some manufacturershowever, it is pointed out that cast resin insulation limits the life of the transformer. The coefficient of thermal expansion of epoxy resin is less than that of copper conductors. The cyclic expansion and contraction as the coils heat up and cool down can eventually cause the resin to crack. It is also noted that the VPI-type transformer can better cope with such processes and therefore last longer. In the end, the final choice is up to the power engineer.
Liquid vs. Dry
Liquid-filled transformers tend to be more efficient than dry-filled ones, so they have a longer service life. In addition, liquid is a more efficient medium for cooling local high temperature areas in windings. Plus, liquid-filled devices have better overload capacity.
Thus, a 1000 KVA dry-type transformer at half load has a loss level of about 8 kW, and at full load about 16 kW. At the same time, the same “thousander”, but liquid, has about half the waste. An oil "two-thousander" at half load incurs losses of 8 kW, and at full load - 16 kW. Its dry counterpart is characterized by costs of 13 and 26.5 kW, respectively. This means that it is dry-type transformers that hold the dubious lead in terms of losses. At the same time, their price is higher than that of liquid ones.
Due to more intense cooling of the windings, liquid devices have smaller dimensions (depth and width) than dry devices of the same power. It mayinfluence the required area of transformer substations (especially built-in ones), and hence the cost of the entire facility. So, a typical dry-type transformer 1000 KVA has a depth of 1.6 m, and a width of 2.44 m. At the same time, a similar oil transformer at a close depth has a width of about 1.5 m. But this type, however, has a number of disadvantages.
For example, fire protection is more important for liquid transformers when using a flammable coolant. True, dry transformers can also catch fire. A misused liquid-type device may even explode.
Depending on operating conditions, liquid-filled products may require a drip pan to collect any coolant leaks.
Perhaps when selecting transformers, the transition from a clear preference for dry type to liquid type is between 500 kVA and 2.5 MVA, with the first type preferably used to the lower limit of the range, and the second above it.
An important factor in choosing the type is the installation location of the transformer, such as inside an office building or outside, as well as servicing industrial loads.
Dry-type transformers over 5MVA are readily available, but many are liquid-filled. For outdoor installation, this type is also predominant.
A few words about ventilation
When the transformer is equipped with a blower fan, the load can be significantly increased. So, for cast windingsthis function can raise the continuous load capacity up to 50% above the nominal load. For VPE or VPI types, the increase in power in this case can be up to 33%.
For example, the power of a standard 3000 kVA cast-wound transformer, when equipped with a blower fan, increases to 4500 kVA (by 50%). At the same time, a 2500 kVA VPE or VPI type with a fan will raise it to 3.333 kVA (by 33%).
However, you must always take into account that the presence of a blower reduces the overall reliability of the system. If the fan fails when operating with blowing under a load higher than the rated one, then there is a real risk of a severe accident, as a result of which you can lose the entire transformer.
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 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 includes Russian, Italian, Chinese and Korean brands. Basically, constructive analogues of well-known Russian brands are offered: TSZ, TSL, TSGL. How much does such a dry transformer cost? The price of a typical "thousander" varies from 900 thousand to 1 million rubles.
The second group includes German and French manufacturers. They offer grades of the DTTH, GDNN, GDHN series. How much will such an imported transformer cost? The price of the same “thousander” will be from 1.5 to 2 million rubles.
