The Difference Between Dry Transformers And Oil Transformers

Dry Transformers

Today, dry transformers are becoming more and more popular. The main arguments of business owners who prefer dry transformers to oil ones are the great economic and safety benefits of such power plants. Is it really so? Let’s try to understand this in this article.

Characteristics of oil and dry transformer systems

The most common reason why utility company owners prefer dry transformers over oil transformers is the desire to save. It is widely believed that dry transformers do not need as frequent inspection and repair as oil transformers, due to the lack of replacement of the insulating liquid. However, this opinion is wrong and inattention to care for this type of equipment can have disastrous consequences.

Dry transformers are also claimed to be safer and more stable than oil transformers and more efficient. In fact, the question of the differences in power plants of different types is more complicated, and it is worth considering a number of characteristics of each of them.

The main difference between dry and oil power transformers is the insulation system. Both types use solid winding insulation, but their principle has differences.

Oil transformer insulation

The insulation system of oil transformers consists of liquid and solid insulation, which is also divided into external and internal.

The external insulation is outside the tank and is created due to air gaps between the winding inlets, between the inlets and the housing, etc. Internal insulation is divided into the main (insulation of the winding to the grounded parts of the magnetic circuit, the tank, and between the windings) and longitudinal (between different points of the winding: turns, coils, layers).

Internal insulation by design is divided into 3 types:

Solid continuous

Purely oily


The isolation function of oil transformers is to protect the windings, the oil part of the bushings, as well as the bends and auxiliary devices (e.g., switches).

In modern transformer technologies, oil barrier insulation is often used in combination with separate sections with solid insulation, that is, combined. Such a system provides the most effective, complete, and multifunctional insulation.

Advantages and disadvantages of oil transformers

The design and insulation dimensions of oil transformers are designed so that the transformer can cope with lightning and other surges.

In order to minimize the voltage on the longitudinal insulation, the bonded winding technique is used, in which the turns are connected to each other in a certain order. In this case, a problem may arise with controlling the longitudinal insulation voltage level, as the turns have different potentials. In this case, the reduction of overvoltage is achieved by increasing the operating voltage and, in connection with the rapid development of technologies based on a high voltage and the increase in the power of the devices, the requirements for stability and resistance also increase. windings and insulation systems.

In addition to constantly monitoring the voltage level, the task is also to achieve long-term energy strength. Intensive heating occurs in transformer windings and magnetic circuits; therefore, insulation must have effective and continuous cooling.

Oil barrier insulation has high dielectric strength and provides intensive and efficient cooling through circulating oil.

Dry transformer insulation

Dry transformers are a modern technology in which the basic principle of insulation is air cooling. This type of installation is used in areas that require greater security for people: metallurgical companies, oil industry, pulp, and paper production, mechanical engineering, the power supply of public buildings, and others.

Dry power plants are available from 20 kW to 2500 and above, and can have both blown and natural cooling.

The higher and lower voltage windings (HV and LV) in dry transformers are enclosed in a special protective housing. Such transformers are installed in dry and closed places where the humidity does not exceed 80% because due to the interaction with air, the insulation becomes additionally moistened, and excessive humidity causes rapid wear of the equipment and irreparable consequences. To reduce hygroscopicity, special impregnating varnishes are used on the windings.

Dry transformers come in 3 different types:

Open winding

Monolithic winding

Cast winding

Open winding transformers:

They are impregnated with resin using vacuum pressure. The thickness of the insulating coating is 0.2 mm. For insulation, high-strength porcelain insulators and insulation profiles (made of fiberglass, metal, and other durable materials) are used.

Monolithic winding transformers:

Created with high vacuum epoxy resin injection technology. Such insulation allows the installation of transformers where environmental and fire safety are especially important.

Cast winding transformers:

The insulation of the HV and LV windings is done with aluminum foil and a dielectric film. The coils are reinforced with fiberglass, then the insulation is dried and coated with epoxy.

Advantages and disadvantages of dry transformers

Each type of dry transformer has advantages and disadvantages.

Compared with oil insulation, air insulation is weaker. The requirements for the protection quality of the windings of a dry transformer are consequently much higher.

The open winding transformer design forms special channels for heat removal, where convection is created, which is sufficient to cool the system. However, due to active interaction with the external environment, there are many risks and difficulties in maintaining the transformer and maintaining its operating condition.

Transformers with the cast and monolithic windings have high electrical and mechanical resistance, as well as environmental safety since they do not emit harmful and dangerous substances. But their disadvantage is that the mass of material for their insulation is enormous and such transformers can have inhomogeneities (both physical and energetic), which leads to partial discharges. Also, cooling high voltage windings can be difficult, sudden changes in temperature and overheating cause plant failures.

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