How to choose a transformer:
Dry vs oily for industry

The transformer is the heart of any industrial electrical system. The correct choice between dry and oil type depends not only on the reliability of the power supply, but also on the fire safety of the facility, maintenance costs and the possibility of integration of modern energy storage systems (BESS). For Ukrainian industrial enterprises, which work in conditions of an unstable network and frequent outages, this issue becomes critical.

How a transformer works: basic principles

A transformer is a static electrical device that converts an alternating current of one voltage into an alternating current of another voltage of the same frequency. The principle of operation is based on electromagnetic induction: an alternating current in the primary winding creates an alternating magnetic field in the core, which induces EMF in the secondary winding.

In industrial systems, transformers perform a key function: they reduce the voltage of the distribution network 6-10 kV to work 0.4 kV (380/220 V), which is used by the company's equipment. The reverse conversion is necessary when releasing excess energy from SES or BESS to the grid.

"The choice between a dry and oil transformer is not a matter of fashion or brand. It is an engineering decision determined by the conditions of the location, fire regulations and plans for the development of the facility for the next 25+ years." -- Chief engineer, BESS Ukraine.

Dry transformer: advantages and limitations

In a dry transformer, the insulation and cooling of the windings is provided by air or solid insulation (epoxy resin, cast compound). The windings are not immersed in liquid, which fundamentally changes the safety and maintenance characteristics.

Oil transformer: an industry classic

In an oil transformer, the windings and magnet wire are immersed in transformer oil, which simultaneously performs two functions: insulation and cooling. The oil removes heat from the windings, transferring it through the walls of the tank or radiators to the environment.

Comparison table: dry vs oily

A systematic comparison of key parameters for industrial transformers with a capacity of 250-2500 kVA:

Total cost of ownership (TCO) for 10 years, 630 kVA

When calculating TCO, it is necessary to take into account not only the initial cost of the transformer, but also the costs of maintenance, construction and installation work, fire protection systems and oil disposal:

When to choose a dry transformer?

A dry transformer is the optimal choice in the following scenarios:

• Accommodation inside the building: Shopping centers, business centers, hospitals, data centers, underground parking lots. Wherever an oil-fired transformer is prohibited by fire regulations or impractical.
• Container KTP of BESS: When integrating the transformer of the energy storage system in one container, the dry type eliminates the risk of fire from oil leakage near the lithium-ion batteries.
• Objects of limited area: Smaller fire protection distances allow the transformer to be placed in tight technical premises.
• Food, pharmaceutical industry: No risk of product contamination with transformer oil.
• Facilities with minimal staff: Does not require qualified maintenance of the oil industry.

When to choose an oil transformer?

The oil transformer remains the best choice for:

• Large industrial enterprises: Capacities over 2,500 kVA, where the price difference is tens of thousands of dollars, and outdoor placement is not a problem.
• Rural area and agricultural sector: External placement, availability of space for oil collection pits, minimum noise requirements.
• Distribution substations 35/10 kV: A standard solution for network infrastructure. Technological maturity, availability of spare parts and service.
• Budget projects: When CAPEX is a critical factor and operating conditions permit outsourcing.
• Objects of sharply variable load: Foundries, welding shops, rolling mills - oil cooling better tolerates short-term overloads.

Integration of transformers of BESS

The energy storage system (BESS) is connected to the grid through a transformer, and the choice of transformer type directly affects the architecture of the entire energy node:

• Dry + BESS in one container: A compact solution for peak shaving and backup power. The transformer, inverter and batteries are housed in a single 20' or 40' container. The minimal footprint is ideal for urban facilities.
• Oil + separate BESS: Traditional scheme for large industrial facilities. Transformer on an open site, BESS in a separate container of its own air conditioning system. Requires additional cable routes.
• Two-transformer scheme: A separate transformer for the BESS (often dry 250-630kVA) and a main transformer for the load (oil). Provides independence and redundancy.

When designing a BESS of a transformer, it is critical to take into account transformation losses. Each transformer adds 0.5-1.5% losses. In a system of two transformers (mains + BESS), total losses can reach 2-3%, which reduces the overall efficiency of the storage system.

Conclusions of BESS Ukraine

For a modern industrial facility in Ukraine, the choice of a transformer should take into account not only the current needs, but also the perspective of the integration of BESS, SES and smart energy management systems. Our recommendations:

• For new objects of BESS: Dry transformer 400-1600 kVA. The higher initial price is offset by savings on construction and installation work, fire protection systems and maintenance.
• For the modernization of existing oil: Assess oil condition (chromatography) and remaining life. If the transformer is less than 15 years old -- leave the oil one, add the BESS separately.
• For projects with a limited budget: Oil transformer with the prospect of replacing it with a dry one during expansion. Design KTP with a reserve for future modernization.

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Frequently Asked Questions about transformers and KTP

Can an oil transformer be installed inside a building?

Technically yes, but with significant limitations. According to PUE (Rules for the arrangement of electrical installations) and DBN, an oil transformer inside the building requires: a separate room with fire-resistant walls (REI 150), an oil collection pit for 100% of the oil volume, an automatic fire extinguishing system, forced ventilation and an emergency oil drain. The cost of arranging such a room often exceeds the difference in price between an oil and dry transformer, which makes the dry version more economically advantageous for indoor placement.

What transformer power is required for a 200 kW BESS?

For a BESS with a capacity of 200 kW, a transformer with a nominal capacity of at least 250 kVA is recommended (taking into account a power factor of 0.8-0.9 and a margin of 10-15%). If the BESS is connected to an existing utility transformer, its residual power must be checked. For example, if a facility consumes 400 kVA of a 630 kVA transformer, then for a BESS 200 kW of residual power (230 kVA) will be sufficient for charging, but caution is required when discharging the BESS and full load at the same time.

How much does a turnkey KTP of a transformer cost in Ukraine?

The cost of a complete turnkey transformer substation (KTP) depends on the capacity, type of transformer and configuration. Approximate prices for 2026: KTP 250 kVA with oil transformer -- $15,000-25,000; KTP 630 kVA of oil -- $25,000-40,000; KTP 630 kVA of dry -- $35,000-55,000; KTP 1000 kV of dry -- $50,000-80,000. The price includes a transformer, KTP housing, switching equipment (RU 10 kV and 0.4 kV), protective automation, installation and commissioning. Contact us for an individual calculation for your object. And for the supply of any industrial equipment, we recommend contacting our partner B2B Engineer.

Is it necessary to replace the transformer when installing a SES or BESS?

Not always. If the existing transformer has sufficient power reserves and works in bilateral mode (can transfer energy both from the network to the object and vice versa), then replacement is not required. However, old transformers (before 2005 years of release) may have increased no-load losses, which reduces the efficiency of the BESS. In this case, it is advisable to consider replacing it with a modern energy-efficient class Ck transformer (with low losses). For SES to be issued to the network, accounting is additionally required in accordance with the license conditions of the NCRECP.