The choice between a dry-type and an oil-immersed Custom Transformer is a project decision, not a product preference. In a direct head-to-head, dry-type cast-resin transformers are normally selected for indoor, fire-sensitive or close-to-load locations, while oil-immersed transformers are normally selected for conventional distribution and power substations, larger capacity ranges and outdoor installations. The wrong starting point is to ask which type is 'better'. The right starting point is to fix the installation location, the applicable fire strategy, the electrical duty and the substation format, then let those constraints select the insulation and cooling architecture.

Custom Transformer here means an engineered-to-order unit: the rating, vector group, impedance, tap changing, cooling, enclosure and interface are fixed for the project rather than taken from a catalogue. Apex Power Systems (Nanjing) Co., Ltd. is an international trading and supply-partner company for power transformers and substation equipment; it is not a manufacturing factory, was established in 2024, and supplies through audited manufacturing partners. That distinction matters in a head-to-head evaluation because the buyer is not only choosing dry-type or oil-immersed - the buyer is also choosing how the supplier will audit, test, document and deliver the unit.

Problem Definition: The Type Decision Is a Freeze Point

The most expensive mistake in a custom transformer project is to freeze the transformer type before the installation context is fixed. A dry-type unit may be technically compliant but unsuitable if the project has no indoor electrical room and needs a large outdoor power transformer. An oil-immersed unit may be technically compliant but creates oil-handling and maintenance obligations where a dry-type unit could be placed close to the load.

The decision also changes the enclosure and interface. A Dry Type Transformer for Data Center may sit inside the building, while an Oil Immersed Transformer may be integrated into a Box Type Substation, Prefabricated Compact Substation, European Box Substation, American Box Substation or Containerized Mobile Substation. Each choice changes civil works, footprint, testing, documentation and commissioning. For that reason, the comparison should be run before the enquiry is issued, not after the drawings are approved.

The same logic applies when the package includes Complete Switchgear, Medium Voltage Switchgear, Gas Insulated Switchgear (GIS), Air Insulated Switchgear (AIS) or a Ring Main Unit (RMU): the transformer type must be coordinated with the switchgear, protection and automation interface. The transformer is not an isolated item in a substation package.

Industry Background: Why Both Architectures Remain in Demand

Demand is not uniform, which is why both architectures remain relevant. According to Fortune Business Insights, the global transformer market is estimated to reach USD 80.8 billion in 2026. Grand View Research projects the global distribution transformer market to grow from USD 26.0 billion in 2026 to USD 47.7 billion by 2033 at a CAGR of 9.1%. The U.S. data center substation market was valued at USD 3.44 billion in 2024, according to Grand View Research, and the box-type substation for photovoltaics market is estimated at USD 4.54 billion in 2026. These are different duty profiles: data centers and commercial buildings concentrate dry-type demand indoors, while distribution grids, PV plants, wind farms and energy-storage projects use both oil-immersed and dry-type units depending on site layout.

Efficiency rules also shape the decision. China's GB 20052-2024 'Minimum allowable values of energy efficiency and energy efficiency grades for power transformers' took effect on 1 February 2025. In the United States, the DOE 2024 rule for distribution transformers requires a transition towards amorphous electrical steel starting in 2029. Neither rule selects dry-type or oil-immersed by itself, but both make loss evaluation part of the comparison. A Low Loss Energy Efficient Transformer specification should be checked against the destination market's rules before the design is frozen.

Detailed Solution: A Direct Decision Rule

Use the following rule as the first filter. If the transformer will be installed indoors, close to occupied areas, or in a location where an oil-free design is preferred, start with a dry-type unit. If the transformer will serve a conventional distribution or power substation, needs a larger distribution capacity, or will be installed outdoors in an oil-immersed substation package, start with an oil-immersed unit. Then verify the electrical duty and the destination-market compliance before either choice is confirmed.

Where both types are technically feasible, the decision should be made on five project criteria: installation location and fire strategy; required capacity and voltage class; substation or enclosure format; maintenance capability and oil handling; and destination-market certification. A generic preference for 'dry' or 'oil' is not a selection method.

Step-by-Step Breakdown: How to Run the Head-to-Head

Step 1: Fix the installation location and fire strategy

Record whether the unit will be installed indoors or outdoors, in an electrical room, a commercial building, a tunnel, a data center, an industrial plant or a substation yard. For a Dry Type Transformer for Data Center or a unit in a hospital, school, subway or high-rise building, the oil-free construction of the SCB12-SCB18 Series is the natural starting point. For an outdoor distribution substation, a Box Type Substation or a Prefabricated Compact Substation, the oil-immersed S13/S14/S15 series is the natural starting point.

Step 2: Define the electrical duty

Fix the capacity, HV and LV voltage, frequency, vector group, impedance voltage, tap-changing requirement and cooling stage. For a Medium Voltage Transformer or High Voltage Transformer, the vector group and impedance must be stated on the enquiry because they affect parallel operation and protection. The dry-type SCB12-SCB18 range covers 10 kV up to 2,500 kVA and 35 kV up to 25,000 kVA. The oil-immersed S13/S14/S15 range covers 10 kV up to 3,150 kVA and 35 kV up to 31,500 kVA; the SFZ-250000/345 covers 250 MVA at 345 kV.

Step 3: Select insulation and cooling

For dry-type, choose F or H insulation class and AN or AF cooling. For oil-immersed, choose ONAN cooling and the oil grade: No. 25 mineral oil, No. 45 low-temperature oil or FR3 natural ester oil. The cooling class and temperature-rise limits define the permissible loading, so they must be checked against the ambient temperature and altitude of the site. Do not compare a dry-type AN rating with an oil-immersed ONAN rating without confirming the cooling stage and temperature-rise basis.

Step 4: Select the enclosure or substation format

A dry-type unit may use IP00, IP20 or IP23 protection and sit in an indoor electrical room. An oil-immersed unit may be supplied as a standalone transformer, or integrated into a Box Type Substation, Prefabricated Compact Substation, European Box Substation, American Box Substation or Containerized Mobile Substation. The prefabricated cabin substation YBM/ZGS11/ZGS13 is a 10/35 kV package with transformer capacity of 500-50,000 kVA; the transformer can be oil-immersed or dry-type. It integrates HV switchgear, transformer, LV switchgear, compensation equipment, automation system, AC/DC power supply and environmental control, with IP54/IP55 protection and an ambient temperature range of -40°C to +50°C.

Step 5: Match compliance to the destination market

For IEC projects, the dry-type SCB12-SCB18 series is IEC 60076-11 compliant, and the oil-immersed range is supported by IEC 60076 testing. For North America, liquid-filled distribution transformers rated above 750 V are certified to UL requirements for the United States and CSA standards for Canada, supported by the IEEE C57.12 series. The manufacturing base holds UL Certificates of Compliance covering XPLH7 transformers, distribution, liquid-filled type, over 750 volts, certified for Canada, evaluated to CSA C2.1-06 (1st Ed.) and CSA C227.4 (3rd Ed.). For utility projects, the purchasing utility's own specification and the IEEE C57.12 test requirements also apply.

Step 6: Plan factory tests and documentation

Write the test programme into the purchase order. For oil-immersed and dry-type units, the factory acceptance programme should include ratio, winding resistance, no-load and load loss, insulation, temperature rise and lightning impulse as applicable. Apex controls quality risk by requiring witnessed routine and type tests at factory acceptance and by arranging third-party inspection where required. The same discipline applies to the 250 MVA/345 kV power transformer, which is supported by a KEMA Labs Type I Inspection Report.

Step 7: Plan installation and commissioning

Dry-type units are maintenance-free and moisture-resistant, and can be energised after shutdown without pre-drying. Oil-immersed units require oil processing and oil testing: the installation sequence includes vacuum treatment, oil filling under vacuum, dielectric strength and moisture checks, ratio and winding resistance tests, protection checks, tap-changer operation and trial operation under load. The installation method for a prefabricated cabin substation is transport as a complete unit and crane lifting with on-site connection, which reduces site work in regions with limited infrastructure.

Use Cases: Where Each Type Fits

Data centers and AI computing parks

For indoor MV/LV distribution inside a data center or AI computing park, the dry-type SCB12-SCB18 transformer is the recommended starting point because it contains no insulating oil, is suited to fire-sensitive locations and handles non-linear, harmonic-rich IT load. For the main incoming supply, an oil-immersed power transformer or a prefabricated substation is the recommended starting point because higher capacities are available. Where the site must be energised quickly, a containerized prefabricated substation removes most site civil works. This is the practical meaning of a Dry Type Transformer for Data Center specification: dry-type inside the building, oil-immersed or prefabricated at the incoming supply.

Commercial buildings, hospitals and transport hubs

High-rise buildings, commercial centers, hospitals, schools, subways, stations, airports and tunnels are dense, fire-sensitive environments. The dry-type SCB12-SCB18 series is designed for indoor electrical rooms and commercial buildings, with low noise, no oil pollution and close-to-load installation. The oil-immersed S13/S14/S15 series remains appropriate where the project has a dedicated distribution substation or outdoor transformer area and the oil-immersed unit can be integrated into the substation design.

PV, wind and energy-storage projects

The New Energy Transformer SC10/SZ18/SZ20 is available in oil-immersed or dry-type configuration. Its LV output range of 0.4-1.14 kV is compatible with inverters from different brands, and the prefabricated cabin configuration can shorten construction time by up to 60%. A 40,000 kVA energy-storage project in Bulgaria has been delivered. For a Box Substation for Solar Power Plant, the choice between oil-immersed and dry-type should follow the site layout, fire strategy and local substation practice; both configurations are available in the prefabricated cabin package.

Distribution grids and industrial plants

Urban and rural distribution-grid construction and upgrades, industrial plant auxiliary power, commercial buildings, residential communities, hospitals, schools and municipal utilities are the core applications of the oil-immersed S13/S14/S15 series. The series is also used in transformer packages for renewable-energy substations. Reference projects include a 15 MVA pad-mounted transformer project for an American transmission company in the United States, a 132 kV power transformer for the Seville steel-plant substation in Spain, and a 345 kV fully-insulated power transformer project for a US-Georgia power transmission company.

Mining and hazardous locations

Underground coal mines, underground metal mines, tunnel construction and explosion-proof areas in chemical plants require a different answer: the KBSG/KBSGZY Series mining explosion-proof dry-type transformer. It is a dry-type unit in a flameproof welded steel enclosure, with rated capacity 50-6,300 kVA, HV 6/10 kV, LV 0.4/0.69/1.2/3.3/3.45 kV, H-class insulation, AN cooling and Exd I marking for underground coal mines. This is not a general dry-type versus oil-immersed comparison; it is a hazardous-area selection where the enclosure and certification drive the decision.

Comparison Table: Dry-Type vs Oil-Immersed Custom Transformer

Table notes: The capacity, cooling, insulation, protection and application facts above are drawn from the SCB12-SCB18, S13/S14/S15 and SFZ-250000/345 product data. The UL/CSA statement applies to liquid-filled distribution transformers rated above 750 V for North America. The GB 20052 and DOE 2024 references are regulatory context, not a claim that one transformer type is automatically compliant.

FAQ

Conclusion: Select the Constraint, Then the Transformer

The dry-type versus oil-immersed decision becomes simple once the constraints are fixed. Start with the installation location and fire strategy, then the electrical duty, then the enclosure or substation format, then certification and testing. Dry-type cast-resin units are the natural choice for indoor, fire-sensitive, close-to-load and data-center applications. Oil-immersed units are the natural choice for conventional distribution and power substations, larger distribution capacities and outdoor substation packages. Where both are technically feasible, compare the quoted losses, maintenance capability, oil handling and destination-market certification - not a generic product preference.

Apex Power Systems (Nanjing) Co., Ltd. supports this selection as an independent trading and supply partner, not as a factory. The company was established in 2024, exports to the EU, USA and Middle East markets, and supplies through audited manufacturing partners. For a project inquiry, use the contact details below and request the current catalog.

Next Step: Request a Project Review

Send your dry-type or oil-immersed Custom Transformer requirement with the installation location, capacity, voltage class, vector group, cooling and destination market. Apex can review the specification, confirm the applicable compliance route and arrange factory acceptance testing through the audited manufacturing partners.

Contact: Simon | Email: xuke@apexps-nj.com | Tel / WhatsApp: +86 132-0157-1341

Address: Room 1001, Xinke Building, No. 209 Pubin Road, Jiangbei New Area, Nanjing City, Jiangsu Province, China

Website: apexpowerlink.com | an-ele.com

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