In modern power systems, transformers are the core equipment for electrical energy transmission and distribution. Among them, the dry‑type transformer stands out for its safety, environmental friendliness, and low maintenance, making it widely used in high‑rise buildings, data centres, rail transit, and other critical facilities. So, what exactly is a dry‑type transformer? How does it differ from traditional oil‑immersed transformers? This article provides a comprehensive answer.
A dry‑type transformer – also called a non‑oil transformer – uses solid insulating materials (rather than insulating oil) as the dielectric medium. Its core and windings are not immersed in oil; instead, cooling is achieved by natural air convection or forced air circulation.
The insulation system of a dry‑type transformer typically employs epoxy resin, insulating paper, or Nomex® paper, which are specially treated to offer high dielectric strength and thermal endurance. These materials reliably isolate voltages during operation and ensure safe, dependable performance. Because no insulating oil is used, dry‑type transformers completely eliminate the risk of oil leaks, making them safer and more environmentally friendly.
The operating principle of a dry‑type transformer is based on Faraday’s law of electromagnetic induction. When an AC voltage is applied to the primary winding, it produces an alternating magnetic flux in the core. This flux links with the secondary winding and induces a voltage according to the turns ratio.
In simple terms:
The AC current in the primary winding generates a changing magnetic field in the core.
This changing field induces an electromotive force (EMF) in the secondary winding.
By adjusting the turns ratio between the primary and secondary windings, the transformer can step up or step down the voltage.
The solid insulation (epoxy resin or insulating paper) effectively isolates the high‑voltage and low‑voltage conductors, preventing direct voltage transfer and ensuring safe system operation.
Based on construction and manufacturing processes, dry‑type transformers are classified as follows:
Open (Ventilated) Dry‑Type Transformer
The core and windings are exposed to the ambient air, suitable for clean, dry indoor environments. Cooling is by natural air (AN) or forced air (AF).
Enclosed Dry‑Type Transformer
The active part is housed inside a sealed enclosure, isolated from outside air. This type is used in harsh environments, such as mines or tunnels.
Cast‑Resin Dry‑Type Transformer
Windings are completely encapsulated in epoxy resin or other resins, offering a compact structure and small footprint.
Cast‑Resin Dry‑Type Transformer (CRT)
Windings are fully sealed in epoxy resin, providing excellent protection against moisture, dust, and mechanical stress. Ideal for indoor, industrial, and humid environments. Typical models include the SCB series (e.g., SCB13, SCB18).
Vacuum Pressure Impregnation (VPI) Transformer
Varnish or resin is impregnated into the windings under vacuum and pressure, forming a durable insulation layer with good heat dissipation and structural stability.
Vacuum Pressure Encapsulation (VPE) Transformer
Features a thicker protective coating than VPI types, offering superior protection against moisture and environmental contaminants.
Natural Air Cooling (AN) – relies on natural convection; suitable for normal load conditions.
Forced Air Cooling (AF) – uses fans or blowers to increase airflow, supporting higher load capacity.
Combined AN/AF Cooling – operates under natural cooling, switching to fan‑assisted cooling when temperature rises.
Compared with traditional oil‑immersed transformers, dry‑type transformers offer the following significant benefits:
Fire Safety – No flammable insulating oil means zero risk of fire or explosion, allowing installation close to load centres.
Eco‑Friendly – No oil degradation or leakage, thus pollution‑free and aligned with green sustainability goals.
Low Maintenance – No need for periodic oil testing or replacement, greatly reducing maintenance costs and effort.
High Adaptability – Can operate normally at 100% relative humidity, with excellent moisture and corrosion resistance.
Compact Size & Low Noise – Smaller footprint than oil‑filled units, suitable for space‑constrained locations, and operate quietly.
Strong Short‑Circuit Withstand Capability – High mechanical strength, low partial discharge, and outstanding reliability.
| Feature | Dry‑Type Transformer | Oil‑Immersed Transformer |
|---|---|---|
| Insulation medium | Solid insulation (epoxy resin, etc.) | Insulating oil (mineral oil) |
| Appearance | Core and windings visible | Only the tank is visible |
| Safety | Fire‑proof, explosion‑proof, high safety | Fire and leakage risks exist |
| Capacity range | Typically up to 1600 kVA (some up to 35 kV) | Available in all ratings and voltage levels |
| Typical applications | Indoor, crowded areas, strict fire‑protection requirements | Outdoor, independent substations |
| Maintenance cost | Lower | Higher (regular oil quality checks required) |
| Cooling method | Natural or forced air | Oil circulation cooling |
Dry‑type transformers are preferred where safety and environmental protection are paramount:
High‑rise buildings & commercial complexes – indoor load‑centre distribution
Data centres – critical power distribution for IT loads
Rail transit – metro stations, airports, and railway facilities
Hospitals – redundant power supply for operating theatres and ICUs
Industrial plants & chemical factories – harsh fire‑safety requirements
Renewable energy – offshore wind power, energy storage systems, etc.
Mines & tunnels – special environmental demands
When selecting a dry‑type transformer, pay close attention to the following key parameters:
Rated Capacity (kVA) – typically ranges from 30 kVA to 3150 kVA; choose based on total load with a 10‑20% margin.
Voltage Levels – common HV sides: 6 kV, 10 kV, 20 kV, 35 kV; LV side: 0.4 kV (400 V).
Losses – including no‑load loss and load loss; higher efficiency classes mean lower losses.
Short‑Circuit Impedance (Uk%) – typical values: 4%, 6%, 8%; used to limit short‑circuit current.
Insulation Class – common thermal classes include B, F, H, etc.
Dry‑type transformers must comply with stringent international and national standards, such as GB/T 1094.11‑2022 (Power transformers – Part 11: Dry‑type transformers) and GB/T 10228 in China, as well as IEC 60076‑11 globally.
The global dry‑type transformer market was valued at approximately US$7.12 billion** in 2025 and is projected to reach **US$11.87 billion by 2033, growing at a CAGR of about 6.7%. Driven by surging demand from AI data centres, renewable energy expansion, and urbanisation, dry‑type transformers are moving beyond traditional building distribution into high‑value infrastructure segments such as rail transit, offshore wind, and energy storage.
Jiangsu Xinhong Electrical Equipment Co., Ltd. (Xinhong Electrical) is a large‑scale enterprise integrating R&D, manufacturing, trade, and service. We specialise in producing epoxy resin cast‑resin dry‑type transformers covering capacities from 30 kVA to 10,000 kVA and voltage ratings up to 10 kV and 35 kV – meeting the needs of mainstream distribution systems.
Our dry‑type transformers are distinguished by:
Safety & Reliability – oil‑free design, fire‑proof, explosion‑proof, and pollution‑free operation.
High Efficiency – low‑loss design that meets or exceeds national energy‑efficiency standards.
Epoxy Cast‑Resin Process – superior moisture, dust, and short‑circuit withstand capability.
Customisation – tailored solutions for specific customer requirements.
Full Product Range – from commercial buildings to heavy industrial projects, we have you covered.
Whether for high‑rise building power distribution, data centre backup, rail transit, or renewable energy installations, Xinhong Electrical delivers safe, reliable, and high‑performance dry‑type transformer solutions.
With their outstanding safety, environmental friendliness, energy efficiency, and low maintenance, dry‑type transformers have become indispensable components in modern electrical infrastructure. As global electricity demand continues to rise and environmental regulations tighten, the application scope of dry‑type transformers will only expand further.
For more information about dry‑type transformers, product selection, or customised solutions, please contact Xinhong Electrical – we are here to provide professional power solutions tailored to your needs.
