In-Depth Analysis: Core Differences Between Oil-Immersed and Dry-Type Distribution Transformers and Key Selection Considerations

As the terminal hub of power transmission, the choice of distribution transformer type directly impacts the investment cost, operational safety, and long-term benefits of the entire power supply system. Among them, oil-immersed transformers and dry-type transformers are the two most mainstream categories, and the choice between them is often a core issue in project planning. This article will provide an in-depth comparison of their technical characteristics to help you make a scientific decision that best meets your actual needs.

1. Fundamental Differences in Insulation and Cooling Medium

This is the most essential difference between the two, leading to all subsequent characteristic variations.

• Oil-Immersed Transformers: Typically use mineral oil or synthetic ester as the insulation and cooling medium. The oil not only has high insulating strength but its heat capacity and fluidity also make it an excellent coolant, efficiently transferring heat generated by the windings to the tank’s radiators and dissipating it through natural air convection or forced fan cooling.
• Dry-Type Transformers: Use solid insulating materials, commonly employing epoxy resin casting (Cast Resin) or non-epoxy vacuum pressure impregnation (VPI) processes. Their cooling medium is air, relying on natural air circulation or forced ventilation from fans for heat dissipation.

2. Direct Comparison of Safety and Installation Environment

• Fire and Explosion Protection Performance:
  • Dry-Type Transformers: Because they contain no flammable insulating oil, they offer excellent fire and explosion protection performance. Even under extreme fault conditions, there is no risk of oil spraying or explosion, hence they are widely classified as fire-proof equipment.
  • Oil-Immersed Transformers: Insulating oil is a combustible material. Although modern products feature fully sealed designs equipped with pressure relief valves and safety channels, potential risks still exist. A leak could potentially cause a fire.
• Installation Environment Requirements:
  • Dry-Type Transformers: Due to their safety, they can be installed directly indoors at the load center, such as in office building floors, basements, subway stations, hospital operating layers, data center server rooms, etc., close to the electrical equipment. This significantly reduces low-voltage cable length and line losses.
  • Oil-Immersed Transformers: Usually require installation in a dedicated transformer room or an outdoor pad-mounted enclosure (kiosk). The construction of a transformer room requires consideration of fire protection, explosion venting, oil containment, and other facilities, increasing civil engineering costs and space occupancy.

3. Economic Analysis: Initial Investment vs. Operating Costs

• Initial Procurement and Installation Costs:
  • For the same capacity rating, the manufacturing cost and initial purchase price of oil-immersed transformers are usually lower than those of dry-type transformers.
  • However, installation costs must be considered comprehensively: Dry-type transformers are simple to install and can be placed directly. Oil-immersed transformers may require additional civil works (transformer room, oil pit), fire protection systems, potentially making the overall installation cost higher.
• Operation and Maintenance Costs:
  • Efficiency and Electricity Costs: Dry-type transformers generally have lower no-load losses, while oil-immersed transformers often have an advantage in load losses. Actual electricity costs depend on the user’s load factor. For scenarios with long-term low load operation (e.g., <30%), dry-type or high-efficiency oil-immersed transformers (like S13, S14, amorphous alloy) are more energy-efficient.
  • Maintenance Costs: Oil-immersed transformers require regular checks of oil level, oil temperature, oil sample testing, and preventive testing, resulting in higher maintenance workload and costs. Dry-type transformers are virtually maintenance-free, requiring only cleaning and periodic checks of tightness, leading to very low maintenance costs.

4. Capacity and Voltage Application Range

• Oil-Immersed Transformers: Dominate in the high-capacity field, easily reaching tens of MVA (thousands of kVA) or even higher. Voltage levels can also cover higher transmission and distribution levels.
• Dry-Type Transformers: Limited by the cooling efficiency of air and solid insulation technology, the single-unit capacity is usually smaller (generally most economical below 2500kVA or 3150kVA), and voltage levels are mostly used in medium and low-voltage distribution networks (35kV and below).

5. Key Points for Selection Decision-Making

Choosing between oil-immersed and dry-type should be based on a comprehensive decision matrix:

1. Safety is the Top Priority: For locations inside buildings, densely populated areas, and places with extreme fire safety requirements (e.g., airports, subways, data centers, commercial complexes), dry-type transformers must be chosen. This is mandated by electrical codes in most countries.
2. Consider Total Cost of Ownership (TCO): Don’t just look at the purchase price. For indoor installation, calculate the total cost of a dry-type transformer (“equipment price”) versus an oil-immersed transformer (“equipment price + civil works + fire protection system”). Also, simulate the total electricity cost over 20+ years for different transformer models based on annual electricity consumption and load profile.
3. Evaluate Load Characteristics: If the load factor is consistently low over the long term (e.g., <30%), prioritize models with low no-load losses, such as amorphous alloy oil-immersed or dry-type transformers, to achieve long-term energy savings.
4. Balance Capacity Requirements: For larger capacity needs exceeding 2500kVA, if installed outdoors, oil-immersed transformers are usually a more economical and reliable choice.

Conclusion:
Oil-immersed and dry-type transformers are not about which is better overall, but each dominates specific application scenarios. Safety regulations, installation space, total capacity requirements, and lifecycle cost are the four pillars for making the right choice. Understanding their fundamental differences means you can not only meet the basic functional needs of the project but also build a power system that operates reliably and cost-effectively for decades to come.