Choosing between oil-filled and dry-type custom transformers is one of the most important decisions when designing a transformer for high-voltage applications. The insulation system influences safety, installation requirements, maintenance, and long-term performance. Because every application has different demands, there’s no single right answer.
This guide compares oil-filled vs. dry-type custom transformers to help engineers specify the right insulation medium for a custom project.
The Critical Role of Dielectric Strength and Thermal Management
The insulation medium serves two functions: preventing arc discharge between conductors and moving heat away from the windings. How well it performs each function determines the transformer’s operating limits.
When to Choose Oil-Filled for High Power Density
Mineral oil has a dielectric constant roughly twice that of air. It also transfers heat away from windings more efficiently than solid or gaseous insulation. These two properties make oil-filled designs the preferred choice for high-power-density applications where the transformer must operate at elevated voltages for extended periods.
Oil-filled units handle:
- Sustained high-voltage loads. 300 kVDC and above.
- High KVA ratings. Thermal management would otherwise require a much larger dry-type enclosure.
- Compact-footprint applications. Installation simplicity is a secondary concern.
At Stangenes Industries, several of our high-voltage isolation transformers, including units rated for 600 kV pulse and 800 kV pulse service, use oil as the insulating medium precisely because of these demands. You can review the full specs on our isolation transformers page.
The Case for Dry-Type: Safety and Maintenance Advantages
Dry-type transformers use solid insulation—such as cast epoxy and resin-impregnated windings—or air insulation instead of a liquid dielectric. This matters most in environments where fluid containment is a concern.
Key reasons engineers specify dry-type:
- No oil spill risk. Medical facilities and cleanroom labs cannot tolerate fluid leaks near sensitive equipment or patients.
- Lower fire risk. Dry-type units don’t carry the combustion risk associated with oil-filled designs.
- Reduced maintenance. No oil sampling, no leak inspection, no fluid disposal requirements.
- Regulatory fit. Many medical and indoor industrial environments require dry-type or epoxy-cast construction to comply with facility safety codes.
For high-voltage dry-type custom transformers, epoxy cast construction provides excellent mechanical protection. It seals the winding geometry, resists moisture intrusion, and maintains consistent performance across temperature swings, which is a real advantage in labs where ambient conditions vary.
SF₆ (sulfur hexafluoride) gas insulation occupies a middle ground. It offers high dielectric strength without the fire risk of oil, which is why it appears in several of our isolation transformer models rated for isolation voltages from 150 kVDC up to 500 kV.
Balancing Size, Weight, and Environmental Factors
Installation requirements also shape the decision between oil-filled vs. dry-type custom transformers:
- Oil-filled transformers are heavier and typically require secondary containment for potential leaks, making them better suited for industrial machinery than for space-constrained environments like medical imaging suites or university labs.
- Dry-type transformers are lighter, easier to install indoors, and don’t require containment systems. However, they offer lower power density, so achieving the same voltage isolation often requires a larger transformer.
For aerospace and defense applications, weight is often the governing constraint. A custom design with cast epoxy insulation and optimized winding geometry can sometimes achieve the required isolation voltage at a lower weight than a comparable oil-filled unit.
A few other factors worth considering:
- Ambient temperature range. Oil-filled units self-regulate better at sustained high loads. Dry-type units need careful thermal modeling for continuous duty cycles.
- Altitude. Air-insulated dry-type designs derate at high altitudes due to reduced air density. Oil and epoxy do not.
- Service life and repairability. Oil-filled units can sometimes be reprocessed; epoxy-cast units generally cannot be rewound in the field.
Consult with a Custom Manufacturer for Optimized Performance
The oil-filled vs. dry-type custom transformers comparison is ultimately about balancing competing design priorities—voltage, power density, installation constraints, safety, and maintenance—to identify the best-fit design for the application.
Stangenes Industries engineers high-voltage dry-type custom transformers for medical imaging systems, epoxy-cast units for physics research labs, and oil-filled designs for pulsed power applications requiring 800 kV isolation.
If you’re in the early stages of a custom transformer project or re-evaluating a design that’s hitting performance limits, talk to our engineering team. Contact us to discuss your application, or request a quote to kick off a custom project.