Oil-Filled vs. Dry-Type: Selecting the Right Insulation for Custom High-Voltage Transformers

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.

High-Voltage Divider Accuracy: Standards and Calibration for Diagnostic Equipment

The medical sector relies heavily on the uninterrupted performance of advanced diagnostic tools like X-ray systems, CT scanners, and oncology linear accelerators. These sophisticated machines require precise internal electrical control to generate consistent radiation and imaging outputs. Within these high-voltage systems, subtle variations in voltage supply can degrade image clarity or affect diagnostic accuracy. Consequently, design engineers must integrate robust monitoring systems to continually oversee power delivery.

To maintain precise operational control, modern medical equipment manufacturers depend on specialized sub-assemblies built to strict technical guidelines. These components must meet the rigorous performance demands of medical laboratories and clinical environments, backed by engineering support that helps development teams navigate complex power management requirements safely.

Learn more about how strict calibration protocols and advanced system design optimize high-voltage diagnostic applications.

The Impact of Temperature Coefficients and Parasitic Capacitance on Ratio Accuracy

Diagnostic equipment requires precise voltage scaling to function safely. Engineers rely on precision high-voltage dividers for testing to step down high electrical loads into measurable signals. However, environmental factors and inherent electrical properties can disrupt these measurements. Understanding the science behind high-voltage measurement helps engineers account for physical disruptions in sensitive diagnostic applications.

Temperature coefficients dictate how much a resistor’s value changes as heat increases during continuous operation. Unmanaged thermal drift can shift the output ratio and compromise the integrity of the collected data. Medical diagnostic systems generate significant heat, so internal voltage dividers must maintain stability to avoid measurement errors.

Furthermore, parasitic capacitance occurs when unintended electric charge stores between internal components or between the divider and surrounding grounded structures. This stray capacitance distorts high-frequency signals and transient responses, leading to inaccurate readings in fast-pulsed systems. To counteract these effects, engineers implement active shielding techniques and select specialized resistors with low temperature coefficients. Adherence to high voltage divider ratio accuracy standards mandates that these physical and environmental variables remain controlled to achieve reliable, repeatable data in the field.

Calibration Standards for Diagnostic Tools in Labs and Medical Facilities

Medical diagnostics and physics laboratories require consistent equipment validation to maintain operational safety. Regular calibration verifies that a divider steps down voltage accurately across its entire specified scale without deviation. Calibration protocols involve comparing the divider’s output against a certified, traceable reference standard.

Facilities utilize ISO/IEC 17025 testing and calibration to validate equipment performance. This standard establishes the core requirements for the competence, impartiality, and consistent operation of independent testing laboratories.

Laboratories can perform rigorous accuracy tests on electronic standard voltage dividers up to 800 kV. These extreme voltage tests confirm the equipment maintains accuracy under maximum electrical load. Proper calibration includes calculating the exact measurement uncertainty and establishing a chain of traceability to international measurement standards.

Stangenes Industries maintains ISO 9001:2015 certification to support consistent manufacturing quality across all equipment. We produce a diverse product line for diagnostic equipment, including but not limited to:

• Voltage dividers
• Electromagnets
• Transformers
• High-voltage pulse modulators
• Power supplies

Our engineering team can guide customers through complex design requirements, customizing any off-the-shelf product for unique applications.

Partner with Stangenes for Certified Diagnostic Precision

Engineers require dependable hardware to satisfy strict high voltage divider ratio accuracy standards. Stangenes Industries manufactures precision high-voltage dividers for testing alongside custom electromagnets and transformers. We fulfill orders from single prototype units to small- and medium-scale production runs to match your supply chain needs. Our team applies deep engineering expertise to customize products to fit demanding applications in the medical and commercial sectors.

Contact us to discuss your diagnostic equipment specifications, or request a quote to begin your next engineering project.