A Practical Guide to Dry Type Transformers: Selection, Types, and Maintenance

In the world of power distribution, safety is paramount—especially when the equipment is located inside a hospital, a shopping mall, or a high-rise office building. You cannot risk an oil leak or a chemical fire in these environments.

This is where the Dry Type Transformer shines.

Unlike their oil-filled cousins which sit in outdoor substations, dry type transformers use air for cooling. They are safe, reliable, and require minimal maintenance. However, choosing the right one (CRT vs. VPI) and understanding their temperature limits is critical for longevity.

This guide covers the essentials of dry type transformer technology, selection, and care.

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Table of Contents

• 1. What is a Dry Type Transformer?
• 2. The Two Main Types: CRT vs. VPI
• 3. Dry Type vs. Oil-Filled: Comparison
• 4. Understanding Ratings: Temp Class & Cooling
• 5. Maintenance Checklist
• 6. Conclusion

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1. What is a Dry Type Transformer?

A Dry Type Transformer is a magnetic device where the core and windings are not immersed in an insulating liquid (like oil or silicone). Instead, the windings are kept cool by the natural or forced circulation of air.

Why use them?

• Safety: Zero risk of oil leaks, soil contamination, or explosion.
• Location: Can be installed indoors, close to the load (reducing cable costs).
• Maintenance: No oil sampling or tank pressure checks required.

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2. The Two Main Types: CRT vs. VPI

Not all dry transformers are built the same. There are two competing technologies:

1. Cast Resin Transformer (CRT)

• Construction: The primary and secondary windings are completely encapsulated in a solid block of Epoxy Resin under vacuum.
• Pros: Ideally suited for harsh environments. The resin blocks moisture, dust, and corrosive fumes. High short-circuit strength ("The Tank").
• Cons: Heavier. Poor heat dissipation (epoxy is an insulator). Coils cannot be repaired; if one fails, it must be replaced.

2. Vacuum Pressure Impregnated (VPI)

• Construction: The coils are made of foil or wire and then submerged in a polyester varnish under vacuum pressure.
• Pros: Excellent heat dissipation. The coils are repairable. Generally lighter and smaller ("The Flexible One").
• Cons: More susceptible to moisture and harsh industrial chemicals than CRT.

Selection Rule:

• Use CRT for chemical plants, coastal areas, or damp basements.
• Use VPI for standard commercial buildings and clean industrial floors.

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3. Dry Type vs. Oil-Filled: Comparison

Feature	Dry Type Transformer	Oil-Filled Transformer
Cooling Medium	Air	Oil / Mineral Oil
Location	Indoor (Safe)	Outdoor (Fire Risk)
Size	Larger (Air is a poor insulator)	Compact
Maintenance	Low (Cleaning & Tightening)	High (Oil Analysis, Leak checks)
Fire Hazard	Very Low / Self-Extinguishing	High (Flammable Oil)
Life Expectancy	20-25 Years	25-30 Years

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4. Understanding Ratings: Temp Class & Cooling

When reading the nameplate, two specs matter most.

Insulation Class

This dictates how hot the transformer can run before the insulation melts.

• Class F: Max hot spot temp 155°C.
• Class H: Max hot spot temp 180°C.
• Tip: A Class H transformer running at Class F temperatures will have a significantly longer lifespan (often called "Reserve Capacity").

Cooling Method (AN / AF)

• AN (Air Natural): Cooling via natural convection. This is the base rating (e.g., 1000 kVA).
• AF (Air Forced): Cooling via fans installed at the bottom. Turning on fans can typically boost capacity by 33% to 40% (e.g., up to 1400 kVA). This is great for handling peak loads.

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5. Maintenance Checklist

While they are "Low Maintenance," they are not "No Maintenance." Dust is the silent killer of dry transformers.

Annual Checklist:

1. De-energize & Ground: Safety first.
2. Clean: Use compressed air or a vacuum to remove dust from the windings and cooling ducts. Dust acts like a blanket, causing overheating.
3. Inspect: Look for cracks in the resin (CRT) or discoloration in the varnish (VPI) which indicates overheating.
4. Tighten: Vibrations loosen bolts over time. Torque check all electrical connections.
5. Test Fans: Manually trigger the fan controller to ensure all fans spin freely.

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6. Conclusion

The Dry Type Transformer is the heartbeat of modern indoor power distribution. Whether you choose the rugged Cast Resin (CRT) for a humid factory or the efficient VPI for an office tower, understanding the environment is key to the right selection.

Sourcing Power Components? Need replacement fans, temperature controllers, or medium voltage switchgear components? Visit Aichiplink.com to search for industrial electrical supplies.

 

 

 

 

 

Written by Jack Elliott from AIChipLink.

 

AIChipLink, one of the fastest-growing global independent electronic   components distributors in the world, offers millions of products from thousands of manufacturers, and many of our in-stock parts is available to ship same day.

 

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