Premium Membership ♕

CYBER WEEK OFFER 💥 Save 20% on PRO Membership Plan and Video Courses with the coupon CYM23 and learn from experienced engineers.

Home / Technical Articles / Do dry type transformers cost less than oil-filled?

Comparison to dry type transformers

To many industrial and commercial organisations dry type transformers are seen to be advantageous compared with oil filled units. They avoid the perceived fire risk of oil filled transformers and can thus be accommodated inside a building or even on the roof of an office block.

Are Dry Type Transformers Really Cost Less?
Are Dry Type Transformers Really Cost Less? (photo credit:

Savings can be made on LV connections by installing the transformer integral with 415V switchgear.

There is no need to house the transformers in bunded enclosures or make any other provision for spillage as would be the case if a transformer filled with any type of liquid dielectric were installed.

All of these advantages as well as the apparent benefits of low maintenance and high reliability are energetically marketed by manufacturers and cast-resin insulated dry types, in particular, are being installed in increasingly large quantities.

Most users of dry type transformers are aware of the extra initial costs and accept these as the price to be paid for greater convenience. What users often do not realise, however, is that dry type transformers are less energy efficient and can involve significant increases in running costs.

Architects, in particular, may be very conscious of the benefits arising from the installation of dry type transformers without having to live with the running costs and the potential user would do well to recognise this and examine available options with great care.

KVA for kVA a dry type transformer will always be larger than its oil filled counterpart. This is because larger electrical clearances are required when air is part of the insulation system.

GEAFOL dry type power transformer
GEAFOL dry type power transformer (photo credit:

In addition air is less efficient than oil as a cooling medium, so that, although a higher temperature rise is usually permitted, larger cooling ducts will be required. Increased size means greater no-load loss and, because the transformer runs hotter, winding resistance will be increased, thus increasing I2R losses too.

All of this is regardless of the class of dry type transformer used. In addition cast resin transformers often use aluminium as a winding material because its coefficient of thermal expansion is closer to that of resin and this provides a better quality of encapsulation. This in turn leads to bulkier windings which have still higher losses.

Finally, of course, the additional burden placed on ventilation plant by the imposition of transformer losses must not be overlooked.

Reference // Electrical Energy Efficiency by Copper Development Association

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

eighty seven  −    =  eighty one

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge