Right Choice of Dry Type or Liquid-Filled Transformer

Home / Technical Articles / Transformers / Right Choice of Dry Type or Liquid-Filled Transformer
Right Choice of Dry Type or Liquid-Filled Transformer
Right Choice of Dry Type or Liquid-Filled Transformer


1. Two Types of Transformers
2. Cooling and insulating system
3. Losses
4. Operating Life of Transformer
5. Maintainability
6. Repairability
7. Core/Coil Reclamation and Recycling
8. Operating Sound Level and Noise Pollution
9. Footprint
0. Conclusion

Two Types of Transformers

Information on the pros and cons of the available types of transformers frequently varies depending upon what information is made available by the manufacturer. Nevertheless, there are certain performance and application characteristics that are almost universally accepted.

Basically, there are two distinct types of transformers: Liquid insulated and cooled (liquid-filled type) and non liquid insulated, air or air/gas cooled (dry type). Also, there are subcategories of each main type.

For liquid-filled transformers, the cooling medium can be conventional mineral oil. There are also wettype transformers using less flammable liquids, such as high fire point hydrocarbons and silicones.

Liquid-filled transformers are normally more efficient than dry-types, and they usually have a longer life expectancy. Also, liquid is a more efficient cooling medium in reducing hot spot temperatures in the coils. In addition, liquid-filled units have a better overload capability.

There are some drawbacks, however.

For example, fire prevention is more important with liquid-type units because of the use of a liquid cooling medium that may catch fire. (Dry-type transformers can catch fire, too.) It’s even possible for an improperly protected wet-type transformer to explode.

And, depending on the application, liquid-filled transformers may require a containment trough for protection against possible leaks of the fluid.

Arguably, when choosing transformers, the changeover point between dry-types and wet-types is between 500kVA to about 2.5MVA, with dry-types used for the lower ratings and wet-types for the higher ratings.

Important factors when choosing what type to use include where the transformer will be installed, such as inside an office building or outside, servicing an industrial load.

Dry-type transformers with ratings exceeding 5MVA are available, but the vast majority of the higher-capacity transformers are liquid-filled. For outdoor applications, wet-type transformers are the predominate choice.

The flowing Table shows losses in dry type and oil filled type transformers:

Table: Comparison of Losses: Oil type and dry type

(Oil Transformer) LossesDry Type Transformer Losses
KVAHalf Load (W)Full Load (W)KVAHalf Load (W)Full Load (W)

Purchases of transformers are often based on the first cost (without any consideration of long-term economics) when transformer evaluation and purchase decisions are not made by the end-user.

This is particularly true when agents or electrical contractors make purchase decisions on the basis of temperature rise and low first cost for commercial and industrial end-users buying dry-type, pad-mounted transformers.

These agents or contractors may have little incentive to take into consideration any economic factors other than the transformer’s first cost. End-user concerns about higher first costs discourage OEMs and contractors from offering or recommending the more expensive, efficient options to customer who do not specifically request them.

Go to Content ↑

Cooling and insulating system

Because air is the basic cooling and insulating system for dry-type transformers, all dry-type transformers will be larger than liquid-immersed units for the same voltage and capacity (kilovolt/kilovolt-ampere) rating.

When operating at the same flux and current density, more material for core and coil implies higher losses and higher costs.

Dry-type high voltage transformer insulation system
Dry-type high voltage transformer insulation system – Glass polyester laminate insulation sheet

These trade-offs are inherent in the design of dry-type units, but dry-type transformers have traditionally offered certain fire-resistant, environmental, and application advantages for industrial and commercial situations.

Recent advances in liquid-filled units are reducing some of these (dry-type) advantages.

When purchased on the basis of lowest first cost, dry type transformers typically have significantly higher operating losses than the more efficient liquid filled transformers.

For this reason the major utilities seldom purchase dry type transformers. Because dry-type insulation systems lack the additional cooling and insulating properties of the oil-paper systems, for the same rating the dry-type transformers tend to be more costly, larger, and have greater losses than a corresponding liquid-immersed unit.

Go to Content ↑


2500 kVA transformer
2500 kVA transformer

Combined Losses at 100% Loading

Above graphic shows combined losses at 100% loading based on:

Load Losses (kW)16.3821.0018.52
No Load Losses (kW)2.667.007.55
Total Losses (kW)19.0426.0728.00

Above values are typical.

Go to Content ↑

50% Loading

At 50% loading, the no-load loss remains the same, and load loss is reduced by the inverse square:

Load Losses (kW)4.104.635.25
No Load Losses (kW)2.667.007.55
Total Losses (kW)6.7612.1812.25

Go to Content ↑

Costs Of Transformer Losses

Costs Of Transformer Losses – Transformer Energy Consumption:

Energy Costs = $0.06/kWh (Conservative Value)
8760 hours = 24hrs/day * 365 days per year

Total Losses (kW)6.7612.1812.25
KWH Billing Rate:x$0.06$0.06$0.06
Annual Hours:x876087608760
Annual Cost of Energy due to
Losses @ 50% Load:


Excess Annual Energy Costs:Base$2,849$2,886
10-Yr* Excess Energy Costs:Base$28,490$28,860

*Simple costs, assumes no interest rate or escalating energy costs

Go to Content ↑

Additional Cost Of Transformer Losses

Additional Cost Of Transformer Losses – Air Conditioning Energy Consumption:

Energy consumption by the transformer is not the only energy factor. Transformer losses are dissipated as heat, which must be removed from a controlled temperature environment by air conditioning.

Illustrated below are calculations to convert transformer losses into increased air conditioning energy consumption.

1kW = 3415BTU/Hr
1Ton Air Conditioning = 12000BTU/Hour
1Ton Air Conditioning = 1.7kW power use

Total Losses (kW)6.7612.1812.25
BTU/HR per ton A/C:


A/C (tons): =1.923.473.49
kW power usage per ton A/C: x1.71.71.7
Annual Hours of Operation (h):x876087608760
Annual energy usage (kWh):=286495161951916
kWH billing rate:x$0.06$0.06$0.06
Annual Cooling Costs:=$1,718.94$3,097$3,115
Excess Annual Cooling Costs:base$1,378$1,396
10-Yr Excess Energy Costs:base$13,782$13,960

Go to Content ↑

Operating Life of Transformer

Typical dry-type lifespan: 15-25 Years
Typical liquid-filled lifespan: 25-35 Years

The retirement age of transformers removed from service for a variety of reasons ranges from 14 to 35 years; the average is 25 years. However, the average life of liquidimmersed transformers that remain in service is 30 years or more.

Because liquid-filled transformers last longer than dry-type, they save on material, labor to replace, and operational impact due to outage to replace.

Go to Content ↑


Recommended annual maintenance for a typical dry-type transformer consists of inspection, infrared examination of bolted connections, and vacuuming of grills and coils to maintain adequate cooling and prevent buildup of flammable material.

Cleaning of the grill and coils may require the undesirable requirement of de-energizing the transformer, often leading to no cleaning. Omitting the cleaning decreases the transformer efficiency due to decreased airflow and creates a fire hazard.

Preventive maintenance for a liquid-filled transformer may consist of drawing and analyzing an oil sample. The oil analysis provides a very accurate assessment of the transformer condition – something not possible with dry-type transformers. Omitting the preventive maintenance does not decrease transformer efficiency or create a potential fire hazard.

Less-flammable liquid-filled transformers provide the best opportunity to enable maximum efficiency with the least maintenance, and provide the best diagnostics for repair/re-use rather than unforeseen failure/disposal.

Go to Content ↑


“Coils in liquid-type units are much easier to repair than coils in dry-type transformers. Cast coils are not repairable; they must be replaced.” – Moran, Robert B. Guidelines for transformer application designs. Electrical Construction and Maintenance, May 1996.

When a transformer fails, a decision to repair or replace the transformer must be made. Liquid-filled transformers, in most situations, can be economically repaired at local independent service repair facilities.

Liquid-filled transformers provide the best opportunity to repair existing equipment rather than dispose and replace.

Example: 2500kVA Transformer – Purchase and Maintenance

Purchase Price:$35,000$60,000$38,000
Operating Life (years):353025
Annual Maintenance:none6 hours6 hours
Annual Maintenance:none$360$360
Outage Required for Maintenance:N/AYesYes
Fire Hazard if not Maintained:NoYesYes
Annual cost to purchase and maintain:$902$1,693$1,376

Go to Content ↑

Core/Coil Reclamation and Recycling

Feature: Liquid filled transformers allow easier core/coil reclamation
Materials & Resources Benefit: Easier to recycle

Utility companies who use most of the liquid-filled transformers typically replace the coils on old transformers and continue to use them for a large percentage of their old substation transformers. The small distribution transformers are disposed/recycled when they reach an end of life.

When it comes time to decommission a transformer, recycling offsets the need for new material and provides a positive cash flow. Most components of liquid-filled and dry-type transformers can be recycled. Cast resin type transformers are an exception. Because of their construction, the materials in cast resin type transformers can be difficult and uneconomical to recycle. When a cast coil fails, the entire winding, encapsulated in epoxy resin, is rendered useless and typically ends up in a landfill.

This wastes the resource and creates additional costs for disposal, plus long-term liability exposure to the original owner.

In contrast, liquid-filled transformers can be easily recycled after their useful life. The transformer fluid can be reconditioned and used again, and the steel, copper, and aluminum can be completely and economically recycled, providing a positive cash flow.

The scrap values and disposal costs for a 2500 kVA transformer are shown below. Positive cash flows are shown in parentheses.

2500kVA Transformer

Dry TypeCast ResinLiquid Filled
Dielectric Fluid$0$0$500
Core and Coil$1100$100$1200
Tank and Fitting$400$100$400
Disposal Costs$0$400$0
Total Costs (or Savings)$1500$200$2100

Operating Sound Level and Noise Pollution

Feature: Liquid filled transformers have a lower operating sound level
Indoor Environmental Quality Benefit: Less noise pollution

Transformer types comparison - Operating sound level
Transformer types comparison – Operating sound level

Decibels is a logarithmic function, and sound pressure doubles for every three decibel  increase. Research shows that decibel levels over 60 can reduce a person’s attention  span.

A study by the American Society of Interior Designers showed that office  productivity would increase if workspaces were less noisy.

Go to Content ↑


Feature: Liquid filled transformers have a smaller footprint
Materials and Resources Benefit: Smaller equipment reduces building size demand

Typical cost per square foot: $25/SF

kVALiquid:Dry:Difference: $25/SF:
750kVADepth:4.6 ft5.5 ft
Width:4.6 ft8.0 ft
Sq Ft:21 ft244 ft223 ft2$575
1000kVADepth:5.2 ft5.5 ft
Width:4.8 ft8.0 ft
Sq Ft:25 ft244 ft219 ft2$475
1500kVADepth:6.3 ft5.5 ft
Width:4.4 ft8.0 ft
Sq Ft:28 ft244 ft216 ft2$400

A smaller building also has the benefit of requiring less lighting and ventilation.

Go to Content ↑


Use of liquid-filled transformer(s) for commercial and industrial facilities is an innovative design practice. A dry-type transformer is the standard solution for providing power in this type of design.

A total owning cost evaluation of both dry-type and liquid-filled transformers will show the lowest total owning cost choice is the installation of less-flammable liquid filled transformers.

Go to Content ↑

Resources: Best Practice Manual for transformers – Devki Energy Consultancy Pvt. Ltd.; Application for LEED Innovation & Design Points – Transformer Technology:  Liquid-Filled vs. Dry-Type

SEARCH: Articles, software & guides

Premium Membership

Premium membership gives you an access to specialized technical articles and extra premium content (electrical guides and software).
Get Premium Now ⚡

About Author


Edvard Csanyi

Edvard - 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 fascilities. Professional in AutoCAD programming. Present on