Right Choice of Dry Type or Liquid-Filled Transformer _ EEP

7/31/2019 Right Choice of Dry Type or Liquid-Filled Transformer _ EEP

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Ri ght Choi ceof Dry Typeor Li quid-FilledTransformer | EEP

ineering-portal .com/right-choice-of-dry-type-or-l iquid-filled-transformer

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Right Choice of Dry T ype or Liquid-Filled Transformer (on photo Dynapower Corporation

transformers)

Right Choice of Dry Type or Liquid-FilledTra nsformer

Content

1. Two Types of Transformers2. Cooling and insulating system3. Losses4. Operating Life o f Transformer5. Maintainability 6. Repairability 7. Core/Coil Reclamation and Recycling8. Operating Sound Level and Noise Pollution9. Footprint0. Conclusion
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Two Types of Transformers

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

Basically, there are two distinct types of transformers: Liquid insulated and cooled ( liquid-filledtype ) and non liquid insulated , air or air/gas cooled ( dry type ). Also, there are subcategoriesof 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 andsilicones.

Liquid-filled transformers are normally more efficient than dry-types , and they usually havea longer life expectancy. Also, liquid is a more efficient cooling medium in reducing hot spottemperatures 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 liquidcooling medium that may catch fire. ( Dry-type transformers can catch fire, too. ) Its even

possible for an improperly protected wet-type transformer to explode.

And, depending on the application, liquid-filled transformers may require a containment trough forprotection 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 forthe 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 anindustrial load.

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

The flowing Table shows losses in dry type and oil filled type transformers :
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Table: Comparison of Losses: Oil type and dry type

(Oil Transformer) Losses Dry Type Transformer Losses

KVA Half Load (W) Full Load (W) KVA Half Load (W) Full Load (W)

500 2465 4930 500 5000 10000

750 3950 7900 750 7500 15000

1000 4360 8720 1000 8200 16400

1500 6940 13880 1500 11250 22500

2000 8155 16310 2000 13200 26400

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 basisof temperature rise and low first cost for commercial and industrial end-users buy ing dry-type,pad-mounted transformers.

These agents or contractors may have little incentive to take into consideration any economicfactors other than the transformers first cost. End-user concerns about higher first costsdiscourage OEMs and contractors from offering or recommending the more expensive, efficientoptions to customer who do not specifically request them.

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Cooling and insulating system

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

insulation sheet

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

These trade-offsare inherent inthe design of

dry-type units, but dry -typetransformers

have traditionally offered certain fire-resistant, environmental, and application advantagesfor 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-typeinsulation 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 greaterlosses than a corresponding liquid-immersed unit.

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Losses

Combined Losses at 100% Loading
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2500 kVA transformer

bove graphic shows combined lossesat 100% loading based on:

Liquid: Cast: Dry:

LoadLosses(kW)

16.38 21.00 18.52

NoLoadLosses(kW)

2.66 7.00 7.55

TotalLosses(kW)

19.04 26.07 28.00

bove values are typical.

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50% Loading

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

Liquid: Cast: Dry:

Load Losses (kW) 4.10 4.63 5.25

No Load Losses (kW) 2.66 7.00 7.55

Total Losses (kW) 6.76 12.18 12.25

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Costs Of Transformer Losses

Costs Of Transformer Losses Transformer Energy Consumption:

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

Liquid: Cast: Dry:

Total Losses (kW) 6.76 12.18 12.25

KWH Billing Rate: x $0.06 $0.06 $0.06

Annual Hours: x 8760 8760 8760

Annual Cost of Energy due toLosses @ 50% Load:

= $3,553 $6,402 $6,439

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

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Additional Cost Of Transformer Losses

dditional Cost Of Transformer Losses Air Conditioning Energy Consumption:

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

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


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Constants:1kW = 3415BTU/Hr1Ton Air Conditioning = 12000BTU/Hour1Ton Air Conditioning = 1.7kW power use

Liquid: Cast: Dry:

Total Losses (kW) 6.76 12.18 12.25

BTU/HR/KW: x 3415 3415 3415

BTU/HR: = 23085 41595 41834

BTU/HR per ton A/C: 12000 12000 12000

A/C (tons): = 1.92 3.47 3.49

kW power usage per ton A/C: x 1.7 1.7 1.7

kW: = 3.27 5.89 5.93

Annual Hours of Operation (h): x 8760 8760 8760

Annual energy usage (kWh): = 28649 51619 51916

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

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Operating Life of Transformer


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

The retirement age of transformers removed from service for a variety of reasons ranges from 14to 35 years; the average is 25 years. However, the average life of liquidimmersed transformersthat 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.

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Maintainability

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 adequatecooling and prevent buildup of flammable material.

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

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

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

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Repairability

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

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

Example: 2500kVA Transformer Purchase and Maintenance

Liquid Cast Dry

Purchase Price: $35,000 $60,000 $38,000

Operating Life (years): 35 30 25

Annual Maintenance: none 6 hours 6 hours

Annual Maintenance: none $360 $360

Outage Required for Maintenance: N/A Yes Yes

Fire Hazard if not Maintained: No Yes Yes

Repairable: Yes No Yes

Annual cost to purchase and maintain: $902 $1,693 $1,376

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Core/Coil Reclamation and Recycling

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

Utility companies who use most of the liquid-filled transformers typically replace the coils on oldtransformers 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.


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When it comes time to decommission a transformer, recycling offsets the need for new materialand 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, thematerials in cast resin type transformers can be difficult and uneconomical to recycle. When a castcoil fails, the entire winding, encapsulated in epoxy resin, is rendered useless and typically endsup in a landfill.

This wastes the resource and creates additional costs for disposal, plus long-term liability exposureto 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 Type Cast Resin Liquid 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


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Transformer ty pes comparison - Operating sound level

Decibels is a logarithmic function, and sound pressure doubles for every three decibel increase.Research shows that decibel levelsover 60 can reduce a persons attentionspan.

A study by the American Society of

Interior Designers showed that officeproductivity would increase if

workspaces were less noisy.

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Footprint

Feature: Liquid filled transformershave a smaller footprint

aterials and Resources Benefit: Smaller equipment reduces building size demand

Constants:Typical cost per square foot: $25/SF

kVA Liquid: Dry: Difference: $25/SF:

750kVA Depth: 4.6 ft 5.5 ft

Width: 4.6 ft 8.0 ft

Sq Ft: 21 ft 2 44 ft 2 23 ft 2 $575



Sq Ft: 25 ft 2 44 ft 2 19 ft 2 $475



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Sq Ft: 28 ft 2 44 ft 2 16 ft 2 $400

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

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Conclusion

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

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-flammableliquid filled transformers.

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esources: Best Practice Manual for transformers - Devki Energy Consultancy Pvt. Ltd.; Application for LEED Innovation & Design Points - Transformer Technology: Liquid-Filled vs. Dry-Type
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Right Choice of Dry Type or Liquid-Filled Transformer _ EEP

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