The Basics of Large Dry-Type Transformers _ Content Content From Electrical Construction & Maintenance (EC&M) Magazine

Forum Subscribe About Us Magazine Issues Newsletters Advertise Shop

Jun 1, 2007

Tweet COMMENTS 0

Advertisement

The Basics of Large Dry-Type TransformersKeith Lane | Electrical Construction and Maintenance

What you need to know when selecting a transformer for a critical electrical distribution system

Like all of the electrical distribution equipment serving critical systems,

transformers are a key component. That's why it's crucial to evaluate your

decision from every angle before choosing a unit. To help you narrow the playing field,

let's take a look at some of the most important factors to consider when specifying or

purchasing primary dry-type transformers, which typically feed a large industrial plant,

data center, or other critical facility.

Photo1.This12,470Vto480V,3-phasevacuum

pressureencapsulated(VPE)transformerservingalargedatacenterisdualrated

Hot Topics Comments New Topics

Advertisement

Make an Energy Transformation

Tip of the Week: Voltage Drop and the NEC(NFPA 70)

Tip of the Week: Use NFPA 70E to Guide YourSafety Program

Curing Customer Confusion, Part 1

Tip of the Week: Properly Allocate Your MotorMaintenance Resources

Join the Discussion on EC&M TalkJoin the Discussion on EC&M TalkJoin the Discussion on EC&M TalkJoin the Discussion on EC&M Talk

01096922100@ @ 0235682820 @

last reply by marvill201 in Electrical Talk Pics &

Videos

01060037840 !! 0235699066 !! !!


Videos

01060037840!! 0235699066 !!


Videos

01060037840++ 0235699066 +! " ++


Videos

01096922100 !! 0235682820-- --


Videos

HOMEHOMEHOMEHOME > > > > CONTENTCONTENTCONTENTCONTENT > THE BASICS OF LARGE DRY-TYPE TRANSFORMERS > THE BASICS OF LARGE DRY-TYPE TRANSFORMERS > THE BASICS OF LARGE DRY-TYPE TRANSFORMERS > THE BASICS OF LARGE DRY-TYPE TRANSFORMERS

3RecommendRecommend

Capacity The kVA rating of the transformer.1.

Voltage rating Primary and secondary voltage of the transformer.2.

Insulation system The sum of the maximum ambient temperature, plus the

average winding temperature rise, plus the differential between the average

winding temperature rise and the highest temperature of the winding.

3.

Core and coils Wound cores or stacked laminations (copper or aluminum

conductors).

4.

Winding insulation system Open-wound, vacuum pressure impregnation,

vacuum pressure encapsulated, encapsulated, and cast coil.

5.

Photo GalleriesPhoto GalleriesPhoto GalleriesPhoto Galleries

Announcing EC&M's2015 Top 10 Electrical Design FirmsIt's that time of year again -- when EC&M unveils thehighly anticipated 2015 Top 40 electrical design firms inthe country, ranked specifically by electrical designrevenue.

EC&M White PapersEC&M White PapersEC&M White PapersEC&M White Papers

NEC Design Ops & Maintenance Contractor Safety Power Quality Training Basics

Products

REGISTER LOG IN

The Basics of Large Dry-Type Transformers | content content from Elec... http://ecmweb.com/content/basics-large-dry-type-transformers

1 of 5 6/7/2015 10:56 PM

at2,000kVAandfanassistedto2,666kVA.Theright-handsideoftheenclosurecontainsthemedium-voltageswitches,themiddlesectionisthetransformer,andthefarleftsectioncontainsthesecondary480Vbreakers.

Large dry-type distribution transformers are typically fed by medium-voltage power

systems (12.47kV, 13.3kV, or 13.8kV) and feature a secondary voltage rating of 480V,

3-phase (Photo 1). Some of the larger common sizes of dry-type transformers available

today include 500kVA, 750kVA, 1,000kVA, 1,500kVA, 2,000kVA, 2,500kVA, 3,000kVA,

3,750kVA, 5,000kVA, and 7,500kVA.

Photo2.Two45kVAopenwounddry-typetransformersinastackedconfiguration.

As noted above, there are several different construction methods used in the manufacture

of dry-type transformers, resulting in various designs that make installation in many

different environments possible. The key difference in the design of these units is in the

insulation of the windings. Dry-type transformers can have their windings insulated in a

number of different ways.

Sponsored by

Advertisement

EC&M Learning Center

Understanding the 2014 NEC,Volume 1 (Articles 90 to 480)This resource has proven itself in thefield and in the exam room. This latestedition will provide you with a rock-solidfoundation...

Understanding NEC Requirementsfor Grounding vs BondingOne of the most confusing areas of thetrade continues to be grounding andbonding, which is why this book needs tobe in the hands...

Changes to The NEC 2014Don't let the scale of the code changesintimidate you, this book will get you upto speed on the most essential...

BROWSE ALL TITLES

VIEW MORE WHITE PAPERS

Workplace ElectricalSafety: What the NewLanguage Means forYouThe recent changes toNFPA 70E and CSA Z462

have the potential if not the intent to change the way weapproach workplace electrical safety programs....

More

EC&M WebinarsEC&M WebinarsEC&M WebinarsEC&M Webinars

VIEW MORE WEBINARS

ON-DEMAND:Understanding YourOptions: AFCIProtection and the2014 NEC 5Thursday, June 12,

2014

The 2014 National Electrical Code has expanded theoptions to provide Arc Fault protection for branch circuitsMore

EC&M TVEC&M TVEC&M TVEC&M TV

BROWSE ALL VIDEOS

New CertificationAnnouncement fromNFPA...

More


2 of 5 6/7/2015 10:56 PM

Atypicalcastcoiltransformerconsistsofthelow-voltagebus(1),clampbolts(2),high-voltagebus(3),low-voltagewinding(4),core(5),high-voltagewinding(6),tap(7),tapterminals(8),transformerbase(9),liftingchannels(10),airchannel(11),high-voltagewindingconductors(12),andlow-voltage

E-MAIL*

COUNTRY*

Sign-up to receive our free newsletters

eTrain - (Bi-Weekly) View SampleProduct NewsWire - (Monthly) ViewSample

ElectricalZone - (Weekly) View SampleMRO Insider - (Bi-Weekly) View SamplePQ NewsBeat - (Bi-Weekly) ViewSample

Illumination Insider - (Bi-Weekly) ViewSample

CodeWatch - (Bi-Weekly) View SampleG-Biz - (Varies) View SampleElectrical Testing - (Monthly) ViewSample

Enter your email above to receive messages aboutofferings by Penton, its brands, affiliates and/orthird-party partners, consistent with Pentons PrivacyPolicy.

Connect With Us

Open wound

The standard dry-type transformer follows a dip-and-bake construction method.

This is accomplished by preheating the conductor coils and then, when heated,

dipping them in varnish at an elevated temperature. The coils are then baked to

cure the varnish (Photo 2).

Vacuum pressure impregnation (VPI)

This technique applies the varnish coating in interchanging cycles of pressure and

vacuum. The VPI process uses polyester resin. The coils are then cured in an oven.

The VPI process is better than the standard dry-type transformer because it

includes pressure in addition to vacuum. This process allows better penetration of

the varnish in the transformer coil. These units offer an increased resistance to

corona.

Vacuum pressure encapsulated (VPE)

This method is usually superior to the VPI process. Several dip processes are added

to the construction process to encapsulate the coil assembly after which the

coatings are cured in the oven. These transformers feature better protection from

harsh and wet environments than their VPI type counterparts (Photo 1).

Encapsulated (sealed)

Encapsulated transformers are standard open-wound distribution transformers

encased in an electrical-grade silica and epoxy, and totally enclosed in a heavy-duty

style enclosure.

Cast coil (molded epoxy sealing)

These units incorporate coils encapsulated in epoxy by a molding process. The

transformer coils are solidly cast in resin under a vacuum in a mold. The

manufacturing process locks the windings in a strong epoxy resin with high

dielectric strength, protecting the transformer from severe operating environments

(Figure).


3 of 5 6/7/2015 10:56 PM

conductors(13).

Each of the winding insulations noted above is specifically suited for particular

environments. Therefore, it's important for you to understand where best to use each

type. For example, it costs approximately 50% more to specify a cast coil transformer over

a VPE or VPI type unit. Therefore, your final selection can have a significant impact on

the overall cost of a project.

When improved resistance to corona (i.e., electrical discharges caused by the field

intensity exceeding the dielectric strength of the insulation) and increased mechanical

strength of the windings are required, you should use a VPI-type transformer.

Use a cast coil transformer when additional coil strength and protection is needed, such

as in unforgiving environments like chemical process plants, construction material

factories, and for outdoor installations. These types of harsh environments include

substances that can devastate the windings of other dry-type transformers, including salt,

dust, caustic gases, moisture, and metal particles. Additionally, cast coil transformers

have improved abilities to withstand heavy electrical surges, such as recurring but short-

duration overloads experienced by transformers serving industrial processes. Cast coil

transformers typically have the same increased levels of BIL as oil-filled transformers,

while still providing ample protection of the transformer coils. Cast coil transformers are

also being used in some harsh locations that were previously thought best served by

liquid-filled transformers.

The engineer has a choice between a VPI/VPE transformer or cast coil transformer for

critical environments and harsh environments. The cast coil is typically accepted as the

best transformer in these types of environments. Some manufacturers, however, indicate

that the cast resin in the cast coil transformers can be detrimental to the life of the

transformer. The coefficient of expansion of the epoxy is less than the copper windings.

The cyclical expansion and contraction with the heating and cooling of the coils can

eventually crack the resin. These manufacturers indicate that the epoxy encapsulation in

the VPI transformer can better deal with this expansion and contraction; therefore, if

offers longer life. In the end, the final choice is left to the design engineer.

In addition to the capacities noted earlier, the electrical design engineer can specify a

fan-assisted transformer that can add significant capacity to the rating of the transformer.

For a cast coil transformer, the addition of the fans to cool the transformer during heavy

loading can add another 50% to the rating of the transformer. For a VPE or VPI

transformer, an additional 33% kVA capacity can be added to the rating of the

transformer. For instance, a standard cast coil dry-type transformer rated for 3,000kVA

can have a second rating when fan assisted to 4,500kVA (a 50% increase in the standard

rating). On the other hand, a VPE or VPI transformer rated 2,500kVA can have a second

rating when fan assisted to 3,333kVA (a 33% increase in the standard rating).

Remember: When using the fan-assist rating of a transformer, you are relying on moving

parts to supply the rated capacity. For critical installations, you may only want to rely on

the fan-assist rating for unusual overloads that may occur under certain operations,

including:

For critical environments where uptime is essential, a system that relies on the

fan-assisted rating for normal operation may reduce overall system reliability.

The electrical design engineer must always evaluate cost implications and system

reliability when designing electrical distribution systems. This type of evaluation becomes

even more important in critical environments. The transformer is only one component in

complex electrical distribution systems. As you can see, the design engineer must be

armed with the most accurate information in order to make the proper transformer

selection.

Lane is a registered professional engineer, licensed electrical administrator, RCDD/NTS

specialist, TPM, LC, LEED A.P. and serves as director/vice president engineering for

The extra current required to recharge UPS batteries after the UPS system has been

used during a power outage before the generators begin to come online.

1.

Planned maintenance bypass that placed a short-term condition of extra load on a

specific transformer.

2.

Other short-term overloads of the electrical distribution system.3.


4 of 5 6/7/2015 10:56 PM

Site FeaturesSite FeaturesSite FeaturesSite Features

Author Guidelines

RSS

Sitemap

Site Archive

Subscribe

View Mobile Site

EC&M CorporateEC&M CorporateEC&M CorporateEC&M Corporate

Privacy Policy

Terms of Service

About Us

Advertise

Contact Us

Follow UsFollow UsFollow UsFollow Us

Search ecmweb.comSearch ecmweb.comSearch ecmweb.comSearch ecmweb.com

Subscribe to the print magazineSubscribe to the print magazineSubscribe to the print magazineSubscribe to the print magazine

Electrical Wholesaling Electrical Marketing Transmission & Distribution World

Copyright 2015 Penton

Ecmweb.comEcmweb.comEcmweb.comEcmweb.com

NEC Design Ops & Maintenance Contractor Safety Power Quality Training Basics Products

Related EC&M SitesRelated EC&M SitesRelated EC&M SitesRelated EC&M Sites

Advertisement

Tweet

SASCO Design/Build Electrical Contractors and Consultants in Cerritos, Calif., San

Francisco, and Woodinville, Wash.

Please Log In or Register to post comments.

Related ArticlesRelated ArticlesRelated ArticlesRelated Articles

Mounting a Dry-Type Transformer

How to Select the Right Energy-Efficient Dry Type Transformer

Unconventional Transformer Connections Part 3

Using Single-Phase Transformers to Create 3-Phase Systems

Transformer Secondary Conductors 5

3RecommendRecommend


5 of 5 6/7/2015 10:56 PM

Documents

The Basics of Large Dry-Type Transformers _ Content Content From Electrical Construction & Maintenance (EC&M) Magazine