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Transf ormer windings forces due to the magnetic f orces (photo by ALSTOM)

http ://ele ctrical- eng ine ering -p orta l.co m/wh en- tra nsfo rmer- has- a- stomach -p ain -a nd- wa nts- to Ma y 13, 2013

When a transformer has stomach pain and wants to

Edvard

About stomach pain

During normal operation, transformer internal structures and windings are subjected tomechanical forces due to the magnetic forces. These forces are illustrated in Figure 1.

By designing the internal structure very strong to withstand these forces over a long period of

t ime, service life can be extended. However, in a large transformer during a through fault (faultcurrent passing through a transformer), forces can reach millions of pounds, pulling the coils upand down and pulling them apart 60/50 times per second.

Notice in Figure 1 that the internal low-voltage coilis being pulled downward, while the high-voltage windingis pulled up, in the opposite direction.

At the same t ime, the right-hand part of the figure shows that the high- and low-vo ltage coilsare being forced apart.

http://electrical-engineering-portal.com/when-transformer-has-a-stomach-pain-and-wants-tohttp://electrical-engineering-portal.com/power-transformer-construction-windingshttp://electrical-engineering-portal.com/power-transformer-construction-corehttp://electrical-engineering-portal.com/when-transformer-has-a-stomach-pain-and-wants-to

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Figure 1 - Transf ormer Internal Forces

Keepin mindthattheseforcesare

reversing 50/60 times each second. It is obvious why internal structures of t ransformers mustbe built incredibly strong.

Many times, if fault currents are high, these forces can rip a transformer apartand causeelectrical faults inside the transformeritself . This normally results in arcing inside thetransformer that can result in explosive failure of the tank, throwing flaming oil over a wide area.

There areprotective relaying systems to protect against this possibility, although explosivefailures do occur occasionally.

How to prevent pain

Through Fault Short Circuit withstand considerations

The windings are subject to bot h radial and axial forces related to t he current and f luxinteractions. Radial forces in the inner winding (normally the LV winding) are in compressionwhile the out er winding (normally the HV winding) fo rces are in tension.

Design of the windings and bracingmust consider the magnitude of these forces andprovide adequate strength to withstand t hem without signif icant mechanical deformationwhich could result in a dielectric failure.

The picture below is an example ofa free bucking mechanical failure of an inner windingresulting from radial fo rces in compression on the winding. Note, even though there ismechanical failure, there wasnt a dielectric failure of this winding.

http://electrical-engineering-portal.com/transformer-winding-faults

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Free bucking mechanical failure of an inner winding resulting f rom radial f orces in compress ionon the winding

Flux

f ields are dependent of the balance of the ampere turn distribution of the HV and LV windings.When the ampere turns of the HV and LV windings are equal and balanced, the only forces areradial.

DETC taps (De Energized Tap Changer) in the HV windings and LTC(Load Tap Changer)operat ion result in changes in the ampere turn distribution result ing in axial forces. If the HV andLV windings are not aligned axially or one winding is physically shorter than the other, ampereturn balance is significantly af fected and axial forces are magnified.

http://electrical-engineering-portal.com/using-tap-changers-to-match-the-system-voltagehttp://electrical-engineering-portal.com/importance-of-transformer-inrush-current

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Transf omer core fo rce and f lux

Autotransformers, low impedance, motor starting duty,transformers with multiple voltages by reconnecting thetransformer windings in series and parallel configurations,three winding transformers with dual secondary windingsfor start up or unit auxiliary service at power plants allcan result in increased axial and radial forces during ashort circuit and require special consideration.

Construction of power transformer (VIDEO)

Cant see this video? Clickhere to watch it on Youtube.

Primary and assembly process (VIDEO)

Cant see this video? Clickhere to watch it on Youtube.

References:

Presentation on Short-circuit Forces by H. Jin Sim, CTO of SPX-Waukesha

Transformers Basics, Maintenance and Diagnostics U.S. Dpt. of the Interior Bureau ofReclamation

The Design and Performance of Circular Disc, Helical and Layer Windings for PowerTransformer Applications - David L. Harris, P.E.

EEP recommends

http://www.youtube.com/watch?v=LslbnRbemtIhttp://www.youtube.com/watch?v=JXTg6z4JgA0http://electrical-engineering-portal.com/autotransformer-connection-explained