Use of Autotransformer in Transmission Station 1

[rockman7892]

I have been involved in a few transmission station projects recently where an Autotransformer is used to step voltage between sub-transmission (or distribution) and transmission portion of substation. An example is use of a 33MVA Autotransformer to step between 46kV sub transmission and 115kV transmission.

Autotransformer is listed as 115kV GND Y - 46kV GND Y but also appears to have a delta tertiary winding. Is there a reason for use of this tertiary winding in most applications?

Is there a particular reason why Autotransformers are used as opposed to traditional 2-winding transformers for these transmission applications? Assuming it has to do with lower cost of Autotransformer? Is there a reason for use of this tertiary winding in most applications?

Thanks

 

[cranky108]

The tertiary delta winding is for stability during ground faults, and to handle harmonics.
An auto transformer usually cost less than a two winding transformer, and is smaller, because it only has one power winding.
Most companies don't use the tertiary for customer load, but they may use them for station power, or adding reactors, but usually at higher voltages.

 

[xnuke]

The delta winding is a stabilizing winding: Link

Autotransformers are less expensive due to the power gain associated with their turns ratio. On a much smaller scale, I've used two 1 kVA boost transformers in open delta to serve up to 16.5 kVA of load.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[prc]

1) For tertiary winding- Please see javascriptpenindex(450,450,'

http://www.tipmaster.com/includes/refinfo.cfm?w=450&h=450').

At the end, I had enclosed a tutorial paper.
2) Auto-transformer connection becomes economical when the primary to secondary voltage ratio is three or less. As the ratio goes down savings in price and losses as compared to an equivalent two-winding transformer increase. Ideally, auto connection is used only up to a voltage ratio of two.

 

[prc]

sorry, the link is thread238-437739