high voltage transformers 1

[ukzn]

Can someone please help. I would like to know, why most HV transformers are connected Star-Delta, either on the primary or on the secondary. Does it have to do with the fault levels, or not. Why not just connect it Star-Star.

 

[waross]

A delta primary/star secondary distributes unbalanced loads on to the supply better.
A three legged, three phase transformer core will often over heat unless it has a delta winding to accomodate third harmonic magnetic flux, and unbalanced flux due to unbalanced loading.
Some transformers have a delta tertiary winding for this reason.
respectfully

 

[dpc]

Another advantage of delta-wye configuration is that ground faults on one side are not seen as ground faults on the other side - no zero sequence current can flow through the transformer. This greatly simplifies relay coordination and improves selectivity on ground faults.

 

[tommom]

See if you can find a copy of an old GE paper - The Why's of the y's. The paper shows why y-y transformers are not generally used - although if they are shell type the 3rd harmonic problem is not present, but with core-type or three separate transformers, you end up with large 3rd harmonic voltages and bad regulation.

 

[davidbeach]

Most HV transformers are Wye-Wye since essentially all HV transformers are used in the bulk power transmission system. Distribution, at Medium Voltage, may use Delta-Wye or it may use Wye-Wye; lots and lots of three-phase Wye-Wye transformers in use between the MV distribution system and the LV services. Quite often the HV-MV transformers that separate the transmission system from the distribution system are very often Delta on the high-side and Wye on the low-side.

 

[ukzn]

thanks for the feed back guys.

 

[RalphChristie]

Some info on different transformer connections:

http://www.beckwithelectric.com/powerlines/powerlines-37.html

In addition to the previous posters:
Winding configuration: A lot depends on application and maybe in a lesser extent on previous history. Like David said, star-star transformers are still used. Another application for star-star trsfs are as station transformers in Power station auxiliary systems.

Star connections are usually preferred on the LV-side for grounding issues. To prevent zero-sequence currents flowing through the trsf, the HV-side is normally connected in delta (like stated by dpc)

On higher voltages windings (windings subjected to a voltage of 66kV and higher) are connected in star, because, in a star-connection, the winding isolation can be degraded towards the neutral point. (More Insulation = more money) In the case the secondary might be delta (preventing through-flow of zero sequence currents) or star, depending on the application.

Regards
Ralph




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