Deconnecting H0 on a Yg-Yg transformer 4

[JPGQC]

Hi all,

Anyone knows a good reference that can explain what would happen to fault current and 3rd harmonic levels if the H0 of a Yg-Yg transformer is deconnected?

 

[dpc]

Disconnected.

Standby for waross. In the meantime:

http://www.geindustrial.com/publibrary/checkout/White Papers|GET-3388B|generic

 

[jghrist]

With H0 disconnected, there is no zero-sequence path, so the secondary has no ground source, except through a phantom delta on some core types. There will be no significant Ø-N fault current or balanced 3rd harmonic currents. See thread238-257503

 

[cranky108]

Some transformers are not insulated near the H0 bushing to handle the voltage shift of a phase to ground fault. The result could be a flash over under the oil from the winding/bushing/leads to the transformer shell.

Need to look at this very carefully.

 

[alehman]

Why would you want to do that?

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[waross]

dpc;
Thanks for that reference. Nothing left for me to say. Another star.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JPGQC]

Thanks everyone

Alehman: Why do I ask? Because someone here was looking for some reference on that topic. He has a client wanting to find the cheapest way to deal with their Yg-Yg transformers, since the local utility doesnt accept this configuration.

Thanks again!

 

[JPGQC]

dpc: thanks for the link and sorry for the spelling mistake, but english is not my first language and
I cant find how to edit post.

Anyone?

 

[alehman]

JPGQC: My question was because what you proposed seems unusual and generally is not advisable.

This will effectively remove the earth reference for the low-voltage winding. Third harmonics are zero sequence so there will be a resultant 3f voltage that appears on the neutral.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[waross]

Consider a zig-zag transformer on either the primary or the secondary to provide the needed neutral reference point.
A wye/delta transformer or bank may also work.
A simple analogy.
Consider first a 120:240 volt panel with 120 volt resistive loads. If this panel looses the neutral connection, the voltages across each load group will be in the inverse ratio of the loads.
Now if the resistive loads are replaced with 120 volt transformers and the neutral connection is lost, saturation becomes a factor. As the voltage across the lighter loaded transformer rises to the saturation point, the current increases. The increased current due to saturation tends to limit the voltage swing.
This is further complicated by power factor and, for some three phase transformers, the phantom delta effect.
It is generally not a good idea to float the primary of a wye delta bank unless there is a zig-zag transformer or a delta connected winding somewhere in the installation to provide a neutral point.

Bill
--------------------
"Why not the best?"
Jimmy Carter