Function/usefullness of transfomer interleaving shield?

[ahandle]

Hi,

Our standard transformer specification requires "An earthed, interleaving shield shall be installed between the primary and secondary windings" for electrostatic reasons. A vendor has responded that they normally do not apply this to two winding transformers and to comply would require major changes to the original design.

Is anyone able to please provide their view on the value of interleaving shields? The transformer in question is a 10MVA 33kV/3.3kV Dyn11 transformer.

Many thanks in advance.

 

[Marmite]

Interleaved shields are a method of improving the impulse level by reducing the capacitance of the winding to earth. There other methods such as line end and dummy strand, which are cheaper and more practicable. A designer would normally try to minimise interleaving.
I'm not sure why you would want to tie the hands of the designer with such a prescriptive specification. I would specify the required electrical parameters including system BIL, and the specification you want it built/tested to such as IEC, IEEE etc. Leave the detailed design upto the vendor.
Regards
Marmite

 

[ScottyUK]

Might be an engineer's way of steering the job toward a specific preferred vendor without overtly doing so.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[dpc]

I've only seen electrostatic shields on small dry-type transformers used for control power. Never on a 10 MVA unit.



David Castor

http://www.cvoes.com

 

[jimbofitz]

Do you specify a shielding ratio? Some end-users like to filter out swtiching and other HF noise through a shield.

 

[prc]

Marmite,the shield you are referring to is diffenent.Transformer engineers used to provide such an earth shield outside the winding, connected to line voltage to get better voltage distribution in transformer windings.This was the practice 60 years back to reduce winding capacitance to earth.Invention of interleaving the disc winding by English Electric in late1940's changed the scenario and interleaved windings or coupling shield ( both for increasing the series capacitance of winding and nothing to do with earth capacitance)became the norm for HV and EHV transformers.

The shield ahandle referring to is the earthing shield (isolating shield)between HV and LV winding to avoid transfer of harmonics anf HF noise from the connected load to the electric source.This used to be required for transformers feeding to thyristor drives etc.May be Skosgura can guide us further on the topic.

 

[pie314]

Ahandle -- This is sometimes called an "Electrostatic shield". and is usually required when the secondary winding is subjected to arcing. For instance most precipitation transformers require this shield. It is usually a 10 mill copper sheet placed over the secondary winding and then grounded. ( Of course properly insulated)

 

[prc]

pie314, What is the precipitation transformer ? Where it is used ?Typical rating and details.

 

[pie314]

Precipitation transformers are typically less than 100 KVA and are Rectifier transformers used to remove smoke particles.
Transformers used to test other transformers sometimes require a shield also.

 

[rcw retired EE]

"Precipitation transformers" = electrostatic precipitator transformer. Output is high voltage DC after the rectifier.

 

[waross]

Some small (less than 25KVA) transformers for isolated circuits in a hospital operating room require a grounded shield between the primary and secondary winding to prevent primary voltage from being applied to the secondary in the event of a transformer failure. These are often supplied as part of a panel with leakage monitoring and alarms included.


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

 

[prc]

Thank you pie 314, wilson and waross ! I used to call them ESP transformers.