Circuit switcher fault interrupting

[Mbrooke]

How is the primary interrupting capacity determined when clearing secondary faults? My understanding is the normal 25,000 amp value does not apply like it would for say a primary bushing flash over.


See page three:

http://www.sandc.com/edocs_pdfs/EDOC_001825.pdf

 

[jghrist]

Doesn't the page you referenced answer your question? If a secondary fault with an infinite primary system bus exceeds 4,000 A on the primary side, then it exceeds the interrupting rating. For instance, the primary fault current on a 15 MVA base rated 8% impedance, 115 kV transformer is 941 A. It is within the rating. The fault current for a 100 MVA, 8% impedance, 115 kV transformer is 6,276 A which exceeds the 4,000 A rating.

 

[Mbrooke]

I guess I am wondering why the interrupting rating is low for a secondary fault yet so high for a primary fault?

 

[jghrist]

The higher X/R ratio causes higher transient recovery voltage (TRV).

 

[Mbrooke]

That would make sense then. Does S&C's equation apply to all faults (like L-G) as long as the true primary current does not exceed 4000 amps?

 

[jghrist]

Note that the limit is not on actual fault current. It is on the current that would be in the primary for a 3Ø secondary fault with no system impedance (infinite source). A single phase fault will result in equal or lower primary fault current (depending on the transformer connection) so will not be the worst case.

 

[Mbrooke]

Connection is delta wye grounded.

Would parallel operation of the secondary have any effect on interpreting?