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Circuit switcher fault interrupting

Circuit switcher fault interrupting

Circuit switcher fault interrupting

(OP)
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





RE: Circuit switcher fault interrupting

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.

RE: Circuit switcher fault interrupting

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

RE: Circuit switcher fault interrupting

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

RE: Circuit switcher fault interrupting

(OP)
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?

RE: Circuit switcher fault interrupting

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.

RE: Circuit switcher fault interrupting

(OP)
Connection is delta wye grounded.

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

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