Dual Interrupting Ratings of 115kV Mark IV Circuit Switchers
Dual Interrupting Ratings of 115kV Mark IV Circuit Switchers
(OP)
A couple questions for Substation design folks, but first some background: I am performing the P&C design for a brownfield 115/12.47kV Sub (in fact, it is a complete redesign, all existing relays are getting scrapped). The 115kV part of the station has: two busses using a S&C Mark IV as a tie; Bus 1 connects to a Utility Line (protected by a breaker), an outgoing line (protected by a breaker), and a 115/12.47kV transformer (protected on the high side by another Mark IV, and a breaker on the low side); Bus 2 connects to another Utility and another Outgoing Line (both protected by breakers). Among the P&C schemes being implemented: the transformer has a differential, and both Bus 1&2 are treated as a single bus and are protected by the same bus differential scheme.
The Mark IV's have a 3000/7000 A interrupting rating. The 7000 A interrupting rating, according to available S&C literature, is only effective when "the total connected length of all lines on the source side of the Circuit-Switcher is NOT less than 15 miles," otherwise the 3000 A rating should be used. According to some limited fault duty data we have received, the available fault current from Utility is about 1300 MVA (~6300A per phase) on a 3 phase fault and from any one line the phase current is above the 3000 A level on a 3 phase fault. Now the questions.
1.) What electrical phenomena contribute to these duel interrupting ratings on the Mark IV CI's? (My research has revealed several possible contributions, I'm hoping a knowledgeable person can pare that down)
2.) If the cumalitive source side line distance is not known, is their another method to determine which rating should be used for implementation of the Mark IV's?
Thanks!
The Mark IV's have a 3000/7000 A interrupting rating. The 7000 A interrupting rating, according to available S&C literature, is only effective when "the total connected length of all lines on the source side of the Circuit-Switcher is NOT less than 15 miles," otherwise the 3000 A rating should be used. According to some limited fault duty data we have received, the available fault current from Utility is about 1300 MVA (~6300A per phase) on a 3 phase fault and from any one line the phase current is above the 3000 A level on a 3 phase fault. Now the questions.
1.) What electrical phenomena contribute to these duel interrupting ratings on the Mark IV CI's? (My research has revealed several possible contributions, I'm hoping a knowledgeable person can pare that down)
2.) If the cumalitive source side line distance is not known, is their another method to determine which rating should be used for implementation of the Mark IV's?
Thanks!





