Available Fault MVA from separate source

[hidalgoe]

Consider: Two separate 69kV lines feeding two 12.47kV lineups through 20MVA transformers. The lineups are connected through a N.O. 1200A tie breaker. The fault duty at one lineup is 100MVA and 105MVA at the other lineup. If conditions on one line are inadequate the load will be transferred to the other line in a closed transition process.

What is the fault duty at the interconnecting breaker during the closed transition process when the tie breaker is closed?

 

[jwerthman]

If the two 69-kV lines are essentially decoupled from each other (which they probably are not), then the fault MVA with the bus tie closed will be approximately the sum of the individual contributions. If the 69-kV lines cannot be considered decoupled, then the total fault MVA will be less than the sum and you need to know more about the line impedances and the equivalent impedance ahead of the lines. In any case, the fault MVA with the bus tie closed will never exceed the sum of the individual contributions.

 

[peterb]

Agreed, but note that the sum of the two fault levels is NOT the current that the tie breaker will see. The feeder breakers will be subject to the summated fault level, but the tie breaker will only ever see the larger of the two contributions.

 

[brupp]

True that the tie breaker will only see the larger of the two fault contributions as a maximum. However, any feeder from that bus will see the sum (or nearly so, as described above). This is a common problem and is usually ignored if the closed transition is for a very short duration. I prefer to either make the transition automatic so that it will be limited to .25 seconds or less or, if manual, only give the operator 10 or 15 seconds to complete the operation before automatically tripping the tie breaker again. The reasoning is that the chance of a downstream fault during this switching operation is quite remote.