Subtransient Reactance
- Thread starter mull982
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- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
The term is used in the context of analyzing a fault in a system that provides power to large motors. The analysis is essentially nothing fancier than an application of Ohm?s Law, though the mathematics is more complicated than most of us will ever use. The time period of interest is immediately after the fault, when the system responds to a sudden increase of current (i.e., flowing through the fault point), and before the motors have a chance to start slowing down. During that time, the system will respond as though the motor had a value of resistance (reactance, actually) that is different that you see while the motor is running, and different than you see several seconds later. The value of sub-transient reactance will influence the total amount of current that the motor will contribute to the fault point.
I do not remember the exact time period of interest, but it is much less than one second.
I have no information on typical values for any particular motors.
Someone else will have to take up the answer from here.
I do not remember the exact time period of interest, but it is much less than one second.
I have no information on typical values for any particular motors.
Someone else will have to take up the answer from here.
Here is my take. I have no references to back this up. But it sounds good.
When analyzing a system for fault current, one starts with a bolted fault, V phase = 0V.
All spinning motors on the line have stored energy in the rotating mass. The motor stator is still excited and acts an induction alternator, dumping the rotating energy to the fault.
I'm a bit surprised you have seen references for typical large motors being 16.7% This translates to a large motor contribution to a fault as being 6 X FLA. Most fault calcs I have seen show large motor contribution as 4 X FLA. This should translate to a subtransient reactance of 25%.
My limited understanding is charlie b is correct on the time duration.
I've now told you more than I know. It's time for me to stop.
I'm hoping some of our whizes will jump in and do a little education. This one is interesting.
carl
When analyzing a system for fault current, one starts with a bolted fault, V phase = 0V.
All spinning motors on the line have stored energy in the rotating mass. The motor stator is still excited and acts an induction alternator, dumping the rotating energy to the fault.
I'm a bit surprised you have seen references for typical large motors being 16.7% This translates to a large motor contribution to a fault as being 6 X FLA. Most fault calcs I have seen show large motor contribution as 4 X FLA. This should translate to a subtransient reactance of 25%.
My limited understanding is charlie b is correct on the time duration.
I've now told you more than I know. It's time for me to stop.
I'm hoping some of our whizes will jump in and do a little education. This one is interesting.
carl
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
QUICK! Copyright that statement, before some Inspector starts using it as an official motto! :grin:coulter said:I have no references to back this up. But it sounds good.
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