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Fault clearing time?
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Well, that's a rather general question, to which I can only give a general answer....
Primary protective devices should operate as quickly as possible, with backup devices picking up shortly thereafter (allowing time for primary protection to pick up first so as not to trip out more of your system than necessary to isolate the fault).
If you can provide more details about your specific system, I'm sure that I (or other forum members) can provide a more specific answer.
Primary protective devices should operate as quickly as possible, with backup devices picking up shortly thereafter (allowing time for primary protection to pick up first so as not to trip out more of your system than necessary to isolate the fault).
If you can provide more details about your specific system, I'm sure that I (or other forum members) can provide a more specific answer.
Assuming there is no generation on your system, the general answer is as follows -
- Primary protection will/should operate instantaneously, typically somewhere around say 20 mS
- Backup protection will be time delayed to coordinate with downstream protection; typically, this could be of the order of say 300-400 mS to give an adequate grading margin.
The time delayed protection must be set to protect the downstream equipment. Transformers and cables have thermal withstand rating curves, and the protection must operate faster than these curves while operating slower than the downstream primary protection.
If there is generation involved, then there are additional concerns re stability and critical clearing time. At this moment, I am working on a stability study for a client's 69/13.8 kV system and the "right" answer for them is 340 mS critical clearing time for 13.8 kV faults - any longer fault duration will cause the generators to become unstable relative to the utility system.
- Primary protection will/should operate instantaneously, typically somewhere around say 20 mS
- Backup protection will be time delayed to coordinate with downstream protection; typically, this could be of the order of say 300-400 mS to give an adequate grading margin.
The time delayed protection must be set to protect the downstream equipment. Transformers and cables have thermal withstand rating curves, and the protection must operate faster than these curves while operating slower than the downstream primary protection.
If there is generation involved, then there are additional concerns re stability and critical clearing time. At this moment, I am working on a stability study for a client's 69/13.8 kV system and the "right" answer for them is 340 mS critical clearing time for 13.8 kV faults - any longer fault duration will cause the generators to become unstable relative to the utility system.
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