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Fault Current & Instantaneous Protection setting

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Would someone explain how the (impedance/Rating) of the generating system effects the way a bus can be connected(two transformers feeding two busses in parallel for redundancy of Primary supply)? I always assumed that the fault Current rating of the bus (being supplied by the feeder/transformer) as well as the impedance of the transformer was what decided the “Instantaneous Protection Setting “of the Transformer/Feeder. Also would you suggest a text books/Internet sites which give me some more info on the philosophy of Protection & Discrimination.
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  • 1 year later...

It all depend on your protection philosophy. If you are talking about O/C protection:


Instantaneous setting = immediate operation


If you want to coordinate, you'll have to allow downsteam breakers to clear the fault before the upstream breakers operate. Normally we use instantaeous settings at the primary side of a transformer. The setting is 125% of the maximum through-fault current. If there is a fault in the secondary side of the trsf, your secondary side have to take it out before the primary side take action. (Primary side use curve) But if the fault-current is higher than the through-fault current, the fault have to be on the primary side, and the primary breaker can operate imediatelly. The same philosophy can be used on long feeders.




Network Protection & Automation Guide (downloadable) at the website of Areva:




(Copy and paste whole address)





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