Sum of currents in protection relay

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In a differential protection relay there is a setting of a threshold for the sum of amps (or the squares). In what way would that be of interest? The relay is an Areva P521.
Also, the relay can display the ratio between the positive sequence current and the negative. What information does that give me?
 

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Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
 
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In a differential protection relay there is a setting of a threshold for the sum of amps (or the squares). In what way would that be of interest? The relay is an Areva P521.
Don't know of characteristics of this one but threshold settings are of particular importance for relays in longitudal differential protection where relays are connected to secondaries of CTs (Block diagram) . Threshold setting corresponding to range 10-30% of a nominal current usually offer sufficient protection level and selectivity for all sorts of faults. But setting the threshold too low, say 0.05In or less, would require almost perfect adjustment of CTs and may cause undesired relay activation and tripping when faults occur elsewhere in the network.
Also, the relay can display the ratio between the positive sequence current and the negative. What information does that give me?
Level of unbalance in the system (which doesn't necessarily mean fault)
 

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