Hey guys, if i have 2 relay ANSI 50/51 with different curves, how can i gain selectivity if they cross at a point for a certain current value?

i was thinking that its possible to gain selectivity by adjusting the instantaneous units beforethe cross. Or it is impossible to use different curves in coordination, what i mean is, it is obligatory to use the same curve in coordination always.

any idea??

 

What are you going on about? A picture is worth several thousand words.

 

for example, in this image the SI curve cross with with the EI curve at a certain current value
if i use instant units before the cross between the curves, can i gain selectivity?? or it is obligatory to use the same curve for all the relays in the coordination procedure

hope with the pic is better to understand what i'm trying to ask

 

Sorry, the image is too small to read and extract useful information.
EDIT: I was able to download and expand the image.
I'm not sure I understand what the chart is showing. What kind of relays are these?
What is the meaning of selectivity in this context?
Also, what is a coordination procedure?

 

all right, those are relays for power systems. Those relays have the ability to depending the topology of the power system, trigger instantanously or with time delay (i can configure how i need it depending on a certain criteria). So, the coordination procedure is to gain selectivity, that refers to isolate an area with a fault. You can think that a relay has the trigger control of a breaker in a substation.

Is it more clear??

 

It is only sort of clear. Certainly in a power distribution system the Circuit breakers (relays) closest to a fault should operate first, so that a small fault does not darken a large area. I had this exact discussion with my local power company 2 years ago and they made some adjustments and as a result there were far fewer large-sector blackouts ever since. So definitely having the fastest response in the protection closest to the faults is the way to go. That would be those breakers farthest towards the loads in a grid arrangement. I hope this explanation makes sense.