I am confused 1

[devilskm]

The equation for calculating the time delay for NI type curve of any relay is
T = AD / ((I/Ipu )^B – 1)
And we know that for NI, we have A=0.14 and B= 0.02.
Suppose I am using this curve for any Electrical Load having FLA of 100A. Setting the pick-up at 120% of FLA, Ipick-up will be 120A.
From where i will get the value of "I"? I am doing the study on SKM, should I consider the Busbar 3P fault as "I" for all the loads connected to this switchgear or it shall be the SLG fault on that particular busbar or anything else. I am using SKM for the study. Let me know which value of “I” shall i choose.

 

[ScottyUK]

I is the value of current at the point in circuit where the measuring CT(s) is/are located. The magnitude of the current depends on circuit conditions, e.g. overload, short-circuit, earth fault. etc.

Quite honestly it sounds like you would be better spending some time understanding the fundamentals of protection schemes before diving in to modelling them in SKM. Software, good as it is, will still produce nonsense results if you feed it the wrong information. There are some great references for protection in David Beach's faq238-1287 - once you achieve a sound understanding of the principles then you are in a position to start modelling protection schemes.


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If we learn from our mistakes I'm getting a great education!

 

[devilskm]

Thanks Scotty, I am working on your advise.

So, which value of I shall be taken for ANSI function 51, 3P fault at that point or SLG fault at that point?

 

[ScottyUK]

You should set your protection pick-ups as low as possible consistent with ensuring that nuisance tripping is minimised. Obviously you want your instantaneous protection to pick up for a full 3-phase short but in most instances it should be set much much lower than that. The setting depends on your loads and your source.

ANSI code 51 is overcurrent, so the setting is determined by the rating of whatever the relay is protecting, e.g. a busbar, a cable, a transformer, etc.

Seriously, stop tinkering with SKM for now and learn how protection works. The last thing anyone in the field needs is a relay setting schedule which is based on incorrect assumptions and bad data. That's how equipment explodes and people get killed. Think about it.


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

Also see thread238-281559
Eventually leads to the same thing Scotty posted.

Rafiq Bulsara

http://www.srengineersct.com