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Hi gents!


We are recently working on a substation having 132/11kV system with 36MVA transformer (Y-Y with treasery winding). There are differential, O/C, U/V, O/V, E/F and all other transformer protections is there with individual protection for different feeders (S/C, E/F, O/C).


I have following quires:


1. What is the difference b/w unrestricted and restricted earth fault protection, how they differ and what are there selecting criteria according to application, which one provided more accurate and complete protection?

2. What is the setting procedure for differential protection?

3. Selection criteria of protection relays setting i.e. O/V, U/V, O/C and S/C.

4. What is directional O/C relay, how they differ from no directional relay, is there any advantage for using directional relay?

5. Plz info me any web site given me details about protection relays (cables feeder protection, …)?

6. Is any one having info about E-Tap software?



Best regards


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  • 1 month later...
  • 2 months later...

I would suggest that you visit following sites


http://www.geindustrial.com/pm/notes/artsci/ - go to transformer protection


http://www.ee.polyu.edu.hk/staff/eekkli/BE.../Ch3_OC2003.pdf - very good - about relaying




For last one you have to register (free) - then go to NPAG relay technology


It is a lot of places - but there is good information


Good luck


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  • 5 months later...

Hello tahira,


1. Unrestricted earth fault is used for feeder protection,not for limited zone & less sensitive. Restricted earth fault is used for transformer protection ,it is also called ground diffrential & limited to zone , not will trip beyond the zone.

Restricted is more accurate protection.

2. Setting procedure of diffrential depends on both side Pri/Sec current & CT ratio ( Suppose in case of transformer Delta to Star)

Star side I = 1600 A & CT is 2000/1

Delta side I = 52 A & CT is 75/1

1600/2000=base current

52/75=base current

those settings intered into relay includind system parameters

actually diffrential relay sense ratio/deffrence between both side in term of % according to settings + error in base current ratio, in this relay no any time setting is required it take action on immediate effect as sense diffrence.

3. Selection criteria of protection relays depends system designe/load/criticality /as per current carrying capacity of equipment/short ckt values/i.e down stream side settings should be lower than up stream side.

4. Directional relay is better than withour directional relay it woks/sense through one direction only.

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