Instantaneus ground setting

[jairo]

If I set the ground tap = 0.3 I phases and the instantaneous is set to 30 times but the max setting at the relay is 20 times.
Is a good practice set the ground tap in a bigger value (0.6 In ) and then set the instantaneous to 15 times for use the instantaneous setting?

Thanks

Jairo

 

[RRaghunath]

Jairo,

0.3pu setting itself is high for earth fault protection.

What is the expected maximum earth fault current and

what is the CT ratio / any CBCT?

Typically, the earth fault protection (Time delayed) should be set at 10% of the expected max. current magnitude. Instantaneous element interferes with downstream protection coordination and hence is not used unless it is a motor feeder or transformer feeder with delta connected primary.

Hope the above is helpful.

 

[RalphChristie]

Agree with rraghunath's comments.

What kind of earthing method do you use? This will determine what magnitude of earth fault-current you can expect during an earth fault.
An instantaneous element especially for earth fault protection are normally responsible for a lot of nuisance trips and makes grading almost impossible. I would rather use a IDMT-curve or a time-delayed setting for earth protection.

If you have to use the instantaneous element:

What kind (electromagnetic, electronic etc.) and brand (CDG, ABB, Reyrolle etc) of relay do you use?
For some of the relays the inst. setting is independant of the O/C and E/F-setting (most relays I've worked with and actually all the electromechanical relays I've seen), but rather dependant on the nominal current. (1A or 5A) In this case you'll have to change your CTs to a bigger size.
In some cases (like the Siemens 7SJ50) the inst. setting is dependant on the O/C and E/F setting and then you can do what you have described.
But, like rraghunath pointed out, an ideal E/F-setting would be at 10%, in my opinion 60% is much to high.

Regards
Ralph