Stabilising Resistor in REF Protection

[bejoyeleengineer]

We generally provide a Stabilising Resistor in a High Impedance Type REF Protection Scheme, to de-sensitise the relay from operating from through fault conditions - in case a CT Saturates. This resistor is calculated such that under an external fault condition (that is for full fault level), the voltage developed by the un-saturated CT shall not circulate more current than the setting in the relay - thus preventing the relay from oper\ating for an external fault.

Now, my doubt is, will the relay not be de-senisistised upto this full fault level, even during an internal fault? Normally for an internal fault, the magnitude of the fault current would be much lesser than a full external fault. But, by providing the resistor, we have already de-sensitised the relay from operating even for a full fault current. How then, the relay gets sufficient current even after the resistor, during an internal fault?

 

[bejoyeleengineer]

Hello,
Is there no one who could give an answer or was not the question clear

 

[don_resqcapt19]

For the most part this is a NEC forum. There may be a few members who are engineers that can answer your question, but you may be better off posting this question at http://www.eng-tips.com in the the "electric power & transmission & distribution" forum.

 

[SG-1]

For the external fault all the CTs in the path will all see the same current. Most of which will cancel the other out.

I do not think the sum of the output of the CTs will be nearly as great as an internal fault.

In the case of an internal fault there is no CT opposing or canceling out the fault current flowing through the CT in the fault path. The CT output should be much higher.

I only see MV differential schemes. We do not use any resistors in the way you describe. The relay setting itself is used for this.
So, my answer may be all wet.

 

[SG-1]

Here is a previous thread on nearly the same topic.

http://forums.mikeholt.com/showthread.php?t=124484