the basis of using differential relay instead of distance relay in short transmission lines

[zaboli]

Hi everybody
I would be thankful if anyone explain me about the scientific or standard reference about using of differential relay instead of distance relay in short lines.some specialists believe that SIR>>>5 (approximately 10) is the basis of selecting 87 instead of 21.I would be grateful that someone explain me that if there is any standard,reliable reference or even an essay about this.
best regards

 

[cranky108]

I would use a test of the minimum omic reach of the relay, and the the calculation/CT/PT error of the circuit as a basis for the decision to use distance, or differential.

Why make some kind of rule like the SIR?

So what text book did the SIR>>>5 come from?

 

[HamburgerHelper]

Here is an SEL paper covering talking about this. When I have seen differential specified, it was due to like cranky said, the line was too short to be confident in the reach or the line was short enough that running fiber for differential wasn't very expensive. A high SIR, i suppose is the exact same as week infeed.

https://cdn.selinc.com/assets/Liter...6661_TutorialCalculating_MT_20150210_Web2.pdf

 

[HamburgerHelper]

I suppose SIR is more so a measure of how the relay voltage will drop and how fault currents will change along depending on the fault's location on the line. So, I guess it is different than week infeed. High SIRs indicate that there will be a substantial voltage drop and the fault current won't change much wherever it is on the line.

 

[crshears]

For us relaying/protection newbies, what's SIR? I've only been operating power systems for a few years [32], and I've never heard the term before.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[mgtrp]

It's the source impedance ratio, which is the ratio of the impedance of the source to the impedance of the line. I'm sure a quick bit of googling will fill in the rest

 

[davidbeach]

Source Impedance Ratio. The ratio of the source impedance to the line impedance. The higher the ratio, the shorter the line, at least electrically. With a very high SIR, essentially all of the voltage drop to the fault occurs on the source side of the relay and there is very little change is fault current from one end of the line to the other. For an SIR of 1, the voltage drop at the relay is only half the prefault voltage and there is considerable variation in fault current over the length of the line.