Is Calculating SIR Using Only Positive Sequence Components Justifiable?

[Luichip]

I would like to get opinions when calculating Source Impedance Ratio (SIR) on a Power System.

Usually when calculating a three Phase fault it only takes into account positive sequence components, obtaining the source impedance ratio via the ratio of the source positive sequence component divided by the positive sequence component of the protected line.

Also, when calculating a single line to ground fault, we take all the sequence components of the source and add them together, and divide this by the SUM of the positive, negative and Zero sequence impendances of the line being evaluated.

The calculated fault location is at the terminal were the relay is evaluated, and does not consider the line in service so that the Thevelin equivalent impedance calculated would be from the system only.

I see that performing a single line to ground fault and breaking down into sequence components and getting the SIR for that, is very different that for a three phase fault that only have positive sequence components.

My QUESTION is: Is there a benefit or assumption that would justify to calculate the SIR only on based only on Positive sequence components ? since I consider is ignoring things/effects like grounded impedance, mutual couplings, etc.

Thanks,

 

[anorlunda]

The art and science of protective relaying is a broad subject.

You are not clear as to your intent. Is it a distance relay? Is it local protection? Your question can not be answered without knowing your purpose.