Source Impedance Ratio (SIR) Calculation

[spectrohelioscope]

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

Usually I have seen a three Phase fault** and only taken into account the positive sequence components of the Source and divide this by the positive sequence component of the protected line.

Also, I have seen calculating a single line to ground fault**, and taken 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 fault is at the terminal were the relay is being evaluated, and the fault does not consider the line in service so that the Thevelin equivalent impedance with the voltage source is only from the System.


Now, 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,

Luichip

 

[521AB]

The new specification IEC 60255 for line distance protection will define SIR. For phase-ground faults you need, to my opinion, to consider also the earth-return impedance, which is function of positive sequence impedance and zero sequence impedance. It is common to assume negative sequence impedance = positive sequence impedance. You will also see that by simply putting the three sequence components circuits in series.