Line resistance 2

[Danilo917]

Gents,

I have a line impedance of 0.9+j0.1(positive phase sequence) on a 15km transmission line, can i ignore the 0.9 resistance in the calculation of fault current using per unit method? or is this still relevant given that length of transmission line?

many thanks,
Danilo

 

[davidbeach]

Something very wrong with those numbers. For a line the X component is going to be much larger than the R component. For a cable they'll be of similar magnitude, but never with the R so much larger than the X.

When the R is so small that X and Z are essentially the same it can be easier to ignore the R when doing hand calcs, but since modern tools, such as MathCAD, handle complex numbers easily there's no reason to ignore either part.

 

[krisys]

The line mentioned by Danilo appears to be 415 V line. It is quite possible that at that voltage the resistance may be dominating the reactance.

David is right for the lines at transmission voltage level.

With the modern scientifIc calculators, it has become very easy to compute the complex numbers like
simple arithmatic operation. So now a days I stopped using per unit method.

 

[PHovnanian]

The line mentioned by Danilo appears to be 415 V line.

The "15km transmission line" probably led most of us to believe otherwise.

 

[davidbeach]

I wouldn't worry much about fault current at the end of a 15km 415V line; pretty sure that it isn't at 415V. I've never encountered any line/cable at any voltage level with a line angle that low.

 

[Danilo917]

Thanks Gents for your replies.

its 132kv line and the impedance is .9+j1.1 ohms..sorry for the error.. im doing a per unit method at the moment to initially compute the fault currents.

is it recommended to ignore the R component of the line?

regards,

Danilo

 

[davidbeach]

That is vaguely possible for a cable, but not conceivably possible for a line. If you choose to ignore the R component, you'll only be using 77.4% of the line impedance. That doesn't seem very accurate. The traditional rule of thumb of only considering X applies only to lines, not cables, and then applies for very high line angles. At 75 degrees (a repetitively low line angle) with an X of j1.1, you'd only have an R of less than 0.3; a third of your value. Given the tools at hand, I never ignore R. But I can certainly understand why my predecessors who did it all with slide rules and log tables would ignore R and treat voltage and impedance as being at 90 degree angles to each other.

 

[Danilo917]

Thanks..im thinking maybe by ignoring the R, the calculation is a bit conservative since the short circuit capacity of the line will increase. I just dont from the point of view of protection system if being conservative makes the system more reliable and secured..what do you think?

 

[davidbeach]

Your system, not mine; your lawsuits, not mine; I wouldn't ignore it. It's so easy to include, I just don't get why you'd consider ignoring it?

 

[marks1080]

Don't ignore it. And you can't make something more secure AND more reliable at the same time... There's always a trade-off between the two. If you were to ignore it in this case I would suspect that could cause issues for over/under tripping with protection settings for adjacent zones (think in-feed's for adjacent zone faults). This will make your system less reliable and potentially less secure.

 

[krisys]

If Danilo is asking for the purpose of setting the distance relay, then it is totally not acceptable.
Unless the X/R ratio is less than at least 3-5 times, one should not ignore the value of resistive component.

 

[Danilo917]

Thanks Krisys..i will use it for the setting of distance relay..I agree with all of you to include the value of resistive component. I have a follow up question regarding angle displacement:

Is it possible to have at the 132kv source an available MVA (positive phase sequence) with an angle 10deg less than zero phase sequence angle, say 1000MVA at 70deg (positive sequence) and 1200 at 80deg (zero phase sequence)? Would it have an impact on the setting of protection relays?

regards,

Danilo

 

[marks1080]

As long as the phase and ground elements of the relay are set properly it shouldn't make a difference. If the line is really short, you may want to consider a differential pilot-wire scheme, as opposed to distance. Distance settings for short lines can get tricky, especially when catoring for things like cold-load pick-up, or in-feed fault currents.

Just a side note - angles of 70 and 80 deg. for zero and positive seq. fault conditions are pretty typical. Do some reading up on Maximum Torque Angle (MTA). The best way to really understand distance protection is to learn how the old electromechanical relays were implemented. The same philosophies are still used today for our electronic relays.