NERC PRC-23 Line Loadability

[PeterVenkman]

I have been looking at the standarded PRC-23, which is in place to prevent loading and operator actions from tripping a relay. I have a long 345 kV line that has enough line impedance to limit the fault current far below out neighbor we are interconnect to short term thermal rating of the line. I am supposed to evaluate the loading of the line with 0.85 percent voltage and 30 degree power factor angle.

The part I don't get is that in the worksheets they draw a line from origin at 30 degrees representing the load and from that you get Zrelay30, which is point the line intsects the zone protection. Why isn't the 30 degree load line drawn so that it intersects the impedance point that represents the end of the line?

http://www.nerc.com/docs/standards/sar/Relay_Loadability_Reference_09Jan07.pdf

My artistic rendering of what I am talking about.

http://ge.tt/58RbA1s

 

[pwrtran]

It perhaps because of zone3, the over reach of the normal configuration of the line, which is a typical 200%.

 

[PeterVenkman]

Why does the nerc load go through the origin instead of the point that represents the end of the line?

 

[davidbeach]

Because the relay is at the origin, not at the end of the line.

 

[PeterVenkman]

At maximum loading, won't the impedance point be near the line impedance?

 

[pwrtran]

Load impedance is much less than fault impedance

 

[pwrtran]

Make a comment on my previous reply - "Load impedance is much less than fault impedance "
Normally it is true but in some case where it is not true, then the load encroachment will be use to overcome the issue

 

[pwrtran]

Sorry, the fault impedance is less than the load impedance, I meant. What I am doing today :-(

 

[davidbeach]

As seen from the relay, a fault at the end of the line has the line impedance to the fault, lots of X and a bit of R. Load, on the other hand is mostly R with a bit of X thrown in for grins and giggles.

Figure out your emergency rating, add 50%, give it a 30 degree angle, assume 85% nominal voltage and plot what the relay sees. Then draw a line from the back to th origin, probably comes out pretty close to 30 degrees.

 

[cranky108]

david is correct, but a little over simplestic. You also need to read attachment A because it has a few hooks.

Most of us use use the first requirment to meet the standard for lines, although there are twelve other ways to meet the standard.