Loadability of a Distance POTT Protection Scheme - NERC PRC-023-4 Guidelines unclear 1

[Distribution73]

Hi
I am trying to understand how to calculate the loadability of a distance scheme that uses a basic POTT (permissive overreach, looking into the forward direction with no modifications such as echo, weak infeed logic etc) scheme. I am focusing on a simple case of a two ended line with no taps.
I would have thought that in this scenario the distance relays could not trip due to loading, simply because I dont think that both relays could sense de load in their forward direction. That is. One end could see load encroaching its tripping area, but the other end would see that same load on the reverse zone. However reading the NERC "Implementation Guidance for PRC-023-4 System Protection and Control Subcommittee" this doesnt appear to be the case.
I attach a link to the document:
Link
Appendix B, and in particular Figure B-1 shows the MHO operating curves and how this scheme can operate on load.
I have seen a similar figure in the 4th Edition of the Blackburn book "Protective Relaying - Principles and Applications".
Any insight on how the relay loadability is calculated for this type scheme and how figure B-1 is constructed would be much appreciated.
Thank you very much in advance for your help!

 

[jwatty]

Figure B-1 is simply superimposing the remote relay reach looking back towards the local relay, with Zl being the line impedance. Most of the time your pilot zone 2 is also going to be used for nonpiloted step distance (operating in something around 15-30 cycles), so more in-depth calculations beyond just looking at the reach of the local relay is unnecessary. The discussion along with figure B-1 indicates if piloting on POTT instead of step distance, the loadability will inherently increase (as you're implying). The problem is it's typical to do both, in case the pilot channel fails.

 

[cranky108]

If you are operating a POTT scheme with no backup, like step distance, then you might be fine. But that sounds risky.

But I believe the loadability has to do with the method that you choose for calculating it, and which rating you use.

 

[davidbeach]

I've never seen it be a problem. As mentioned, you're probably also using that element for tripping as part of a step distance scheme so it has to meet PRC-023 on its own. If that's not the case you can always turn on load encroachment or set the fault detectors above the PRC-023 minimum current.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Distribution73]

jwatty, cranky108 and davidbeach,
Thank you very much for the feedback!
I am curretly studying the loadability of a line where the utility is using a zone 2 POTT scheme with no delayed tripping (step distance). The scheme
is intended as a backup for other prot. functions (such as line diff) and I dont have any insight on why the have chosen such approach. I can see that in other cases they normally include an additional delayed tripping in comms assisted tripping schemes, but not in this case.
With regards to the figure B1 in the NERC document, I can see thanks to jwatty comments how it is constructed. However it appears to me, that for such scheme to trip in point "Y", relay A should see load in its first quadrant (R pos, X pos), whereas relay B should see it in its second quadrant (R neg, X pos). This is something I cannot understand for loading conditions where both relays will see approximately the same Voltage and the same current but flowing in opposite direction.
What is your view?
Thank you!

 

[jwatty]

PRC-23 is inherently focused on not always stable conditions - you're looking at heavy overloads (150%) and depressed voltages (85%), so inherently some buses will have higher voltages than others and you're likely to see some power swing behaviors. As power flows transition from one flow direction to another, or bus voltages suddenly sag you'll get some weird transient conditions that relays will detect.

 

[Distribution73]

Hi
Interesting. That's something I hadn't considered. Thank you very much for the feedback.

 

[RRaghunath]

Most of the times, it is the Zone-3 element with its large reach that malfunctions when the line is heavily loaded.
Load encroachment check and curtailment of Zone-3 reach is very critical during protection study, to prevent such occurrences.
Of course, this applies whether the distance scheme is POTT or PUTT.
I can't imagine malfunctioning of time delayed Zone-2 element if the reach is set as 120% of the protected line (there is some possibility if the reach is set as 100% of protected line + 50% of the following line and the following line is a long one)

 

[cranky108]

I believe the zone 3 comment is in reference to a reverse zone (in the case of POTT) that is used for stability, and assumes it can be used for tripping.

Years ago, someone removed the tripping for all our zone 3s, but enabled tripping of our zone 4s for about the same function. So we would not have a zone 3 problem.
Which is sort of a joke, as many of NERC documents still refer to zone 3, and miss that other zones could act in the same manor.

I believe there is software that can look at powerflow reversals and make a prediction if you will have an issue.

 

[HamburgerHelper]

Jwatty,

Those are stable conditions. What they don't want is for the lines to trip out before an operator can implement a mitigation. PRC-026 deals with non-tripping during stable power swings. You will see voltage and currents way out of whack during system temporarily or during something like a blackstart.

Davidbeach,

I don't think that this is applicable in most cases. Your mho's circles from either station is going to be torqued close to 90 degrees, making it hard for the two circles to overlap on the NERC 30 degree loading NERC requirement. Only in the overlapping region will the POTT scheme operate. You would be defeating the point of having a scheme in the first place if one or both sides are giving permission because of load. I think this is a non issue if you have mho's circles torqued at 90 degrees and they aren't ridiculously large to overlap on the NERC 30 degree loading line.
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If you can't explain it to a six year old, you don't understand it yourself.

 

[davidbeach]

It takes a long line to have load encroachment set less than the zone 2 reach. We have one (plus our end of a couple of interconnections) where the line is long enough to have a zone 2 reach sufficient to have part of it blocked by load encroachment. Interestingly enough though, that no longer affects the POTT scheme because or PRC-026. Now that line uses a zone 2 mho in the step-distance, partially blocked by load encroachment, but has to use a quad in the POTT scheme to stay out of the PRC-026 stable swing region.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

@Cranky: SEL calls Zone 3, Zone 4. So I've always taken it as NERC not applying