Reverse Z3 elements in DCB scheme

[marks1080]

I'm working on an old, crude DCB scheme.... The zone 3 elements are 67, not 21. When I think about it... I start wondering if the 67 is actually more secure than a 21. I would never think of using 67 on a new job. When using a Z3 21 you must coordinate with the remote end Z2 21. I guess if you're using a 67 there's no need for coordination?

Anyway.... was just pondering this for a few moments when working on this ancient scheme... I'm temporarily hooking into a new IED and will use the old PLC gear until I get newer teleprotections installed. So my local terminal new IED will use 21 based Z3 elements to supervise my DCB signal, while the remote end uses 67 for its Z3. Anyone have any comments? Is there a solid reason why 21 is better as the supervising Z3 element vs a directional O/C 67? Or... have i just missed something painfully obvious here?

 

[cranky108]

Some zone 3 were not set to trip, so coordination was a moot point.

Some DCB ground schemes used a 50 element to start, and a 67 to stop.

 

[marks1080]

Ya I just noticed the existing system uses a 67 for Z2 ground. So does the remote end. Makes coordinate my new 21 Z3 element easier.

I'm more curious as to why the industry has moved away from this and more towards using 21 elements for all zones. Is it just because it's easy to do with IEDs? I'd like to see some arguments as to which method has the most security.

 

[HamburgerHelper]

I think the 21 elements give you a more defined reach if your backwards looking zone 3 backing up the lines behind you. For DCB schemes, you really just care about the direction being correct for blocking if you are not doing anything else. Schemes that have a 50 start and a 67 stop, do it this way because the 50 will pickup faster than the 67 because it doesn't have to determine the direction. The block signal will start immediately until direction can be determined.



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If you can't explain it to a six year old, you don't understand it yourself.

 

[Mbrooke]

Back then all sorts of things were done. To my knowledge 67 offers no benefit over 21- at least not today when we can accurately measure line impedance and accurately calculate system power swings/depressed voltage conditions. IMHO 21 is countless times more secure in terms of protective relaying coordination then any 67 could or ever would be.

 

[cranky108]

21's do have a problem with restive faults, which is why I don't use them without the 67.

On the other hand, I don't use DCB schemes. I like to use a POTT as it reduces the chance of a false trip on a communications problem.
Then again, we do have communications that don't follow the power line paths. Some companies don't have that.

DCB schemes are good if you are using PLC communications, or have three terminal lines.

 

[HamburgerHelper]

The only other thing that I think could be gotten from 67 elements is how it determines directionality. You might have more options with the 67.

I don't think there is any advantage that is gotten by 67 elements over 21s. If you make your 67 sensitive, it will have a long reach just like a 21 element. You would get better resistive coverage with a 21 element than a 67 if you used quadrilateral elements and stretched it out on the R-axis.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[marks1080]

Thanks all for the comments. Pretty much in line with what I was thinking. For this particular application I used the 21 & 67 together. The remote end ONLY uses a 67 with a pretty low pickup... coordinating a 21 with the remote end 67 was not practical. the reverse reach would have to be so long you'd see back into the remote station through another path. I tend to believe that purely in terms of directionality 67 is more secure than 21. Not to say a 67 based scheme is more secure than 21. I can't find any reason not to use both with a modern IED.