Line differential protection 2

[sadgol]

hello everybody
As we know according to technical data and catalogs,new distance relays can protect even very short lines(e.g less than 3km),so if in practis they can protect a very short line corectly,can we use them instead of line differential relay?

 

[davidbeach]

Yes, but why? Distance protection still has the long line-short line problem. To get decent clearing times over the entire line you still need communications, so why not include differential on that comm channel?

 

[sadgol]

Thank you so much for your reply.It's very kind of you to tell me distance problem for protection of very short lines?According to cataloge,distance could do this but I don't know what is going on in practise.

 

[slavag]

Hi Sadgol.
As David saied, long and short line have same problems with 21 Line distance protection.
1-st zone of 21 is covered only 85% of line with "0" time.
If you would like provide a fast clearing of fault on the whole line, you have provide some kind of communication logic: PUTT or POTT.
For this you need communication link.
More .. for short line possible problem of R/X calculation and minimum sensetivity of setting and need put a bigger size of CT.

More easy today install 87L line diff protection.
You have today a not expensive range of relays with FO, etc communication links.

Best Regards.
Slava

 

[lz5pl]

sadgol, I had once a case to calculate and after that to set-up three 21L protections of 1,5 km 33kV cable lines. And to make the things worse - CT's were /5A secondary current and there was no communication between both ends. Honestly - it was really "unforgettable" experience - and I hope I will never face similar case :-(
With the possibility to use OPGW on overhead lines or usual FO-cable for cable lines I cannot see why not to use 87L protection with it's 100% selectivity for short circuits on the entire line. And of course almost every modern 87L relay has also 21-function as a back-up, so time-graded zones for next lines can also be used.
At least for new connections in our country 87L protection is widely used - even in my two current projects - RED670 from ABB and 7SD61 from Siemens.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[sadgol]

Thank you so much daer Slavag and lz5pl,hope you the best.

 

[sadgol]

following to my question,would you tell me less than which distances we should use line differential protection instead of distance relay,e.g less than 10km or 5km or something eles.

 

[slavag]

Hi Sadgol.
It's not so correct Q.
Please send more information about your project.
What is a type of line: Voltages, current.
Are it transit line between two SS, or it's week end line, etc.
Are you need autoreclosere functionality or not?

Best Regards.
Slava

 

[davidbeach]

I'd use differential on any line I can get differential. Distance is a good back up. The biggest problem with distance is where you have lines of different lengths; a distance relay looking though the next bus at lines of very different lengths will have compromises made in its settings. Differential as the primary protection makes the whole line the zone of protection and excludes the rest of the system. Distance will always trip for out of zone faults if they persist long enough and therefore makes a good backup protection, but getting good selectivity can be a challenge if line lengths have significant variation.

 

[sadgol]

Hi slavag
Thank you so much for your help.
Our line:cable line,4km,132kv,800A,without autorecloser,there are two line just between two substation,on the other hand this is end substation and is not in a ring of substations.

 

[slavag]

Hi.
OK, next Q, are thode two lines operated in parallel?
Are you have coupler on this end substation?
What is a load type on this substation, I meant after Xfmr/Trafo.

It's short cables, subtransmission voltage level.
87L , diff line protection is best solution here, w/o any Q's.
BTW, for HV cables, recommended always use 87L. For O/H lines possible only 21, distace protection.

About back up protection, wait to your answers.
Maybe possible use a simple O/C relays 51/51 and E/F relays 51N/50N, or if it;s paralleling lines directional O/C, E/F protection 67, 67N.

Best Regards.
Slava

 

[sadgol]

hi dear slavag
our lines are parallel,we have coupler.
It's clear that for short circuit lines it's so better to use line differential relay but there is an important parameter that we should consider,"PRICE".if we use line diff. we should use fiber optic cable and this increases the price.so we prefer to useline diff.only when we realy need and couldn't use distance.
It is very interesting for me that I see you ask meabout some parameters to determine using diff or distance relay,could I ask you how do you underestand this by these parameters?is there any calculation?
Best regards

 

[slavag]

Hi Sodgol.
See, 69-150kV it's some "grey zone".
Possible in your case use only ditanse protection and 50/51 O/C protection as back-up protection, if price is very critical in your case. BTW 4km of FO today isn't so high, but is not so cheaper, about 10$ per meter with installation ( of course is depemd on few parameters).
Possible use more simple twisted paire cable ( pilot wire diff protection). On end side you can put directional O/C, you need in case of coupler will closed and [possible feed SC on the line by current from parallel line. Or use on other side distance too, instead .
with 0 time you can choice first zone of feeded side distance protection, will covered 85% of cable and with some time grading second zone , that covered bus and trafos and must selectivity with trafo protection on the LV side.

It's more or less rules, but of cource is depend on countries/utilities standards. In lot of companies, cables is protectecd always by the diff protection.

Best regards.
Hope , that help
Slava