Hypothetical question (line differential protection)

[mbrooke]

Is it possible or practical to protect all lines in a system (power grid) with differential protection only?

 

[Bugman1400]

It is not possible in all cases. Not all lines have communication over to the other side, hence, power line carrier. It is also not practical in all cases. For example, radial circuits could not use line diff (LD) principles since there is no source at the other end. Also, LD is not always the best protection scheme. There are several ways to provide LD protection. I believe all the relay mfrs have their own special way of applying this principle (RFL-Charge Comparison, SEL-Alpha plane, etc.).

 

[mbrooke]

It is not possible in all cases. Not all lines have communication over to the other side, hence, power line carrier.

How much would it add to capitol costs if all none radial lines were equipped with PLC or OPGW for communication? My understanding (though humble) is that most none radial 115 and 345kv lines have some sort of communication scheme already in place to aid conventional protection such as permissive overreaching transfer trip.

It is also not practical in all cases. For example, radial circuits could not use line diff (LD) principles since there is no source at the other end.

These lines would (and do) receive standard 50/51 relays, however, theoretically do you really need a source? If you have a CT on the other end (both ends) and communication, would it still not be possible to implement differential protection?

Also, LD is not always the best protection scheme. There are several ways to provide LD protection. I believe all the relay mfrs have their own special way of applying this principle (RFL-Charge Comparison, SEL-Alpha plane, etc.).

What would be an example where another scheme be preferred over LD?

 

[mbrooke]

As is any none radial line on a breaker and a half substation requires communication for remote tripping should the center breaker become stuck?

 

[mivey]

Is it possible or practical to protect all lines in a system (power grid) with differential protection only?

Possible? Yes, if you communicate with all endpoints and properly account for losses. I would think do-able for small systems.

Practical? Very unlikely. You still need other things like overload protection and even state analysis for bigger systems.

 

[mivey]

How much would it add to capitol costs if all none radial lines were equipped with PLC or OPGW for communication? My understanding (though humble) is that most none radial 115 and 345kv lines have some sort of communication scheme already in place to aid conventional protection such as permissive overreaching transfer trip.

True. Some type communication system will be in place.

If you have a CT on the other end (both ends) and communication, would it still not be possible to implement differential protection?

Yes. A net zero state is the target.

 

[Sahib]

As is any none radial line on a breaker and a half substation requires communication for remote tripping should the center breaker become stuck?

Non-unit type protection for the center breaker?

 

[mbrooke]

Possible? Yes, if you communicate with all endpoints and properly account for losses. I would think do-able for small systems.

Practical? Very unlikely. You still need other things like overload protection and even state analysis for bigger systems.

What would make it impractical though? For larger systems over load protection is generally inherent when the system is designed around peak load flows that also into account various n+1, n-1-1, N+2, ect contingencies (a line, transformer or bus outage does not result in any remaining elements going above the LTE rating, for example). Though I could see inverse time current protection providing conductor protection if a line was pushed beyond its STE during extreme contingencies.


My understanding is that (though I could be wrong) that differential protection becomes preferred as primary relaying the larger or more complex the system especially at 345kv.

Non-unit type protection for the center breaker?

I do not know anything about this. How does it work?

 

[mbrooke]

Also just to add when I say system with just differential protection I am only referring to 500, 345 and 115kv transmission lines. 34.5 sub transmission and 12.47kv distribution lines are operated radially and protected by conventional time over current relays so these are excluded in this thread.

 

[Sahib]

I do not know anything about this. How does it work?

Non unit type of protection includes time graded over current protection;current graded over current protection and distance protection. Such non unit type protections don’t have pilots.

 

[mbrooke]

Non unit type of protection includes time graded over current protection;current graded over current protection and distance protection. Such non unit type protections don’t have pilots.

But you would still need communication in the event the middle breaker fails to trip for a fault in breaker and 1/2?

 

[Sahib]

But you would still need communication in the event the middle breaker fails to trip for a fault in breaker and 1/2?

Why do you not consider operation of any upstream breaker, beyond LD protection, in that case?

 

[mbrooke]

Why do you not consider operation of any upstream breaker, beyond LD protection, in that case?

That is the point, when the center breaker fails the line on the other side needs to be tripped at its remote terminal or generator. Any back feed into the faulted line through the failed breaker needs to be stopped.

 

[Sahib]

My understanding is that (though I could be wrong) that differential protection becomes preferred as primary relaying the larger or more complex the system especially at 345kv.

Pilot wire schemes are usually economical only for short distances with the restriction of voltage drop due to impedance of the pilot wires. These in turn cause energy loss.

To overcome the drawbacks of all conventional protection communication schemes used and to maintain high reliability and lowering of energy loss, it is necessary touse accurate communication media like wireless communication protocol between the relays.

 

[Bugman1400]

How much would it add to capitol costs if all none radial lines were equipped with PLC or OPGW for communication? My understanding (though humble) is that most none radial 115 and 345kv lines have some sort of communication scheme already in place to aid conventional protection such as permissive overreaching transfer trip.

OK, now you're starting to confuse protection schemes. You cannot use LD with PLC. So, what's your question....can you use LD everywhere or can you use a pilot scheme everywhere?

These lines would (and do) receive standard 50/51 relays, however, theoretically do you really need a source? If you have a CT on the other end (both ends) and communication, would it still not be possible to implement differential protection?

You technically can but, it does not offer any faster response than a 50 element or a Zone 1 that reaches the entire line and half way through the xfmrs.

 

[Bugman1400]

As is any none radial line on a breaker and a half substation requires communication for remote tripping should the center breaker become stuck?

It is typical but, not mandatory. There are other ways with dealing with Breaker Failure at substations that do not have a Breaker Failure scheme. For example, many utilities use a forward Zone 3 at the remote end that reaches to the end of the adjacent line to monitor the middle breaker. Also, you can use reverse Zone 3 on all the other terminals in the local sub.

 

[Bugman1400]

Also just to add when I say system with just differential protection I am only referring to 500, 345 and 115kv transmission lines. 34.5 sub transmission and 12.47kv distribution lines are operated radially and protected by conventional time over current relays so these are excluded in this thread.

This clear things up for me as to what you are asking. Where I was getting confused was for most 115kV and some 230kV lines. For me, the problem I have with your philosophy on LD is that many 115kV and 230 kV lines have tapped loads off the line to feed residential, commercial, and industrial loads. The number of taps could be many. This makes it extremely difficult for an LD scheme. A POTT or DCB scheme would work just fine for this.

However, typical 500 and 345kV lines are transmission only where there is only two or three terminals. In these cases, I believe you are completely correct about an LD or some other pilot scheme. I think most utilities have standardized some type of pilot scheme (LD, POTT, DCB, DCUB, etc.) for both the primary and backup line relays.

 

[Bugman1400]

Pilot wire schemes are usually economical only for short distances with the restriction of voltage drop due to impedance of the pilot wires. These in turn cause energy loss.

To overcome the drawbacks of all conventional protection communication schemes used and to maintain high reliability and lowering of energy loss, it is necessary to use accurate communication media like wireless communication protocol between the relays.

This not true in the US. There are several reasons for pilot schemes. Short line distances and the inability for the relay to determine and in-zone fault are only one factor. A short line distance is also relative to the voltage level and SIR. Source impedance is another main factor. If the source impedance is low (due to generation plant or main grid source) then pilot schemes are typically used to lessen the impact of a fault...no matter how long the line is.

I will say that pilot schemes are starting to be more common on new transmission lines for obvious reasons. Using OPGW on new construction is standard at most utilities.

 

[mbrooke]

Why do you not consider operation of any upstream breaker, beyond LD protection, in that case?

You have a point here. I assume you mean over reaching zones?

Pilot wire schemes are usually economical only for short distances with the restriction of voltage drop due to impedance of the pilot wires. These in turn cause energy loss.

Correct, but if fiber optic pilot communications is employed, much of the old limitations are mitigated.

To overcome the drawbacks of all conventional protection communication schemes used and to maintain high reliability and lowering of energy loss, it is necessary touse accurate communication media like wireless communication protocol between the relays.

Correct. Although I would argue communication would use more energy then a simple copper pilot wires. but in any case those losses are negligible and well spent imo.

 

[mbrooke]

OK, now you're starting to confuse protection schemes. You cannot use LD with PLC. So, what's your question....can you use LD everywhere or can you use a pilot scheme everywhere?

Sorry, I meant LD. :ashamed1: Although I did not know PLC does not support LD. I guess I should ask what is PLC is used for?

You technically can but, it does not offer any faster response than a 50 element or a Zone 1 that reaches the entire line and half way through the xfmrs.

Correct, and if using a communication (POTT) scheme once zone 1 is cleared the remote terminal can be tripped by communicating that a breaker has opened tripping instantly afterwards eliminating the time delay in none communicating schemes. However, where LD comes in is easy of setting, application, and possible faster tripping of all involved breakers. No need to do extensive impedance calculations on fault current in relation to distance and guessing what values to plug in.

It is typical but, not mandatory. There are other ways with dealing with Breaker Failure at substations that do not have a Breaker Failure scheme. For example, many utilities use a forward Zone 3 at the remote end that reaches to the end of the adjacent line to monitor the middle breaker. Also, you can use reverse Zone 3 on all the other terminals in the local sub.

Good point! That should have been obvious now that I think about it. Though to be fair to myself every breaker and half I've seen had a breaker failure scheme.

This clear things up for me as to what you are asking. Where I was getting confused was for most 115kV and some 230kV lines. For me, the problem I have with your philosophy on LD is that many 115kV and 230 kV lines have tapped loads off the line to feed residential, commercial, and industrial loads. The number of taps could be many. This makes it extremely difficult for an LD scheme. A POTT or DCB scheme would work just fine for this.

Well, that depends on the utility. Some utilities will tap directly off of 115kv lines many times, while others loop the lines in and out of each substation leaving only 2 and 3 terminal 115kv lines. In this discussion we assume that all 115 and 230 kv lines loop in and out of each transmission to distribution substation.


However, that is not to say it can not be done. Could the summed currents not take into account a inverse time current curve? SEL offer relays that can integrate line taps into the differential application. Both SEL and GE have papers on it, though I can not find the SEL at the moment.


http://store.gedigitalenergy.com/faq/Documents/L90/GER-3978.pdf

However, typical 500 and 345kV lines are transmission only where there is only two or three terminals. In these cases, I believe you are completely correct about an LD or some other pilot scheme. I think most utilities have standardized some type of pilot scheme (LD, POTT, DCB, DCUB, etc.) for both the primary and backup line relays.

They do, often primary is LD with secondary as POTT or DCUB. But would it not be possible to have two redundant relays each with their own communication channel doing LD?