When is a transmission line too short for distance protection? 3

[veritas]

Hi

I have a 4.6km 110kV transmission line where the client wants current diff and distance as the dual main protection schemes. My gut feeling says rather double main current diff with distance backup. How does one technically prove that a line is too short and not suitable for distance relaying?

Thanks.

 

[davidbeach]

4.6km certainly doesn't sound too short. What is the secondary impedance of the line, what is the minimum setting of the relay? Diff is always nice, but a good distance based POTT scheme doesn't need to appologize for much.

 

[veritas]

David

Certainly a suitably overreaching element such as zone 2 to initiate the permissive trip signal has its merits but what if the comms is down? Then a stepped distance scheme is all that remains.

The relay minimum impedance setting is 0.5ohms sec. The required zone 1 settings is 1.8ohms sec. Does this mean all is well? I'm wondering about accuracy and voltage available to the relay?

I'd be interested to hear what the practice is in other utilities regarding the cut-off point for distance with respect to line length.

Thanks.

 

[cranky108]

Change the CT ratio to increase the secondary line impedance.

Why do you think you need a zone 1? Zone 2 is over reaching, so that's all you need. I guess a zone 3 is required for some added stability, but not for tripping.

 

[veritas]

cranky108

Yes, CTR increase will increase relay sec impedance. I'm not really pushed here to get the desired setting into the relay. My concern is more about relay performance. I believe the SIR is important, the SIR for this application is about 2 but I am not too sure how to use it.

Also, if there is a comms fail what would you trip on then if there is no Zone 1?

 

[inpran]

Veritas,
If communications fail, you still trip the breaker at your end and ensure that the fault is not being fed from your end. If the fault persists, and there is a relay at the remote end, then the remote end breaker too will trip.
The communication is in place just to ensure that the remote end trips almost at the same instance as your breaker does.

 

[veritas]

inpran

If the comms is down what protection element will you use to trip your local end? If not Zone 1 how are you guarenteed that this element will not trip for faults beyond the line end?

Thanks.

 

[inpran]

Veritas,

When there is a fault in the line, your distance element will still be active even when your communications are down. This element will in turn trip the breaker at local end.

However, I have come across relays that will block the primary protection, distance or differential, when communication is down. In such a case, the back up protection such as a non-directional or directional over-current or earth fault relay will operate and trip the breaker at local end. Please ensure what relay you have and how it will behave when communication is down from the relay manufacturer/catalogue.

Proper protection co-ordination should take care of tripping for in-zone fault alone and not beyond it.

 

[cranky108]

May be you should look at redundit communications, not different type of relaying.

Besides, if the line is that short, then communications shoulden't be an issue.

 

[spectrohelioscope]

Veritas,

Based on the 4.6km at 110KV with a SIR of 2 I do not concider this a short line. Typically a short line can be a line with a very high SIR usually above 10... and even some relay manufactures line SEL have in their relays what they called a CCVT transient compensation settings that you can set to Y(Yes) or N(No) to prevent misoperation of the distance elements in short lines (High SIR>>>5) during a fault condition to mis-calculate the impedance to the fault for the first couple of cycles and cause the relays distance element to over-reach due to the existing transient.

In my experience I have seen that short-lines less than 2 miles, or with a SIR higher than 10+ have been concider short lines, and typically with double 87L and some directional time-overcurrent elements will be enough to backup some remotes terminals (Coordinated).

I hope this helps,

Thanks.

LAP

 

[veritas]

spectrohelioscope

This is more what I am looking for. Thanks for that. Here where I am the CCVT compensation settings are all disabled as a standard in the SEL relays. Maybe something to look into.

Regards and thanks again.