138kV 1/2km Line with Distance Protection Relay 2

[ppinuelas]

Hello all,

I am not really that familiar with distance relays since the type of work I do is not really transmission lines, but the project that I am working on we need to add a 1/2 km 138kV transmission line that is going to feed a new substation. Originally, I was thinking on protecting this short line with a delay overcurrent relay 51/51N when I got a client comment back asking why not use distance protection relay, my first thought was because is a really short line, so my question is how short can the line be to use a distance protection relay? and if we can use it in this applications what are the advantages over the overcurrent relay?

The reason that the client came up with this comment is because he does not want to be moving settings in the future when we add more load to the line.

Any comment is welcome.

Thank you,

- Pablo

 

[davidbeach]

I'd go with current differential as protection for the line itself and use distance for the backup protection of adjacent lines. Overcurrent on transmission should be avoided if possible. As for protecting the line itself with distance, what is the impedance of the line (in secondary ohms) compared to the minimum relay reach setting? Can you even set a zone 1 at less than the line length?

 

[ppinuelas]

Thanks for the quick answer David.

The thing is that the line is going to end at the new substation, where we are stepping down from 138kV to 34.5kV having a switchgear in the secondary of the transformers. In this switchgear is where we are going to add loads in the future.

Like I said I only know the basics of distance relays so I do not know what you mean with the impedance of the line in secondary ohms or the minimum relay reach setting. I do understand the zones on the distance relay that is why my question on how short can the line be to use it.

 

[Sharpie8]

If it is a new line there will be negligible cost impact of including OPGW for the OHEW to provide a fibre link between the two substations, so David is spot on, current diff is definitely the better solution. By using a numerical relay you could use a backup overcurrent element to protect the line conductors themselves, set somewhere around the overload capability of the line. That way it will be load independent as exceeding the O/C setting is exceeding the capability of the line conductors so increased load would require increasing the line capacity.

Also, as David suggested, Distance protection may not even be possible, depending on how the line impedance compares to the relay's minimum zone 1 reach setting. I think if distance was possible you almost certainly would want to use a PUTT or POTT scheme as well, which makes differential protection even more appealing for this application.

 

[collies99]

You can probably use distance in the line feeds directly into a transformer since the transformer impedance is likely to be much higher than the line impedance. Z1 will likely reach into the first turn.

Most short line avoid the use of z1 and instead use z2 timed for coordination with the transformer protection.

if radial line, then you can even have strickly O/c without high side breaker at the new site.

Line differential or pilot wire are similar but need communication and breaker st both end and 21 are for non-radial lines.

 

[veritas]

I've just finished a similar project where I used line differential for a 0.5km line with distance backup. As each line was really a transformer ended feeder, the Zone 1 was set to look halfway into the trfr (no trfr HV breaker). Zone 2 was set to look right through the trfr as remote backup. Distance only comes into play with loss of comms.

I would also steer clear of OC backup for, eventhough it is a possible solution, it is not usually the best one. It is dependant on the source impedance and in my case there were two lines in parallel both being trfr ended feeders making it even more tricky for OC backup - the backup OC relay would see different current depending on whether the LV bus-section breaker was closed or not.

You may not have two lines at this point in time but maybe in future.

Another snag of the OC relay is that you need to carefully determine it's settings as it would see different currents for different faults on the LV side of the trfr.

 

[bacon4life]

Is this radial? Or is there distributed generation connected to the new substation? If this is truly radial, you only need line relays at the source substation. If there is distributed generation connected to the new substation, you will need a whole lot more than overcurrent relays.

Distance protection requires 138 kV potential transformers. If the substation does not have PTs, this can be a pretty significant cost addition.

Line differential and PUTT/POTT requires relaying at both substations and a robust communication path between the substations.

Overcurrent or distance protection would either need to be slowed down to coordinate with the transformer protection or would have intentional mis-coordination. On many of our 115 kV lines with multiple tapped transformers, both our zone 1 distance protection and the transformer differential will operate for a transformer fault. The transmission line then recloses to restore the adjacent unfaulted transformers. With a very short line like yours, the minimum distance the relay can see might include part of your transformer so you would have mis-coordination. Transformer faults are much rarer than line faults, so it might be an acceptable trade-off to avoid investing in a communications path.

If you have multiple 138/34 kV transformers, Current differential and PUTT/POTT schemes allow for fast line clearing without having to coordinate with the transformer protection.

As other stated, phase overcurrent relays are difficult to coordinate and must be reevaluated when the upstream system configuration changes. Even as backup elements, they are troublesome.

Sharpie
In general, relays should not be set to protect transmission line conductors from moderate overloads. Rather, System Operators are supposed to monitor the transmission system and take action to alleviate the overload. As a result of blackouts exacerbated by load sensitive relays, NERC standard PRC-023 requires overcurrent relays to be set above 115% of the highest 15 minute rating or above 150% of the highest seasonal 4 hour rating.

 

[Badiya]

For protecting 500m of line, you can use
- simple differential relay (one box relay and measuring/control cables wired to CTs and Tripping cables wired to CBs at both ends of line).
- Pilot wire relay- Two relay boxes mounted at both ends, connected to each other via Pilot wires
- Line Differential relay - Two devices mounted at both ends , connected to each other via fibre optic

Regards