High Voltage Distribution Loop Coordination

[System Protection]

I have proposed high voltage (230kV) loop feed, leaving the station on one terminal and coming back to the same station on a different terminal.
There are multiple distribution stations tapped on the line.

Traditionally this would have been served radially and protection would be very simple.

However, there are proposals to add breakers to each of the distribution stations to split the loop into several series line segments.
This would prevent service interruptions for line faults.



Couple issues I see here.

First the lines between stations are very short, less than half a mile.
This leaves line differentials as the only option.

However, backup coordination is going to get messy.
Overcurrent elements are way too slow.
Impedance elements will be unreliable due to the line length and regardless we will have remote stations Z2 elements racing the Z2 elements of the line section relays.

Solutions:
Keep them as two separate radially feeds with a N/O breaker/switch in between. This makes protection simple and fast with no comms necessary.
This is my preferred direction. I don't like the idea of a distribution loop possibly affecting coordination of actual transmission lines.

Or have the impedance elements of the station terminals cover the entire line with a short delay. This way the line differentials only have a short amount of time to clear the fault before the entire loop is taken out to prevent any type of backup tripping from remote stations.

Thoughts?

Figured this would be a good discussion anyway as "Line Splitting" is more common now where in the past direct tapping was the norm. Although my scenario is a bit different and something I have never seen before.

 

[bacon4life]

Compared to the impact of always tripping several stations on a radial feed, only tripping the whole line during loss of communication seems like an attractive tradeoff.

For loss of communication anticipated to last 30+ minutes, it may be reasonable for the operations department to open the middle breaker in the loop to improve coordination.

In my area line differential via fiber is pretty much the standard design for any new 115 kV line, so I am a bit surprised at having concerns about line differential for a 230 kV line.

If you are in the USA, there can be some significant NERC compliance drawbacks to converting from radial to loop configurations.

 

[System Protection]

The main concern is not impacting any remote stations for failing to clear.
The simplicity and speed of clearing a radial line is hard to beat.
Not to mention the equipment, labor, and engineering costs associated with creating multiple line segments.

I don't have any concerns for line diffs, the sections are just too short to use impedance elements as a backup.
Except for a delayed impedance zone that encompasses the whole loop.

I don't believe this qualifies as BES as it just loops back to the same station.

 

[davidbeach]

A pair of line diff relays at each end of each segment, with backup POTT based on distance elements. Both on different fibers. Piece of cake.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[davidbeach]

And if you're really paranoid about loss of comms, activate both 87L channels on the relays and run the first channel directly and the other, a hot standby, around the loop the other direction. Or run the A the short route and B the long route with the hot standby taking the other route. No single point of failure compromises the protection.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[System Protection]

I am paranoid!
It is a good practice in protection.

The POTT and 87L makes perfect sense.

I see no reason to not have backup impedance elements with a short delay to ensure no possibility of a remote relay operation.