Distance Protection Zones Settings

[burntcoil]

I have an outgoing spare feeder in a 34.5 Metal Clad Switchgear. The customer want to use this feeder as an Emergency incoming feeder for another 34.5kV Switchgear.

On the upstream side, the custormer has asked for Distance Protection Settings. My question is that does I need to set Zone 2 and Zone 3 in this case, because in this case I have only one incomer and manyoutgoing feeders. So whether Zone 1 settings is enough or Zone 2 and 3 are required to be set? If they are required to be set, which outgoing feeder i will choose because there are more then one outoing feeder (as we set Zone 2, 120%).

Thanks in advance.

 

[inpran]

Burntcoil,

Zone 2 and Zone 3 are back-up protection (part of your protection co-ordination.)

Suppose the out-going feeders have been already provided with both main and a back-up protection, then zone 2 and 3 would be redundant. In addition, zone 2 and 3 should have sufficient time delay to allow the main protection in the outgoing feeders to operate.

Further, I can only assume here as the details such as length of line, mode of communication between upstream and downstream feeders etc aren't provided, that there will not be any auto-reclosure in the line feeder. Your zone 1 and zone 2 settings will only be for determining the exact fault location.

Also, zone 1 is set at 80 % in order to account for your Instrument Transformer errors. zone 2 is set at 120 %, considering the shortest line in the next station. Zone 3 at 150%, considering the longest line of the station next to it.

All depends on the protection co-ordination required / envisaged.

 

[cranky108]

I got confused. Do you have a diagram?

 

[lume7006]

I have one question:

Are you sure that the customer is willing to install at distance protection at a 34.5 kV feeder?

It may works as Inpran explained, however, typically the protection is done by overcurrent devices, reclosers and maybe, differentials.

Distance protection may work well as indicated by Inpran, but you need to take attention to the rest of the protection scheme and configuration of the relay.

 

[burntcoil]

Attached is the roughly drawn Single Line for the system. There is one spare breaker in each switchgear # 1 and 2.

The Client want to feed switchgear#2 from the spare breaker of switchgear #1 in case when both the incomers of Switchgear#2 are down and the switchgear#2 will be fed through its spare feeder as shown by red line. The distance between these two switchgears is about 1000m and are connected through direct burried cable.

ABB REF542plus distance protection relays are installed on both the spare breakers. I am planning to Enable Zone 1 for Siwthcgear#1 feeder, disable Zone-2 and 3, set it in the forward direction to protect the 80% of the feeder and for Switchgear No.2 spare feeder (which will be an incomer in this case) I will enable Zone1, Disable Zone-2 and 3 and set it in the backword direction so that incase of fault in the remaining 20% of the line, the relay on Switchgear#2 feeder will operate.

I want to ask

a). Is above approach correct in this scenario.
b). Do I need 50/51 and 50N/51N protection when I have already set distance protection. Which one will be my primary protection?

 

[isquaredr]

Distance is primary protection whilst 50/51 & 50N/51N used as backup. I would be very suprised if you could achieve an accurate 80% zone 1 setting based on a 1km length of ug cable though, this type of protection is better suited to transmission lines.

 

[burntcoil]

Why? Accurate 80% protection is not possible? Any reason?

 

[isquaredr]

Have you calculated phase-phase & phase-earth impeadances, the difference between 80% & 100% I think would be marginal, could the tolerence on your distance relay take you past you 100% figure and into other protected zones with the possibility of coordination issues.

 

[defreitas]

The difference in impedance for 800m of cable (80% setting) and 1200m (120% setting), would be so small that I doubt you will be able to achieve the desired accuracy on that protection relay.



Dan de Freitas

 

[cranky108]

It dosen't depend on the length of cable, but the impedance. I've done this coordination with a shorter length for 115 kv overhead.

At 34.5 kv the wire is typically smaller, so the impedance is larger. And yes people do have problems coordination distance on 34.5kv because they are to cheep to buy breakers. Most companies want to use reclosers or fuses at 35 kv because of lower cost.

On the other hand the old form 3 reclosers you could add a distance relay to operate the control. Not sure of the new reclosers.