Utility Recloser settings calculation on 27.6 and 44 KV feeders

[pajce]

Hi all,
I have recloser installed downstream of Transformer Station. LDC I work for have guidlines for calculating relay settings , but I can't seem to find guidlines for revising of the relay (in the Station) settings when we introduce recloser downstream. This is not coordination question, my question is: how do I calculate setting for 50b and 50Nb elements? Without recloser we have criteria that 3ph fault current at the feeder end is devided by 2 and this would be Pick Up for 50b relay in the TS. It also need to be higher then 2 x max load on the distribution feeder. This devision by 2 gives margin for inaccuracy of CT etc and for fault resistance. (is fault resistance always purely resistive?)
For 50Nb we have criteria that TS relay Pick Up need to be less then 20% of phase-ground fault at the feeder end, and also need to be higher then 2 x unbalance on the feeder.

Now, my question is : is there any guidline what happens when we introduce recloser downstream of this relay in order not to disturbe Customer with A/R 's, normally 50b and 50Nb settings need to be revised, as zone of protection (feeder end) is now place where recloser is installed???

How do we cater for fault resistance there? Is it fair to say that fault resistance on 27.6 and 44 kV is 4 OHMs?

Thanks for any feedback on this number of questions

 

[jghrist]

Cooper Power publishes Electrical Distribution-System Protection which has a good section on recloser applications and coordination. It is geared toward a North American system, but the general principals should apply.

For one thing, you would change you setting criteria for your 50 relays to consider the minimum fault at the recloser location because the recloser will provide a fast trip beyond that point. You have to be careful with recloser/relay coordination because if there is no time delay in the 50/50n, then it will operate for faults beyond the recloser and the recloser will also operate. The recloser will count that as an operation. To get around this you need what is called "sequence coordination" which is explained in the Cooper book.

Wouldn't you use the minimum ground fault for setting the 50n relay?

 

[pajce]

Thanks JGHRIST.
For 50Nb we have criteria that TS relay Pick Up need to be less then 20% of minimum phase-ground fault at the feeder end, and also need to be higher then 2 x unbalance on the feeder.
For 50N we use 25% of the same fault value.

 

[pajce]

Jghrist:
Where can I find this Cooper Electrical Distribution-System Protection ?

 

[wbd]

Here is the link for the Cooper book. It is a box on the right side of the page that will take you to ordering info.

http://www.cooperpower.com/Library/

 

[magoo2]

Fault resistance is highly variable. I wouldn't just blindly use 4 ohms. On higher voltage systems in particular, you're less likely to get a significant fault resistance.

 

[pajce]

magoo2:
on higher voltage arc is longer and is resistance of the arc. Is it not?

 

[jghrist]

Arc voltage per unit length is relatively constant and is generally independent of system voltage. The maximum arc length is longer on higher voltage systems. With constant voltage, the arc resistance is inversely proportional to the current.

That said, magoo2 is also right because it takes a higher resistance to be "significant" on a higher voltage system.