Paralleling of Substation Transformers Serving Spot Networks

[kartracer087]

So I ran a quick study in SKM to see what happens if you run a substation with (2) transformers that step 138kV down to 13.2kV. The substation I experimented with had a bus tie breaker between the two transformers. The substation was serving a 2-transformer spot network with each transformer connected to a different 13.2kV bus. When I ran a short circuit calc faulting one of the primary feeders with the substation 13.2kV tie breaker closed, I was getting in the range of 6,000A backfeed on the 480V side of the spot network. Of course, I ran this again with the bus tie open at the station, and the result didn't suprise me. I saw that the fault current backfeed on the 480V side of the secondary network went up as high as 30,000A. Now with this being said, does this create serious concerns that if you ran a station with an open bus tie that you could potentially have very strong fault currents flowing on the secondary bus backfeeding the network? I am wondering because I notice that some utlities have spot networks from separated (not tied) bus sections, and my first thought is "well that's probably not good" having that kind of magnitude flow through the secondary side each time there is a fault on the primary side. What are your thoughts and experiences with this, and has it been generally acceptable if you do have those levels of fault flow through the secondary side on a primary backfeed? Maybe it isn't that big of a deal and the secondary buswork and equipment has historically been able to cope. But then again, what about network protector fuses, surely if those saw those kinds of fault current, they would melt? Edit: well, I guess if you had an instantaneous overcurrent relay at the substation that would clear the fault in 5 cycles your network protector and the substation 13.2kV feeder breaker can beat some of the larger protector fuses. But still..

 

[bacon4life]

Network protectors are designed to open to interrupt current flowing from the LV side to the MV side.

Network protectors operate very fast, and are designed for very high fault currents. Perhaps check out this article that mentions 2 cycle clearing times:

https://www.eaton.com/content/dam/e...y-network-arc-flash-case-study-wp024002en.pdf

 

[cuky2000]

Why a SC of 6 kA for a single transformer raise 5 times (30 kA) for paralleling two transformers?

For two identical transformers the SC in parallel is expected to be around 12 kA.

 

[waross]

Please post a one line diagram.
Do you have any wye/delta transformer connections, or delta tertiary windings?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[kartracer087]

With distribution sub bus split (tie open)

With distribution sub bus closed (tie closed)

Fault is on Bus 004. With the bus tie closed, you can see the fault current returning on the primary is much lower, about 62A. With the tie open, its orders of magnitude higher at 647A. And as you can see, the secondary bus needs to withstand roughly 18kA every time there is a fault on the primary feeder. Is this a big concern to have this much fault current flow on the secondary side each time there is a primary fault?

The way I see it, its better to operate a network with a closed bus tie at the distribution substation. However, the primary fault currents will be higher as a result.

Conversely, if you operate the system with a split bus at the distribution substation, you will have generally lower primary fault currents, but significant network backfeed current if a primary fault occurs, of which the secondary bus and connections must endure.

Thoughts?

PS Cuky2000 you are correct, the numerical figures example I gave originally was for a larger example with more transformers than the one I just posted here and the example in my original write-up. However, conceptually my question remains about withstanding high secondary backfeed currents.