Subtransmission Vs. Distribution

[SubHuman]

I was wondering if there are any codes/regulations regarding the use of a subtransmission line as a distribution line. Also, if anyone has any thoughts or concerns about this practice?

I work for a municipal electric department. Our subtransmission voltage is 23kv. Our distribution voltages are 4160, 12470 & 23,000. We have plans to use an existing 23kv distribution line to also act as the subtransmission line that will be feeding two substations.

Personally, I don't like this idea. But, I'm here to try and get some hard facts concerning this practice, whether positive or negative.

Thank-you!

 

[jim dungar]

Utility 'distribution' systems are not subject to the NEC, but you might find something in the NESC.
I think it would be more of a protective relaying issue more than any thing else.

 

[bob]

I work for a municipal electric department. Our subtransmission voltage is 23kv. Our distribution voltages are 4160, 12470 & 23,000. We have plans to use an existing 23kv distribution line to also act as the subtransmission line that will be feeding two substations.

There are many utilities converting their 4 kv systems to a higher voltage. With added loads the 4 kv systems can not handle these additions. Usually the 23 kv systems used as a "transmission system" do not have a neutral. So you will need to install neutral on these lines used for the conversion. Also the fault currents will be much higher. You will need to run fault current studies to determine if the existing 4 kv conductor is able to handle the fault currents. The 23 kv distribution lines are constructed more like distribution systems rather that transmission systems so you will need to develop new standards of construction. I would assume that you will not work this new 23 kv line hot. Do you have existing 4 kv and 23 kv load studies? Where are you located?

 

[SubHuman]

Okay, let me rephrase...

Okay, let me rephrase...

I'm so tired, I can't even think straight right now. But...

Here is the real concern...

We currently have two feeders exiting one of our delivery points. Both of these feeders are 23kv. One of them is residential, and the other is heavily industrial. But, to date, they have been used merely for distribution.

Feeder #1 ------------------------------------------>

Feeder #2 ------------------------------------------>

Now, without changing the distribution aspect of these feeders, we are going to tie them in like this, in a closed loop...


Feeder #1 -------> Sub 1 ------->Sub 2 ----------> Feeder #2
(both subs 1 & 2 are using
this as subtransmission)

So, what used to be 2 seperate 23kv distribution feeders with regulation, will NOW be a closed loop subtransmission line (23kv), with both residential and industrial customers tapped off of this loop.

It exits the originating substation...goes to a second substation...goes to a third substation...then back to the originating substation. But, all of the original customers are still on this converted line.

Hope I explained better...but I need sleep

 

[djohns6]

These two circuits are radial now ? What kind of protection do you have on them ?

 

[SubHuman]

Radial to Loop

Radial to Loop

Currently, the two feeders in question are radial...as are all of our distribution feeders. They are both 23kv, ending at tie poles with open GOABs.

They are currently protected with standard overcurrent relays, as well as SEL relays. Both of these feeders are regulated at the substation.

The new relays that we are installing are Siprotecs, which are pretty much full-range programmable. These will be replacing almost all of our existing relays.

In the new layout, these two feeders will be looped, to include being the primary feed for two substations. Thus, my curiousity about this plan. I don't like it at all, and I hope to get some information concerning these plans.

Thank-you.

 

[substation guy]

we use 12470, 34500, 69000 we have subs fed w 34500 which would be "subtrans" i guess

 

[bob]

If you have them connected in a loop and have a fault at the midway point,
theoretically both feeders will feed 50% of the amps into the fault which may not be enough to trip the breakers. I would leave it as is with a normal point.

 

[SubHuman]

My bad...

My bad...

I understand what each of you are saying. Unfortunately, I don't seem to be asking the question in a way that makes sense, as far as expressing my concerns. I'll post again, once I figure out a better way to illustrate my problem.

A huge thanks to all who have replied

 

[jghrist]

Two concerns are voltage regulation and protection.

Subtransmission systems are normally unregulated. They might not be suitable for serving distribution transformers because the service voltage won't stay within acceptable limits. If your lines are regulated, there may be circulating current problems when they are tied together.

Overcurrent protection is more complicated on looped systems because there are two sources of fault current. You will probably have to use directional relaying to protect the lines.

We have a municipal utility client that uses 25 kV for both subtransmission and distribution. Unregulated subtransmission lines feed regulator stations that have distribution feeders. Distribution transformers are not served from the unregulated subtransmission feeders.

 

[72.5kv]

To make this new set work, both end at where the line originate would have to have distance protection and some kind of communation channel between them. A fault between feeder 1 and feeder 2 would have to be cleared by tripping both ends, basic overcurrent would not work. This similar to 69Kv or 115Kv line with tap stations between but just at a lower voltage.

 

[72.5kv]

The protection scheme at the substation would have to change also, now a breaker failure scheme would have to be added as a backup to both terminal in an event they fail to operate