Sectionalizer Vs. Loop-Feed Tx

[eeee]

It has been brought to my attention that for a 3 phase operation, a loop feed transformer with internal switch would be better than using a secionalizing center and manhole combination since we would not have a way to break the primary circuit with the sectionalizer center and manhole combination (thus isolating the various circuits during a fault).

We a passing a primary through the sectionalizer to a far end transformer and also producing two other outgoing secondaries with the sectionalizer and transformer combination laying side by side that feed circuit breakers on houses.

I was told the sectionalizer and manhole combination is ok if the transformer is single phase.

Is this question possible to interpret, or is more explanation necessary.

I don't see why it makes a difference now that it is single phase, although I am going to stare at this resolution some more.

 

[jtester]

Re: Sectionalizer Vs. Loop-Feed Tx

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I'm not sure I understand your sectionalizing/manhole installation. Could you be a bit more specific.

Loop feed transformers are very useful in single phase applications, even with no internal switch. If you use load break elbows on a loop feed transformer, either single phase or 3 phase, you have switching flexibility at very little expense.

I look forward to additional information on your post.

Jim T

 

[eeee]

Re: Sectionalizer Vs. Loop-Feed Tx

Ok,

I have a 13KV, 3 phase primary comming from a substation that enters a manhole then ends on a sectional center (sectionalizer) located right next to the manhole. From the sectional center a primary leg traverses back to the manhole then to a 3 phase transformer servicing a bank 100 yards away. Another primary leg also traverses from the sectional center back to the manhole then to a single phase 208/120VAC transformer located right next to the manhole. From the 208/120VAC transformer, one secondary feeds one house and another secondary feeds three more houses within 100 yards proximity. Now the sectional center has load break elbows in them, probably dead front.

The previous design was feeding a 3 phase loop feed transformer with an internal switch so that the circuit feeding the bank and the two circuit feeding the houses could be isolated in case of a fault.

As soon as it was learned that a single phase transformer was being used, it was decided that the use of the sectional center would be ok and maybe even a better design.

Note that I am indicating a loop feed transformer in my new explanation and not a feed through transformer as before. I believe both are the same thing semantically in terms, but also note that the internal switch was the mechanism that was required to get the isolation in combination with the dead front load break elbows, before it was decided to go with the sectionalizer.

 

[jtester]

Re: Sectionalizer Vs. Loop-Feed Tx

If I understand correctly, one design would have had a three phase circuit to a loop feed transformer, and then only a single phase circuit from it to the single phase transformer.

I wouldn't do this as a general rule. There is something uncomfortable about extending single phase from a three phase transformer. It would prohibit providing a loop feed to the three phase transformer, which is always nice for added reliability. If you see no need for a loop feed to the transformers, then electrically there is nothing wrong with what I believe you explained.

Jim T

 

[eeee]

Re: Sectionalizer Vs. Loop-Feed Tx

I believe the 3 phase transformer had an internal switch and was being used to feed a 3 phase circuit through to the 3 phase transformer servicing the bank, so I don't see where I am loosing my feed through capabilty here since I am pulling a single phase off of it. I also believe the 3 phase transformer was generating the 208/120VAC single phase to the two circuits feeding the houses without the need for a single phase transformer. I don't see why you would need a single phase transformer here?

The way the design is now, an additional component, the sectionalizer is required and this is a defecit since more structure is visible to the neighborhood. The advantage is that no longer a 3 phase transformer is required, but rather a 1 phase transformer is being used. Don't know if this is cheaper. It depends on the cost of a single phase transformer and sectionalizer against the cost of a three phase loop feed transformer witha n internal switch.