Isolation transformer for fault reduction

[mbrooke]

Do others think this is a good scenario where an isolation transformer would be preferred over other means of fault reduction?

A transformer being used instead of a reactor in a 34.5kv subtransmission tap feeding 34.5kv distribution in an area with 34.5kv taking hold as a new standard. The trafo provides voltage regulation via on board tap changer without neutral phase shift (delta-wye instead of 3 wye connected 32 step tap regulators which sometimes give issues on long lines), does not pass zero sequence current, does not force a new neutral on the subtransmission, and- drum roll- limits fault current down to 10ka. The iso unit can be placed in the same yard as the 34.5kv-4.8kv or 34.5kv-12.47kv trafos.

VS

I can take 3 pole banks of the appropriate size and wire them wye grounded primary- closed delta secondary for my zero sequence source, add 3 32 step voltage regulator and then add a recloser after that getting my full T to D package at a lower cost. I could, and I've seen it done that way- but how would I control fault current in this case. Reactor in series?


Also of note/benefit since the power transformer can be ordered with primary taps and reconfiguration- it can be converted 69kv at a latter date.


Thoughts?

 

[Ingenieur]

the std method is an auto transformer

 

[mbrooke]

the std method is an auto transformer

Elaborate on the setup-

 

[Ingenieur]

Elaborate on the setup-

you should be familar with these
it's the standard method, google Cooper voltage regulators

 

[mbrooke]

you should be familar with these
it's the standard method, google Cooper voltage regulators

Yup- I mentioned them- 32 step regulators. Are you saying that I could use the Z in them to limit fault current? I guess I could if the Z is high enough. However the neutral position would still be a concern...

 

[mivey]

Do others think this is a good scenario where an isolation transformer would be preferred over other means of fault reduction?

A transformer being used instead of a reactor in a 34.5kv subtransmission tap feeding 34.5kv distribution in an area with 34.5kv taking hold as a new standard. The trafo provides voltage regulation via on board tap changer without neutral phase shift (delta-wye instead of 3 wye connected 32 step tap regulators which sometimes give issues on long lines), does not pass zero sequence current, does not force a new neutral on the subtransmission, and- drum roll- limits fault current down to 10ka. The iso unit can be placed in the same yard as the 34.5kv-4.8kv or 34.5kv-12.47kv trafos.

VS

I can take 3 pole banks of the appropriate size and wire them wye grounded primary- closed delta secondary for my zero sequence source, add 3 32 step voltage regulator and then add a recloser after that getting my full T to D package at a lower cost. I could, and I've seen it done that way- but how would I control fault current in this case. Reactor in series?


Also of note/benefit since the power transformer can be ordered with primary taps and reconfiguration- it can be converted 69kv at a latter date.


Thoughts?

Run the money and see. Reactors are more efficient and cheaper than transformers but they may not be the cheapest route overall (or be the most practical long-term solution). But since you have to put in a transformer anyway you are comparing incremental costs and efficiency monies.

Sometimes it is not always about money, at least one parties money, and delivery time and unit availabilty play a role.

How much fault current with the standard unit?

For distribution, because of load variance between phases, I prefer feeder regulators over tap changers or bus regulation. Generally works better from an operational standpoint. If the source is not well regulated, then a tap changer would be added but I still like the feeder regs also.

 

[mbrooke]

I can skip the power transformer in theory as I can take a bank of 3 250kva units, wire them wye gorunded / delta and derive my neutral that way.

 

[Ingenieur]

34500/10000 = 3.45 Ohm xfmr Z
what is the xfmr mva?

what was the 10 kA reduced FROM?

 

[mbrooke]

34500/10000 = 3.45 Ohm xfmr Z
what is the xfmr mva?

what was the 10 kA reduced FROM?

About 18-20ka at the tap- the reason:

The subtranmission line is fed from a 34.5kv substation with 3 60MVA 8% Z trafos in parellel- assuming infinite transmission source that gives 37,653 amps at the bus. In reality its less but when all is said and done the sub tranmission line even at the tap half mile away is still in excess of 10ka.

 

[Ingenieur]

what size is the isolation xfmr?

so you have 20 kA at the proposed iso xfmr location?

 

[mbrooke]

Here are the nameplates to the 3 115-34.5kv trafos at the supply substation that account for the high fault current on line in question:

 

[mbrooke]

what size is the isolation xfmr?

10/12.5MVA

34.5kv going to 34.5kv

I do not have the name plate to the ISO trafo, but I can order any Z possible.

 

[Ingenieur]

ok, you have 20 kA at the prim of the proposed iso xfmr
what size is the proposed iso xfrm?

 

[mbrooke]

ok, you have 20 kA at the prim of the proposed iso xfmr
what size is the proposed iso xfrm?

10/12.5 MVA

 

[Ingenieur]

use 10 mva as base
base Z 119 Ohm

fault mva = sqrt3 x 34.5 x 20 kA = 1195
3x60 mva xfmr: Z = 0.92 or 0.53 1ph
system Z = 34500/20000 = 1.725 Ohm or 1 1ph

Z req to get to 10000 = 34500/10000 = 3.45 or 2 1ph
so iso xfmr must be = 3.45 - 1.725 = 1.725 or 1 1ph
iso pu Z = 1/119 x 100 = 0.84 %
makes no sense to use iso xfmr vs clr's
if anything at all is even needed
you have iso at the 3x60's

 

[mbrooke]

use 10 mva as base
base Z 119 Ohm

fault mva = sqrt3 x 34.5 x 20 kA = 1195
3x60 mva xfmr: Z = 0.92 or 0.53 1ph
system Z = 34500/20000 = 1.725 Ohm or 1 1ph

Z req to get to 10000 = 34500/10000 = 3.45 or 2 1ph
so iso xfmr must be = 3.45 - 1.725 = 1.725 or 1 1ph
iso pu Z = 1/119 x 100 = 0.84 %
makes no sense to use iso xfmr vs clr's
if anything at all is even needed
you have iso at the 3x60's

Thanks

Any idea how much the CLRs cost? Remember that I still need V-regs and a way to derive a neutral.

 

[mivey]

Thanks

Any idea how much the CLRs cost? Remember that I still need V-regs and a way to derive a neutral.

That is what helps the iso case. You are just tweaking the Z a bit on a device you are buying anyway. Most of the capital cost is already sunk and thus gets thrown out of the cost comparison.

 

[GoldDigger]

That is what helps the iso case. You are just tweaking the Z a bit on a device you are buying anyway. Most of the capital cost is already sunk and thus gets thrown out of the cost comparison.

But the cost of (increased) energy loss in the transformer is an ongoing expense which is not found in the case of the series reactor.
As stated by me earlier, the cost balance calculation has to include operating cost (including efficiency and not just maintenance) as well as initial cost.

 

[mivey]

But the cost of (increased) energy loss in the transformer is an ongoing expense which is not found in the case of the series reactor.
As stated by me earlier, the cost balance calculation has to include operating cost (including efficiency and not just maintenance) as well as initial cost.

Yes.

The full initial cost for the reactor but only delta cost for the transformer.

 

[Ingenieur]

since he has to derive a neut his choices are limited
iso or grounding xfmr
or get the supplier to give him a grounded supply
unless this is going to another substation for step-down, 34.5 kv seems odd for residential distribution

I am going to try and refrain from offering opinion on complex topics with limited information which are part of some ones job and involves 100's of 1000's of $'s
he should consult with an engineer familiar with the subject matter
this stuff could come back to bite a PE in the @$$