Bundled single phase conductors serving three phase load.

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11bgrunt

11bgrunt

Pragmatist
Location
TEXAS
Occupation
Electric Utility Reliability Coordinator
The engineered design has three 167kVA oil filled transformers to serve an oilfield load. The secondary voltage is 480/277Y. There are three 4/0 aluminum quadraplex overhead service wire runs between the transformers and the customer's load risers.
SOP has been to pull the A, B&C phases from each service run and connect to the load. Two runs of quadraplex or ten runs, A, B&C is pulled from each run. This can become inconvenient with multiple service runs and can look like a rat's nest on both ends when complete.
A contract crew built this job with the designed transformers and the three runs of 4/0Q service wire.
On this build, each run became a single phase to the load. All three insulated aluminum conductors supported by the ACSR were connected to only one of the three phases. This does look cleaner and was simpler to build.
History shows that if these conductors were in a rigid steel pipe, overheating will occur because no cancelling can take place.
The code does allow one conduit to carry only one phase, but the conduit must be non-ferrous, usually PVC.
The concern here is that no current derating was built into the load calculation and the conductors may build heat similar to being inside a rigid steel conduit.
This is not the standard method of construction and this one may get through, if there is no threat to the design.
Does anyone have experience with similar construction and was there an issue?
500kVA.png
 
I think you just bought a main...
One drop. One overhead.
Then distribute. A disco is your greatest friend. In one spot.
Kiss, no offense intended...
I see now... one conductor to three cond.
Uummhh, no
 
The risk is not from derating due to bundling, but rather transformer coupling between the isolated phases and the surrounding steel conduit.

This is why the isolated phase installation is permitted with PVC conduit. You don't get transformer coupling to plastic conduit.

In the case of this overhead install, I would be concerned about induced current circulating in the ACSR messenger cables. While not a conduit, you have a ferromagnetic conductor in the unbalanced magnetic fields of the isolated phase installation.

Are the messenger wires joined at both ends or are they insulated?

Jon
 
hillbilly1 said:
Since the messenger wire is not encircling the conductors, I believe there would be no heating even if it was bonded. (All cable being same phase.)
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Why do you think magnetic fields are shaped like that? My understanding is rudimentary, but I would think the magnetic field would be strongest in the center of the wires.
 
winnie said:
The risk is not from derating due to bundling, but rather transformer coupling between the isolated phases and the surrounding steel conduit.

This is why the isolated phase installation is permitted with PVC conduit. You don't get transformer coupling to plastic conduit.

In the case of this overhead install, I would be concerned about induced current circulating in the ACSR messenger cables. While not a conduit, you have a ferromagnetic conductor in the unbalanced magnetic fields of the isolated phase installation.

Are the messenger wires joined at both ends or are they insulated?

Jon
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All ACSRs are bonded together and connected to the system neutral. Y-Y.
 
I don't know if there is code that prohibits this installation.

Given that it would be desirable to permit this sort of installation in the future you might want to call it an experiment. In other words accept the installation and the risk that it might fail, and then put effort into evaluating it.

The system you describe, with the ACSRs bonded at both ends will certainly have transformer coupling and circulating currents in the messenger cables. This certainly means more heating.

But this doesn't mean there is a problem.
I can't tell you how much coupling will occur nor how much current will flow. It might be worth loading the system up and putting a clamp meter on the individual messenger cables.

Jon
 
My concerns were also over inductive effects on messenger wire, particularly the steel strand that give it support strength.

And yes bonding them together at the ends would allow undesired circulating currents during otherwise normal operation.
 
I am not clear on alot, but- are oil fields subject to city and county? Or is it weird railroad/carnival exempt?
 
rambojoe said:
I am not clear on alot, but- are oil fields subject to city and county? Or is it weird railroad/carnival exempt?
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In most of these cases where an oilfield could exist, I suspect the power company is the JHA.
 
If state and/or local government adopt some version of the NEC without ammendment, then an oilfield would be subject to NEC and city or county. Of course a state or local govt could ammend the code or have additional laws stating that oilfields are covered by other law. They could ammend their version of the code so that it does not cover oilfields, just like it does not cover mines and railroads. I suppose that might be the case in some places. Presumably they would apply some other law or regulation, just as states typically do for railroads and mines. I suppose enforcement might be lax.

A utility is generally not an AHJ, at least not on the customer side of service equipment.
 
This will not be an issue, the ACSR would be no different than a grounded or grounding conductor in a PVC raceway which code allows. There will be no heating effects and marginally lower available fault current at the service point.
 
jaggedben said:
If state and/or local government adopt some version of the NEC without ammendment, then an oilfield would be subject to NEC and city or county. Of course a state or local govt could ammend the code or have additional laws stating that oilfields are covered by other law. They could ammend their version of the code so that it does not cover oilfields, just like it does not cover mines and railroads. I suppose that might be the case in some places. Presumably they would apply some other law or regulation, just as states typically do for railroads and mines. I suppose enforcement might be lax.

A utility is generally not an AHJ, at least not on the customer side of service equipment.
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In absence of laws that dictate specific codes/inspection procedures, POCO still has discretion of what they may not want to energize for liability reasons.
 
mbednarik said:
This will not be an issue, the ACSR would be no different than a grounded or grounding conductor in a PVC raceway which code allows. There will be no heating effects and marginally lower available fault current at the service point.
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But that grounding conductor in the PVC raceway probably won't have the steel strand the ACSR has, so magnetic effects are going to be different.
 
I bet he is a sub for the poco.
O.p.... Are you connecting to the xfrmer? Are you trying to use up 4/0 in the yard?
 
rambojoe said:
I bet he is a sub for the poco.
O.p.... Are you connecting to the xfrmer? Are you trying to use up 4/0 in the yard?
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Don't understand the question. The three single phase 167kVAs are on a transformer rack.
The POCO engineered the three runs of quadraplex that will connect to the transformers and then to the customer's risers.
 
1) Are these conductors installed under the NESC (POCO standards, used for things such as service drops) or NEC (National Electrical Code, used for building wiring)?

-Jon
 
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