Running Substation UG Conductors in One Conuit or Three

[jscaufi]

I have been asked a question by a coworker at a utility about whether it better to install each phase of conductors from a distribution substation transformer in its own separate PVC conduit or to put all three phases in one conduit. Generally, we always see each phase plus the neutral in conduits served by our own customer site distribution transformers. 5 sets of cable, 5 conduits. However, this is in a substation application and our operations group wants to utilize three separate conduits )PVC with rigid (ferromagnetic) elbow outside of the substation fence to the UG riser. They want to do this for eas of pulling since it is 1000 mcm cable. They would pull in a ground wire with each conduit. Each conductor has a concentric neutral (1/3rd or 1/9th of phase, not sure). My thought was there could be a problem with heating the elbows since there would be little cancelation of magnetic field for each phase because the other phases would be in conduit 6" and 12" away depending on which phases you are referencing. There would be about 300-400 amps of line current which is about half the peak of the substation transformer. Any comments and references would be welcomed.

thanks
j

 

[jghrist]

Running each phase in a separate conduit will increase shield losses and heating. Running all phases in the same conduit will increase heating because the other cables are heat sources and will be closer. It would be best to calculate the ampacity each way and decide, taking ease of construction into consideration. It may be that the ampacity is limited by the riser anyway. Will there be three separate riser conduits?

 

[dpc]

The steel elbow is definitely out if they want single phase per conduit. The elbow will heat up and possibly melt the PVC or damage the cable jacket and insulation.

In UG distribution circuits, three separate conduits, one for each phase, is a common practice, since there will be single-phase loads and taps. Generally a fourth empty conduit is installed in case a cable failure damages the conduit to the extent that a new conductor cannot be pulled. But this all has to be PVC, or non-magnetic conduit.

Within a substation, I don't see a reason for this, and would run all three phases in each conduit. Then steel conduit can be used.

 

[jghrist]

The steel elbows slipped past me. They are obviously out of the question for single phase cables. If the elbows are RGS, are the risers also RGS? If so, you would need all three phases in the riser as well.

 

[jscaufi]

I believe they are only making the elbows RGS. PVC to rigid elbows to PVC. I believe once it gets to the riser, all the conductors will be in the riser. I will check to be sure as this is one of the initial steps in researching the idea. Thanks for all the quick responses.

 

[jscaufi]

Ooops, I also see I spelled conduit incorrectly in the thread subject. sorry.

 

[davidbeach]

Any continuous steel loop that surrounds anything less than an equal amount of all three phases means that you can't have phase per conduit. The RGS elbows will destroy the insulation of the conductor inside due to severe overheating. Just don't do it.

 

[jscaufi]

Thanks. Finally having some luck in my searches and with your helpful suggestions.

Here is a link from the UK.

http://www.roltesting.com/UserFiles/File/eddy.pdf