Overhead vs. Underground Distribution

[EETD]

Hi all,

I have a client who wishes to consider the cost change associated with changing from underground distribution to overhead for the following system:

1)Utility Transmission Voltage, 69kV
2)Plant Distribution Voltage, 4.16kV
3)Main Plant Switchgear:
- Main/Tie/Main (N.O. Tie)
- 4000A Main/Tie Bkrs
- 2-1200A Bkrs & 2-2000A Bkrs on one side of Tie
- 3-2000A Bkrs on other side of Tie
- System is low resistance grounded at main substation transformer secondary.
4) All but 1-1200A Feeder need to be distributed into the plant, the main switchgear is not centrally located.
5) Conductor sizes for underground installation of each feeder have been determined as follows:
1200A Bkr - 5 Sets 750kCMIL with #4/0 Gnd
2000A Bkr - 7 Sets 750kCMIL with 250kCMIL Gnd

Questions:

1) Obviously the conductor sizes and number of sets per phase can be decreased for an aerial installation. However, with the requirement to distribute six 5kV feeders into the plant...would a pole/tower structure installation even be practical? Possible?

2)Is this voltage practical for overhead distribution at these loads?

3)Does anyone know of where I might possibly find published cost comparison information (% difference, Overhead vs. Underground) for a system similar to this?

Your thoughts and opinions are appeciated.

Many thanks,

EETD

 

[Marmite]

I've never heard of anyone wanting to dismantle an existing underground system and put it overhead.
Aside from reduced reliability of supply and increased risk of contact, which would be difficult to justify in this litigious risk assessed World we live in, overhead lines don't exist, to my knowledge, at that voltage carrying that sort of current. You would need to construct a line of transmission line standards to support the number and size/weight of conductor required. The 400kV double circuit transmission towers in my country with quad 400mm ACSR conductors per phase are only good for about 2500A per circuit. Overhead line engineering is predominantly mechanical/structural engineering, and you would need some very strong towers. To use overhead lines economically you would have to increase the voltage substantially.
Also, to get overhead switchgear with that kind of rating you would need to buy transmission switchgear.
All in all its an uneconomic and technically unsound suggestion.
Regards
Marmite

 

[Laplacian]

We have a mix of 5KV overhead and underground distribution systems with 1200A feeders and switches. The overhead portion is a PITA to maintain and reliability is terrible compared to the underground section. Poles rot and need outages for replacement, insulators foul with cooling tower drift and need replacement or cleaning, copper to aluminum transitions were done incorrectly, insects nest in overhead switches causing them to jam, etc.

Sure duct banks can collapse, but this is much less frequent.

For aboveground feeders, I would use a cable tray system (with structural steel supports) and stick with copper conductors with fully instulated and shielded cables for replacing failed underground feeders only.

Why would you wholesale replace a functional underground system?

 

[EETD]

Marmite & Laplacian,

My apologies. The installation is part of a plant expansion project, so it would be an entirely new distribution system.

Thank you for your opinions.

Regards,

EETD

 

[dpc]

At most facilities we work at that have OH distribution, most would like to get rid of it. The OH would probably have a lower first cost than a good UG system, but it's always up there in the way. These OH lines and poles are constantly getting hit by trucks, fork-lifts, cranes, etc and this really puts a dent in overall reliability.

One statistic to keep in mind - #1 cause of fatalities involving electricity in the US - contact with overhead power lines.

As suggested, you can run overhead cable tray using metal-clad cable for a little extra protection.

 

[slavag]

Hi.
DPC,it's #1 not only in US.
I know only two plants ( in my work area of course)with OH lines (22kV), number of faults !!!!!!!!!.
Regards.
Slava

 

[jghrist]

As noted by others, 1200A is probably too much to consider for OH 4 kV lines. If the distance is large, it is a lot for OH or UG. I would think you would have voltage drop problems. You should consider distributing at 12.5 kV if the distances are large.

If the distance is not large, it won't be worth the cost of the UG-OH transitions.

Generally, for normal loads, say 400-600A, UG will be about three times the cost, but for short runs, the UG-OH transitions will eat up the savings.

 

[LMBT]

One of the plant I work with, built between the 1930s and the 1950s, has about ten 4000A rated 13.8 kV overhead feeders with lengths between 50 and 1500 meters.

These lines are built on steel towers with 4x1431MCM conductors for each phase. Some sections, with 2 or 1 conductor per phase, are on wood poles.

2000A and 1200A overhead feeders can be done (in your project at 4.16kV), but expect it to be quite expensive, depending on the lengths involved.

I would consider distribution at a higher voltage (12.5, 13.2, 25kV or even 69kV), depending on sizes and locations of your loads. Could you tell us more about your loads and xfr sizes?

 

[EETD]

lmbt,

I believe the main substation transformers are each 25MVA 69/4.16kV.

There are several secondary unit-subtations connected to this system ranging from 750 to 2000kVA, 4.16kV/480V.

There are several large 4.16kV motor loads ranging from 250Hp to 6000Hp.
Largest Motor Loads:
- 2-4000Hp Wound Rotor Ind Motors
- 1-6000Hp Wound Rotor Ind Motor
- 1-4000Hp Ind Motor w/VFD
- 1-2250Hp Ind Motor w/VFD
- 1-2000Hp Ind Motor w/VFD

EETD