Undergound vs Overhead Power Distribution

[gattie]

The major problems that Eastern USA is having due to freak conditions, poses the question - Is there a program to replace
Heavily wooded town distribution O/H lines with underground cable?
There is a huge cost difference, but the costs & disruption due to the present catastrophe could be partly outweighed if the underground option is adopted.
One also often sees pictures of dangerous downed live conducters on the ground spluttering away for ages.
Is there no sensitive earth fault protection available to guard against this dangerous condition?
Just a thought!

 

[MikeHalloran]

I'm not convinced that underground distribution is in any way superior. Help me understand any factors that favor it.

WRT overhead distribution, it used to be relatively trouble- free in SoFla, where the power utility engaged contractors to clear flora from around the wires on a more or less continuous basis. They typically showed up in my neighborhood twice a year.

Then an odd thing happened; the contractors stopped coming.
Then an inevitable thing happened; there was a hurricane.
... after which it somehow became the government's responsibility to clear the flora from the powerlines.



Mike Halloran
Pembroke Pines, FL, USA

 

[dpc]

UG distribution is much more expensive to install than overhead. The underground cables will eventually fail, creating their own outages. The decision to go UG is generally based more on aesthetics than reliability.

Detection of downed overhead power lines is problematic. The feeder's ground fault detection scheme must account for the normal levels of neutral current that can flow from load current imbalance. If there were a foolproof way to detect downed power lines via overcurrent relays, it would already be in wide use. New algorithms and detection schemes are being introduced all the time, so this may improve over time.

The storm that moved through last Friday was a rare event. Decisions must be made as to what level of reliability is required and what people are willing to pay to get this.

 

[cranky108]

For my understanding I have a few questions.
1. Did the trees only fall on the power lines?

2. Is the soil sutable for underground power lines?

3. Is the replacment of UG lines acceptable every 30 to 40 years (and the outages that go with them)?

4. What is wrong with the utilities tree trimming program?

5. Are the people who live there willing to pay more for UG lines?

 

[MikeHalloran]

1. Trees fell _everywhere_. ... but in prior decades they had been trimmed so they couldn't reach the powerlines when they did fall.

2. The water table is about three feet down.

3. Nobody would stand for complete replacement of UG lines, with associated trenching and disturbance. They probably wouldn't even stand for partial excavation associated with pushing pipes underground by slant drilling technology.

4. The utilities just flat don't have a tree trimming program anymore. Apparently it was a cost saving. ... like not painting bridges.

5. I'm not willing to pay more for UG lines; I think it's a crazy-ass stupid idea, at least for SoFla.


Mike Halloran
Pembroke Pines, FL, USA

 

[cranky108]

Mike, it sounds like UG is not for you.

However, your answer to 2, could have a few questions. Here you can hit rock before you go 3 feet down, and the frost line is about at that point. Sounds like you are in a better position for UG, but wood poles maybe not.

Number 4 is troublesome. If an electric utility dosen't have a tree trimming program, then who ever regulates them should be fired.
That would be like not replacing blown fuses, or not replacing bad poles.
Or me not mowing my lawn.

 

[MikeHalloran]

WRT 4, I'm beginning to think that the utility stopped the tree trimming in anticipation of having their infrastructure seized in furtherance of smart power and deregulation.



Mike Halloran
Pembroke Pines, FL, USA

 

[dpc]

In the case of the recent storm in the eastern US, I'm not convinced that more aggressive tree-trimming would have made a big difference. Unless they just clear-cut the entire easement - and this would never happen in a residential area.

 

[davidbeach]

While very much not on the eastern seaboard, or much of anything east, we could never sufficiently tree trim to prevent all vegetation related outages. To do so, you'd have to start at the center line of the pole line, offset about 8-10 feet to both sides, and then clear everything above a 45 deg slope from those edge lines. There'd be he!! to pay. In a recent rate case the state decided we had too much money allocated for tree trimming. Now, there's a problem. I figured we should figure out where every commissioner in our territory lived, and make sure not to do any tree trimming in their area. After a few years they'd be begging us to do more tree trimming.

Undergrounding lines is a decidedly mixed bag. Overhead lines have far more faults than underground lines, but most overhead faults are temporary and essentially all underground faults are permanent. We reclose for overhead lines but never reclose for underground lines. While air is not a strong dielectric, we need lots of clearance, it has one wonderful property - it is entirely self healing. There is no solid dielectric I'm aware of that is self healing.

Sure, in a vicious storm there is going to be a heck of a lot of damage, but in a normal year the average customer is far better off with overhead lines. What we have underground is mostly because we have to, and when something goes wrong it takes far longer to find the problem and deal with it than where we have overhead lines.

 

[gattie]

Yes Chaps
Was a time when Essential Services - Electricity, Water & Telephones were Government non-profit organisations!
Now we are in the hands of greedy Bankers, Shareholders & Stockbrokers.
All profit driven repair services have been privatised, expensive & poor.
Al la Enron!
Now we look forward to the rewards of Smart Metering!
In South Africa, we generally do not have the same problems with large trees falling on O/H lines.
Underground cabling is used where possible.
For overhead lines in residential areas the use of sensitive earth fault protection is encouraged for public safety reasons.
If u/g cable fault location is a problem, the use of thumper & acoustic equipment is quite successful.

 

[jghrist]

Sensitive earth fault protection is not applicable to the typical US distribution feeder that has single-phase fused taps and Ø-grd connected transformers.

 

[cranky108]

However, agressive use of fault indicators is useful in older part of our system. And as UG is reaching between 30 and 40 years of life, in some places, we are doing more cable replacment and cable injections.

Problems we are seeing is cable injection fluid runs down hill, and can leak into equipment boxes. And older UG cable was installed without conduit (direct burried).

In the US, some utilities are still goverment owned, and non-profit. And the utility board answers to the people they serve. The problem is they don't understand the business completly, and are more swayed by astetics, than real technical problems.

With invester owned utilities, they usually answer to some commission, as well as a stock holders board. And what can happen is neather one may understand the business.

The problem with sensitive earth fault detection is it dosen't always work, and isen't selective. At best I have seen figures of it working properly at 50%. It also dosen't coordinate with fuses, reclosers, etc.

What I have seen work on OH lines is a shield wire. They do reduce lightning outages in places with higher isocronic (sp?) levels. The problem is there is an assumption that the lower wire is a neutral, and therefore safe.

At least in some parts of the world UG distribution won't have the unknown customer tap problem.