Reliability of underground power lines?

[N9WOS]

This is one of the more exhaustive studies.
www.scc.virginia.gov/comm/reports/report_hjr153.pdf

Early underground residential circuits failed at a much higher rate than
expected,
but experts now believe that new underground circuits will last at least 30
years,
while overhead circuits will last 40 years or greater.

Why doesn't that statement instill confidence? What about all that cable
that is already in the ground? The "experts" were surprised by the much
higher than expected failure rate of first gen cables. But we should
believe them now? With all the worthless expert "opinion" and "consensus"
that we have seen over the years, you will just have to put me on the
skeptic side of things.

Put then in the ground, cross your fingers, and pray...

The potential benefits, both to the utilities and to the economy, resulting
from the >elimination of tree trimming maintenance,

I have news for them. When you have direct burial, or conduit buried cable,
you still have to keep large growth/deep rooted trees from establishing
themselves. The roots will disturb the underground line. When a tree blows
over, if it's roots are around the line, then it will be ripped out with the
tree. A conduit won't help any.

 

[Josepi]

Our 14Kv cables are installed in duct banks with concrete around them for
major runs down streets. backyards get direct burial cables and this could
be a problem.



I have news for them. When you have direct burial, or conduit buried cable,
you still have to keep large growth/deep rooted trees from establishing
themselves. The roots will disturb the underground line. When a tree blows
over, if it's roots are around the line, then it will be ripped out with the
tree. A conduit won't help any.

 

[Josepi]

Two questions.

1) Are you sure the primary is 38kV underground?

2) What area are you in?

Thanx


Back to my own neighborhood, there haven't been any underground failures of
the 38kv but a neighbor lost her transformer. The first replacement had a
"bushing failure" and had to be replaced again. They were without power
for about 5 days because of the transformers.

 

[vaughn]

In the sense that homes numbered in the thousands usually don't go out for
days at a time, that's true.

It seems like John doesn't live in hurricane country.

Vaughn

 

[[email protected]]

It seems like John doesn't live in hurricane country.

Vaughn

Or Ice Storm country.

 

[Josepi]

That should give 66kV phase to phase. Very HV for underground system,
especially in older installations as the insulations were quite expensive,
at that level.

Two questions.

1) Are you sure the primary is 38kV underground?

Pretty sure. I often "chat" with the guys installing/repairing equipment.
That's the number I remember.

2) What area are you in?

Semi-rural east central Virginia.

 

[Josepi]

We ran a lot of 2400/4160 through subdivisions underground. That was
switched over to 8.0/13.8kV later in an effort to save transformer and line
losses.

I have heard of some utilities running 16.1/27.6kV but mostly overhead
except for u/g dips to avoid obstacles. 27.6kV starts a whole new
technology. Corona problems start and faults result in massive explosions so
once at that level the sky is the limit. 44kV is talked about but I have not
seen it used u/g.

Lots of 115kV, 230kV and 500kV is used here for cross country transmission
on towers. These voltages are rumoured to have some u/g cryo circuits in
use, somewhere??


4800 maybe ?

John T.

That should give 66kV phase to phase. Very HV for underground system,
especially in older installations as the insulations were quite expensive,
at that level.



Pretty sure. I often "chat" with the guys installing/repairing equipment.
That's the number I remember.

Semi-rural east central Virginia.

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[Josepi]

The big difference is big cost. I hate seeing old cables almost 10" in
diameter using old insulation technologies laid in a trench. It wasn't too
long ago this was all done with cloth fibres and insulating oil.

When you run through a current transformer the sheild has to be bared or the
CT doesn't work. Same at all the ends. 38kV takes some HV technology that
2400 doesn't need. Corona rings, and special connections for example. Then
there is that "sheild" as you called it that would more likey be a
concentric neutral. You need room for the stress cones. You know how much
space those will take up in a transformer compartment not to mention the
explosion vents required on the cabinets and human safety clearance issues.

10A of load? I doubt they would increase the costs of insulation that high
for 10A. That's a crock and it probably doesn't happen. Fault levels are not
determined by the load capacity of the wire but rather the source impedance
and the conductor impedance.


What difference does it make?

The HV cables are individually "shielded." You can see this at any place
the HV "goes underground." It doesn't really matter whether another phase
is in the same trench. The highest voltage to ground/neutral is the 38kV.

The important questions are: 1) Can nominal 50/1 turns ratio transformers
at that voltage be designed to be relative cheap and reliable?; 2) Can
cable systems (to include field repair splicing & repair) be designed to
cheaply and reliably handle the 38kV to ground?

At those voltages the currents would typically be less than 10 amps.
Faults are quickly cleared so the high voltage doesn't present any extra
public risk.