Technical Feasibility of Underground AC Transmission Cable Grid

[mbrooke]

Hypothetical question: Cost aside, would it be technically possible to operate an underground transmission grid consisting of solid dielectric cables? Burying all transmission lines 500kv and below on the east coast, west coast, UK or European Union as a discussion reference.

Would shunt reactors at each substation be enough to compensate the immense amount of reactive power? Would switching transients and risks of generator pole slipping make it simply physically impractical?

Somehow I have a feeling there are way more theoretical limitations than I am imagining atm. However, the idea of an entire system 120 volts to 500kv being placed underground with brick GIS substations has an allure to me. Outages would go down I would think.

 

[LarryFine]

First thoughts, in no particular order:

Every inch of a pathway would have to be trenched or bored instead of a pole or tower set at regular distances apart.

Buried cables would be susceptible to damage from rocks, ground movement, careless digging, etc.

The cost-per-foot/mile/etc. would be much higher in both labor and materials compared to overhead lines.

 

[bwat]

I know you said "cost aside", but it's hard to get past the cost difference between underground and overhead. If you've ever done a cost estimation for a new MV utility feed where overhead vs underground was evaluated it is quite significant at times. Even for small and short MV services it can be tens of thousands of dollars for underground vs next to nothing for overhead.

I'd probably agree that outage frequencies may go down... but the duration of the outages when they do occur could go in the opposite direction, so your outage-hours could be relatively the same or worse. And now everything was just more expensive.

 

[drcampbell]

Pirelli seems to think so. They buried a 380 kV cable in 2004.

https://www.pirelli.com/archivePdf/this_is_pirelli/investor_relation/news_pdf/vienna_15_01_2004eng.pdf

 

[electrofelon]

What is the highest voltage commercially available MV/HV cable that is available? What would a 500 KV cable look like?

 

[mbrooke]

What is the highest voltage commercially available MV/HV cable that is available? What would a 500 KV cable look like?

Like a large co-axial, literally.

 

[drcampbell]

They sometimes have a helical insulator to cut down on losses, and a gas fill -- nitrogen or sulfur hexaflouride -- to maintain dielectric strength.
This type is often used for connecting ratio & television transmitters to their antennas. I don't know whether they're also used for electric power transmission & distribution.

This example is 5 inches in diameter and rated for 12 kV.

HJ9HP-50 - Dielectric

HJ9-50, HELIAX® High Power Air Dielectric Coaxial Cable, corrugated copper, 5 in, black PE jacket

www.dielectric.com

 

[LarryFine]

This type is often used for connecting ratio & television transmitters to their antennas.

When I saw the pic, I thought of the coax lines I've seen used at cell towers we've wired power and grounding for.

At a tour of a TV station years ago, we saw solid copper pipe inside solid copper pipe used as a transmission line.

 

[electrofelon]

Like a large co-axial, literally.

I know silly, I am familiar with mv cables. I'm using 15kv CN on a job right now. What I mean is how much dielectric would be required? What would the OD be?

 

[electrofelon]

They sometimes have a helical insulator to cut down on losses, and a gas fill -- nitrogen or sulfur hexaflouride -- to maintain dielectric strength.
This type is often used for connecting ratio & television transmitters to their antennas. I don't know whether they're also used for electric power transmission & distribution.

This example is 5 inches in diameter and rated for 12 kV.

HJ9HP-50 - Dielectric

HJ9-50, HELIAX® High Power Air Dielectric Coaxial Cable, corrugated copper, 5 in, black PE jacket

www.dielectric.com

View attachment 2556997

Us hams call it Hardline

 

[electrofelon]

Looks they so make 500kv cable:

Telecom, Energy & Cables Solutions | Prysmian Group

Telecom, Energy & Cables Solutions | Prysmian Group

www.generalcable.com

 

[mbrooke]

Looks they so make 500kv cable:

Telecom, Energy & Cables Solutions | Prysmian Group

Telecom, Energy & Cables Solutions | Prysmian Group

www.generalcable.com

But of course!

I wish we had the old emojis back.

 

[mbrooke]

They sometimes have a helical insulator to cut down on losses, and a gas fill -- nitrogen or sulfur hexaflouride -- to maintain dielectric strength.
This type is often used for connecting ratio & television transmitters to their antennas. I don't know whether they're also used for electric power transmission & distribution.

This example is 5 inches in diameter and rated for 12 kV.

HJ9HP-50 - Dielectric

HJ9-50, HELIAX® High Power Air Dielectric Coaxial Cable, corrugated copper, 5 in, black PE jacket

www.dielectric.com

View attachment 2556997

Thats similar to a super conducting cable a few POCOs tried:

http://mydocs.epri.com/docs/PublicMeetingMaterials/MRNYPKPLTGV/18-MCCALL-ABOUYAALA-HYDRA_Update.pdf

 

[electrofelon]

500kv with a 750 mcm conductor is about 4.6 inches OD. Weight is 8.2 pounds/foot.

 

[mbrooke]

500kv with a 750 mcm conductor is about 4.6 inches OD. Weight is 8.2 pounds/foot.

Its good stuff


BTW, try 2,500 kcmil. 750 is kind of small.

 

[electrofelon]

2500mm is 6in OD, 25 lbs/foot

 

[mbrooke]

2500mm is 6in OD, 25 lbs/foot

You've got real cable

 

[electrofelon]

You've got real cable

Ain't no 'Fischer Price my first Electrical cable' for sure

 

[mbrooke]

Ain't no 'Fischer Price my first Electrical cable' for sure

Neither is the splice, ain't not ideal twister.

 

[LarryFine]

I wish we had the old emojis back.

Steal them from other sites and paste them. Go crazy!