Experience with Subterranean Transmission Lines (161kV+)

[alphanumericname]

Hi everyone,

Does anyone have experience with underground high-voltage transmission lines? If so, and you are comfortable with answering some questions, I am looking for more info on any or all of the following:

1. What circumstances necessitated the use of underground lines?
2. What is the method of insulation? (Fluid, gas, dielectric cable, other?)
3. Were the lines buried in a trench, installed in an access tunnel, submerged beneath a body of water, or another method?
4. Do your lines require the use of specialty equipment at both ends of subterranean sections of the line, or do you immediately exit the ground and transition to overhead or enter a substation?
5. What has the actual maintenance cost been since install? How does it compare to your overhead lines?
6. Have you had any failures? If so, how long after construction before a failure occurred? How difficult was it to remediate?
7. Would your utility consider installing underground lines again?
8. What was your construction and/or maintenance crews' experience with underground lines?
9. Are there any emerging/innovative technologies that would be worth waiting on before going forward with a subterranean TL project?
10. What professional groups or societies have groups focused on underground transmission lines? Do they offer trainings or information sessions?

Thank you!

 

[ema157]

We have some 220 kV transmission cables.

1. When required in a high density urban area, or when a property developer does not want the existing overhead lines (the developer has to pay for the undergrounding).
2. XLPE.
3. Direct bury in ground, with concrete joint bays.
4. If underground section of a line, there are cable transition structures with cable sealing ends and surge arresters. For substations, the same or terminate into GIS.
6. Yes, the cable joints.
8. We used OEM cable jointers. We have a contractor with specialist cable teams.

 

[cuky2000]

1. Congested areas with obstacles for overhead line. Occasionally crossing HV short line is more convenient than having overhead crossing. Meet electrical and maintenance clearances is easier with UG line for special cases.
2. Agree that XLPE is today a good choice for HV underground application. Oil filled cable bring other environment and liability issues.
3. In the US the most popular installation method is concrete encased ductbank with splicing vaults. Europeans prefer direct burying of UG cables. Direct bury of cable is les expensive but less mechanical protection and lower reliability.
4. Cable termination, surge control, cable attachment, link box, etc is require. In many instance transition station from OH to UG is needed.
5. Installation cost of UG is around 7 to 10 times the OH cost. Similarly the O&M cost is higher and useful life is less than OH lines.
6. UG Cable rate failure is less than OH lines because are less exposed to the elements. However, the repair time is longer and not foreclosing is acceptable in UG line.
7. The first choice for most utilities are OH line. However in large cities UG may be the only choice available such as in NYC,
8. Many utilities have specialized crew and equipment to operate and maintain UG cables. Others, use specialized contractors
9. Trench-less technology such as directional drilling could save significant resources and is more environmental friendly
10. IEEE, IEC, CIGRE among other professional organizations have a chapter dedicate to the UG Transmission lines.

 

[7anoter4]

Around 1970, I was a young engineer in a vast Siderurgica [steel manufacturing] complex.
I was responsible for 110 and 10 kV distribution maintenance [with some 130 technicians, most of them].
The complex contained 4 blast furnaces, a steel plant, and 4 thick sheet metal rolling mills [and many auxiliary installations].
40 km of 110 kV 750 mm^2 copper paper insulated-oil filled [2 at] single-core cables ran between production units.
The complex was supplied by 2 double-circuit OVHD lines of 110 kV and 150 MW gas-turbines, using the gas produced in steel furnaces.
From the central substation of 40 [ 110 kV] switchgears, the cables ran underground, partially in 3 m high tunnels and partially in a trench up to 13 secondary 110 kV substations dispersed in about 1-0.5 km [average]. Except for some junction boxes, all the terminations were indoor mounted in the substation.
I'll try to answer your questions now.
1. Since the entire surface of 3 sqr.km was filled with equipment and heavily transporting roads
no overhead lines were possible. 400/110 kV overhead lines were all around[outside] the complex.
2. The cable insulation was paper oil-filled [low pressure approx 2 at.].
3. Most of the cable run was in tunnels, on steel cable racks.
4. I worked in this complex for no more than 7 years. No maintenance cost -except for some cable tray repairs at the beginning.
5.In some tunnels due to summer to winter temperature difference the cable tray columns [pillars] connection screw was damaged and need to be replaced.
6.A very well-trained crew [Siemens] people [Siemens was the supplier of cable and accessories- terminations and junction box -even the terminals.]
7.no relevant
8. Our people, design, construction, and maintenance crews were highly prepared.
9.It was 50 years ago. I am not up to date.
10. My actual occupation, in the past 50 years, was Power Station design and cable manufacturing.