# Underground cable

1. Dec 31, 2008

### raeesi

HI

why underground cables take continuous relatively high charging currents, while overhead line take less charging current

Underground cables have a better voltage regulation than overhead lines due to the lower inductance (how) formula plz

Last edited: Dec 31, 2008
2. Dec 31, 2008

### Staff: Mentor

Welcome to the PF. It would help us if you could post some links to the information you are asking about. Your questions are general enough that we would have to first guess what you are asking about, and then try to come up with answers.

Are you referring to HV AC Mains power distribution? If so, which level? What voltages? What distances?

What is the context of your question? Is this for homework or coursework? Or are you looking to design your own home-brewed power distribution grid?

3. Dec 31, 2008

### raeesi

I am talking about 2Km for 33KV and it is HV AC

4. Dec 31, 2008

### Staff: Mentor

Okay, and where are you getting your inductance and current numbers from?

5. Dec 31, 2008

### raeesi

there are no numbers I am talking in general and I mentioned 33Kv and 2Km by guess

1-They are less liable to damage by storms or lightning.
2-Underground cables have a better voltage regulation than overhead lines due to the lower inductance they have as a result of the compact, well insulated construction.

1-the cost of underground cables is much higher than that of equivalent overhead line. High voltage cables are 8 to 15 times more expensive than equivalent overhead lines.
2-underground cables take continuous relatively high charging currents. This can cause high I2R losses as well as high dielectric losses.
3-underground cables have a limited power transfer capability caused by lower thermal limits in cable insulation.
4-cables are more liable to permanent damage following a flash-over, this is because while air, which is the insulating medium for overhead lines, is restorable, cable insulation cannot be restored if damaged by a flashover.

6. Dec 31, 2008

### mheslep

Without knowing more about underground power cables I'd speculate that the greater charging current is simply due to a higher line to line capacitance per unit length. The distance between conductors is relatively small in the underground case and the dielectric constant, whatever it may be, is greater than air: $C = \epsilon_{r} \frac{A}{d}$

7. Dec 31, 2008

### dlgoff

I agree that this is the reason. After he first posted I was looking around and found this PDF on the subject of how cable ratings are dependent on charging current.
http://www.contactenergy.co.nz/web/pdf/our_projects/waikatowindfarm/june2008/R01_HMR_Connection_to_220kV_Main_Grid_Undergrounding_Study_Attachment1_Appendix7and8.pdf

8. Jan 1, 2009

### raeesi

thanks my friend I got some idea about charging current

9. Jan 3, 2009

### mheslep

I wonder if the new push for DC power lines will see much of a economic improvement in the underground domain since the charging current goes to zero after the line is energized.

10. Jan 3, 2009

### stewartcs

I didn't know there was a big push for DC power lines.

Probably not since there is more infra-structure required to get the AC to DC.

CS

11. Jan 4, 2009

### mheslep

For a given voltage, it has always been more economical to transmit electrical power as DC rather than AC because of the greater line losses when using AC, going back to battles between Edison and Tesla. DC transmission also eliminates the challenging problem of synchronizing line frequency and phase between remote locations.

The main problem for DC has been in stepping the voltage up and down. At the time of Tesla's victory the AC transformer provided the only practical, no-moving-parts way of doing so. Technology gradually came to the rescue, and the first commercial HVDC line was installed in '54. Today high power semiconductor thyristors and FETs make DC voltage changes practical and increasingly economic, though it still doesn't compete with AC at the end points. Thus at the moment HVDC is only used on very long lines where the power savings pays for the end equipment; underground and undersea cables are even more attractive for DC for the reasons discussed up thread. Recently alternative energy sources such as wind and concentrated solar, where the source is variable and often far removed from the load, have stepped up the buzz on HVDC.
See e.g.

12. Jan 5, 2009

### stewartcs

I found a little more info on the pros and cons of AC vs. DC: