Return current through General Mass of Earth 1

[abrahamJP]

Hi,

I have in mind this question for years,now I got this forum to ask this question.

When we model a simple earthing system as in attached PDF,we assume Earth current return to source even when source earth is kilometers away.

I understand whatever the current go to earth through earth electrode,it get dissipated as heat and finishes it journey in nearest earth electrode.If this is true how earth current return back to source in models.

If a transient fault like Lightining happen,I can think of such a flow of current.But why in normal faults,how we
assume current goes through general mass of earth??

 

[stevenal]

Not true. Energy is dissipated, not current. There is no end to the journey; current returns to the source and keeps on going until the circuit is opened.

 

[racookpe1978]

So, this would allow a long-distance, high-voltage DC line to be single-wire (return current through the earth to source) but that no junctions or slipts be allowed, right?

That is, a long distance DC high volt line needs to be point-to-point transmission
(AC generator -> ac-ac transformer -> ac-to-dc converter-> dc long distance line-> dc-to-ac converter, ac-ac transformer),
and not a source by itself of a DC-supplied network with many dc-ac conversion stations along the dc route. True?

 

[cranky108]

In theory. What can happen is something else.

Soil has a good deal of impedance between one clod and another. But given there are millions of clods, it tends to act like a very low impedance path. If the local soil is, say rock or sand, you will find it difficult to make contact with enough clods to get a good connection at the ground rod. If there just happens to be a pipeline, or rail road in the area, you could find they can conduct a significent amount of current, which the owners of said pipeline don't like.

 

[dpc]

At one time the DC intertie running between California and Oregon had a reduced rating for operating with an earth return. To my knowledge, this has never actually been done. There are a lot of environmental concerns.

 

[TheBlacksmith]

"But given there are millions of clods," sounds like where I work! Given the high degree of reliability expected and the environmental concerns, this theory will probably remain unused. Just look at the electrical gremlins that infest a 10 year old metal car.

 

[mgtrp]

Given the high degree of reliability expected and the environmental concerns, this theory will probably remain unused

If you mean the use of earth for a return path, this has been in use almost since the beginning of HVDC systems back in the 60's.

 

[TheBlacksmith]

I stand corrected. My experience is in automotive and shipboard systems up to 15 kV.

 

[cranky108]

Gee, my 25 year old truck dosen't have electrical gremlins. It has a few bad parts, which consist of moving parts, and silcon assemblies. But the return path is not the problem.

The problem with high voltage dc with earth return is in the grounding. Or getting enough ground contact to accept the current. The current will tend to dryout the local ground grid soil, at which point the grid resistance will increase. There are things you can do to increase the ground contact, like use charcoal, or some high metal supliment for the soil, but it is the same thing as increasing the ground grid size (I think it is cheeper than increasing the ground grid).

 

[davidbeach]

My understanding of the Pacific DC Inter Tie is that the southern grounding electrode is in the Pacific Ocean, 10's of miles west of the Sylmar Converter Station and that the northern grounding electrode is in the Columbia River, not far from the Celilo Converter station.

 

[stevenal]

I believe it is true that that most SWER DC links are point to point, but I fail to see why network arrangements couldn't be made to work.

 

[HowDidYouBreakThat]

They're doing it in Alaska.

https://www.denali.gov/dcpdb/Data/attachments/Polarconsult_HVDC_Project_Comments - Wies.pdf

 

[rcw retired EE]

According to a BPA Publication on the Celilo Station (circa 1970):

The Northern Ground Terminal is at Rice Flats a few miles south of Celilo directly under the line. It is a 1700 foot radius circle of #4/0 awg copper connecting 1,067 cast iron anodes each 18" diameter x 60"long. Total wire is 10670 feet. Anodes and wire are buried 6 feet deep and surrounded with a 2'x2' bed of carbon coke. Three 1700' +/- long "spokes" of 1000MCM copper connect the circle to DC Intertie tower 8/1 that is in the center of the grid.

GE did testing on the ground system prior to system energization. "Telluric currents were noted, as was cathodic protection where it was present. 300 amps of current were circulated in the earth between the ground electrode at Rice Flats and the ocean electrode in Santa Monica Bay. It was assumed that the effect of 1800 amps could be extrapolated linearly from these results. ... With the location chosen, it was anticipated that there would be no insurmountable problem with the existing pipelines and other metallic structures in the ground."

I wonder if the anodes are still there 40 years later. I don't know if the line ever had to operate single pole with ground return.

 

[stevenal]

Wiki says:

The grounding system at Celilo consists of 1,067 cast iron anodes buried in a two foot trench of petroleum coke, which behaves as an electrode, arranged in a ring of 3,255 m (2.02 mi) circumference at Rice Flats (near Rice, Oregon), which is 10.6 km (6.6 mi) SSE of Celilo. It is connected to the converter station by two aerial 644 mm2 steel-reinforced aluminum (ACSR) cables, which end at at a strainer situated at 45.4975865°N 121.0646206°W.

The Sylmar grounding system is a line of 24 silicon-iron alloy electrodes submerged in the Pacific Ocean suspended in concrete enclosures about one meter above the ocean floor. The grounding array is 48 km (30 mi) from the converter station and is connected by a pair of 644 mm2 aluminum cables.

 

[MKFPE]

Google "Carson Pollaczek" (not in quotes) or "Carson's equations" (in quotes) for lots of information on this subject for AC power systems.

 

[abrahamJP]

Thank you for all replies .It help me to have a feel on this
complex subject .To know it further,I need to go through Carsons
equations and experince on how this is acheived in different parts
of world

 

[PersonalProfile]

I may have missed the issue, though this may be of interest:

http://en.wikipedia.org/wiki/Single-wire_earth_return

Regards,
Lyle

 

[tlrols]

PDCI ground return operation is possible and does occur on rare occasions. It is NOT desirable and swift action is taken to cut PDCI path flows and fix the problem that caused blocking/ground return operations. Ground return works, but as one might imagine there is an unpredictable element associated with that sort of operation. Two wire operation is certainly preferred.