I am trying to understand ground current in a power distribution system.
The basics, of course, are simple: in the power grid, the "hot" wires carry high voltage, while the return path (the neutral wire) is connected to ground, so ground forms part of the circuit.
Some people seem to claim that this ground current is harmful. Be that as it may – I am not trying to argue about that question, but rather want to understand how ground current really flows.
The only source I could find was by Duane A.Dahlberg, and to be frank, I could not make heads or tails of it. Other than that, I have not been able to find any reputable source of information.
One of the claims I read was that somebody standing on the ground could experience a potential difference of 0.5 V between his right foot and his left foot. That seems very hard to believe.
I am most interested in how ground current would flow related to a 500kV overhead line, about 100 miles long.
Specifically, what I would like to know.
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What voltage and amperage (order of magnitude) is the actual ground current? Obviously, in a 220kV line, most of the 220kV should go into the load. How much is lost on the actual wire, and, what is the voltage between ground on one end of such a transmission line to ground on the other end?
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Does the ground current travel along the surface, or does it penetrate into the ground?
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How does the type of material affect ground current (e.g., dry soil, rock, ground water, etc.)
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Are there differences between AC and DC power lines with respect to ground currents?
Best Answer
Your question contains one very common misconception, and that is that large amounts of current flowing through ground is normal.
In this diagram, notice the lack of a particular path for current flow into or out of ground. As long as everything remains balanced and a power line isn't touching the earth or a tree, there is no path for current to travel through ground (except some minor leakages).
Here is a wye connected four wire system. Notice how the ground is connected directly to the neutral point in the center, and the neutral (N) line has no coil. In this system, the neutral line is the return path for all single phase loads - that is, anything that is connected from a phase to the neutral. Phase to phase connections are still available, of course. But current flow through ground is still not normal.
Which is a great thing - if you put a current transformer on the ground connection, you now have a high reliability mechanism to detect if the system is grounded. This is a standard feature on the power grid.
Now, lets say a high voltage power line has been broken and is laying on the ground. At 500 KV, there is definitely going to be some current flow through the earth. And as with all current flow, voltage drops when current flows across a resistance. Starting from the 500 KV at the end of the line, and reaching zero back at the nearest system ground connection, means that there can be a huge difference between one foot and the other in that vicinity. In the industry, we call this step voltage differential, and it can be lethal. It's the reason why you may have heard that you need to shuffle away from lightning strikes and power lines, rather than walk; that keeps your feet close to each other and prevents a current flow from leg to leg.
On the off chance that a line has been downed and grounded, the current flow will radiate away from the point of earth contact to whatever path is most favorable for current flow. If the topsoil is recently wet, it will tend to stay there. If everything around is fairly dry, there may not actually be much current flow at all, and it will radiate in nearly every direction as the voltage charges the ground. It will flow through ground water, if it can get there.
As far as the difference between AC and DC grounding events, the physical characteristics of DC make it more likely that it does not find a good path back to the nearest system ground, which makes it more likely to have a potentially lethal step voltage differential.