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Considering a monopolar HVDC system, with the ground as the return path.

I was wondering if the current path is physically speaking like the red one or the green one in the picture? I think it is like the green one, although if there are living beings in the middle, they can be thunderstruck by the current. The green one considers that the earth has a charge.

(It's strange to say that nobody has explained me this thing until now, and I'm a student at Politecnico di Milano, I'm following the Smart grid path and I am in the fourth year. Unluckily, these such things usually are not explained.) Do you know the answer?

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Marcus Müller
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Samuele Benito Di Gioia
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    It's like the red one. – Andy aka Mar 20 '21 at 11:57
  • @Andyaka are you sure? Is it not dangerous for living beings in the middle of that path? – Samuele Benito Di Gioia Mar 20 '21 at 12:05
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    Yes I'm sure and yes, it can be dangerous under certain circumstances and that is why return conductors are used or the earth return is used in remote and mainly uninhabited areas. – Andy aka Mar 20 '21 at 12:20
  • thank you so much @Andyaka !! – Samuele Benito Di Gioia Mar 20 '21 at 12:22
  • @Andyaka HVDC engineer here. Not only that, if the ground conditions aren’t favorable enough in the first place, ground return isn’t even an option. – winny Mar 20 '21 at 12:59
  • @winny cheers - do you work for a big company like GE maybe? – Andy aka Mar 20 '21 at 13:00
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    Samuele, the red and the green drawings are identical: unless you assume that the whole current is used to charge a large capacitor made of "planet earth", the charges must somehow travel. – Marcus Müller Mar 20 '21 at 13:09
  • @Andyaka Something like that. – winny Mar 20 '21 at 13:11
  • I know a Chinese Eng who has installed 2GV dc distribution lines with PD skills that rival or surpass ABB, one PhD who quit ABB to become the Transformer magazine editor. – Tony Stewart EE75 Mar 20 '21 at 13:11
  • @winny how is that ground return conductor implemented, e.g. in a buried or a submarine cable? I could basically imagine either two equally well-isolated identical conductors, or, considering the ground return should, if nothing goes terribly wrong, be relatively close to earth potential, so maybe something coaxial with "hot" on the central conductor and "return" on the outer, with only "light" (for whathever light would be compared to megavolt insulation) isolation around? – Marcus Müller Mar 20 '21 at 13:11
  • They are differential lines and earth moisture path is unreliable. – Tony Stewart EE75 Mar 20 '21 at 13:12
  • re the planet earth [capacitor](https://electronics.stackexchange.com/questions/554285/current-path-into-the-ground#comment1445273_554285): that's actually been considered, but as matching circuit for ELF transmitters (you take two seas as electrodes, call denmark an electrolytic capacitor with a bit of leakage current, and get on with it). – Marcus Müller Mar 20 '21 at 13:13
  • Even transatlantic voltage drop on marine cables is huge. (KV) – Tony Stewart EE75 Mar 20 '21 at 13:14
  • @TonyStewartSunnyskyguyEE75 yeah, that's expected... one of the hard parts of getting data from Europe to the US east coast is that the amplifiers that you need every couple hundred kilometers need power, and unlike light in a fiber, there's a power loss that's not only 0.2 dB/km when transporting it from shore to amplifier. – Marcus Müller Mar 20 '21 at 13:16
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    @MarcusMüller Exactly like that coaxial you describe. It also gives favorable, if you can call it that, hard short circuit to low impedance ground (0 V) via the cable and not local earth (dirt) if someone digs into it or drop and drag an anchor across it. – winny Mar 20 '21 at 13:27
  • Niiiice! Thanks! Makes a lot of sense, you get all the E-field on the inside, and not somewhere you'd have to control somehow else. – Marcus Müller Mar 20 '21 at 13:29
  • thank you all so much for your help! I really like this platform because I can communicate with people with high experience and knowledge like you, and I need it since I am only a simple student, graduated in electrical engineering but I'm still a student of Smart Grid so I still don't know a lot of things that can be acquired mainly by working in the field. Thank you again! – Samuele Benito Di Gioia Mar 20 '21 at 18:41

2 Answers2

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I think it is like the green one, although if there are living beings in the middle, they can be thunderstruck by the current. The green one considers that the earth has a charge.

We generally assume that the charge of Earth is neutral. For a DC link as you describe current leaving the ground at the feed end and travelling through the transmission line must enter the ground at the other end.

As you probably suspect already, the current will not travel in a direct path back to the feed end but will spread out a bit like the magnetic lines of flux on the bar magnet and iron filings experiment.

A bigger engineering problem is that the electrolysis of any ground connections. With an AC electrode there might be some chance of acceptable lifespan but for DC it will be eaten away very quickly.

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Figure 1. Consideration of ground return and metallic conductor return is given in Ireland's Grid West Project HVDC Technology Review section 2.2.2.1.

This report also points out problems with corrosion of third-party buried metal pipelines and magnetic saturation of transformers. It might be worth a read. (It might be worth my while reading it too as it passes close to my home.)

My understanding is that for the reasons listed above that a return conductor is a better engineering solution. This may initially seem wasteful but there's a benefit: the voltage can be split into a symmetrical supply arrangement. If, for example, a 1 MV DC is required then the supply would be configured as ±500 kV. The savings come in the reduced insulation costs and, presumably, easier handling during installation.

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*Figure 1. Route map with options.

At the moment a project to lay the 575 km Celtic Interconnector between Knockraha substation in Cork, Ireland, to the La Martyre substation in Finistère, France, is proposing ±320 kV or ±500 kV for transmission. Capacity will be 700 MW and will cost €1 billion. (The Republic of Ireland's peak demand is in the order of 5000 MW so this is > 10% of that.)

Transistor
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It's the red one.

And actually you can have big potential differences on the ground. In the case for example of very high voltage powerlines fallen in the ground, you can actually get electrocuted by just walking close to them, as the potential difference between your two feet could be high enough to create a path through you.

For electric charge to flow you need a close circuit of some sort and a potential differential. The ground being a potential path.

One could imagine the ground to be ionized to some extent, as your green drawing, but that would be minimal.

Damien
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  • "*For electron to flow ...*" Please don't reinforce the notion that all electric current is electron flow. There are enough confused contributors to the site already. In the ground it is quite likely to be positive ion flow as well. – Transistor Mar 20 '21 at 19:08