Discussion in 'Homework Help' started by Cerkit, Sep 26, 2013.

1. ### Cerkit Thread Starter Active Member

Jan 4, 2009
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Hi. I wanted to know if in a solid earthed neutral system where there is a neutral earth resistor at a primary substation, when there is a fault on an overhead line for example and a conductor is on the ground does the current flow through the mass of the earth back and through the neutral resistor to the source?? Is that correct that the current flows through the earth (the soil) ??

Jul 18, 2013
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Yes. Earth ground is considered a uniform conductor and when ground rods are set in place for sub stations or towers, transformers, an earth ground resistance check is done of the area in order for the ground resistance to be of an acceptable level.
There was a time in the UK where the Dist transformer had the star point grounded and a service ground was not supplied Just a local earthing system such as water supply or ground rods, a resistance check had to be performed from local earth gnd to the transformer.
Max.

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3. ### WBahn Moderator

Mar 31, 2012
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One thing that might make this easier to understand is that if you have a grounded transformer at one point and your fault-to-ground at a point that is, say, 100km away and you have a fault current of 1000A, that does not mean that you've got this narrow channel of 1000A current running through the Earth for 100km. Think of the Earth as a huge -- I mean HUGE -- ready source of electrons that can be moved around a tiny bit very easily. Now consider that 1000A is 1000 coulombs per second but that is the same as 1 coulombs per millisecond or 1 million coulombs per 1000 seconds. If we have enough charge available, we can support very large currents using lots and lots of electrons that barely move. When a current enters/exits an Earth ground connection is spreads out so far and wide that the number of electrons available to participate in the current is mind-bogglingly huge.

In effect, the Earth acts as an effectively infinite capactor connecting the two points 100km apart with the two "plates" being the local ground around each connection. The charges involves stay local to the connection, just like in a capacitor, but the "current" travels between the two via the changing electric field.

But even that electric field is weak. Let's say that you have a one million volt line at a distance of just 1km. That's an electric field of 1000V/m. The electric field between the terminals of a 9V battery is around 3000V/m.

Last edited: Sep 26, 2013

Jul 18, 2013
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Incidentally on the subject of transmission lines, high voltage transmission from Generator stations is now done with a conversion of AC to DC, the DC is converted back to 3 phase AC at the line end.
Apart from lower line losses, another advantage is that previously with lightening strikes on AC distribution towers, the spike caused by the strike would travel right through to the end user with usual disastrous results.
Now the spike is detected at the DC-AC conversion end and eliminated.
Max.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Looks like your country has or intends to go exclusively DC transmission. In my country we are still predominantly AC transmission. I know DC has been used here where two AC systems are interconnected via a longish undersea cable link.

I'm interested in your comment regarding lightning strikes. AC systems seem to have been quite robust given the frequency of lightning strikes normally encountered in their operation. Electrical transmission engineers have gone to considerable lengths to mitigate potential disasters due to strikes. In what sense are DC systems more robust with regard to lightning strikes? I would think one still has to mitigate their impact in DC systems.

Last edited: Sep 26, 2013

Jul 18, 2013
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The area I live in generates more electricity than we actually need, 15 Hydro stations and 2 Thermal, 6000Mw total.
This does not include the wind farms.
I have both friends and family that are engineers working for the Hydro Co, and have obtained this information just in conversations with them, One of them also travels world wide as a consultant.
From the information I gathered that the lightening strikes do not pose such a problem in the DC transmission, due to the ability to sense any disturbance when detected at the DC to AC conversion point.
I do not have specific details, but I could get more next time I see them.
Max.

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7. ### WBahn Moderator

Mar 31, 2012
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I don't think (my understanding, but not something I can back up) we are looking at going DC in any particularly big way, but using DC for long haul transmission has a lot of advantages. My understanding is that one of the biggest is actually non-technical -- the cost of obtaining and maintaining the right of way for a DC monoline is considerably less than a three phase AC system. Then of course you have the construction costs themselves. Then you have the AC line losses. Furthermore, you can run the DC line at even higher voltage (over a million volts last I heard many years ago) and reduce IR losses even more. And on top of that you don't have to synchronize the two grids in order to bridge them.

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Thanks. I guess I was interpreting this as a trend in Canada rather than North America.

Jul 18, 2013
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One of the reasons it is done here now is lowering the losses due to transmission line lengths we have, the amount gained is quite considerable.
Max.

10. ### WBahn Moderator

Mar 31, 2012
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I can see it as a greater fraction of long-haul in Canada given the physical size and sparse population. The U.S. will be slower to adopt for much of anything beyond very long haul because we already have an extensive traditional distribution network in place.

11. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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I, for one, would love to see 60Hz high-tension lines done away with. They muck with my AM radio reception (I commute under a major distribution network here in S. Fla).

12. ### WBahn Moderator

Mar 31, 2012
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I'm guessing you're not gonna see a big push to completely overhaul the power distribution network just to improve AM radio reception.

Jul 18, 2013
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Especially with the prevalence of FM, Internet radio and Cable stations.
Max.

Apr 26, 2005
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