# DC Transmission - Third Wire?



## Big John (May 23, 2010)

Happened to be driving through NH the other day with some guys from work when we went by a transmission tower with the biggest insulators I've ever seen.

Being a bunch of electrical nerds we got to talking about it. It sure looked like DC, because there were two main cables on the huge insulator sets. What threw us was there was a third smaller insulator and cable smack in the middle of the structure. It wasn't a static line, because it was at least the size of a traditional 69kV insulator, though still much smaller than the other two.

Looking it up, I found pictures of the 300kVDC "Northern Pass" transmission line that goes right through that area, and you can also see that third wire on those pictures, so I'm thinking that's exactly what we saw. 

Anyone know what that middle wire would be for?


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## donaldelectrician (Sep 30, 2010)

Big John said:


> Happened to be driving through NH the other day with some guys from work when we went by a transmission tower with the biggest insulators I've ever seen.
> 
> Being a bunch of electrical nerds we got to talking about it. It sure looked like DC, because there were two main cables on the huge insulator sets. What threw us was there was a third smaller insulator and cable smack in the middle of the structure. It wasn't a static line, because it was at least the size of a traditional 69kV insulator, though still much smaller than the other two.
> 
> ...




Ground return on bipolar hvdc system ?

don


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## Celtic (Nov 19, 2007)

donaldelectrician said:


> Ground return on bipolar hvdc system ?
> 
> don


I agree:










*High-voltage direct current*
[wikipedia]





...although I work on AC only, so I might be wrong.


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## Big John (May 23, 2010)

So apparently it's called the electrode line, and as best as I can tell that conductor is unused during bipolar operation because the load is split between both outside conductors at +300kVD and -300kVD to make a 600kV circuit.

So that line does one of two things, and I'm not sure which: It either runs part of the distance of the transmission line and eventually connects to a remote ground-electrode. In the event of a fault on either cable, this would allow the system to continue to run at half voltage, using the remaining conductor and ground-return. This would be a monopole setup.

Or that cable is actually a solid conductor run all the way between both converter stations, and also during monopole operation, runs close to ground potential as a return path, but is not actually enabling ground return.

Sorta, kinda, maybe?


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## wildleg (Apr 12, 2009)

If it was intended to be used for 1/2 I would think it would be sized 1.5 times the other conductors (doesn't appear to be). I'm thinking maybe it's just a reference for monitoring. (wag) If you know someone at the power company up there why don't you find out and post for us ?


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## erics37 (May 7, 2009)

What's the advantage of DC transmission?


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## Big John (May 23, 2010)

wildleg said:


> If it was intended to be used for 1/2 I would think it would be sized 1.5 times the other conductors...


 I don't follow where the 1.5 comes in? But you're right, it does seem undersized to be a return path from one end of the transmission line to the other. Which makes me think it is actually just going to the remote electrode: It would only have to carry the ground-return for a short distance and wouldn't have to be upsized for voltage drop.


> ...If you know someone at the power company up there why don't you find out and post for us ?


 I only know a crew who was doing the SCADA system for a DC line, if one of them can explain it, I'll post their answer.

Regarding the benefits of DC transmission, I can think of two major ones: 

The line losses are less than those found on AC. The very high voltage allows for comparatively low current, so voltage drop is not nearly as big a concern as it would be on a DC distribution circuit. And for very long distances there are none of the reactive or radiative losses that you get when alternating a current over the same length of wire.

The other advantage is that it allows you to tie in different sections of the electrical grid where the frequencies are not exactly in synch, because you are electronically creating the 60Hz frequency at the load end, so it does not matter what the line end is connected to. Given that the line in those pictures is an intertie to Canada, it wouldn't surprise me if this was also the case.


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## Big John (May 23, 2010)

And what helped me confirm it was even DC to start with was this map of our grid, for those who haven't seen it. You can see the DC link running from the Canadian border down to Massachusetts.


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## wildleg (Apr 12, 2009)

big john said:


> i don't follow where the 1.5 comes in? ....


 
3/2 = 1.5


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## Celtic (Nov 19, 2007)

erics37 said:


> What's the advantage of DC transmission?


Not simply DC...but HVDC



> An updated, high-voltage version of DC, called HVDC, is being touted as the transmission method of the future because of its ability to transmit current over very long distances with fewer losses than AC. And that trend may be accelerated by a new device called a hybrid HVDC breaker, which may make it possible to use DC on large power grids without the fear of catastrophic breakdown that stymied the technology in the past.


*High-Voltage DC Breakthrough Could Boost Renewable Energy*


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## Meadow (Jan 14, 2011)

Any body find out yet?


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## 8V71 (Dec 23, 2011)

I wonder if it could be a voltage sense line to keep the voltage on the far end stable regardless of load. This is commonly used on DC supplies with long runs of wire to monitor the voltage drop and automatically adjust the supply up or down. There is almost no current in this wire, only voltage.


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## chrisfnl (Sep 13, 2010)

erics37 said:


> What's the advantage of DC transmission?


Transmission lines have capacitance and inductance (all cables do, expressed in value per length, but typically it's negligible in distances not measured in kilometers or miles)

Xc = 1/(2[pi]fC)

Xl= 2[pi]fL

If f = 0, as in DC, then Xc and Xc equal 0.

Huge savings in terms in reactive losses in a DC transmission line.


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## gottspeed (Mar 8, 2010)

Anybody have pictures of the gates they use to rectify from ac? They must be massive...


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## Jerome208 (May 10, 2013)

I'm pretty sure this is the right answer:



Big John said:


> runs part of the distance of the transmission line and eventually connects to a remote ground-electrode. In the event of a fault on either cable, this would allow the system to continue to run at half voltage, using the remaining conductor and ground-return. This would be a monopole setup.


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## donaldelectrician (Sep 30, 2010)

Jerome208 said:


> I'm pretty sure this is the right answer:


Mayby in part . 

I took Big Johns name for the wire , Electrode Line , and looked in the

glossary of a HVDC New Zealand inter island upgrade .

electrode lines
HVDC circuits connecting the a.c./d.c. converter stations at Benmore and Haywards to their low resistance solid earthing points.* HVDC electrode lines are insulated from their supporting steel and wood pole structures and although these circuits are nominally at earth potential, electrode line voltages can increase significantly under certain loading conditions.* Special earthing procedures apply to the application of portable earthing to HVDC electrode lines.

I would also think under certain Fault conditions also .


Don


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## bobelectric (Feb 24, 2007)

Maybe an interconnecting comunication cable.


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## vinister (Apr 11, 2012)

gottspeed said:


> Anybody have pictures of the gates they use to rectify from ac? They must be massive...


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## Awg-Dawg (Jan 23, 2007)

I dont know what that is, but I want my picture taken with it.


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## jza (Oct 31, 2009)

That is absolutely insane.


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## Awg-Dawg (Jan 23, 2007)




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## triden (Jun 13, 2012)

chrisfnl said:


> Transmission lines have capacitance and inductance (all cables do, expressed in value per length, but typically it's negligible in distances not measured in kilometers or miles)
> 
> Xc = 1/(2[pi]fC)
> 
> ...


DC is also great for linking multiple distribution systems together. With DC you don't have to worry about a frequency unbalance causing a fault. It's a great way to share power with different countries/systems.


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## triden (Jun 13, 2012)

Awg-Dawg said:


> I dont know what that is, but I want my picture taken with it.


That's a valve hall. It's a big-ass load of thyristors used to rectify the AC waveform to HVDC.


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## Cl906um (Jul 21, 2012)

Around her the power company runs fiber optic in the static line. They are currently adding these to existing lines to communicate as well. Just kinda wanted to ask what the odds are for dc lines to get struck by lightning?


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## xaH (Jun 21, 2013)

Seriously awesome thread. Major :nerd::nerd::nerd: factor too.


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## Celtic (Nov 19, 2007)

cl219um said:


> Around her the power company runs fiber optic in the static line.


They do the same here ..the fiber is communications between points [like switch yards ]



cl219um said:


> Just kinda wanted to ask what the odds are for dc lines to get struck by lightning?


Same odds as it is for AC lines :jester:


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