HVDC line grounding system question

I posted this to alt.engineering.electrical and it was suggested I post this here since this group is more active than alt.engineering.electrical and with knowledgeable people.

(somewhat edited)

There is a long HVDC power line from northern Quebec Canada to Ayer, Massachusetts USA. It operates at either +/- 375 kV or +/- 450 kV depending on source. As I understand it, it is grounded at exactly one point, near Saint-Claude, Quebec. You can see it on this Google satellite view:

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where the power line ROW runs diagonally on the right side, and the actual grounding point is the weird circular shape at the upper left. Additionally, multiple conductors (6) run from the ROW to the circle. The street view at
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near the grounding poing shows the huge towers as well as 4 conductors, two at the top of the towers and two others on wooden poles. They are on rather substantial insulators.

I don't think the lines on top of the towers are lightning arrestors. They are insulated from the towers while lightning lines aren't. In addition, near the south end of this line (near Ayer, Mass.), this line has an insulated conductor as well as a pair of (uninsulated) lightning arrestor lines.

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the other two on the wooden poles seem to >carry too high a potential to being ground wires. they probably could be >a MV feed for a nearby town.

They are not a feed for a town.

I followed these lines to the circular ground point using Google Satellite View zoomed in. It showed enough detail to see the towers/poles (and their shadows) and the lines themselves. They are two of the 6 conductors which go there. Two others are the two conductors on top of the pylons. It appears the other two are also the two conductors on top of the pylons but from the ones continuing past the point where the lines go to the circle.

See:

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the actual HVDC line goes from top right to the bottom, and 6 individual lines go to the weird circular area to the top left.

Does anyone know any details for this rather odd setup? Why multiple grounding conductors, and any details of the circular structure? Do the grounding conductors sometimes carry current during normal operation, perhaps with the line at half power with one side of the HVDC conductors out of service and the grounding line carries the return current? Meaning it's more like the neutral wire in home electrical wiring, carrying current and connected to the actual ground at only one point?

Reply to
Michael Moroney
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To me, that circular feature looks like a radial array of buried ground rods, covering sufficient area to be an adequate earth terminal for such a high power DC power line. I see four radials, but there may be more. The ring may also be a buried ground rod.

Joe Gwinn

Reply to
Joe Gwinn

There are two 'earths'. The main earth electrode for power transmission is located some KM away from the generating station, or main receiving terminal, to reduce local corrosion in equipment. The grounding point is constructed within treated coke packing, which has a defined impedance and low decomposition characteristics.

The station earth is connected to the neutral point of converters at substations, as part of local overvoltage surge protection.

The 'insulated' wires you see above the transmission line are actually surge arrested, not insulated, and provide a distributed surge protection to the system.

RL

Reply to
legg

Old unipolar SCR based under sea HVDC systems have grounding electrodes at each shore and only one conductor runs from coast to coast. Those SCR based systems also require a line potential reversal if energy is to be transferred in the opposite direction.

Newer IGBT based systems are usually bipolar and two conductors are required. What is the point to ground the midpoint except electric safety ? In these systems one conductor is always positive and the other negative, regardless in which direction energy is transferred.

Reply to
upsidedown

Any idea what the resistance of the sea and earth are as measured for this applications?

What might the ground currents be into the coke pile?

Reply to
Cydrome Leader

I+ + I- .

Des Cantons / Windsor circuit ~770A under worst case single line fault. Normally

Reply to
legg

A few things I have learned since posting this:

The grounding area is spread out to decrease the "step charge" voltage (voltage between two points on the ground) below a certain level. I saw a diagram for a circular grounding system (unsure if it was the same one I mentioned) and there were multiple cables leading to the grounding circle, to different points evenly spaced along the circle, to ensure the current is dissipated evenly. Important when an irresistable force is pushing hundreds or even thousands of amperes into the ground.

A big issue is electrolytic corrosion where metal can be eaten away by the current. Not as much of an issue with grounded monopolar system (connect the ground to the negative terminal) but is for bipolar system where the ground can be either the positive or negative terminal when operating in monopolar mode. Apparently they can be run this way for days under a fault situation. Probably the reason for the coke (carbon) mentioned. p.s. look at the car battery terminals of an older car and notice the positive terminal is more corroded.

The ground should be somewhat away from human activity since currents can affect underground water pipes, oil pipelines etc.

Reply to
Michael Moroney

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