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Imagining the Supergrid: HVDC Loops for High Penetration of Renewable Energy

I do a lot of thinking about what practical steps are needed to break our addiction to fossil fuels, and this leads me to think that electric power transmission is the (not very sexy) key enabler for non-dispatchable energy sources like wind and solar to become the basis of our energy economy. Local and especially off-grid wind and solar generators are very unreliable (because the wind and sunlight available vary so much), and if power is to be available 24/7, energy storage and/or back-up generation are required. In aggregate, the cost and environmental damage from all the storage and backup generators that would be needed to enable off-grid renewable energy-based electrical systems to replace our grid are much higher than if distant generators can share their power via a supergrid.

The case for a supergrid is very sensibly made by several organizations, including Global Energy Network Institute and Friends of the Supergrid for example; one of the key problems with the supergrid concept is that the full benefits are not obtained until the system is complete, because the crux idea of a continental-scale supergrid is to be able to support inter-regional transmission on a massive scale (hundreds of gigawatts, GW must be transmitted thousands of km) so that the aggregate reliability of wind and solar are greatly improved, because generators in different weather regions can share capacity. Since weather systems are typically ~ 2500 km (~1500 miles) across, the supergrid does not begin to fulfill its potential until it is quite large, after many billions of dollars have been invested. This factor is very much holding up practical movement towards a supergrid, and what we are currently getting instead is a patchwork of transmission upgrades that are economically inefficient point-to-point connections that will not later fit in as components of a future supergrid. The fact that all these new power lines cannot later serve as components of a supergrid actually decreases the probability that a supergrid will ultimately be built. I argue therefore that it is quite important to start building power system upgrades that will make sense as parts of a supergrid in the future. This implies that a common operating voltage for the supergrid must be defined (probably between 600-800kV), and that high capacity lines that serve multiple power taps (multi-terminal HVDC) become the new norm.

(To consider all the options and trade-offs for creating a supergrid requires a deep dive into power transmission technology, for which I recommend reading Appendix A of my NYSERDA grant application on “Using Electric Pipelines to Create a Regional HVDC Grid.” I hope to tease out this information in a series of blog posts on this site, beginning with this post about the next logical steps towards a supergrid, but for those who are sufficiently interested, this linked document should be very helpful.)

One of the paramount properties of an electric grid is reliability. In fact, the “three R’s” of an electric grid are reliability, redundancy, and repairability. Redundancy has been formalized in a set of rules that have been accepted worldwide by the North American Electric Reliability Corporation (NERC); one of the most fundamental rules is that the grid must be able to withstand the sudden outage of any given power line without experiencing a system collapse; this is known as the “n-1 rule” and limits the maximum power that can safely be carried by any single grid-connected power line. Typically, two independent connections are required before a large amount of transmission can flow between two points on the grid according to the n-1 rule. This has huge implications for the idea of incrementally building a supergrid, because the large power lines that are needed to create a practical supergrid can not carry their full rated power until enough of the supergrid is completed to provide at least two independent connections between any two major power taps (points where power is sent or received) before that much power can safely be transmitted. At present, HVDC connections are almost exclusively point-to-point connections, which are severely limited by the NERC n-1 rule as to how much power can be transmitted. This limitation can be relaxed if instead of power lines, we consider power loops, with on the order of six or more power taps per loop. This is because of the unique property of a power loop that it provides two independent connections between any two points on the loop: a clockwise connection and a counterclockwise connection. In order to be able to take advantage of this intrinsic redundancy, circuit breakers are needed between every next neighbor set of power taps. In the case I presented as an example in my 2009 NYSERDA (New York State Energy Research and Development Authority) grant application Using Electric Pipes to Create a Regional HVDC Grid, I presented this map:

 A high-voltage DC loop

Which shows seven power taps at Albany, New York City. Atlanta, Saint Louis, Chicago, Akron, and Buffalo (three are within New York, because I was applying to a New York agency for funding). Logically, a loop of this size would be tied into the AC grid at far more places on the loop than shown in the map above, but that implies higher transmission capacity for the loop than I was contemplating in that 2009 document. The loop would not only link large cities, but also remote energy sites, such as wind farms, geothermal energy sites, solar installations, pumped storage and other remote energy storage sites, and maybe also conventional power plants of various kinds that are remotely located. (What I did not consider in detail at that time was the need for DC circuit breakers between each set of power taps; you can read about that problem here.)

The importance of self-redundant loops is that they are the natural “unit cells” of the supergrid. Each loop is self redundant if there are enough circuit breakers (one for each power tap, located between the power taps).

I am not alone in advocating for the importance of HVDC loops. Steve Eckroad of EPRI (Electric Power Research Institute) also filed a US patent on stabilizing an urban area against a rolling blackout using HVDC multi-terminal loops.

 
Roger Faulkner's picture

Thank Roger for the Post!

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Roger Faulkner's picture
Roger Faulkner on Mar 27, 2013 12:20 pm GMT

This was my first post on this site, and I had hoped to get some feedback. Is anyone out there? Is there actually anyone who would like to see a series of posts explainung the supergrid better?

Roger Faulkner's picture
Roger Faulkner on Mar 27, 2013 1:36 pm GMT

Thanks Jim. Your link was not visible to me, but the argument is quite similar to that of www.geni.org which is (as far as I know) the oldest organization arguing for a supergrid. I met the organizer of GENI, Peter Meisen in 1991, and we were both initially inspired by R. Buckminster Fuller; in the last years of his life, Bucky came to think that a supergrid was the most important innovation for raising living standards worldwide.

Nathan Wilson's picture
Nathan Wilson on Mar 28, 2013 7:05 am GMT

"...what we are currently getting instead is a patchwork of transmission upgrades that are economically inefficient point-to-point connections...."

The present grid acts like a thin sheet of copper covering the nation, with thicker areas where the current flow is higher.  The fact that it is implemented as three separate regions (Eastern, Western, and Texas) with controlled power flow between them does not matter much.  The fact that the voltages vary on different segments also does not change the applicability of the copper sheet model.  The envisioned supergrid would be much the same, but with much thicker copper reinforcing the grid’s high current areas.

Why do you believe that the point-to-point connects are economically in efficient?  Making the copper thickest in areas with high current flow is the most economical solution.

"... connections that will not later fit in as components of a future supergrid. ..."

Why do you believe this statement is true?  Under the copper sheet model, when the existing (230-500kV-AC) grid is overlaid with a super grid (at 765KV and up), the large number of nodes on the older lower voltage grid will serve to feed current into a small number of super grid nodes (the HVDC nodes are much more expensive than 230 & 500kVAC transformers).

“… The fact that all these new power lines cannot later serve as components of a supergrid actually decreases the probability that a supergrid will ultimately be built..."

I don’t think so.  Almost all grid upgrades are made in response to predicted load and planned generation assets (in other words, cost reductions in renewable power will be the biggest factor that drives the supergrid).  However, there are advantages to be had (lower cost and smaller land footprint occupied by power lines) when major grid capacity growth is done in big chunks rather than in small increments (e.g. a 765kV line can do the work of four 345kV lines).

Here is a study done for my local transmission company:  Quanta SPP EHV Overlay Study - 2008

The study was done for the Southwest Power Pool (which manages power transmission in Oklahoma, Kansas, and portions of several surrounding states) and evaluated several grid expansion options, which were intended to support the wind energy expansions envisioned by the AWEA and DOE NREL.  The options studied do interconnect with adjacent regions (i.e. to form a national grid), but the power flows supported are limited to the forecasted demand (i.e. it won’t get built if no one wants to pay for it).

Roger Faulkner's picture
Roger Faulkner on Mar 28, 2013 9:33 pm GMT

One cannot predict where the greatest capacity will be needed in a high wind penetration senario. If you consider for example the joint planning document EWITS (there are several versions), there are 7 long 800kV HVDC lines from the Midwest to the East Coast. Each line is essentially a point-to-point connection to export wind power from the midwest to the East, to help states meet their renewable energy standards. These are LCC lines, perhaps having up to four power taps, but not connected together. Wind power from Minnesota cannot flow to Georgia for example through these lines; it gets dumped on the AC grid near NYC, But suppose the wind is strong in Minnesota but weak in Arkansas...one must rely on the AC grid to move the power from NYC to Georgia, but it is too crowded to allow that. Fewer miles of transmission line can be used to link the entire region if multi-terminal HVDC is used in a loop format, and the unique property of self-redundance in such a loop relaxes the n-1 redundancy rule.

 

I K's picture
I K on Mar 29, 2013 10:19 pm GMT

Your elpipes idea seems interesting,
The way it is presented leads people to think you will need to first install a pipe and then run your cable through it. You should try to market it or change it to be a way to lay cables in existing pipes. So for instance the USA has quite an extensive natural gas pipeline.  Use a crawl or drag robot to pull a line through an existing natural gas pipeline rather than bury a new line or build overhead lines.

Some countries also have large long distance water pipes/tunnels which could possibly take the same.  Other than going through existing pipes is there a real advantage to buy burying a normal pipe?

Potentially install a new HVDC or AC cable through the pipe when it is in downtime?

Roger Faulkner's picture
Roger Faulkner on Mar 29, 2013 10:55 pm GMT

Crawling a conventional cable into an existing pipe does have merit, but the pipeline will need to be shut down during this process. If anything needs to be repaired, you lose two energy conduits at once. And most importantly, crawling a conventional HVDC cable into an existing pipeline does not answer the primary problem elpipes are designed to solve: to enable higher capacity transmission underground than is feasible at present with any cable; indeed, even higher capacity than is feasible for an overhead power line.

The main thing limiting the maximum transmission capacity of a cable is quite simple, that cables must be wrapped on a reel for transport. Truck-transportable cables are thus limited to maximum voltage of 325kV at present, and about 200MW per cable pair. Subsea cables, because they are wrapped on much bigger reels (~30 m diameter) can be much thicker, and so can carry more power (~2.2 GW per cable pair, at 600kV). Elpipes are segmented conductors that hook together like the cars of a train, which enables far more conductor to be used, and therefore enables higher transmission capacity. In this case, the “track” on which the “elpipe train” runs is the inside of a pipeline. The pipeline is pretty much identical to a gas pipeline, but I do not think in terms of simultaneously using the pipeline for both gas and electricity, since the elpipe nearly fills the pipe.

See this post for some more details.

I K's picture
I K on Mar 29, 2013 11:53 pm GMT

I still don’t understand how the idea would allow you to be able to transmit more power. Are you saying a normal cable is limited by how much weight a truck can carry?

What is the downside in multiple trucks carrying each section? So for arguments sake if one truck can carry 5km, use a second truck to carry the next 5km and fuse the two together and so on.

Also why not just feed a chain or rope through with a crawling robot and just pull your cable through. There will obviously be a limit in that the force of the friction must not be greater than the tensile strength of your cable. If that is a problem you could perhaps oil the cable. Would seem easier than building multiple track wheels and pushing the cable forward.

Roger Faulkner's picture
Roger Faulkner on Mar 30, 2013 11:15 am GMT

IK:

The limitation on max capacity of a cable is based on the minimum bending radius…it is all mechanical engineering! Since a truck transportable cable must bend on a drum that can only be about 12 feet in diameter, that limits both the wire radius and the insulation thickness. Such a cable can be ~4″ in diameter maximum. A cable wound on a 30 meter diameter reel on a cable-laying ship can be thicker, maybe up to 9″ in diameter, with both a thicker wire and thicker insulation. 

It seems so obvious once you see it, but I myself was blown away when I realized that the thing limiting cable capacity is cable diameter, which is in turn determined by the minimum radius of curvature, which is normally for the first turn of cable on the drum.

Thanks for keeping after me to explain this; you made me remember that I did not understand when I first heard this from a cable expert. Simply going to rigid segments removes this limitation. Most conductors in a power plant or a transformer yard are rigid buss bars; in the simplest explanation, an elpipe is a segmented conductor built out of insulated buss bars.

I intend to do a series of postings building up a basic understanding about HVDC technology before delving too much into elpipes on this site because I do not want to be seen as promoting my particular technology; however www.elpipe.com has a lot of more detailed information if you want to “jump ahead.”

I K's picture
I K on Mar 30, 2013 2:00 pm GMT

Oh yes that makes sense you literally can’t wind cable over a certain thickness for a given diameter of drum.

Just trying to think of ways around this. Why not just use sheet conductors rather than pipe or normal cable?  For arguments sake lets say you have a 1cm thick conductor in sheet form and the sheet is 50 cm wide. So your cross section conductor area is 50 square centimetres. If that is not enough, you could have it 1cm thick and 100cm wide to give 100 square cm or even larger and possibly thicker.

You get around the problem of limited thickness of cable relative to drum size.
You have a very large surface area relative to cable or pipe cross section area to get rid of heat more effectively.
You can use cheaper aluminium instead of copper since you are not conductor diameter limited.
You can lay this sheet vertically in a hole or possibly horizontally on an existing train track.
You can have two poles in the same sheet separated by an insulator in the middle

Good idea?

The only limit then would be how much weight your truck or ship can carry.
You are looking at ~30-40 tonnes / km for a 100cm2 area aluminium sheet.
500km = 15,000-20,000 tonnes so a ship could possibly carry both poles on two very wide drums in one trip. No need to fuel multiple sections.

Roger Faulkner's picture
Roger Faulkner on Mar 31, 2013 12:07 am GMT

At high voltage, the overwhelming technical challenge is insulation. The insulation is often where most of the cost is incurred as well. That is where circular cross-section insulators shine: not only is the amount of insulation minimized, but you don’t have sharp edges to contend with, which concentrate the electric field and promote dielectric breakdown.

My elpipes are mostly aluminum (a little copper will be used in the splices), as are nearly all high voltage overhead wires installed today. Most HVDC cables are copper though because the other costs in the cable (insulation, jacketing, installation, splices etc.) swamp the cost of the metal, and copper wires can be smaller in diameter than aluminum wires. Elpipes turn that all on its head…but let me buld up to that! My intention is to put out a series of posts that are basically tutorial…talking about transmission technology from a fundamental point of view, to buld the case for elpipes, but also to promote a better knowledge of transmission in general.

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