Importance of Multi-Terminal HVDC Loops for High Penetration of Renewable Electricity into the Grid

Prepared for The Energy Collective (March 27, 2013). Re-posted here on April 25, 2013. The version below is edited slightly;  if you want to read the original post, go here.

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 availability vary so much), and if power is to be available 24/7, then energy storage and/or back-up generation are required to be as large as the maximum load. 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 for the aggregate reliability of wind and solar to be greatly improved on a continental scale. That is so because for maximum effect the supergrid must be large enough to connect generators in different weather systems. 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 delayed benefits 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 which 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 (because we will have spent so much money on the wrong technologies). 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 500-800kV DC), and that high capacity lines that serve multiple power taps (multi-terminal HVDC) must 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 (pages 20-45) of my NYSERDA grant application on “Using Electric Pipelines to Create a Regional HVDC Grid;” 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 both IEC (international standards) and 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 or generator 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 n-1 redundancy 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:
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, hydropower and 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. Here is an illustration from his patent:
Note that Steve only called for three main loop circuit breakers in his scheme because the available power electronic circuit breakers he envisioned at that time are lossy (~0.5% power loss in the breaker itself) as well as expensive, ~$35/kW, one fourth of the cost of a voltage source converter (VSC) for converting HVAC to HVDC or HVDC to HVAC. Since then though, ABB has announced a breakthrough on HVDC circuit breakers with much lower on state loss (~.005%), but estimated cost about the same as the power electronic circuit breakers that Steve was considering ~$35/kW. My invention that I call a Ballistic Breaker™ can I believe be developed for HVDC applications at much lower cost than ABB’s design. Demonstrating a realistic HVDC loop has become the most important next step in evolution of the supergrid.

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