If we were to have another Carrington event (huge solar mass ejection in 1859) today, we would have massive damage to our electrical grid. The reason for this is that when a large CME hits Earth’s magnetosphere, it compresses the Earth’s magnetic field on the Sun side. As it does, the magnetic field lines cut across long distance transmission lines, inducing a very low frequency current (almost DC).
Transformers that terminate the line on either end are inductive. They have a high impedance at 60Hz (or 50Hz in some countries) that they are designed to operate at but a very low impedance at the low frequencies induced in the lines by Solar CME’s. The actual voltage induced in the lines isn’t large but because the transformers impedance is so low at the frequencies induced, huge currents flow, burning out transformer windings. The longer the line, the higher the voltage and thus currents.
High voltage DC transmission lines are not affected because the voltage induced is only a very small percentage of the line voltage and can readily be compensated for by the terminal equipment.
High voltage DC lines also require fewer conductors and can be operated at 1.414 times the voltage of an AC line using the same insulators owing to the fact that insulators need to be sized for the peak of the AC waveform and not the average. This allows a DC line to carry considerably more power than an AC line using the same insulators.
A DC line can also be operated at a higher current for the same conductors. There are two reasons for this. AC lines can tolerate very little heat because it causes the cable to sag, resulting in a longer path, and thus shifts the phase of the AC power so that it won’t be in phase with other sources. Because DC has no phase concerns, higher temperatures and more sag can be tolerated. Lines carrying AC power also suffer from what is known as the skin effect. The current flowing through the conductor creates a magnetic field that induces a counter current and tends to force the current to the outside of the conductor, thus the inner portion effectively carries little current. Because a constant current flows in a DC line, magnetic lines of force are not moving, no skin effect results.
Long AC lines are effective antennas and radiate away a significant portion of their power, sometimes as much as 20% for very long lines. DC lines do not radiate and so do not loose power to radiation. This is not only good from an efficiency standpoint, it’s also good from a health standpoint. 50HZ and 60HZ AC magnetic fields have been associated with higher levels of leukemia and bone cancers. DC lines eliminate this low frequency AC field.
For those curious as to how a low frequency AC field can induce cancer, AC fields cause ions to spiral as they cross through ion channels in cell membranes. This reduces the efficiency of cell membrane ion transport. Some blood pressure drugs that work by decreasing ion transport across cell membranes have also been linked to slightly higher leukemia and bone cancer rates and so it is plausible that low frequency magnetic fields may be causing these cancers through this mechanism, but I know of no definitive research to validate this hypothesis.
Eliminating this radiation is a good thing. Although the magnetic fields from the wiring in your house is probably much stronger owing to your closer proximity to them. This probably accounts for the weakness of the statistical link between power lines and these cancers. The AC magnetic field radiated from your house wiring obscures the effect of that radiated from power lines because of proximity. Electromagnetic field strength decreases as the square of the distance, the fields generated by the power lines are much greater but you are much closer to your house wiring.
Using DC transmission is one way to inter-tie grids of different frequency or phase. Presently there are three different grids in the United States and phase differences are the reason they can’t be inter-tied with AC transmission links. When the distances get too great, phase shift caused by line sag causes problems.
DC transmission lines become financially economical when their length exceeds approximately 700km. The terminal expenses for a DC line are greater, but the transmission line is less costly, because the same copper and insulators can carry much more DC power than AC. Less land is required because there is no electromagnetic radiation from DC lines and fewer physical lines are required.
DC transmission lines are more efficient when their length exceeds about 300km. There is some loss in the AC/DC and DC/AC conversion process in the terminal equipment, but if the line is longer than 300km, the reduced losses in the line more than compensate for the loss in the terminal equipment. For long lines, the savings can be tremendous.
In the United States, as is many countries, the population centers do not correspond well with the location of renewable energy resources. In the United States, one of the most economical renewable resources is wind power in the mid-west, but demand is largely on the East and West coasts and Great Lakes region. The problem is that grid capacity to get power from the mid-west to these regions is insufficient to non-existent.
As those of you who have followed this blog over the years know, I’ve been advocating the adoption of high voltage DC power transmission for lines longer than 300km for years. In China, DC transmission is already widespread, but in the United States, it’s adoption has been slow. So when the big CME hits, the whole world won’t go dark, much of China and parts of Europe will still have a functional grid.
The good news is there is some movement in the United States to remedy this situation. The Tres Amigas project aims to create a 5000 MW DC-to-DC inter-tie in Clovis, NM, to tie together the Eastern, Western, and Texas grid. This would also provide a place from which mid-west generated wind power could feed into all three grids. Unfortunately, DC would only be used to inter-connect the three grids, transmission to and from this facility would still be over inefficient and vulnerable AC transmission lines. Also, there is no completion date as of yet, this is still a project in the planning and funding phase.
A 500 mile HVDC line that would span from O’Brien County, Iowa to Grundy County, Illinois, would bring power from Iowa, Nebraska, South Dakota and Minnesota where there is much wind generation potential but where existing transmission facilities are already operating at capacity and thus unable to take further power, to Chicago and cities farther East. This line is scheduled for completion in 2017. There is a group opposing it as it will compete with energy produced in Illinois and the East Coast (yeah, we just love our coal soot).
The TransWest Express Transmission Project aims to build a 3000 MW HVDC transmission line from an area near Rawlins, Wyoming, to Las Vegas, Nevada, to deliver power from wind farms in Wyoming to Nevada, Arizona, and California. This project is scheduled to begin construction in 2014 and be operational in 2015.
The Pacific Inter-tie originally went into service in 1970 using mercury ARC tubes for conversion, transmitted 1400 MW from the Pacific Northwest to California. Between 1984 and 2006, a series of upgrades ultimately raised the capacity to 3500 MW.
There are a few other small DC lines in the United States, but the big HVDC build outs are happening in China. I don’t know what happened to this countries ability to get it done, but it seems we are lagging behind the world in so many ways, when we used to be leaders. It seems all of our capital goes into the war machine and now the domestic spying machine instead of needed infrastructure and innovation.