It's often said that a strong solar storm can burn out the biggest transformers which would take months to replace.
If these transformers are so critical, I can't believe they don't have any safety measures. Eg. cutting them off from the grid when they overheat to cool them down, so we would have a short blackout instead of months without elecricity.
Multi-ton transformers damaged by such a storm might take years to repair.
This effect has been observed on a small scale (nasa.gov):
A similar flare on March 13, 1989, provoked geomagnetic storms that disrupted electric power transmission from the Hydro Québec generating station in Canada, blacking out most of the province and plunging 6 million people into darkness for 9 hours; aurora-induced power surges even melted power transformers in New Jersey.
[...]
Another Carrington-class flare would dwarf these events.
The Department of Homeland Security issued a risk assessment in 2011 that said (p. 3):
Recent estimates state that 300 large extra-high-voltage transformers in the United States would be vulnerable to geomagnetically induced currents. Damage to an extra-high-voltage transformer from geomagnetically induced currents could take months or even a year to repair and cost in excess of $10 million.
It's often said that a strong solar storm can burn out the biggest transformers which would take months to replace.
That would be a horrible mistake on the part of the grid operators. Can it do that? Yes, I will show why. Will it do that? Most likely not, since the voltages and currents a Carrington event scale geomagnetic storm induces are similar to the voltages and current present in an electricity transmission system, and regular disconnecting equipment needs to be in place to handle those voltages and currents anyway.
Let's start from what a large solar storm is. The best information we have today about a large solar storm is from Carrington Event. It occurred when we didn't have large-scale electricity distribution networks. But back then, we had telegraph networks. Those telegraph networks gave electric shocks to telegraph operators and caused arcing. This happened because ordinarily telegraph networks handle quite low voltages, but their size is about the same as the size of today's electricity distribution networks that run today at 110-400 kilovolts. Those telegraph networks were not built to handle the 100 kilovolt scale arcs. The electricity distribution networks are built to handle that.
The voltage induced into a current loop is solely dependent on (1) the amount of change in magnetic field, (2) the time in which the magnetic field changes, (3) the surface area of the current loop. That's it. Nothing else required. So a telegraph network and an electricity distribution network will get exactly the same voltage for the same parameters.
Let's start from (1). According to https://www.earthmagazine.org/article/how-strong-was-carrington-event, the change in Earth's magnetic field during Carrington Event was 1760 nanoteslas. We didn't have accurate magnetic field monitoring back then, so this is just an estimate. I picked the highest estimate I could find, there are lower estimates as well.
However, this doesn't help if we don't know how quickly the 1760 nanotesla change happened. I found two sources: an upper bound and a lower bound:
Let's say that it takes about 10 seconds for the maximum change during Carrington event, 1760 nanoteslas, to happen.
Then we need an estimate for the surface area of a current loop. Theoretically you might be able to create a current loop the size of a large continent like North America. However, it's quite unlikely that such a big current loop would form during a geomagnetic storm. Electricity finds the easiest way. Typically transmission networks have several phases near each other, and if one phase conducts electricity one way it's quite likely it will want to flow back in another phase. So let's use the surface area of a small country, Finland, as an estimate for the kind of current loop that might happen. The surface area is 340 000 square kilometers.
According to Faraday's law of induction, the induced voltage is:
340000e6 * 1760e-9 / 10 = 59840 volts
So approximately 60 kilovolts. Note that large electricity distribution networks operate at 110, 220 or 400 kilovolts. So if they have switches for disconnecting the transformers, likely 60 kilovolts can be disconnected so that an arc doesn't form that would maintain the current despite the disconnecting. Also smaller citywide distribution networks operate at lower voltages, but in such a smaller network, the system is likely constructed in the shape of a tree: electricity flows from one location and branches to multiple sub-circuits. Also such smaller networks have very small surface areas. So it's extremely unlikely that the induced voltage in such a case is something the disconnecting equipment can't handle.
Then we need to know the current. If we assume Finland is a circle, then 340 000 square kilometers has a radius:
sqrt(340000e6/pi) = 0.33e6 meters
And a circumference:
2*pi*0.33e6 = 2.1e6 meters
Typical large-scale networks use around 400 square millimeter aluminum conductors. Aluminum has a resistivity of 2.7e-8 Ohm*m. So the current loop has a resistivity of:
2.7e-8*2.1e6/(400e-6) = 142 Ohms
60 kilovolts run through 142 ohms gives a current of about 423 amperes.
If the large current loop is let's say 110 kilovolt loop, 423 amperes would transfer only 46 megawatts. Typical large-scale distribution networks transfer far more power than that, so their current must be higher. 423 amperes is entirely within the realm of "possible to handle".
So where's the problem, then? Didn't I just show that the transformers and switches can handle the voltage and the current?
The problem is that the 423 ampere current is ADDED to whatever current the transformer is handling. Because the 423 ampere additional current is slowly varying, it can be treated as a DC offset. So if the transformer normally handles 1000 amperes, now it has to handle 1423 amperes. This would (1) increase resistive heating in the transformer, possibly overheating it, but most importantly, (2) repeatedly drive the transformer core to saturation where it's not designed to operate, very rapidly overheating it.
But the defense mechanism is simple: monitor the temperature and current in the transformer. Once they exceed the specifications, disconnect the equipment. There has to be switches for disconnecting 1000+ amperes at 100+ kilovolts anyway, so 60 kilovolts and ~400 amperes is easy.
It would be incredibly incompetent for a grid operator to fail to monitor that the transformers are operating within their design limits. It would be incredibly incompetent if there are no mechanism for immediately and automatically disconnect and protect every transformer that's in the danger of being damaged.
However, the Carrington event lasted for few days. During those few days, if a new Carrington event scale solar storm hits the Earth, we will have lots of these automatic disconnections happening. So yes, someone who is worried about a solar storm should have a plan of how to get electricity during those few days of unreliable supply. Gas stations, grocery stores, etc. should be designed to operate without power for few days at least.
But months? Only if the grid operator is incredibly incompetent.
Theoretically months of damage could happen in a medium-size lower-voltage distribution network where everything is done as cheaply as possible and suitable monitoring is not in place. If a suitable current loop forms, it will induce less voltage and current as in the Finland-sized example. But these lower-voltage lower-power networks have lower voltage and current anyway. So theoretically such a medium-size network could suffer from transformer breakages. Whether or not this happens and in how many places it happens is dependent on whether a current loop of suitable area forms: electricity has a tendency to find the easiest path, so in most places, probably a suitable current loop won't form.
But I don't believe that a large country would have a countrywide blackout lasting for months. Most likely, the national power transmission networks supplying the entire country with electricity have suitable safeguards in place.
And definitely, we won't have a planetwide blackout lasting for months.
Also, even the smallest citywide transmission networks with suitable safeguards could be protected from damage. They will cause automatic blackouts lasting from hours to days then, but not transformer breakages that would take months to repair.
If these transformers are so critical, I can't believe they don't have any safety measures. Eg. cutting them off from the grid when they overheat to cool them down, so we would have a short blackout instead of months without elecricity.
I don't believe either. I showed that they current and voltages that happen in a solar superstorm have such magnitude that they can be handled automatically, without any extra equipment needed, if there is simple disconnecting equipment that can disconnect the voltages and currents present in the grid anyway, and if the current and temperature in the transformer is already monitored.
