Animats
8 months ago
PJM issued a geomagnetic disturbance warning, then an action. No emergency actions, and it's already over.
Msg ID: 104606
Message Type: Geomagnetic Disturbance Action
Priority: Action
Effective Start Time: 06.01.2025 09:31
Effective End Time: 06.01.2025 12:25
Regions COMED
A Geomagnetic Disturbance Action has been issued as of 09:31 on 06.01.2025 to protect
the power system from damage or disruptions due to increased geomagnetic activity.
Background:
These messages are from the US east coast power grid control room in Valley Forge, PA sending to people at generating stations and other key control centers. This is a slow-moving event. If the grid was stressed, there would be "Pre-Emergency Load Reduction" and "Conservative Operation" actions ordered. If there was real trouble, there would be many more actions. But things never got beyond preparing for trouble.
A geomagnetic disturbance event in 1989 caused transformer damage leading to outages. The solar flux going between power lines and conductive ground induces DC currents into the ground and lines, so that ground potential is different at different points. This causes partial saturation of transformers, and heating. That wasn't noticed until it was too late. So now, DC current in some key AC lines is monitored continuously, so power levels can be reduced if necessary.
Training materials for understanding this:[1] Start at slide 21.
Background info on how a power grid works.[2] Start with "PJM 101"
idatum
8 months ago
> Times are "Eastern Prevailing Time", which is Eastern Daylight Time right now.
I'm trying to recall when I last ever saw "Eastern Prevailing Time" used.
Can anyone share why it's used?
I see more use of ET over that (for Eastern US) or better yet UTC/GMT.
op00to
8 months ago
It removes ambiguity between ET (which do I mean?), EST (oops I meant EDT), and EDT (oops I meant EST).
I suspect they probably had an issue related to this, and prevailing time seems to work for them.
UTC is probably better, but a little less intuitive for most people.
xhrpost
8 months ago
> So now, DC current in some key AC lines is monitored continuously...
Can power lines have multiple currents in them at once? What would that mean for when the AC phase is moving opposite the DC direction?
andy99
8 months ago
AC is a sinusoid at 60/50 Hz. In principle, adding a DC current is just an offset, so e.g. if you had 1 Amp of AC current it would look like a sine wave 1 Amp high (actually sqrt(2)=1.414 high using the usual convention but that's not important) and the wave would be centered at 0A so go from -1(.414) to 1(.414). If you had a DC current of -1A on top of that, it would just be offset by that amount, so would look like a sine wave with a minimum of -2.414 and a maximum of .414.
Tldr, DC is just like an offset to the voltage or current waveform which is itself a sine wave.
globular-toast
8 months ago
Electrical conductors are basically reservoirs of electrons that can move around freely. Think of a body of water like the ocean. There is a lot of movement happening all at the same time: the tides, the waves, rip currents etc.
A wire is an essential one dimensional conductor; the electrons can move only backwards or forwards. Think of a narrow channel of water like a canal or river. A canal has no current, while a river has a direct current: it always flows one way. But a tidal inlet has an alternating current. The net flow of water is zero, but there is still a constant movement of water, backwards and forwards.
At a tidal estuary both things are happening: there's a DC component caused by net egress of water and an AC component caused by the tide.
The analogy doesn't really work because bodies of water also have capacitance. Wires are more like pipes. But hopefully you get the idea. Another way to think of it is like shouting in the wind, if you know how sound works.
Look into Fourier transforms if you're interested in learning more.
zrm
8 months ago
> https://pjm.adobeconnect.com/p63ultsdb2v/
Apparently my browser does not support some content in the file I'm trying to view and I'm instructed to use, among other things, "Firefox undefined or later". Which may or may not be what I was trying to use to begin with.
Though it seems to work anyway, so okay then.