Comment by Animats | Hacker News Reader

Comment by Animats | Hacker News Reader

Checking the PJM power grid's alerts:

Warning: A Geomagnetic Disturbance Warning has been issued for 01:47 on 09.08.2017 through 17:00 on 09.08.2017 . A GMD warning of K7 or greater is in effect for this period. Additional Comments: SpaceWeatherPredictionCenter Extended original Warning of GMD Intensity of K7 or greater until 9/8/17 21:00 UTC (17:00 EDT).

Warning: A Geomagnetic Disturbance Warning has been issued for 19:21 on 09.07.2017 through 02:00 on 09.08.2017 . A GMD warning of K7 or greater or greater is in effect for this period.

These are just warnings. No alarms or actions yet.

The effect of these events on the power grid is to induce DC currents between grounded points far apart. This reduces the capacity of some transformers by causing partial saturation. PJM monitors those currents at some substations, and will take action if transformers start to saturate. There are routine switching procedures for this.

For long-distance DC lines, it's a win; the system can use the power.

There's an app, "PJM Now", for monitoring what PJM is doing.[1][2] But unless you're in the industry you're going to be bored by 99% of what it shows you.

They haven't taken any geomagnetic disturbance actions yet, beyond issuing the warnings above. If this was affecting anything in the power grid, there'd be actions listed. So far, all is quiet.

(Background: PJM runs the wholesale power grid for the northeastern US and part of Canada. They don't own it; there are lots of different companies that own parts of it, and they collectively own PJM. PJM has control of the transmission hardware. The system is run from a control room in Valley Forge, PA and a backup center somewhere else.

They also operate the market in electricity, like a commodity exchange. Generator startup and shutdown decisions are usually made by the owners of units based on current price info. But when there's trouble, the PJM control center can order "conservative operation", which means the control center will tell generators to start up, shut down, or stand by. When they expect trouble, due to weather, high heat, extreme cold, or equipment outages, there will be orders to make preparations.

PJM had a major blackout caused by a geomagnetic disturbance on March 13, 1989. They're very aware of the problem. They have emergency procedures for when it happens again.

So what PJM says is not PR; it's the control room giving orders. If they're not showing emergency conditions in effect, which they're not today, the geomagnetic disturbance isn't hurting the power grid and isn't expected to do so in the near future. Every time somebody posts about some current geomagnetic disturbance, I check PJM's info. If they're not worried in the control room, there's probably not much to worry about.)

[1] https://play.google.com/store/apps/details?id=com.pjm.pjmnow... [2] https://itunes.apple.com/us/app/pjm-now/id1180491584?ls=1&mt...

How common are long-distance DC lines and what are they used for?

My understanding is that the vast majority of power infrastructure is AC, and long distances are using High Voltage AC lines

Oldschool telephone lines.

https://en.wikipedia.org/wiki/High-voltage_direct_current

HVDC is becoming more common for medium and long distance transmission lines because HVDC doesn't suffer from the "skin effect" where current is carried mostly by the outside of the conductor, and therefore you can transmit more power with less conductor with HVDC. The tradeoff is, it's much easier to step up and step down AC voltage (using solid state transformers)

True, but none of this has changed recently.

The recent change is the fact that high-voltage semiconductors now make step-up/step-down efficient. In many cases, more efficient than purely using transformers (most semiconductor stuff still uses some form of inductive effect--they just do so at MUCH higher frequencies than in the past which allows them to use MUCH smaller components and get much higher efficiencies).

One of the big advantages of AC has to do with switching. If you flip a switch on AC, within about 10 milliseconds the voltage goes to zero as a natural consequence of the waveform irrespective of what current was flowing previously. A switch can interrupt that without much grief.

If you flip a switch on DC, it has to completely stop the current flow in the presence of a large current and/or voltage that will create a conductive plasma. That means a much larger switch for the same level of power.

There is a reason why your ISP has a huge switch and it is often encased in some inert gas. And also why it sounds like a bomb going off if it trips. And why it takes a hydraulic pump to reset.

Side note: The Pacific DC Intertie was the largest in the world until the 3 Gorges Dam interties--and it dated to the Kennedy administration and used vacuum tubes. https://en.wikipedia.org/wiki/Pacific_DC_Intertie

Cool, thanks for the additional info. TIL!

Sf has one of them - https://en.m.wikipedia.org/wiki/Trans_Bay_Cable

On a related note, there was a fascinating article on sf's dc grids by Spectrum https://www.google.com/amp/spectrum.ieee.org/tech-history/da... (pardon the amp link)

My office building taps into it to power the old school loading elevator!

HVDC is used for long-distance transmission where taps aren't necessary, and to connect disparate AC standards. Popular applications are undersea cables, remote generation facilities, and intercontinental or inter-country distribution. It's a lot more efficient than AC over distance as it requires fewer conductors and doesn't suffer from skin effect, so transmission losses are lower. And since you're already doing the AC->DC->AC conversion, there's no need for the connected grids to be synchronized.

As renewables (solar, wind, hydro) become more popular, so too does the need for reliable long-distance power transmission. With fossil fuel, fuels could be delivered to the point of generation - with solar, wind, and hydro, the location of the generator is non-negotiable and so the energy must be transported instead.

So we'll probably see more and more HVDC systems in coming years.

For undersea connections, the water around it means the line has a lot higher capacitance. An AC link has to load/unload that capacity all the time and looses energy to it, a DC link doesn't.

Similar effects happen with normal power lines as well, so it also has benefits on long-distance connections on land. Since you need conversion equipment on all ends, it's typically used only for point-to-point links, not for lines with branches in between - an example might be connection hydro plants in the wilderness somewhere to more populated areas.

For links between non-synchronized networks, since conversion is needed anyways to match between them.

Wikipedia: https://en.wikipedia.org/wiki/High-voltage_direct_current and https://en.wikipedia.org/wiki/List_of_HVDC_projects

From that wiki link "Xinjiang - Anhui (China)" line is 1100kV, 10GW power! That's impressive.

I have not heard much about long-distance DC lines in the United States, where AC transmission lines are incredibly common. However, I have read long-distance DC transmission lines are becoming more popular with alternative energy sources in China, such as the Three Gorges Dam project.

https://www.economist.com/blogs/graphicdetail/2017/01/daily-...

https://en.wikipedia.org/wiki/HVDC_Three_Gorges_%E2%80%93_Gu...

The pacific DC intertie is a notable one here: https://en.wikipedia.org/wiki/Pacific_DC_Intertie

In Europe you start to see some projects like that to connect renewable energy sources to consumption areas. The last notable one has a 1.4GW capacity[0] between France and Spain.

[0]: http://www.eib.org/infocentre/stories/all/2015-february-02/c...

New Zealand has 350kv 1200kw dc link between north and south islands

https://en.wikipedia.org/wiki/HVDC_Inter-Island

> For long-distance DC lines, it's a win; the system can use the power.

Do you have a reference for that? They may be more resilient than AC lines, but I doubt these systems are designed to use power generated by solar events.

Fun examples have been old telegraph operators having their lines work without power connected.

As for recent things I didn't even know long range DC still existed.

HVDC transmission is huge in China. Cheap hydroelectric capacity is in the northwestern part of the country, and the big loads are near the east coast. So there are thousand-mile million-volt DC transmission links.

High-voltage DC is becoming more popular, not less.

The technical advance has been the efficiency of high-voltage semiconductors.

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