PG&E Shutting off power.

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GRA said:
Most streetcar systems ran at around 500VDC.
Here in Australia, they ran/run at around 580VDC, because that's what you get when you mercury rectify 415 VAC three phase (240 V phase to neutral, we skupped the whole 110/115/117/120 V nonsense). The only reason it was DC was that traction electric motors were DC at the time. And of course two wires (one near-ground potential rail and one overhead cable) is much easier for transportation than three live wires.

But AC won because it could cover a much larger area, so was far cheaper in terms of wire,
Yes, because of low frequency transformers.

The other huge advantage of AC is that you can turn it off with a cheap switch. The arc is self quenching because the current crosses zero every 8.3 or 10 milliseconds. Also fuses are cheaper for the same reason. That's a major nuisance for DC over about 24 V.

Edit: yet one more advantage of AC, at least outside of North America where three phase seems to be uncommon, is that induction motors run nicely at utility frequency, at a convenient 1800 or 1500 RPM, less a little. They happily run continuously for years, and don't have brushes that wear out.
 
coulomb said:
GRA said:
Most streetcar systems ran at around 500VDC.
Here in Australia, they ran/run at around 580VDC, because that's what you get when you mercury rectify 415 VAC three phase (240 V phase to neutral, we skupped the whole 110/115/117/120 V nonsense). The only reason it was DC was that traction electric motors were DC at the time. And of course two wires (one near-ground potential rail and one overhead cable) is much easier for transportation than three live wires.

But AC won because it could cover a much larger area, so was far cheaper in terms of wire,
Yes, because of low frequency transformers.

The other huge advantage of AC is that you can turn it off with a cheap switch. The arc is self quenching because the current crosses zero every 8.3 or 10 milliseconds. Also fuses are cheaper for the same reason. That's a major nuisance for DC over about 24 V.


Yeah, when I used to design off-grid systems, the cost of DC rated switches was a lot higher than AC. The other problem with DC, lacking any cheap, efficient means of changing the voltage, was that everyone had to string wires for different voltages for every different load - lights, alarms, machinery etc., darkening the skies with all the wires stacked on poles. The book I mentioned upthread, "The Grid", points out that there were something like 7 different DC voltages in common use at one time in the U.S, all of which were often strung on the same poles: 100, 110, 220, 500, 600, 1,200, 2,000. Not that AC immediately brought about standardization, because IIRR there were also nine different frequencies in use. From memory, they included 25, 30, 33.3, 40, 50, 60, 66.6, 83.3, and 125 Hz.
 
Lots of great points in your post but could you explain this a bit? I don't see how AC vs DC would affect the complexity or cost of a fuse....

coulomb said:
Also fuses are cheaper for the same reason.
 
goldbrick said:
I don't see how AC vs DC would affect the complexity or cost of a fuse....

A fuse is like a switch that operates by melting a conductor. When DC current is reduced, any inductance in the circuit will tend to keep the current flowing. So the fuse will tend to arc, that is, the very air at the gap ionizes and conducts. An arc has a resistance of very roughly four ohms, so if say 10 amps is being interrupted, or the current manages to fall to 10 amps, it's only dropping some 40 volts, or 10% of a typical Electric Vehicle battery. Meanwhile, that 40 volts and ten amps is 400 watts, quite a lot of heat in a very small volume. This heat is enough to keep the air ionized, and this can keep the current flowing until something burns up.

A DC rated fuse will have the conductor surrounded by glass beads or ceramic to keep the gap cool, so the air can't ionize. A 100 amp DC cartridge style fuse is almost an inch diameter and two inches long (pardon the caveman units, but this is a US hosted forum so I try to speak the vernacular). It's heavy and costs some US$10.

Once an arc starts, its self sustaining nature means that the gap has to be increased to ridiculous lengths to extinguish it, like 1 mm per volt, or well over a foot for 400 V. It's like starting a fire, its just best not to let one start at all.

Switches and contactors rated for say 500 V DC have to contain magnetic blowouts or other clever arrangements.
 
goldbrick said:
Lots of great points in your post but could you explain this a bit? I don't see how AC vs DC would affect the complexity or cost of a fuse....

coulomb said:
Also fuses are cheaper for the same reason.

Just search YouTube for differences between AC and DC arc.

All the big fuse I have cut open are packed with glass beads.

I weld with AC and DC for different applications, big differences in the arcs.
 
Thanks for the explanation. I knew about the difference in switches but never connected the dots on a fuse, which is just a one-time use switch in a way. Thanks again, great info.
 
Sigh...

I've been checking https://psps.ss.pge.com/ recently including last night. It had said for my address (in the South Bay) no impact. Now it doesn't. :( It now says this for me. :(
STATUS
October 24, 2019: Due to gusty winds and dry conditions, combined with a heightened fire risk, PG&E may need to turn off power for public safety at this address in the next 36 to 48 hours. As we continue to monitor conditions, please prepare for outages that could last longer than 48 hours. By providing your specific address information in this tool, you are getting a more accurate view than PSPS area map. Get the latest information on this event at pge.com/pspsupdates
Maybe the only good (?) thing is that if it lasts awhile, I won't even be in town. I get on a plane to Japan on Sunday morning.
 
cwerdna said:
I won't even be in town. I get on a plane to Japan on Sunday morning.
So you're the one causing all this climate change :-D

Here in Florida after a hurricane the people who have their power out for only three days are referred to as "the lucky ones". A little perspective can be helpful; living without electricity isn't as nice as with it, but you'll be amazed the extent to which life goes on. A month from now everyone will have moved on and forgotten all about it.
 
Gone all day for a new station opening in Portland but arrived home to a dark house. Power restored at 3 am. Sucks and found that knowing the power is going out is much nicer than being surprised by it coming home.

As it stands, it was less convenient than a normal Saturday would have been but hardly enjoyable.
 
LTLFTcomposite said:
cwerdna said:
I won't even be in town. I get on a plane to Japan on Sunday morning.
So you're the one causing all this climate change :-D

Here in Florida after a hurricane the people who have their power out for only three days are referred to as "the lucky ones". A little perspective can be helpful; living without electricity isn't as nice as with it, but you'll be amazed the extent to which life goes on. A month from now everyone will have moved on and forgotten all about it.
LOL on the first part. So, power went out Saturday at ~8:20 pm. Unfortunately, Comcrap didn't stay up long this time. It went out before the 30 minute mark. (My cable modem and access point are on UPSes.) During the last PSPS, Comcrap stayed up for over 45 minutes.

Yeah, I recall a co-worker (originally from Florida) talking about hurricanes and the power outages.

Unfortunately, I remember see a headline (e.g. https://www.latimes.com/california/story/2019-10-19/pg-e-ten-years-of-power-shutoffs) about this being a thing for 10 years. :( Ugh! I grew up in CA but was out of the state for about 9 years. We never had this sort of thing except for Enron rolling blackout scam when I was residing in another state. They certainly didn't shut off power for these "public safety" or wildfire risk.

The weirdest part about the grid is that my entire neighborhood was out. I went to get something to eat, and surprisingly every single traffic light on an expressway to my destination, including one not far from home was working. I went a few miles and at that point, all the houses to the left and right had power. Where I went to eat, there was no outage at all (a busy commercial area).

I'm waiting at the airport now w/a 2 hour delayed flight. It sounds like the all clear for my area is Monday at 8 am but it sounds like it's for the weather event. They still need to inspect/finish inspections of the power lines before they turn power back on. So, it could be that day or another day. And, if there's damage, that needs to be fixed.
 
Oilpan4 said:
You are better off in an electric car. Driving a gasoline car in heavy, stop and go traffic uses far more gasoline per mile than usual. The reverse is true for an electric. Got 60 miles of range? Santa Rosa to SF is 56 miles, and with heavy traffic will probably get better than 60 miles.
Power is out, so are many gas pumps. And evacuation isn't the time to charge or fill up the gas tank.
 
Several years back there was a big storm threatening Houston and tons of people were trying to evacuate and running out of gas because they were stuck in traffic. The only people making it through we're the ones in Priuses.
 
My wife's Hyundai hybrid could likely run for several days in stop and go traffic running the A/C. When the 2kwh battery gets low it runs the engine about 1 to 2 minutes for every 10 minutes of A/C run time.
I wouldn't want to be stuck in a Houston like evaluation in a leaf.

If the power goes out here and that Hyundai had half a tank of gas it would still go about 300 miles. Then when gasoline is found, or the power comes back on fill back up in 2 or 3 minutes.

While the leaf is on, all accessories off it still draws around 700 watts according to leaf spy it's far from perfectly efficient in stop and go traffic.
 
Oilpan4 said:
While the leaf is on, all accessories off it still draws around 700 watts according to leaf spy it's far from perfectly efficient in stop and go traffic.

So 40kWh / 700W = 57 hours. About twice as long as a gasoline car will idle.

Or do you mean some old LEAF 2011?
 
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