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Old 06-29-2014, 11:07 AM   #41
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... Therefore, these inverters rely on the frequency stability and the "rigidity" or low impedance of the local grid for this to work. The grid-tie inverters are just following the grid. If we ever get to a point where, say 80% of the homes in a neighborhood is on solar, we will get into trouble with instabilities. ...
Interesting, I never thought about that problem!

In simple terms, all these inverters play 'follow the leader', but as you say, if they become the majority, there is no leader - who follows who (whom? I hate grammar!).

Maybe as we approach that point, all newer systems would need to have a separate control signal to lead them? Maybe a higher frequency signal riding the AC line? Maybe an RF control channel, or maybe tap into GPS or any local radio broadcaster for a reference signal?

But then, there are propagation delays in the two separate systems that could be a nightmare!

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Old 06-29-2014, 11:13 AM   #42
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Yes, the problem can be solved with more sophistication that is currently lacking. After all, the current power grid is a distributed system, and the hundreds of power plants manage to synchronize with one another. I am not a power engineer, so do not know the exact mechanism they use.

PS. I believe that "whom" is the right word to use above.

PPS. All grid-tie inverters have the "smart" to turn themselves off if their connection to the grid is broken. And this has to be done extremely fast to prevent power surges that would destroy the electronic appliances inside the home, when the grid is suddenly not there to act as a load.

Stand-alone inverters for off-grid use and little inverters that we plug into cigarette plugs generate their own 60Hz reference which is not synchronized to anything, hence do not have this problem. These act like a voltage source, and not a current source like the grid-tie inverters.
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Old 06-29-2014, 02:49 PM   #43
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I tried buying a solar system but the Martians weren't selling.


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Old 06-29-2014, 03:12 PM   #44
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Power companies use a very expensive gizmo, called the synchroscope to sync generators to the grid. Mesuring phase and voltage differences until both are matching the grid before connecting. Happy homeowners can parrallel small generators by using light bulbs. Honda inverter generators use the inverters to sync, not the engine speed.

Once generators are parralled they will be synchronised and locked to the grid frequency. If there are diiferences in the grid frequency and the local genset frequency and are connected anyway, the grid will sync the oncoming generator, with usually destructive results. Ie. bent drive shafts, or catastrophic failure.

There is a concept of power wheeling where a local generator will be powered more than the need to maintain sync,the resulting leading phase angle will feed more current from that particular generator to the grid. This is the simplified description.
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Old 06-29-2014, 04:01 PM   #45
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And we will all have flying cars! Try doing the math to see how much solar would need to be installed each year to handle a summer day in ten years (roughly 10% of current peak capacity a year), I don't think you would find that to be a reasonable number at all.-ERD50
Try reading up on the current numbers, you'll be surprised. All this from wikipedia:

Germany has 50% of its peak right now, and doubled on average every 1.5 years for the past 20 years.

The US is growing at 70%+ per year. They were installing something like 10% of Germany's capacity in 2012. At least one report forecasts by 2025 that indeed 10% of US electricity will come from PV. That report was from 2008, before the boom really got going.

Spain had 2.7% of electricity production covered by solar in 2010.

Italy was at 3.2% of electricity production covered by solar 2011.

Growth is insane, the breakthrough has happened.

0.38% of the world's electricity is now generated by solar, and the growth rate is something like 40% per year.

One telling chart: Growth of photovoltaics - Wikipedia, the free encyclopedia

Now flying cars .. that's another story ..
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Old 06-29-2014, 04:08 PM   #46
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Thanks for the input so far!

travelover: both good points. on the rush to judgement, I actually only have to sign on as "interested" by the end of the month, then a contract would need to be signed by end of July, after assessment of my roof's suitability (by the non-profit), and specific estimate for my house by the installer.

MRG: we don't get much hail, and the panels are supposed to withstand 1" hail at terminal velocity, but I had not thought about insurance implications. suppose I would need to talk to my home insurer.

scrinch: nice to hear you have had a good experience!
I installed a 4.5KW solar system a last June. Rather than buying the system outright I spent $12K to buy 20 years worth of electricity. At the end of 20 years the unit belongs to the leasing company Sunrun. At the end of 20 years it will cost more for them to remove than it is worth so I am sure they'll let me buy it for a nominal amount. It is saving me between $200-210/month in electricity bills. Honolulu electricity is .$35 KWH 3-5x more than the rest of the country.

I ended up using a local company, a good friends is the VP of Sales and gave me a deal. But honestly I was super impressed with Solar City. There price was lower before the discount, everything was computerized and done via skype and email. FYI Solar City is Elon Musk 3rd company, but he is only Chairmen of the board not CEO.

So if SolarCity does business in your state I would definitely have them give you a bid.
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Old 06-29-2014, 05:12 PM   #47
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Try reading up on the current numbers, you'll be surprised. All this from wikipedia:

Germany has 50% of its peak right now, and doubled on average every 1.5 years for the past 20 years.

The US is growing at 70%+ per year. They were installing something like 10% of Germany's capacity in 2012. At least one report forecasts by 2025 that indeed 10% of US electricity will come from PV. That report was from 2008, before the boom really got going.

Spain had 2.7% of electricity production covered by solar in 2010.

Italy was at 3.2% of electricity production covered by solar 2011.

Growth is insane, the breakthrough has happened.

0.38% of the world's electricity is now generated by solar, and the growth rate is something like 40% per year.

One telling chart: Growth of photovoltaics - Wikipedia, the free encyclopedia

Now flying cars .. that's another story ..
Impressive numbers, but there's a big difference between Germany and the US, and a big difference between 50% and 100%. From what I've just read, even at current levels of ~ 50% peak, Germany needs to export electricity to other countries and has plans in place to shut down some solar if the grid can't absorb the power.

Who would we export to? Germany is about the size of some our states, with several nearby countries to sell to. The economics get worse and worse as you produce excess that can't be used. That's not a problem at low %, it becomes increasingly a problem as you approach 100% of peaks. On sunny days that aren't all that hot, A/C loads won't be so high - that will be a lot of wasted power. I just don't think any utility will go that far.

If we have a safe, cheap storage technology, then solar can expand. But that's not going to be in place within 10 years. Sorry, I'll stick with my original skepticism.

-ERD50
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Old 06-29-2014, 06:02 PM   #48
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Power companies use a very expensive gizmo, called the synchroscope to sync generators to the grid. Mesuring phase and voltage differences until both are matching the grid before connecting. Happy homeowners can parrallel small generators by using light bulbs. Honda inverter generators use the inverters to sync, not the engine speed.

Once generators are parralled they will be synchronised and locked to the grid frequency. If there are diiferences in the grid frequency and the local genset frequency and are connected anyway, the grid will sync the oncoming generator, with usually destructive results. Ie. bent drive shafts, or catastrophic failure.
Yes, I have read about the way a generator is sync'ed up to the grid to match in frequency, phase, and voltage before the switch is thrown. What I do not quite understand is that there appears to be no absolute reference, and would that not cause the entire grid to slow down or speed up in frequency eventually?

Take the analogy of the stock market. Suppose you want to trade Google. How do you know how much a share costs? You look at the last trade of course, and try to sync with the market price by asking or bidding only a few pennies around it. However, there's no absolute value, and with time the stock price will drift up or down. For stocks that are thinly traded, there are "market makers" who take the lead to set ask/bid prices to get the ball rolling. These are the stocks that are most easy to manipulate. But I digress.

With the electric grid, perhaps the big "wheels" like the generating stations at Hoover Dam or Niagara Falls set the pace because they are so big, and other smaller generators have to follow suits.

Quote:
There is a concept of power wheeling where a local generator will be powered more than the need to maintain sync,the resulting leading phase angle will feed more current from that particular generator to the grid. This is the simplified description.
Yes, a synchronous generator will feed more power to the grid if you try to increase its speed, because it is now leading in phase. That increase in power delivery will keep you from accelerating the shaft speed because it demands more input mechanical power for conversion to electrical power. Conversely, if you remove the mechanical power source, the synchronous generator still loafs along with the grid, acting as a motor and an electrical drag.

Anyway, a quick search found this article on the problem of synchronization in a decentralized grid.

Synchronization in a Decentralized Power Grid | SciTech Daily
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Old 06-29-2014, 07:54 PM   #49
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Perhaps, but it would be worth asking your local FD if they are prepared for this.

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They'll catch on by and by. Ours has gotten pretty good about the fancy homes here that have gas pipes and electric wires in the wall.
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Old 06-29-2014, 08:03 PM   #50
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They'll catch on by and by. Ours has gotten pretty good about the fancy homes here that have gas pipes and electric wires in the wall.
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Old 06-29-2014, 08:10 PM   #51
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They'll catch on by and by. Ours has gotten pretty good about the fancy homes here that have gas pipes and electric wires in the wall.
Yes, and there is a shut off for gas & electric. The problem, as I understand it, is there is no way to get to the panels to shut them off.

I don't know if firefighters won't spray water until they've verified the power is off or not. And maybe the concerns over solar panels are over-blown. But it would be interesting to ask, and get input from the local FD, no?

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Old 06-29-2014, 08:39 PM   #52
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I don't know if firefighters won't spray water until they've verified the power is off or not. And maybe the concerns over solar panels are over-blown. But it would be interesting to ask, and get input from the local FD, no?
Yah. It turns out the local VFD isn't totally dim. Why, they even realize that there's power on the lines inside the walls up to the building's cutoff switch.

They're also pretty good at handling other things that are on fire which can't be turned off. Chemical fires (including sodium and sulfur), fuel fires (tricky with water) and vehicle fires (fuel, electricity in new-fangled hybrids, and some Big Damn Batteries.)

Training. We has it.

Primary hazards in photovoltaic systems for first responders:
  • Tripping/Slipping
  • Structural collapse due to extra weight
  • Flame spread
  • Inhalation exposure
  • Electrical shock
  • Battery hazards.
Nothing new here, but the incident commander has to size-up the scene correctly. Oh, and tarping residential systems to de-energize them works well. (Yes, this is a minor item in training.)
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Old 06-29-2014, 08:52 PM   #53
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...

Primary hazards in photovoltaic systems for first responders:
  • Tripping/Slipping
  • Structural collapse due to extra weight
  • Flame spread
  • Inhalation exposure
  • Electrical shock
  • Battery hazards.
Nothing new here, but the incident commander has to size-up the scene correctly. Oh, and tarping residential systems to de-energize them works well. (Yes, this is a minor item in training.)
Right, it is an issue, not a 'non-issue' as was posted.

Maybe these aren't anything major relative to all the other hazards, but it couldn't hurt to ask.


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These guys seem to be taking it pretty seriously:

Fire Fighter Safety and Response for Solar Power Systems


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Old 06-29-2014, 08:58 PM   #54
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Some fire departments also refuse to come near the roof of your house due to electrocution danger.
may be less than real in many communities.

I suppose it couldn't hurt to ask.

If your FD says they'd refuse to work the fire, you may want to ask what else they would avoid, and maybe take a look at their training budget. Might have debigulated that gummint pork a bit too much...
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Old 06-30-2014, 05:53 AM   #55
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One design characteristic of the microinverters that are used to convert the DC to AC on our solar panel array is that they stop converting/outputting power from the panels if local grid power isn't available.

In the event of a house fire for that type of system, as soon as the main power from the grid was shut off as part of the response, the output from the panels would be disabled as well. Might not be perfect but seems helpful for that case.
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Old 06-30-2014, 07:09 AM   #56
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Impressive numbers, but there's a big difference between Germany and the US,...

-ERD50
the biggest difference is price per kw. Germany is about .35 per kw usa is about .12 per kw on average. This is one area that I'm glad the Europeans are leading the way.
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Old 06-30-2014, 07:26 AM   #57
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Old 06-30-2014, 02:39 PM   #58
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Wow, as usual, an incredible wealth of info on this forum. Thanks for all the additional things to think about...I'm keeping a list of questions...I'll post some answers when I have them (sounds like the program got a lot of interest so there is a backlog and may be a couple weeks til they get around to me).
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Old 06-30-2014, 07:47 PM   #59
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With the electric grid, perhaps the big "wheels" like the generating stations at Hoover Dam or Niagara Falls set the pace because they are so big, and other smaller generators have to follow suits.



Synchronization in a Decentralized Power Grid | SciTech Daily

From my days of working at a nuke power generating station, The nuke plants were used as reference. Can't recall the full story, but IIRC the reason was that that Reactors were difficult to modulate for variable loads. The preference was was to keep them at a fixed power level.

Some nukes would be able to elaborate on changing reactor output, ie how fast is and what delta achievable.
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Old 06-30-2014, 07:51 PM   #60
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the biggest difference is price per kw. Germany is about .35 per kw usa is about .12 per kw on average. This is one area that I'm glad the Europeans are leading the way.
It is not common for Europeans to have A/C in the homes. I would not be able to afford A/C if I have to pay $0.35/KWh. In fact, the entire SW might be deserted.
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