Most of these grid-tied inverters don't work the way you might think - they don't create a sine-wave using some internal clock & then sync this with the mains. They use the mains as their clock - no mains = no clock = no output. This is not the same method as would be used to sync a conventional rotating generator onto the system.
Is the "Smart" meter role out being done ready for another Winter of discontent.
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There will be "mains" from other micro-generators.
Maybe a need for some form of wireless or internet driven sync????
Sure - but, as has been said, so long as there is at least one other micro-generation installation connected, there will be an AC 'supply' with which it can synch, won't there?
Kind Regards, John
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There will presumably be some (small) phase differences between different parts of the grid, so that 'national synching' (of phase as well as frequency) would probably not be possible?
Kind Regards, John
Yes, of course, but that's totally different from what Bernard was talking about - which appeared to be a situation in which a group of ('self-sufficient') micro-generating installations were (or could be) totally disconnected from the grid, hence not exporting anything.
Kind Regards, John
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In a power station there is some one to make a decision to turn off or not, but with wind and solar it has to be all automated, so when some thing goes wrong, and voltage drops to under 207 volts per phase, instead of trying as hard as it can to bring the voltage back up, it just gives up and switches off, often needing a manual reset.
So we get the domino effect.
And likely it needs much more power to be able to get it all running again.
I had it to a smaller scale on the Falklands, I was looking after 4 generators, 250 kVA each, we needed at least 2, but rarely needed 3, but we had a problem with fan belts, the governors on the RR engines was electronic, and acted fast, which caused the fan belts to break, this cause engine overheating and it would auto shut down, I would see lights flicker and run to generators, they were still running but under speed, because under speed they could not produce enough power to trip overload, so sitting their at 40 Hz and exhausts glowing red hot, but I could not put on another generator until Hz back to at least 45, so I had to shed load, so quickly opened on breaker, then put on a replacement generator, share the load, then put that breaker back on.
I got into trouble as I had switched off office supply and it seems they had not realised the dip in lights likely would mean loss of power and they needed to save and log off. But point is I had to decide who to switch off, and quickly, so it was the first I came to. Had I not switched some one off, all power would have been lost.
Even with just 4 generators it could take 1/2 hour after loosing them before I had finally got everyone back on, ensuring load was shared each time more load was added. How long it would take with the national grid I don't know.
So we get the domino effect.
And likely it needs much more power to be able to get it all running again.
I had it to a smaller scale on the Falklands, I was looking after 4 generators, 250 kVA each, we needed at least 2, but rarely needed 3, but we had a problem with fan belts, the governors on the RR engines was electronic, and acted fast, which caused the fan belts to break, this cause engine overheating and it would auto shut down, I would see lights flicker and run to generators, they were still running but under speed, because under speed they could not produce enough power to trip overload, so sitting their at 40 Hz and exhausts glowing red hot, but I could not put on another generator until Hz back to at least 45, so I had to shed load, so quickly opened on breaker, then put on a replacement generator, share the load, then put that breaker back on.
I got into trouble as I had switched off office supply and it seems they had not realised the dip in lights likely would mean loss of power and they needed to save and log off. But point is I had to decide who to switch off, and quickly, so it was the first I came to. Had I not switched some one off, all power would have been lost.
Even with just 4 generators it could take 1/2 hour after loosing them before I had finally got everyone back on, ensuring load was shared each time more load was added. How long it would take with the national grid I don't know.
Well no.
As I said above, these inverters don't work by synthesizing a sinewave, sync'ing this with the mains & exporting power.
Their method is a little different & relies on the MAINS being present (and is the reason why you can't use these devices to build an off-grid system). A brief summary is that they find the mains zero crossing point & note whether this is a positive or negative going cross. They then trigger IGBTs to switch the DC (from the panels etc) onto the mains until the voltage of the mains is just below the DC level, at which point the IGBTs are turned off until the point arrives were the mains sinewave has gain fallen below the DC level, at which point the IGBTs are turned on again until the zero cross point. This whole process happens again, on the next half cycle but with reverse polarity. It is by controlling the firing angle of the IGBTs into the 'inverted' zone that the device gets its name. The method is 'cheap' to do and doesn't require a awful lot of electronics to make it work - just a reasonable speed processor to monitor what is happening & adjust the firing angle to maintain the desired current/power.
You might think that this would lead to problems as this devices fires 'chunks' of current only at the start & end of each half cycle, however, since the mains is 'BIG' (the 'infinite bus' concept) compared to a tiny 4kW or so inverter, it can't really change anything. You might get a small voltage rise close to the device but its not significant.
How does a micro-generator tell whether the incoming power is from the real mains or from another micro-generator. ?
Maybe ( I don't know ) the real mains has a high frequency marker signal added to it and if this marker is not there then the micro-generator will shut down,
Thanks. I didn't expect that they would be generating anything approaching a sinewave, but nor did I think that they were anything like as 'crude' as what you go on to describe. I must say that I'm not at all sure that I would intuitively regard what you describe as 'an inverter' (in the sense that I usually think of such) even though, as you point out, it does repeatedly 'invert' the DC!
Yes, I might well have 'thought that', but what you go on to say puts it into (current) perspective. However, what you say seems to rely on the fact that we are talking about "a tiny 4kW or so inverter" - and the situation could presumably change if/when one was talking about a very large number of such 'inverters' simultaneously doing much the same sort of thing?
Kind Regards, John
As I understand, what Adrian has described is a system which relies on there being a zero-crossing point in a continuous waveform something approaching a sine wave, and which therefore would probably not work with what sounds like alternate positive and negative pulses of 'DC' (with gaps between them (i.e. a sort-of square-wave, but with portions of zero voltage as well).
Adrian, is that correct?
Kind Regards, John
That is how I read it. It sounds like with no mains input, there is no crossing point so the output would be dc - unless there is something to prevent that.
Quite - no 'crossing point' as such but, rather, periods of zero voltage in between positive and negative 'DC' excursions ('half square-waves') with very rapid rise/fall times - which the system Adrian described would quite probably not recognise as 'zero crossings.
Kind Regards, John
@JohnW2 , thank you for your interesting contribution. Would it be churlish of me to suggest, therefore, that if my average kettle consumption is 28W, then I should be able to get away with fitting a BS1362 3A fuse? Of course it would be, and this was my point. A cable rated for 100A will get mildly warm at 100A; at 200A it'll get decidedly hot, and at 400A it'll probably melt in less time than it takes to boil my kettle!
This is the state of our distribution network, and as more households migrate to the EV and ASHP type of loads we rapidly approach the scenario where load shedding becomes the norm rather than the exception.
[ I presume that there is either sarcasm of a missing "not" in that third sentence above? ]
As you may possibly understand, it's not really the point, but by trying to be clever, you have got quite close to shooting yourself in the foot
Were we talking about a 1,500W kettle, you would stand a good chance of getting away with a 3A BS1362 fuse, because a typical one of those will take at least 2 minutes (the duration of your kettle's use) to blow at a current of ~6.5A !However, as said, that's not the point. I was talking about averages (over appreciable periods of time - be that a day or whatever), and fuses know only about 'the present', not averages over appreciable periods of time.
If one ignores the complication of things like TV breaks (Coronation street etc.), the variation in times at which different people turn on their kettles will largely 'even things out', so that the demand on the network for kettle boiling in N households will be fairly constant, and in the ballpark of N times the 'average' (over time) for one household - at least if the averages are averages over 'daytime' (day+evening) hours.
Kind Regards, John
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