Two questions about solar panels.

[ericmark]

Two questions about solar panels.

First is the disconnection in the case of power failure. Where a whole street with all houses fitted with grid-tie inverters is disconnected from grid how do they know the connection has failed? One is simple two becomes a little more of a problem but when we reach 10 how does it work?

The second is volt drop we are allowed +10% -6% and I would expect the transformer would sit no load at 253 volt so when one full load the furthest user will still get 216.2 volt. However with grid-tie during the day when all occupants are at work the inverters will want to push power back into the grid. So the transformer would need to sit at 230 volt to allow that otherwise the inverters will all close down due to over voltage.

I am guessing the two are related and the 2 second disconnect and 3 minute reconnect will ensure that either under or over voltage will shut down inverters in turn until last one shuts down.

So although today I may fit solar panels and they all work A1 in a couple of years they may fail on a regular basis although there is nothing wrong with the system.

There are two ways to get solar panels on ones roof. One is you buy the system the other is you do a deal to allow your roof to be used. With the latter when the panel is working all you use is free. But you don’t get the feed in tariff so all feed into grid the installer gets.

Clearly the installer wants to get paid and unlike oneself who has to pay someone to maintain the panels they will have their own team so getting the voltage set at 254 will insure their panel is last to switch off. Which means those who fit their own will have theirs fail more often.

Also a large firm can put more pressure on the DNO if the voltage is regular high and will also have the monitoring equipment to know when the problem exists.

At the moment I am waiting for a report to if my roof is at the right angle for free panels to be fitted. If not then clearly not worth me fitting anyway but if it is then do I go for free panels or fit my own. If in the future there is going to be problems with so many being fitted then best going for free ones. However if I can be assured that it will not fail in the future than better buying my own.

Thoughts please.

 

[pjcomp]

In answer to the first part pof the question, if there's a power failure the inverter disconnects and no power is generated, otherwise whoever's fixing the transformer would get 240V coming the wrong way.

The choice between buying the panels and getting them free: Buy them and they belong to you and you get the feed-in tariff (FIT) from the generating company plus you get the benefit of any electricity generated that you're able to use (if you're out at work all day the pwoer you generate will keep the fridge going; if you're home-based you can use all that you generate and thus reduce your bill for bought-in electricity).

If you go for the free option, you don't have the capital outlay but you only get that bit of free electricity that you generate and can use. The supplier company gets all the benefit of the FIT payment. The real sting in the tail with this is if you try to seel your house - you effectively have a sitting tenant on the roof, and mortgage lenders don't like that sort of thing. Your potential buyer might not like it either, whereas selling a house with income from the FIT payments is a bit more attractive.

The pitch angle of your roof won't make a lot of difference to how much you generate - this is affected more by which direction your roof is facing and whether there's any shading, overhanging trees etc.

PJ

 

[JohnW2]

In answer to the first part pof the question, if there's a power failure the inverter disconnects and no power is generated, otherwise whoever's fixing the transformer would get 240V coming the wrong way.

Yes, that's what one wants to happen - but I think eric is asking essentially the same question I asked a few days ago. If, as he suggests, there are several nearby grid-tie inverters, how do any of them know that the electricity they are seeing is coming from other inverters and not from a (failed) network?

Kind Regards, John

 

[DetlefSchmitz]

Actually the first question is not quite as simple, but I think that the answer is that the external load will exceed the PV generating capability for even that many installations, and therefore the inverters have sufficient clues.

I also wonder if the frequency wouldn't drift out of spec with no external reference, and cause a switch off.

Good question.


(typed without seeing John's answer)

 

[pjcomp]

In answer to the first part pof the question, if there's a power failure the inverter disconnects and no power is generated, otherwise whoever's fixing the transformer would get 240V coming the wrong way.

Yes, that's what one wants to happen - but I think eric is asking essentially the same question I asked a few days ago. If, as he suggests, there are several nearby grid-tie inverters, how do any of them know that the electricity they are seeing is coming from other inverters and not from a (failed) network?

Kind Regards, John

Power goes, each inverter is "aware" of the fact and shuts down, so no generation for other inverters to feed off.

I think it's safe to assume that in the many years panels and inverters have been around (in Germany if not the UK) this one has been tested out and been found to work. I also suspect the Grid wouldn't have accepted panels (and other micro-generation systems like wind and water) if the inverter shutdown wasn't considered foolproof enough.

The fatal flaw in my argument, of course, is the assumption that "they" know what they're doing and this will all work as it should .... oh s**t!!!

PJ

 

[JohnW2]

Power goes, each inverter is "aware" of the fact and shuts down, so no generation for other inverters to feed off.

... but how do any of them become "aware" of the loss of grid electricity, if they are still seeing electricity of the same voltage and frequency as would come from the grid but which, in fact, is coming from other inverters? As has been suggested, the answer might well rely on the fact that it's unlikley (but not impossible) that the solar systems would be able to cope with the load. Voltage would therefore fall and that itself might well cause them to shut themselves down or, at least, would not present enough voltage to other inverters to 'impersonate' a grid supply.

I think it's safe to assume that in the many years panels and inverters have been around (in Germany if not the UK) this one has been tested out and been found to work. I also suspect the Grid wouldn't have accepted panels (and other micro-generation systems like wind and water) if the inverter shutdown wasn't considered foolproof enough.

Yes, I would also assume all that - it is the mechanics of how an inverter reliably detects 'grid power failure' if there are a lot of inverters feeding into an (isolated) segment of the network that I was (am) less sure about.

Kind Regards, John

 

[Lectrician]

On large sites, you could have several inverters hooked up to one supply. They are also usually linked by a serial cable to an IP network terminal. I assumed that this serial link (as well as doing other things), would communicate true mains fail via this link. Iin passing I asked the question. The manufacturers contact said it didn't, and the electronics were able do work out a true power failure without the need to talk to each other.m I assumed as it locks to the frequency of the supply and follows it.

 

[JohnW2]

On large sites, you could have several inverters hooked up to one supply. They are also usually linked by a serial cable to an IP network terminal. I assumed that this serial link (as well as doing other things), would communicate true mains fail via this link. Iin passing I asked the question. The manufacturers contact said it didn't, and the electronics were able do work out a true power failure without the need to talk to each other.m

Interesting - so, even in that situations, the inverters have to 'work it out for themselves'.

I assumed as it locks to the frequency of the supply and follows it.

Hmmm. I still don't really understand how this would work. If you have several inverters, all locked (frequency and phase) to the grid supply, immediately after that supply has failed they will surely all still be seeing electricity of the same frequency (from the other inverters) as they were seeing (from the grid supply) a fraction of a second earlier, won't they? Thereafter, I would have expected the inverters to remain synched (frequency- and phase-wise) to one another, and to remain that way until they ('collectively') started drifting in frequency to the extent that some safety device was triggered. That presumably would not be reliably quick enough to ensure adequately rapid safe disconnection in the event of a grid failure.

I still suspect that the inability of the inverters to service the load (i.e. maintain voltage) would be the most likely thing to result in a realisation (by the inverters) that the grid supply had failed - but I may well be wrong!

Kind Regards, John

 

[westie101]

Hmm backfeeding onto the DNO system during a power failure, as stated the inverters sense mains voltage and do shut down, however we've been at it a few years and have precautions we take to prevent the presence of voltages when we are working on the system.
It's always been a risk from any generation that customers may have that is no connected in accordance with regs.

To my knowledge there has only been one death which was to a linesman in 1987 who fell from a roof after receiving a shock from an illegally connected generator.

As for output volts the voltage limits are set by industry standard and generally inverters will operate up to 259 volts before shutting down.
We are spending a lot of time adjusting volts where this is occurring where we find the system voltage is outside limits.
There is no big business involved, no undue influence, if it's outside limits we correct it - end of!

Clearly the installer wants to get paid and unlike oneself who has to pay someone to maintain the panels they will have their own team so getting the voltage set at 254 will insure their panel is last to switch off. Which means those who fit their own will have theirs fail more often.

Nope, nonsense!

 

[JohnW2]

Hmm backfeeding onto the DNO system during a power failure, as stated the inverters sense mains voltage and do shut down, ..

Indeed, but have you any idea how they do it? In other words, how do they sense loss of grid voltage when there is an inverter next door doing a good impersonation of the grid? Does it, as I've suggested, rely on the fact that the inverters alone will not be able to maintain a reasonable voltage in the presence of the likely loads, or what?

Kind Regards, John

 

[westie101]

Indeed, but have you any idea how they do it?

Nope, and in theory each type could do it in a different way which would be up to the designer (and possibly a trade secret?)

That they do all shut down I can confirm

 

[JohnW2]

Indeed, but have you any idea how they do it?

Nope, and in theory each type could do it in a different way which would be up to the designer (and possibly a trade secret?)

I suppose so, but it would perhaps be a little odd if compliance with a Standard or regulation were achieved by a secret process/mechanism, wouldn't it?

That they do all shut down I can confirm

So it seems, and that's clearly good news for you and your colleagues. Do I take it that you've experienced situations in which (during daylight!) there have been a lot of inverters feeding into a segment of the network that died, and that all the inverters have immediately shut down?

Kind Regards, John

 

[westie101]

Do I take it that you've experienced situations in which (during daylight!) there have been a lot of inverters feeding into a segment of the network that died, and that all the inverters have immediately shut down?

Yes, the latest on Wednesday of this week whilst reducing the voltage at a transformer feeding a number of PV installations on a small housing estate.

 

[ban-all-sheds]

whereas selling a house with income from the FIT payments is a bit more attractive.

Only to those happy to kill old people and make more children go cold and hungry.

 

[Lectrician]

On large sites, you could have several inverters hooked up to one supply. They are also usually linked by a serial cable to an IP network terminal. I assumed that this serial link (as well as doing other things), would communicate true mains fail via this link. Iin passing I asked the question. The manufacturers contact said it didn't, and the electronics were able do work out a true power failure without the need to talk to each other.m

Interesting - so, even in that situations, the inverters have to 'work it out for themselves'.


Kind Regards, John

All the large inverters I see have the serial connections done with cat5e via RJ45 ports on the inverters (yep, serial RS485 on the RJ45). Them inverters have two RJ45's, they link them all together and back to a controller gateway. The gateway then either has a cellular modem or Ethernet connection to ADSL etc.

I guess they would never rely on something like that for critical shutdown signals.