Stumped!

[Darkwood]

Ive also been led to believe the sychronisation of the sine wave is generated at a slight voltage lag to aid the injection into the grid, this and the very small voltage increase together is what give it the pushing force against the incomming grid, is this correct?

 

[yellowvanman]

Ive also been led to believe the sychronisation of the sine wave is generated at a slight voltage lag to aid the injection into the grid, this and the very small voltage increase together is what give it the pushing force against the incomming grid, is this correct?

You're making this more complicated than it is! I don't know whether the inverters do what you are saying, but I'd be surprised if it is, but will stand corrected if anybody knows better.

Looking at it more simply. Lets take Kirchoffs law - sum of all currents at a node = 0.

So lets say the cu, inverter and meter are all connected to a henley block. The inverter is generating some power so it has to go somewhere, so lets say its generating x amps, cu is using y current.

If x > y then the inverter is exporting to the mains at a current (x - y) amps

If x < y then the mains is supplying current to fill the inverter short fall at (y - x) amps

 

[Earthstore]

Well I bet the OP is stumped now.....

 

[snowhead]

Right, explanation no. 49.

To the O.P;

Imagine electricity as a person standing on a staircase.

Now we've got 2 staircases and 2 persons.

The person on the GRID staircase is on step no 4.
So when the sun shines the person on the P.V staircase goes to step no.5 on his own staircase and chucks all his current at the load, your house and beats off the GRID.
Because the GRID now doesn't have to work so hard, he can move up onto step 5.
Not to be outdone, the P.V moves to step 6 and continues to chuck and win.

However, now your nextdoor neighbour turns off his kettle and shower, lightening the load for the GRID allowing him to move to step no 7 chucking all the current he can at your house and winning.
Poor old P.V can't do a thing, no more sun available.
He/she has to sit it out and wait for the Tanning studio down the road to turn on all the sun beds and drag the GRID down a couple of steps.

Summary, whilst ever the P.V can beat the GRID on voltage, it wins.
Of course the movement up and down the stairs is continuous and very quick.

 

[liquidity]

Are these spiral or straight staircases? Any landings?

Sorry; I'll get my coat.

 

[needasparks]

The DNO is now plagued with voltage anomilies on the grid and from what I see this is down to a number of installs in same are (this is a different topic all together).

Offen wondered how the DNO get on now with repairing/replacing supply cables with so much solar backfeeding the system.
We were plagued near us over the last couple of years with numptys driving into the supply poles giving large areas without power wilst the overheads were replaced. Don't know how they would get on now if the cables on the floor were still live.

 

[Earthstore]

If there is no AC supply PV systems shut down so this would not be an issue.
I hope this helps.

 

[needasparks]

If there is no AC supply PV systems shut down so this would not be an issue.
I hope this helps.

Now that i did not know but must still give the guys (in) the ground something extra to think about as would multiple feed in's muddy the waters and back feed other units so they don't turn off.

 

[SolarCity]

Now that i did not know but must still give the guys (in) the ground something extra to think about as would multiple feed in's muddy the waters and back feed other units so they don't turn off.

I don't know exactly how they do it, but inverters will always shut off if the grid is disconnected, regardless of 'back feed' from other inverters.

 

[Glen Powell]

The inverter samples electricity and syncs to the frequency, this can take a little while, the moment the electricity is off from the grid they loose sync, for 2 inverters to sync together is virtually, if not totally impossible as they will have no input electricity to sample

 

[RS-232]

Sorry I couldn’t reply sooner, but thank you all for your kind responses to my question. This is far more complicated than I envisaged and find myself more confused than ever.

I began by drawing out Gavin’s irrigation system on a piece of paper and realized that the analogy didn’t seem to be quite right, since the pumps were feeding the main supply pipe from both ends of the field. In the case of a PV installation, wouldn’t this be different, because the pumps, like the inverter/mains, are effectively connected at the same point on the system? (someone called it a henley block?). This raises the problem of the pumps delivering unequal pressure – if one is higher than the other, then the water might be forced backwards through the weaker pump. I suppose this would be the same as the inverter forcing current back into the mains.

If the supply pipe burst near the pump connections, then I suppose that since there would be no resistance, water would flow from both pumps simultaneously, which is what I didn’t understand about the inverter and mains supplies both feeding the household circuits at the same time. But this would be enough to blow the fuses wouldn’t it?

Maybe a spark of understanding was prompted by yellowvanman’s explanation – when electricians say that the inverter always tries to supply a higher voltage than the mains, does this mean that it has to do so in order to make the voltage which appears at the henley block the same as that from the mains and not higher? If this is the case, then since the two are equal, it sort of makes sense that current could flow from both sources at the same time. But then, I’ve just thought of another problem – if both voltages are equal, how can the inverter drive current into the mains (when exporting)?

Sorry if I sound a bit dim. Perhaps I should stick to gardening and not ask silly questions.

 

[SolarCity]

for 2 inverters to sync together is virtually, if not totally impossible as they will have no input electricity to sample

How does it distinguish between electricity from the grid and the electricity from, say, next doors inverter which is also synced to the grid parameters?

 

[Earthstore]

My take on this sine wave thing is... A true sine wave is from rotating generation, all inverters electrically modify the electricity to make it compatable to a true sign wave, but it is not a true sine wave, some inverters produce a higher quality sine wave than others.
Inverters need a small imput to excite them to make them operate, and at a guess, they need to match the voltage and sine wave that they are conecting to.
A modern car alternator works in the same way, if your car battery is flat, it is a waste of time trying to bump start a car, unlike in the days of a dynamo.

 

[yellowvanman]

Maybe a spark of understanding was prompted by yellowvanman’s explanation – when electricians say that the inverter always tries to supply a higher voltage than the mains, does this mean that it has to do so in order to make the voltage which appears at the henley block the same as that from the mains and not higher?

Yes

If this is the case, then since the two are equal, it sort of makes sense that current could flow from both sources at the same time. But then, I’ve just thought of another problem – if both voltages are equal, how can the inverter drive current into the mains (when exporting)?

The inverter is generating current as it has to have the power its creating used, so the inverter is pushing the current down to the henley block, if the house doesn't take it, there is enough 'push' from the inverter to put it back into the mains to be used by the neighbours.