Stumped!

[RS-232]

Please bear with me, I’m just an ordinary customer and not an electrician, but I’m slowly acquiring a basic idea of the technology, thanks to the helpful posts from the many experts on this forum.

Please could someone spare a little of their valuable time to explain something that I’m struggling to get my head around - how a PV system connected to the mains can actually work in certain instances?

If I understand some older discussions correctly, an inverter always tries to give a greater output voltage than the mains, so that current flows out of the inverter into the house circuits and if there’s any to spare, into the mains (exporting).

If the panels aren’t producing any electricity (at night) then all the power required in the house flows from the mains and none from the inverter.

What I don’t understand is the situation where the inverter is producing electricity, but not enough to supply all the power needed in the house. I can see that some will be supplied from the inverter and some from the mains, because the lights are flashing on both the generation meter and the domestic meter at the same time. But how can this be, because the inverter and mains are tied together at a connector block and if the inverter is giving an output voltage greater than the mains, current shouldn’t be imported from the mains at all, as it would be flowing ‘uphill’?

There must be an explanation for this, since as far as I can see, it shouldn’t work but obviously does!

 

[Earthstore]

It's magic...

 

[Earthstore]

Lets say you are producing 3kw from your pv system and you switch you 4kw kettle on, therfore you are 1 kw short, so you import this from the grid, and the cost of your cup of coffee would be 75% less as you have only imported 1kw.

I hope this helps

 

[Gavin A]

try thinking about it like a situation where a farmer had a pumped irrigation system with 2 pumps operating in parallel with each other pumping continuously into the same set of pipework feeding the fields from both ends.

The irrigation system has lots of timed control valves to open, close and reduce the flow to various bits of the irrigation system depending on the soil moisture levels.

At different points in the day, the water flowing from the same point in the system could come from either pump, or a mix of both depending on which other valves were opened and closed on the system between it and the 2 pumps, but the water would flow continuously with no noticeable change over between the 2 water sources (unless you stuck some red dye in to one source or something). The overall pressure in the system would vary depending on the number of valves open, which would also mean that the flow rate at those valves would vary, but the pressure on a properly designed system would never be so low or high as to make the pump cut out or stop the irrigation from working.

That's basically how it's working except one pump is replaced by the mains water, and the other pump is set up to push the water out at a slightly higher pressure than mains pressure, but can still only supply a relatively limited amount of flow, so if enough of the valves are open the water from the pump will only reach so far into the system before the pressure drops and the mains water takes over. (water regs obviously don't apply in this situation).

 

[Glen Powell]

Gavin I am more confused than ever now lol

I class it as black magic and voodoo,

however the way I try to explain it is that electricity is lazy and will want to travel the least amount of distance, therefore if you are generating electricity and require it the electricity from your PV has the shortest distance to travel, if more is required the electricity from the grid will be asked to give a hand.

If you are generating electricity and do not need it the electricity will then look for the next shortest root which will probably be your neighbour, it will go out through your meter and then into your neighbours house and he will be charged by the utility company for the electricity you send to grid.

I try to call it push / pull technology where electricity will travel through the rought with least resistance.

 

[RS-232]

Thanks for your responses, folks.

Earthstore: Yes, I can grasp this, which is why I try to work out if its cheaper to run the immersion heater during the day and supplement the solar power with mains, or run it on Economy 7 at night. It's not always easy to do this, when the sun keeps going behind the clouds!

Gavin: Struggling with electricity is one thing, but plumbing is a definite no-no. Your explanation is going to need some deep thought.

Glen: Yes, this makes sense, but it's the 'pressure' thing I'm struggling with - how can mains electricity, which is at a lower voltage than the inverter, push against it and force current into the same household circuits? I always thought, from my vaguely-recollected school physics lessons, that current flows from a higher voltage to a lower one.

 

[Earthstore]

To try and help, the slightly higher PV electricity has been used by your kettle so no longer exists in your wires therefore it allows the electricity in from the grid.

 

[Glen Powell]

Most houshold items will have a capacitor in this enables in a way a cleansing of the electricity so if you get iregular voltages it smooths it out, before this point under load the device knows it needs x voltage x current for the desired result so simply asks for it from all local electrical sources The PV source has least resistance so will make its path first and stick to it and the grid electricity will take whatever path is left (electrons jumping electrons).

So your PV is 248v and your incoming main is at 246v it will go through the capacitor and come out somewhere between the two with the current to match (this is not the best way of describing it but its the best you will get out of me on a bank holiday monday)

 

[Darkwood]

Although i dont really do solar panels etc im slightly bemused not by your query RS 232 but by the replies which if anything they even confuse me, what an inverter does that is grid tied is monitor the voltage and waveform of your mains supply and tries to match it as close as possible, the 'free' electric generated from your panel is then fed into the mains supply, it will choose the path of least resistance which will be back into the grid not generally through your household loads. The meters you have had fitted will measure import and export quantities and your bill will be worked out on the net value, their is no magic gateway that tell the free generated electric to commit itself to your house any flow of current is monitored and calculated by the meters so if you import 20kw and export 16kw then you will be billed for the net value of 4kw unless you have some agreed tarriff in place.

 

[Glen Powell]

Although i dont really do solar panels etc im slightly bemused not by your query RS 232 but by the replies which if anything they even confuse me, what an inverter does that is grid tied is monitor the voltage and waveform of your mains supply and tries to match it as close as possible, the 'free' electric generated from your panel is then fed into the mains supply, it will choose the path of least resistance which will be back into the grid not generally through your household loads. The meters you have had fitted will measure import and export quantities and your bill will be worked out on the net value, their is no magic gateway that tell the free generated electric to commit itself to your house any flow of current is monitored and calculated by the meters so if you import 20kw and export 16kw then you will be billed for the net value of 4kw unless you have some agreed tarriff in place.

I agree with the inverter bit, the Net metering side however is totally wrong

 

[Darkwood]

I agree with the inverter bit, the Net metering side however is totally wrong

Well as i say i dont do the installs but i use inverters for other uses, i may be wrong on how the metering side is done and would be more than happy to stand corrected, but as goes electric flow and where it ends up is all down to path of least resistance and if your a long way from the substation it maybe the case it will find its way directly into supplementing your power supply demands, but all you have really done is matched the mains for voltage and frequency as well as syncronising the waveform, once its been coupled up to the mains its destination is purely determined by path of least resistance whether this be sharing the house loads or popping up the mains and helping supplement next doors loads etc etc ... is here nor there but it will be accounted for through the metering system.

 

[Glen Powell]

I am sure I said that in a previous post, you will however find inverters do kick out a slightly higher voltage at peak generation regardless of other factors, the frequency is not in question here as the inverter will not output unless synchronised however there can be deviations between voltages, this is more common on multiple inverter installs are an area where multiple installations are carried out in local area.

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).

Quality of inverter and how it syncs is just as important, path and resistance already covered to the enth degree me thinks.

 

[Darkwood]

Well with the noise been generated into the mains .. i assume their is a high proportion of modified sinewave inverters as opposed to pure sign wave inverters ( cost been the obvious reason) and i can see these having strict guidelines been applied with a clampdown of their use soon as the DNO are already implementing fines to commercial and industrial firms who are feeding excessive noise into the grid regardless of the cause. Domestic dosn't really get scrutinised as its harder to source the issues but im sure an industry clampdown on cheap inverters is around the corner.

 

[Gavin A]

Well with the noise been generated into the mains .. i assume their is a high proportion of modified sinewave inverters as opposed to pure sign wave inverters

you'd assume wrong, there's no way they'd be allowing modified sine wave inverters to be grid connected (well ok, so essentially all inverters are modified sine wave to a degree, but all would be sold as pure sine wave type if off grid).

Personally I've found it easier to understand this and explain it all via plumbing analogies, but maybe that's because I've got some background in that field as well as renewable energy / electrics, and my scenario here was probably a little complex.

 

[yellowvanman]

Where the inverter and mains meet (say at the mains meter) the voltage is one value, to get to that point the inverter has to generate the ac at a higher voltage to take account of voltage drop from its supply cable and so 'push' current down the cable.

Whatever current is coming down the cable on the line (brown) conductor from the inverter has to return back via the neutral (blue conductor) to complete the circuit, so the round trip of current from the inverter is much shorter than the mains round trip from the nearest transformer, so if the electrons are taking the path of least resistance the current from the inverter is used in the house first and if not enough more is used from the mains!

 

[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.