Question regarding info in P.V. training course notes

[kme]

I was tackled by the QS of the solar company this afternoon - he`s currently teaching the PV course to sparx.

In the notes, was the following:

"The inverter is carefully designed to output a slightly higher voltage than the incoming grid voltage, in order to differentiate between the supplies".

He wanted my take on whether two different AC voltages, at the same frequency, could be present on the same cable - I say no. Further, I can`t see the inverter voltage being higher - it cannot "pull up" the grid voltage, FFS!

So he then wanted to know how your current-using-appliances "know" to use your locally generated electricity, rather than the grid.

My initial response was that, being "on-site", that supply had the "path of least resistance" to the load....but that doesn`t seem quite right, either.

So....how does "your" electricity get used, in preference to the grid supply?

e.g. If you & your neighbour are on a shared supply cable, you`ve got 1000W of lighting on, and your neighbour switches on his electric shower - why doesn`t some of "your" electric get used on his shower, wired in 10mm, with minimal impedance - THAT would be the "path of least resistance".....

Your thoughts, guys.

KME

 

[wozz]

The inverter has to put out a slightly higher voltage than the incoming supply otherwise it will never end up back on the grid, you need more EMF to push the meter backwards if you have that type of meter. Without a higher EMF the generation meter will never read anything? Thats how I understood it anyway, rightly or wrongly!

 

[kme]

Now MY understanding was that the generation meters were directional.

However, could we infer from your understanding that every power plant, when started up, pushes out a slightly higher voltage than is already there? What about the ones already running? Why wouldn`t the back EMF inhibit their output?

This "extra voltage". It CANNOT push the grid voltage up; so I`m struggling to get my head around how far up the network this EMF is supposed to push? To the substation? To your meter?

I`m confused. Maybe I`m having a "stupid" moment, or summat...........

 

[sellers]

my take on it is that the inverter runs at a higher voltage, the power produced gets used by the path of least resistance, usually being your equipment but in the case of your neighbour putting on their shower and using your leccy, then theres your 50% sell back being used.

The inverters no way going to push all the way back to the sub station, it will get used up by the other people who live down your street who don't have pv.

 

[Andy™]

similar to connecting to DC batteries together - both 'push' the voltage along the circuit, even though they are at the same voltage.

id imagine you would get problems if one has a higher voltage

'your' leccy gets used because its there at point where its needed.

not sure about the 'path of least resistance' bit, the inverter 'loop' is probably way higher than the grid supply

 

[kme]

Now, see, that is my thinking, too.

If the incoming grid is, say, 240.0V at the inverter, and it decides to push out 240.2V.......

Are we seriously supposed to believe that a (comparatively) tiny inverter can influence the grid voltage???

so farmer giles, with his TPN supply, coming straight off "his" local TX up a pole, is going to up his voltage with an inverter, so, when he isn`t using much, he`ll backfeed the grid with his 10KW at a higher voltage, through his TX, down the 11 (and a bit) KV lines to the next TX, back down to 240.2V.

But his neighbour has ALSO had solar installed. So HIS inverter says "oh. I must put out 240.4V"! Where would this end? Everybody`s inverter pushes the grid up to....what?

I`m with Andy - but the grid `leccy is also "there"..........So why does yours get used in your premises first???

 

[Sidewinder]

Well the grid is an inductive load so it sucks current in...

As it works backwards to a "normal" load so I have been told.

 

[kme]

OK -I can go with that, bud......

keep going, if you would.........

 

[Sidewinder]

Gimme a break!

I'll have to check out my regen drive notes!

Canoe, how does regen work, remind me please!

Same thing really!

 

[SPECIAL LOCATION]

Now ...

I am not Solar sun-bed PVc qualified or anything...

BUT

are we not here talking about the flow of electric current?

Think of a SIMPLE block drawing system

[GRID]-----L(a)----[METER]-----L(B)-------[PV SYSTEM]-------[LOAD]

[GRID]-----N(a)----[METER]-----N(B)-------[PV SYSTEM]-------[LOAD]

I am thinking If we imagine 4 points Live(a) & (B) and Neutral(a) & (B)

So when the SolarPV kit is off the potential difference between the 'L' & 'N' is nominal voltage..

thus current flows when load is switched on all the way from the grid!

Now if the SolarPV is on..

it synchs its AC cycle up to the grids AC cycles..

so when L(a) is at 230v L(B) is also @ a generated 230V

& whatever part of the voltage cycle it is on L(a) = New generated L(B)

and N(a) will equal new generated N(B)..

so the potential difference and thus the current flow through the meter is zero or negligible..

If Mrs homeowner then goes and sticks on the oven & the shower trying to take more juice than the PV can give...

The PV if supplying on its own would have a volt drop problem...

Then the potentials @ L(B) & the N(b)s will start letting current though from the grid...

I always understood it as current flows when there is a potential difference.

No p.d. no current flow?

I could be wrong though..

and I await kayakkid and sideboard to slap me round the head with a wet fish.:CWet Fish

I may think differently once more sober though?

Dunno what the meter makes of all this though cuz best practice guide 3 says some meters have stuff to stop them running backwards...

Others can run backwards but supply company may accuse you of theft!!!

http://www.esc.org.uk/fileadmin/user_upload/documents/industry/best_practice/BPG3_08.pdf

 

[kerching]

My understanding of this is analogous to a hosepipe with water flowing through it. Water will flow as long as there is no resistance [kinks, taps etc]

When this resistance is too high the flow stops.

If you remove the resistance and then force water back down the hosepipe at the same pressure the flow will stop, but the water will still be there. However if you shove a bit more water back down the pipe than that which is coming up the pipe there will be a reverse flow. This flow will eventually drop due to internal resistance of the pipes. Back in the day the voltage available used to be called the 'pressure'. One of my customers used to have 'Medium Pressure' hand sign written on the trunking within the switch room. cannot get a picture as the site was demolished 12 months ago! [ so that was 35 years of my life that I will never get back, spent 6 months ripping out my last 35 years work : headbang ].......... :coat

 

[SPECIAL LOCATION]

My understanding of this is analogous to a hosepipe with water flowing through it. Water will flow as long as there is no resistance [kinks, taps etc]When this resistance is too high the flow stops.

If you remove the resistance and then force water back down the hosepipe at the same pressure the flow will stop, but the water will still be there. However if you shove a bit more water back down the pipe than that which is coming up the pipe there will be a reverse flow. This flow will eventually drop due to internal resistance of the pipes. Back in the day the voltage available used to be called the 'pressure'. One of my customers used to have 'Medium Pressure' hand sign written on the trunking within the switch room. cannot get a picture as the site was demolished 12 months ago! [ so that was 35 years of my life that I will never get back, spent 6 months ripping out my last 35 years work : headbang ].......... :coat

Or the posh word...for elektwicall pressure...

Potential Difference!!!

:coat

 

[Sidewinder]

The other thing that has just entered my addled brain, IIRC, if there is overcapacity in the grid then the frequency increases.

As I understand it they control the output power of the alternators in the power stations by controlling the speed, thus, the power generated, as like a car alternator the faster you go the higher the voltage, but, in a.c. the frequency increases.

So I'll have to check but is this a typical situation of an inductive load as after all the power stations "see" transformers!

The end user when feeding into the grid "sees" transformers, so, going by CIVIL, then for an inductive load the voltage leads the current so if you increase the voltage slightly then the current increases, so if you increase the frequency slightly the generated voltage will lead the grid voltage and thus will "suck" the current into the grid?...

Please be gentle on me as I have done this from a very distant memory and, I have had one heck of a few days, and to de-stress I have partaken in some amber nectar!

Bed now for me, up early in the am to sort out ready for my kidney patient!