how do you tell where current flows?

[SparkyTris]

one does not care (within reason) what the voltages and currents in the line are[/b]

hmmm, yeah, there may be something in that comment indeed.

Plugwash,

the main thing* that the inverter actually needs to be monitor is the input voltage. If the input voltage is too low then the inverter is overloading the source so it should reduce the output voltage. if the input voltage is too high then the inverter is underloading the source so it should increase the output voltage.

yes, this makes a lot of sense. I believe the inverters have to be "tuned" to whatever device they are acting for, in order to do just this. (clearly the windmills or PV's are not linear in operation) Power Point Tracking or something I think, from SMA.

So, from your explanations, I confidently conclude that an export-only windmill, connected in very close proximity to its owner's domestic CU, will in fact be preferentially supplying it's owners' electrical demand.


extending this argument further to my previous rant about export cable capacity;
all the windmills in orkney will preferentially supply Orkney's demand. only when this is done are export amps available.
And, as OFGEM /Scottish & Southern Electric are whinging about their grid's capacity to carry energy south, we can assume that the southbound link is "tight". so there will have to be even more amps available in Orkney's generation pool to overcome the transmission losses southbound.

Please could someone shed more enlightened light on this than is available in my weak brain?

 

[JohnW2]

[extending this argument further to my previous rant about export cable capacity; all the windmills in orkney will preferentially supply Orkney's demand. only when this is done are export amps available.

Maybe I'm missing something about this discusion, but I keep asking myself whether it actually 'matters'. If it were the case that all the power generated by Orkney's windmills is consumed in Orkney (which, at the level of electrons, probably is almost true), then that is surely effectively the same as exporting all the windmills' power directly to the mainland (e.g. via a dedicated line) and then using 'imported' (from mainland) electricity to supply all of Orkney's needs? What actually happens is presumably better in efficiency terms, since the shorter current paths mean less losses.

Kind Regards, John.

 

[SparkyTris]

Of course you are right, there is no such thing as green, pink, brown, rural or nuclear electric, it's all just electrical energy. so it doesnt really matter.
it's mainly a personal thing, wanting to know how the grid works in relation to uGeneration

3 counters as to why I think it does matter though:

1. the PV salesman telling us all that because the sub 4kW solar system connects to the CU spare fuseway, you get to use your own energy first. Unlike in the case of the "nasty" windmills whereby all of your green energy is exported. This was used as a selling tactic, and as demonstrated above, it is not really true. OK, Yes it is true from a metering/invoicing point of view, but that is entirely notional/legal.

2. Many people (in this area), myself included, would have a windmill if we got "first dibs" on its energy and exported the spare. So, on a windy day, the meter would be stationary or turning "backwards" and earning market rate. and on a calm day, the meter would be turning forwards, and Id be paying market rate.
This seems a much more elegant solution: less hardware, less installation, less paperwork, more obvious to a layman.

3. Why are OFGEM whinging about export capacity southbound out of Scotland/Orkney with regard to renewable energy generation here?
I reiterate my previous comment: they don't winge about increased consumption. The net effect of increased generation in rural areas will be initially to decrease amps carried in the national interconnectors, so will actually remove strain on the grid.
Im not a conspiracy theorist at all, but it's beginning to look fishy, you must agree!

unless you can explain to me that the grid is directional, and was designed for carrying amps northward only. (implying that major work would be necessary to carry amps south)

OR, that Orkney/Northern Scotland long ago became electrically self sufficient and the strain is all due to increasing export amps. I dont *think* that's the case.

 

[SparkyTris]

all the power generated by Orkney's windmills is consumed in Orkney (which, at the level of electrons, probably is almost true)

Surely you mean coulombs rather than electrons? I thought electrons dont actually get displaced, rather they just jiggle around a bit

why do you say almost true? I believe Plugwash's arithmetic explanations indicate that this is completely true, given that there is about 60 miles of very old 3-phase ac subsea cable connecting orkney islands' ring circuits to the UK mainland

(not trying to be argumentative, just want to know why from those with more knowledge than me!)

 

[JohnW2]

Of course you are right, there is no such thing as green, pink, brown, rural or nuclear electric, it's all just electrical energy. so it doesnt really matter.

Exactly.

1. the PV salesman telling us all that because the sub 4kW solar system connects to the CU spare fuseway, you get to use your own energy first. Unlike in the case of the "nasty" windmills whereby all of your green energy is exported.

As we've agreed, that is a meaningless argument.

2. Many people (in this area), myself included, would have a windmill if we got "first dibs" on its energy and exported the spare.

Again, except in relation to the (very small) reduced losses if the generated electricity is used close to the source of generation, where the electrons from the windmill end up is irrelevant.

3. Why are OFGEM whinging about export capacity southbound out of Scotland/Orkney with regard to renewable energy generation here?

You'd have to ask them since, as I think we're agreed, it's a daft argument - unless, of course, there is more 'renewable' electricity being generated in Orkney than Orkney's total electricity requirements.

That's how I see it, anyway.

Kind Regards, John.

 

[JohnW2]

all the power generated by Orkney's windmills is consumed in Orkney (which, at the level of electrons, probably is almost true)

Surely you mean coulombs rather than electrons? I thought electrons dont actually get displaced, rather they just jiggle around a bit

I was just talking coloquially. Coulombs don't move, either! Electrons do move, but each electron covers much less distance than the total current flow. Think of a 100m hosepipe full of water. Squirt an extra 10 ml into one end, and 10 ml will come out of the other end. Each molecule of water in the pipe has moved a very small distance, but the 'flow' has moved 100m. However, if the 10 ml of water you squirted in was coloured, if you kept on squirting further 10 ml shots into it, if you were patient enough the coloured water would eventually come out of the other end! As far as I am aware, much the same is true of electrons (not that one can 'colour' one of them to test out the theory!).

[why do you say almost true? I believe Plugwash's arithmetic explanations indicate that this is completely true, given that there is about 60 miles of very old 3-phase ac subsea cable connecting orkney islands' ring circuits to the UK mainland

I still think this is a pretty meaningless and very abstract way of looking at things. In reality, one cannot distinguish between (a) 1A of windmill-generated current being generated in Orkney and used to supply a load in Orkney and (b) 1A of windmill-generated current being exported from Orkney to the mainland and an extra 1A coming from the mainland to supply the load in question. As far as the under-sea cable is concened, those two situations are surely identical?

Kind Regards, John

 

[SparkyTris]

No.
your scenario b) - where 1A of finest orkney wind energy goes south to those who are willing to pay extra for green energy but 1A of cheap nuclear electric comes north to service Orkney will of course not happen due to the laws of physics.

I get that, and the point you are making that there just is no choice. the amps flow according to voltage levels & resistances and that is that.

But, a lot of people here DO think that this is going on.

even the SSE linesmen are telling us that the local grid cannot stand "all these windmills"

the local and national press are on about the iniquity of "us" having all the renewable resource but unable to press on with development of it because the grid can't take the export.

Obviously Im going to have to ask the question of an engineer at SSE. just tell me I am right (or wrong!) about this:
IF the infrastructure can support the county's consumption of X amps WITHOUT any local generation, THEN it can support a local generation of 2.X amps just as comfortably.

I drew attention to the old ac subsea interconnector because it represents an electrically thin (high resistance) interconnection to the UK mainland.

 

[JohnW2]

No. your scenario b) - where 1A of finest orkney wind energy goes south to those who are willing to pay extra for green energy but 1A of cheap nuclear electric comes north to service Orkney will of course not happen due to the laws of physics.

I didn't actually is it would 'happen'(since such a statement is meaningless) but, rather, that as far as the Orkney/mainland cable is concerned, it's the same as if the 'Orkney wind energy' were all used at home.

For the purpose of illustration, say that, in addition to that 1A load, the remainder of the Orkney load amounted to 10,000A. If the wind energy is all used at home to supply that final 1A of load, 10,000 would be flowing under the sea from the mainland to supply the remainder of the load. If, somehow, the 1A of wind-generated load was flowing to the mainland, an additional 1A would have to flow from the mainland (i.e. 10,001A total) to supply Orkney's needs - so, again, what the under-sea cable would see would be 10,001A - 1A flowing in the 'to Orkney'direction - i.e., again,10,000A.

But, a lot of people here DO think that this is going on.

When the recipients don't have the technical knowledge, sales and marketing people/material can appear very persuasive.

People might find it easier to understand if one used water/pipe analogies. If there were limited water supplies in Orkney, and an under-sea pipeline were supplying most of Orkney's needs from the mainland, you'd have a directly analogous situation. Maybe people would find it easier to see that sending Orkney water to the mainland and then having to get more water from the mainland to supply Orkney's needs would be the same of using the Orkney water locally and importing less from the mainland - but in this case, maybe they'd find it easier to understand that one cup of water was the same as any other cup of water?

Kind Regards, John.

 

[SparkyTris]

say that, in addition to that 1A load, the remainder of the Orkney load amounted to 10,000A. If the wind energy is all used at home to supply that final 1A of load, 10,000 would be flowing under the sea from the mainland to supply the remainder of the load.

Agreed.

If, somehow, the 1A of wind-generated load (you mean current?) was flowing to the mainland,

"somehow"? This could ONLY happen if volts were higher on the Orkney side. (?)

which would preclude this:

an additional 1A would have to flow from the mainland (i.e. 10,001A total) to supply Orkney's needs -

happening.

so, again, what the under-sea cable would see would be 10,001A - 1A flowing in the 'to Orkney'direction - i.e., again,10,000A.

??? pls clarify that?

or have I got totally the wrong end of the stick, and if Orkney draws X amps, and exports Y amps then the export lines have to handle X+Y amps?

that would explain everything, certainly would explain why the infrastructure "cannae tak it"


John, the way you apply the water pipe analogy is interesting and I am sure flawed:

sending Orkney water to the mainland and then having to get more water from the mainland to supply Orkney's needs would be the same of using the Orkney water locally and importing less from the mainland

it costs per litre to send orkney water to mainland. and it costs per litre to import water. so there is higher cost for same net water usage. And THAT is just the scenario that OFGEM are stating.

using locally (no export costs) and importing less (less import cost) will be lower total cost for same net usage.

so the only thing that is the same in the 2 cases is net consumption.

 

[JohnW2]

If, somehow, the 1A of wind-generated load (you mean current?) was flowing to the mainland,

"somehow"? This could ONLY happen if volts were higher on the Orkney side. (?) .... which would preclude this:

an additional 1A would have to flow from the mainland (i.e. 10,001A total) to supply Orkney's needs -

happening.

(yes, I did mean 'current' - apologies). I think you are getting tied up by the difference between realities/physics and salestalk. As you say, that 'somehow' could not arise - but it is the very 'somehow' that sales folk attempt to invoke.

You and I know that, at any point in time, current can flow only one directiopn or the other through the Orkney/mainland cable - and the direction of that flow is determined solely by the direction of the (instantaneous) potential difference between the two ends of that cable. I am assuming that the flow is northwards, so the potential at the southern end must be higher. Connecting a few windmills, PV installations or whatever to the Orkney side of the cable is not going to alter that situation. No matter what their off-load voltage might be, connecting them to the Orkney side of the undersea cable is not going to appreciably change the voltage, certainly not enough to change the direction of flow under the sea. What it will do, of course, is slighly decrease the amount of current that has to flow nothwards into Orkney.

John, the way you apply the water pipe analogy is interesting and I am sure flawed: it costs per litre to send orkney water to mainland. and it costs per litre to import water. so there is higher cost for same net water usage. And THAT is just the scenario that OFGEM are stating.

I agree that, in the sense you mention, it's not a very good analogy - but that's because, unlike what is essentially the case with electricity, there is an operational cost in pumping water around.

The crucial point surely remains that, unless the generation of electricity in Orkney ever comes to exceed its needs, there is always going to be a northwards flow in that cable under the sea (an inevitability of physics), and no individual generating facility in Orkney can alter that.

Kind Regards, John

 

[Mikefromlondon]

Personally, if I was concerned about producing my own energy on a wind farm from my land, I would first make sure that it supplied all my needs, and to do this I would use an inverter that provides a steady stream of electricity at 50Hz, and nominal 230V ac, with possible battery back up for short term fluctuations, next, my inverter will monitor what my consumption is, and if there is surplous power available, it can export that by throwing contactors in at the right time pahse wise, if my own current demand increases and my inverter will automatically provide me with extra power and reduce the power it is exporting, the export meter would register all my exported power so that I could be paid for it.

Finally, if my own demand increases, or there just isn't enough wind, then my inverter will take a rest, and allow the grid to take over my household power requirement, and this coukld then be metered as energy used by me for billing by the energy supplier.

What happens when the wind is low to moderate that my generator is only able to produce 50% of the energy that I am using, in this case the export contactor will drop out, and an import contactor drop in and any extra energy or power my household needs will now be added to by the grid, again, it would be essential that this change over happens correctly at precise time to avoid phase clash and blow out.

But I see your point about who should benefit from locally produced wind energy, sure the answer is, the people who have to put up with the nuisance of wind farms should benefit most, and cheaper terrif too.

I am of the opinion that locally produced wind energy should benefit local people, and it should not be minglked with the grid for exportting as i am sure there probably won't be enough to export, so local people should benefit from this and cheaper terrif.

 

[SparkyTris]

John, thanks for explaining.

What it will do, of course, is slighly decrease the amount of current that has to flow nothwards into Orkney

thats just what I think too., I shall have to ask of the grownups what the story is with all this talk of interconnector upgrade. Yes, it might be necessary eventually, but only once local generation has far exceeded local consuption.

The first effect of renewables grid connection in rural areas is to generally (and slightly) lessen load all round.

thanks very much for the argument. albeit rather esoteric.

MikefromLondon
yes nice plan. but do mechanical contactors act fast enough to catch the phase at the right instant?!?!

I wasnt particularly making a point about who should benefit from windmills, though that is indeed a valid issue. In Any case, it appears that - at a physical level (if not an economic level), those that do live close to windmills CAN rest assured that they are using the energy produced by the 'mills!

 

[Mikefromlondon]

I just used a general or a bold term of throwing contactors at the appropriate times, but in essence, this would be carried out by a refined system, the time the contactors take would be irrelavent, as the phase angle of the inverter would be pre-synchronised so that no matter how long contactors take to come on, the pahse angle would be within the specified limits, usually +/- 1 degree! .

 

[JohnW2]

John, thanks for explaining.

What it will do, of course, is slighly decrease the amount of current that has to flow nothwards into Orkney

thats just what I think too.

You really don't have to merely think that - it is surely an inevitability of Kirchoff's laws? If the total current requirement of Orkney is xA, then if all is being supplied by the mainland, then xA has to flow north to Orkney. However, if yA are provided by generation in Orkney, then only (x-y)A will flow north into Orkney, to provide the total requirement of xA. If more than (x-y)A were flowing into Orkney, where on earth would the 'excess' be going? Mr Kirchoff would not be happy

I shall have to ask of the grownups what the story is with all this talk of interconnector upgrade. Yes, it might be necessary eventually, but only once local generation has far exceeded local consuption.

Simple arithmetic suggests that Orkney would have to generate double Orkney's requirements before the situation (in terms of network/undersea cable) even got back to where it would be without any generation in Orkney (the under-sea flow would then be the same, but southwards, rather than northwards). Only if Orkney generation rose to more than double Orkney's needs would the network be more 'stressed' than it would be without any Orkney generation - and I suspect that's not going to happen any day soon!

The first effect of renewables grid connection in rural areas is to generally (and slightly) lessen load all round.

That's certainly how it would seem. I suppose that moderately large generation facilities could theoretically 'stress' the cables in the immediate vicinity but, in general, I agree with you.

Kind Regards, John.

 

[SparkyTris]

The "dream", one supposes, is that eventually, there will be so many windmills (or general "renewable" sources), with so much geographic diversity, that the current flowing from the fossil-fired or nuclear-heated power stations will dwindle down, and some of them can eventually be turned off.