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Topic Title: Meters running in reverse?
Topic Summary: How does current flow out of a solar PV system?
Created On: 12 June 2012 11:33 AM
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 12 June 2012 11:33 AM
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EwanCroucher

Posts: 6
Joined: 24 September 2002

Maybe I'm forgetting all my electrical knowledge as I near retirement but can anyone explain the numerous claims of analogue electricity meters running backwards and "exporting" power?
As I remember it, the speed the aluminium disk rotates is determined by a voltage coil and a current coil producing eddy currents in the disk. The speed of rotation of the disk is proportional to V x A hence power.
What I don't get is, if the main supply is at 240v and the PV system is generating 240v how can current pass through the current coil to generate any reverse torque?
 12 June 2012 01:16 PM
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ectophile

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The PV system monitors the mains voltage and generates its own electricity at a slightly higher voltage. That way, it can "push" the electricity it generates back into the mains. If the generated power exceeds the current usage of the household, then any excess will be exported back through the incoming mains to other households on the same substation.

Not all meters can run backwards. The newer rotating disc meters have a ratchet mechanism to stop this happening, and will simply stop turning when the PV system is exporting power out to the grid.

-------------------------
S P Barker BSc PhD MIET
 12 June 2012 02:13 PM
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EwanCroucher

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Joined: 24 September 2002

Thanks for the reply. Does this mean that if a neighbour or neighbours have similar systems then the voltage ratchets up as they compete to supply?
 13 June 2012 01:05 AM
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alancapon

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Quite possible. Most distribution networks were designed with the feeding substation as the only source, and volt-drops calculated as you got further away from it. With the connection of embedded generation (wind, solar etc), you end up with additional infeeds, and the volt-drops as calculated may not exist. In a worts case scenario, it may actually raise the voltage at the substation to such an extent that the supply is close to its maximum allowable voltage near the substation when all the generation is running.

Regards,

Alan.
 13 June 2012 10:46 AM
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EwanCroucher

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Well, the plot thickens. Has no one thought this through?
As far as I can determine supply companies don't require meter readers to read the generated power in these systems. When asked, our meter reader had never read anything other than import meters.
The Feed In Tariff payments are apparantly made on estimates based on the installation size. The export payments are "deemed" to be 50% of this generated figure, thereby compounding the error.
On top of this, many PV customers also receive a discount on their bills equivalent to exported power if their meters run backwards.

Maybe, in a few years time when Smart Meters are rolled out, they may monitor generated as well as imported power and won't run backwards. Then there will be a lot of unhappy customers claiming to have been mis-sold PV systems!
I suppose the only customers who should be concerned are those who don't have PV since apparantly supply companies add a levy to all bills to cover these subsidies but I think that should be another topic.

Regards,
Ewan
 13 June 2012 01:11 PM
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ectophile

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The amount of power being generated by the PV system is certainly metered. I have a second meter connected to the PV inverter which measures that. The feed in tariff payments are based on what that meter reads - and if my electricity company ever finishes processing the paperwork for my installation, I will be sending them a meter reading every quarter.

My main meter is of the sort that can't run backwards. As a result, there is no way of knowing how much electricity I have exported. It is just assumed that I will use half the energy generated, and the other half is exported. I am (or will be) paid for the exported electricity at wholesale rates.

Since the wholesale price of electricity is a fraction of the feed in tariff, the accuracy of that assumed export is of little consequence.

Notice that the FITs are based on the amount of electricity I generate, regardless of who uses it. So if I run a high-current appliance on a sunny day, not only is the electricity free, but I still get paid the FIT for generating it.

-------------------------
S P Barker BSc PhD MIET
 13 June 2012 04:14 PM
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aroscoe

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It does seem a bit of a shame that we have to have these two meters that can only run one way, one for import and one for export, when a single "smart" meter which could record the net balance over half-hourly (or smaller) timeframes would probably be more useful.

As I understand it though, even the new "smart" meters will still give a "positive" e.g. consumption figure even if your house actually exports energy, or it will register zero and potentially go into an error condition. I guess they just put a mathematical abs() or ">0" function inside the meter software. Reading between the lines, this is for tamper protection so that if someone tries to tamper with the meter to reduce their consumption bill it still always records a zero or "normal" consumption figure.

But, it would be a neater and more efficient system if tamper-protection wasn't an issue and a single meter could record the net balance over time, forwards and backwards.

Andrew

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 13 June 2012 04:25 PM
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aroscoe

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ectophile,

Out of interest, how are your meters connected. Are the import and export meters totally in parallel so they register two totally independent import/export power flows, or is the PV export meter connected "downstream" of the normal "import" meter?

In this latter case the import meter would register (import-export) but truncated to zero if export>import, and I can see how this means that effectively there is a loss of data when export>import because there can be no way of knowing what your consumption is. That would be daft, but is sounds like that may be whats happening, especially in another forum posting set

http://paulaowenconsulting.co....e-from-pv-installers/

Andrew

-------------------------
Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 13 June 2012 08:00 PM
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drhirst

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In any sensible electricity market, the price of electricity will be different at different times. Even if this is not so for you as a consumer, it will be so for your retailer / supplier.
So if the meter records the "net" flow of electricity, this is unlikely to reflect the net value. You may have been exporting at a time of low prices, and importing at a time of high prices, and still record a net zero kWh. Why should your retailer bear the cost of this?
What the meter should really record is the net flow of cost. So when exporting, it calculates and accumulates what your retailer should pay you for what you export, and when importing, it calculates and accumulates how much you should pay your retailer for electricity.
In a fair system (which is very far from what we have), it is reasonable for the retailer / supplier to collect a margin. So it can reasonably expect to charge your neighbours a bit more than they pay you for the electricity you export and they import.
But this is perhaps wishful thinking. We do not have a sensible retail electricity market, and nothing being done to reform it looks like making it any more sensible!

-------------------------
David Hirst
 13 June 2012 08:37 PM
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EwanCroucher

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aroscoe,

The problem with net balance metering (which is what old analogue meters do) is that it means supply companies would buy energy at the same price they sell it. An unlikely situation I think. Also with the present FIT scheme the PV owner also gets the export tariff. Since I'm paying for it via the levy I consider that wrong.
Does anyone have a separate export meter? I understood that the PV system meter just records generated power.

Rgds,Ewan
 14 June 2012 08:38 AM
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ectophile

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The main meter is still connected just as it always was - between the supplier's cut-out and the consumer unit. It is measuring the electricity I consume from the grid, bit doesn't record anything I export.

The PV meter is connected between the inverter and the consumer unit and so records everything generated, regardless of who uses it. The PV meter is wired to a circuit breaker just as if it were a normal appliance.

It would be simpler if I had a main meter that could run backwards, so that my electricity bill would just show the net units I consume. However, the FIT is always based on the amount generated, as indicated by the new meter.

-------------------------
S P Barker BSc PhD MIET
 14 June 2012 09:57 AM
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aroscoe

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ectophile - Hmmm. So there is a period where data is completely lost - When you are generating more than you are using, there is a record of your generation (on the export meter) but the import meter will be truncated to zero. So, if you had a big PV array, generating 10kW on a sunny day, you might as well use all of it at home, because the electricity company has no way of telling the difference between a domestic consumption of 0W and 10kW during this time .... You could run a nice business in charging electric cars during this time, for free!

Also, in reply to Ewan one post ago: Point taken, BUT if my single net meter (capable of running in either direction) can record usage every 15 to 30 minutes, AND I receive a variable tariff stucture from my utility (different prices during the day, according to weather etc, and different prices for import/export) then this could all be reconciled fairly and accurately without any guesswork. Right enough, while people have been talking about this for a few years now, very few domestic customers globally get any kind of flexible/dynamic time-based tariff. However, yesterday I was at a presentation by Scottish Power and they were actually inferring that they are envisaging this coming soon. This is the first time I have heard a utility/supplier in the UK actually enthusing about this.

Andrew

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Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 14 June 2012 10:17 AM
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broadgage

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Originally posted by: EwanCroucher

Thanks for the reply. Does this mean that if a neighbour or neighbours have similar systems then the voltage ratchets up as they compete to supply?


Yes, but only very slightly.
The generated voltage only has to be minutely over the mains voltage for power to flow backwards.
Presuming that the distribution system is correctly designed, then the local increase in voltage is very small and normally a matter of no concern.
 14 June 2012 11:53 AM
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EwanCroucher

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Andrew,
I agree your Smart meter capable of recording accurate imports and exports would provide the solution and also provide the accurate information to properly evaluate the performance of these systems. But then you've thought it through unlike the designers of the FIT scheme.

Maybe I'm just cynical in my old age but I don't think it will happen .
1. Ofgem have published their proposals which would scrap tiered tariffs and standing charges in favour of a standard tariff and a single unit rate. In view of the existing difficulty in comparing suppliers I think a sophisticated time based tariff would be very unpopular.

2. The only company offering smart meters I know of is First Utility. It isn't clear from the website if their meter is capable of time based tariffs or monitoring exported energy. They also charge a premium over their standard meter tariff which is odd since they don't have to pay a meter reader to knock on the door.
Has Ofgem published any requirements for the smart meters?

3 The initial Feed in Tariff scheme was driven by political considerations. What happens to the PV owner with an old reversible meter who already has a 25 year contract for an export tariff linked to RPI?
Regards,
Ewan
 14 June 2012 12:59 PM
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williamjohn

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My supplier (E.On) offerred a Smart meter but I declined as I have heard that a supplier can shed load remotely at a consumer's Smart meter. However, I have also heard that a Smart meter records three readings, kWh from the mains, kWh to the mains and kWh generated by the pV panels. Has anyone else heard this and is it true that the supplier can switch off the supply at a Smart meter remotely?
 14 June 2012 01:48 PM
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aroscoe

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Broadgage: Yes, NORMALLY a matter of no concern. However as Alan point out above, it CAN be if all your neighbours also get big PV installations. In the UK this hasn't come up much yet (although there may be some schemes in N. Wales?) but in Germany it is a recognised problem. There is a good paper at: http://www.iwes.fraunhofer.de/...es/15_Cired_2011.pdf The thing is that the utility normally sets the voltage at the 11kV/400V transformer high, about 104%, which is why most people see about 240V at their house, not the nominal which is 230.9V +-10%. This is so that when houses consume electricity, the voltage tends to drop along the cable under the street (due to its dominant resistive property), and you need to make sure that the guy at the end of the street always sees more than 230.9V -10%. But, this means that there is only 6% headroom left at the top end, and if all the houses start exporting 10kW then the +10% limit can be exceeded, particularly when you bear in mind that it is much more likely for PV power exports to be correlated in time across all the houses (the sun shines on them all at the same time), than it is for peak demands to be correlated from all the houses. Its highly unlikely that everyone in the street takes their shower at the same time!

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Dr. Andrew Roscoe

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 14 June 2012 02:04 PM
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aroscoe

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Ewan,

I agree cynically that it appears that it may never happen. But, there are some really good reasons why it may eventually - due to the amount of reduction in spinning reserve (etc) and hence operational costs which you can achieve at the core of the electrical network. The commercial barriers in the market to dynamic pricing are huge (and it is not my field), because there is such a disconnect between who you pay your bill to and the people who are generating the power and maintaining the transformers and cables. However, there is so much talk about it now, that perhaps it will finally happen.

I've lost touch with it a bit, but the DECC page at http://www.decc.gov.uk/en/cont...ters/smart_meters.aspx seems to be very active, with lots of updates of the consultation. If anyone has time to read all this, great!

Andrew

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Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe
 14 June 2012 02:10 PM
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ectophile

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Originally posted by: williamjohn

My supplier (E.On) offerred a Smart meter but I declined as I have heard that a supplier can shed load remotely at a consumer's Smart meter. However, I have also heard that a Smart meter records three readings, kWh from the mains, kWh to the mains and kWh generated by the pV panels. Has anyone else heard this and is it true that the supplier can switch off the supply at a Smart meter remotely?


I understand that the new meters do have a remote shutoff system. It's intended as an easier way to shut of non-paying customers. The only way at present is to pull the customer's supply fuse, and if the meter is indoors, that means breaking down the customer's door.

I can't imagine it being used at any other time except in an emergency. Electricity customers wouldn't stand for thair supply being turned on and off througout the evening, just because an electricity supplier hadn't bought enough power from the generators.

There has been talk of individual appliances being turned off to save power at peak times, but that's impossible with any equipment available at the moment - there is nothing built into appliances or consumer units to turn individual appliances off.

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S P Barker BSc PhD MIET
 14 June 2012 03:07 PM
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EwanCroucher

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The other reason for having a remote shutoff is to protect people working on the mains network during a fault although I thought this was built into the inverter design.

Ewan
 14 June 2012 08:08 PM
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jcm256

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You can be dammed sure rent-a-roof will have the right sort of meter installed to grab all the FITs they can. I think they leave the homeowner only with a small token amount.


Why is it unfair on all consumers?
FIT is paid for by you - all electricity customers pay a small levy on their bill to fund the scheme, and in its first year more than £10m was paid out. Rent-a-roof schemes mean that 'free solar' companies pocketed a chunk of that cash.
Which? thinks that the profits from rent-a-roof schemes should be shared more fairly between the rent-a-roof company and the householder. We are also concerned that cash intended for householders is going elsewhere.
And we're all paying for the feed-in tariff : the Department of Energy and Climate Change has estimated that by 2020 we'll be paying £11 a year towards the scheme through our bills. We've asked the government to tighten it up to make it fairer.
Jcm I Eng MIET
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