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Topic Title: Micro Generation in Parrallel with Mains - Import/Export Question
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Created On: 27 March 2014 04:21 PM
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 27 March 2014 04:21 PM
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altezzarm

Posts: 6
Joined: 09 August 2012

Quick query bothering me for a while now as iv just been involved in a renewable project.

Scenario: Load (Building) fed via Utility Mains and also on site PV installation.

Import/Export meter arrangement installed!!

PV Array is generating 10Kw

If the demand from the building is 10kw, there should be no power drawn from the grid.

But how does the system know to take the power from the PV supply and not the grid/mains supply? Is this processed through the import export meter?
What if there was no export ?
 27 March 2014 06:06 PM
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AdrianWint

Posts: 271
Joined: 25 May 2006

The laws of Physics!

The meter doesn't make any decisions, it simply records what it sees.

There is a topic of Power Engineering called 'Load Flow'. This deals with the flow of real & reactive power around a system. I'm not going to go into detail here, but, basically, if the power required by the installation is less than that produced by the PV then the building will export back to the mains. If the power required by the building is more than produced by the PV then the building will import from the mains. That's physics and there is nothing you can do about it.

This will happen just the same with or without an export meter. The only difference is financial... if the meter isn't programmed for export then your customer wont get any credit for the power he exports back to the grid - he'll give it away for free!
 27 March 2014 10:05 PM
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ectophile

Posts: 552
Joined: 17 September 2001

The PV inverter will control its output voltage to ensure that it is always pushing current out, even as the load varies.

Other than that, nothing is doing any controlling.

Consider the point in the circuit where the incoming supply, the output of the PV inverter and the line to the load all meet (there must be such a place, somewhere in the system).

Consider incoming currents to be positive and outgoing ones to be negative.

The sum of all currents going in and out of that point must be zero, or the electrons would all be piling up in a heap.

So if the inverter is producing 16A, and the load is 16A, then the current drawn from the supply must be zero.

If the inverter produces less, then the current from the supply would increase so it all adds to zero again.

If the load drops, then the only way it still sums to zero is if some of the current is exported to the supply (i.e. the supply current goes negative).

-------------------------
S P Barker BSc PhD MIET
 28 March 2014 07:45 PM
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jcm256

Posts: 1943
Joined: 01 April 2006

Spain, (Was there last week), will have a new system for pricing electricity from April. not sure would be better than the UK for feed in/ out small scale domestic generation, because it will be priced every hour via intelligent meters (smart meters) the price of MW per hour could go from 0Euro in the early hours of the morning, up to 90 Euros in the evening. You do not see much PV fitted on roofs in Spain, you would think with all the sunshine there would be. Only posted this, as you would need to take into account the dreaded smart meters fitted to every home in UK the next few years.

http://www.computerweekly.com/...smart-meter-roll-out-

Maybe you should stick to 50% rather than try to measure your actual export.

http://www.energysavingtrust.o...icity-you-exportdoomed
 28 March 2014 08:53 PM
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davidwalker2

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Joined: 29 April 2009

I have a place on the Costa del Sol Spain - and there is a lot of PV. All official government buildings have PV as a matter of course. At Ikea near Malaga airport the whole car park uses PV panels to provide shade for the cars, I estimate about 100kW installed. Andalucia has the largest solar generator Andasol1, not PV but a large area of steerable mirrors focused to the top of a tower that generates steam for turbines, has enough storage to work 24hrs a day, around 50MWe
It is true that domestic premises tend to have solar hot water systems rather than PV, but that is historical and there are an increasing number of PV installs.
Edited to correct Andasol1 power. Google "Solar energy Spain" for more info.

David

Edited: 28 March 2014 at 09:09 PM by davidwalker2
 29 March 2014 11:16 AM
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AdrianWint

Posts: 271
Joined: 25 May 2006

Originally posted by: ectophile

The PV inverter will control its output voltage to ensure that it is always pushing current out, even as the load varies.




I don't think its quite that simple with AC. Won't varying the voltage only affect the reactive power and not the real power? The system voltage will be fixed by the infinite bus that is the grid, so the inverter increasing its output voltage to try and 'pull up' the infinite bus will cause an increased reactive power flow.
 30 March 2014 12:40 AM
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ectophile

Posts: 552
Joined: 17 September 2001

Originally posted by: AdrianWint

Originally posted by: ectophile



The PV inverter will control its output voltage to ensure that it is always pushing current out, even as the load varies.








I don't think its quite that simple with AC. Won't varying the voltage only affect the reactive power and not the real power? The system voltage will be fixed by the infinite bus that is the grid, so the inverter increasing its output voltage to try and 'pull up' the infinite bus will cause an increased reactive power flow.


My original reply was somewhat simplistic. Grid-tied inverters have to be pretty clever to match the voltage, frequency and phase of the incoming supply.

-------------------------
S P Barker BSc PhD MIET
 01 April 2014 04:46 PM
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altezzarm

Posts: 6
Joined: 09 August 2012

Thanks everyone for your insight/help. I think Il have to read the thread on "load flow" to get a solid grasp on this. I believe a given load may take the currnet required from either source, but one offsets the other regarding kwh unit charge.
I,e Load demands 5Kw, PV inverter supplying 5Kw. The load may take the current (5Kw) in this instance from the grid, however the 5Kw supplied from the PV inverter will simply offset this by dumping the 5Kw onto the grid.
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