I have been reading some papers and articles about grid-tie inverters fed by PV cells and I can not make a clear idea of the following case:
- Assuming a 220 Vrms single phase grid line.
- A PV array + inverter capabe to draw 3Kw.
- A load (for example an air conditioner) that absorbs 3.5Kw
I am looking for your replies!
Thank you.
Dear Prof. Mauricio Neiff,
In my point of view....
1. Yes. It is possible to draw 3kW power from solar PV system using grid tied inverter if the power generating capacity of solar PV system is above 3kw in that moment (If we are receiving sufficient irradiation from sun).
2. Voltage-phase-frequency are synchronized to connect the solar PV system with the grid. But to control the power flow to the load from grid and solar PV system inverter will act smartly. Inverter will monitor the grid voltage and increase the output voltage of inverter slightly above the grid voltage so that the power drawn from the inverter is controlled. This increase in voltage is controlled based on grid voltage and power output of Solar PV system.
Mauricio,
I will add a little bit to what Maharaja has stated, because I feel he has missed your question (as #2 in your message).
An inverter fed by renewable sources is connected to the grid through a transformer. This offers an impedance, one side of which is connected to the inverter and the other to the point of common coupling. So even in a simple system (without going for an elaborate microgrid control) the inverter side voltage can be maintained at desired rms and angle relative to the utility bus.
This essentially amounts to controlling the inverter end for an appropriate primary-to-secondary load angle across the transformer.
BTW in your example, a 3kW PV-inverter unit will rarely pump all of 3kW into the load. Don't overlook losses within the inverter and its transformer, which can be quite significant sometimes !
-Sanjay
Mr. Maharaja Kamatchi:
Thank you very much for your answer. As you mentioned in your #2 answer, I've read that the output voltage of the inverter must be slightly higher than the grid in order to let the flow of power goes from the Inverter to the load and the grid side. Please observe the image from the attached link. As it shows, the voltage measurement for control is in the Point of Common Coupling (PCC) Am I right to state that "Having a higher voltage output (in my case >220Vrms) is possible because of the existence of the LG inductance" ?
Mr. Sanjoy Roy:
Thank you for your time in answer me. If I am not disturbing you much, please now observe the attached link. I understand tha we can (and should) replace that PCC with a transformer to achieve galvanic isolation. I suppose it should be a 1:1 transformer. Is it important which side my load will be connected to?
By the moment, let's assume that 3Kw is the effective final and neat power capacity of the system since I want to focus my attention to the flow of power between Inverter&Load&Grid.
Thank you for your answers.
http://www.rroij.com/articles-images/IJIRSET-59-g001.gif
Hi Mauricio,
Please don't talk about disturbing me ! You are always very welcome to discuss over the web.
Though theoretically you should be able to connect load on either side of the transformer, I would suggest you to stick to the utility side. There can be several protection issues on the inverter side.
To say the least, as you very well know, the inverter side must always have a floating neutral. And that may not go well for many of the load types, which may require grounded neutrals !!
Hope that helped...
-Sanjay
Mauricio, consider what happens if you connect a load to the grid - without any inverter or anything else. Power flows from the grid into the load, and in doing so the grid voltage will drop slightly according to the 'output impedance' of the grid. In most cases it will be a fraction of an ohm; but it will not be zero. Therefore if you now connect an inverter it will try to drive current into the grid thus raising the grid voltage, again slightly, according to how much current it attempts to drive. The power you are considering in your example is split between the grid and the inverter because of the grid output impedance. If this impedance were zero then the split of power between the inverter and the grid would become indeterminate, as you suggest.
My opinion is that when you connect the pv generator to the grid by connecting its inverter to the grid, may be one of the most convenient control mode is to operate the inverter in output current control mode. In this way you control the power delivered to the grid. When connecting the load to the grid it will take its power demand from the grid. Since you deliver to the grid the PV power and take from it the load power then the load will need only to draw the difference from the grid.
For the current controlled grid connected inverter please refer to the paper in the link:Article Simulation and Implementation of Grid-connected Inverters,
Best wishes
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