Multiple grid tied inverters......

[dmrolyat]

Ok, a question....
So, I have a 2kW hybrid system installed with 8x240W Sanyo hybrid panels and a sunnyboy 3000 inverter.
The AC SWA cable from the inverter in the attic runs to a seperate CU and meter with breakers before tying into the mains cabling with Henley blocks.

Now, as I am looking at having additional 8 panels fitted to a different elevation (back of house facing SW), I appreciate this will be a seperate string and run through a second inverter.

I have been told that the system, identical to the one already fitted, can be installed and the AC output connected to the same AC cabling as the existing inverter in the loft. Therefore retaining just the one SWA cable running to the Consumer Unit. This would allow the same meter to be used and the same Elios system already fitted, rather than having 2 seperate generation meters and monitoring.

So, my question is how does this work in practice?

My understanding of the way inverters worked was that they sensed the input AC voltage and then raised their output above this voltage in order to export to grid. Assuming 2 inverters are connected, essentially in parallel, I would have assumed that as they are each 'sensing' the input AC voltage that they would end up in a never ending loop of increasing voltage. Ie, the first senses the domestic supply voltage say 220v, then increases to say 225 in order to export to grid. Meanwhile the 2nd inverter is sensing 225v and increases it's output to 230v and so forth and so forth.....

Can someone explain how 2 inverters, each with independent strings work together?

Also, is it reasonable to connect them together in this way?

Thanks in advance.

[inetdog]

Ok, a question....
So, I have a 2kW hybrid system installed with 8x240W Sanyo hybrid panels and a sunnyboy 3000 inverter.
The AC SWA cable from the inverter in the attic runs to a seperate CU and meter with breakers before tying into the mains cabling with Henley blocks.

Now, as I am looking at having additional 8 panels fitted to a different elevation (back of house facing SW), I appreciate this will be a seperate string and run through a second inverter.

I have been told that the system, identical to the one already fitted, can be installed and the AC output connected to the same AC cabling as the existing inverter in the loft. Therefore retaining just the one SWA cable running to the Consumer Unit. This would allow the same meter to be used and the same Elios system already fitted, rather than having 2 seperate generation meters and monitoring.

So, my question is how does this work in practice?

My understanding of the way inverters worked was that they sensed the input AC voltage and then raised their output above this voltage in order to export to grid. Assuming 2 inverters are connected, essentially in parallel, I would have assumed that as they are each 'sensing' the input AC voltage that they would end up in a never ending loop of increasing voltage. Ie, the first senses the domestic supply voltage say 220v, then increases to say 225 in order to export to grid. Meanwhile the 2nd inverter is sensing 225v and increases it's output to 230v and so forth and so forth.....

Can someone explain how 2 inverters, each with independent strings work together?

Also, is it reasonable to connect them together in this way?

Thanks in advance.

Let's say that there is an effective resistance of .1 ohm between the inverter output terminals and the utility terminals, and a nominal voltage of 120V.
Now to deliver 10A back to POCO, one inverter will have to have 121 volts at its terminals.
When you connect two inverters at 10A each, the voltage at the point where the two inverters are connected together will be 122V, feeding 20A through .1 ohm.
Whether you get to that stable point by solving the equation for the stable condition or you get there by adding up an infinite number of catch-up increases
of 1 + 1/2 + 1/4 +1/8..... = 2, you arrive at the same point.
The sum of an infinite number of steps can still be a finite number if the steps keep getting smaller fast enough.

This is one of the cases where theory and practice meet nicely, and the wiring setup you describe is certainly the simplest and least expensive way to go.

[dmrolyat]

Thanks inetdog....
So, are you saying that connecting 2 inverters together in this way is normal and not a problem?

[inetdog]

Thanks inetdog....
So, are you saying that connecting 2 inverters together in this way is normal and not a problem?

Absolutely.
Just make sure that the wiring from the inverters through the production meter all the way to the main panel is large enough to handle twice the current. Or else have the wiring updated as necessary by an electrician.

[dmrolyat]

I think the Existing SWA is 4mm. The installers said that they installed this as standard on all 4kW and below domestic installs, so would be more than adequate adding the second inverter.
Does this sound ok?

Yes, but I did not actually run the numbers.