How do grid connected inverters determine frequency and phase

[AussieBruce]

Hi,  I’m after some information on how renewable energy inverters (solar, wind) remain on frequency and in phase with a grid load. Wikipedia doesn’t go deep enough for what I’m after. Presumably the principle of operation is that  when feeding a substantial grid, the inverter output won’t be able to substantially affect the connected waveform, so problems with feedback instability aren’t significant. Is it as simple as a programmed device such as a micro or an FPGA detecting the + and – crossing points following a small amount of filtering, and using this to synthesize the pattern of switching signals that generate the chopped waveform, based on the required output power level? Or is something more sophisticated involved, such as a PLL that generates a tracking waveform that’s then used for controlling the output switching? 

If there's a such a thing as a device that does all this, a type number would be useful, the datasheet will tell all.  Thanks for any info.

 

[Faringdon]

If there's a such a thing as a device that does all this, a type number would be useful, the datasheet will tell all.

I did a search on this some time back, and couldnt find any such device, so i think either of your  stated methods is fine, just as long as you can stay in phase within +/- x%....so yes, the phase detection thing is just a role your own, in either of the ways that you describe.

It amazed me how rough the whole thing is....because if too many people in a neighbourhood are inverting into the mains, then the mains will rise too high, and any other residents who want to invert into mains, wont then be able to....because the mains would be too high, and all GTI's detect the mains voltage b4 starting and wont start unless mains is less than 253VAC or so. The governments were going to install hugh battery banks everywhere to mitigate this, and so that more people  could invert into mains when they want to.

[David Hess]

Is it as simple as a programmed device such as a micro or an FPGA detecting the + and – crossing points following a small amount of filtering, and using this to synthesize the pattern of switching signals that generate the chopped waveform, based on the required output power level? Or is something more sophisticated involved, such as a PLL that generates a tracking waveform that’s then used for controlling the output switching?

Those are essentially the same thing.  A reference waveform that is phase locked to the power line is used to modulate the output current.  Using the reference waveform allows the inverter to ignore noise on the power line.

It amazed me how rough the whole thing is....because if too many people in a neighbourhood are inverting into the mains, then the mains will rise too high, and any other residents who want to invert into mains, wont then be able to....because the mains would be too high, and all GTI's detect the mains voltage b4 starting and wont start unless mains is less than 253VAC or so.

The inverter can also shift the phase of its about a limited amount to increase its output power without raising the peak voltage.