How to design a single phase inverter (5kW)

[Faringdon]

I am wondering, if a grid tied inverter was to produce a sinusoidal current in phase with the mains voltage, and push that into the mains, then as long as the mains voltage did not get raised outside of 230+/-10%,  then no-one would be able to complain about this?...not even the regulatory bodies?

[David Hess]

The regulations for grid-tie inverters specify voltage, phase, frequency, and power factor limits. By combining them, the inverter can drive more power into the grid without exceeding the maximum voltage.

[Faringdon]

The regulations for grid-tie inverters specify voltage, phase, frequency, and power factor limits. By combining them, the inverter can drive more power into the grid without exceeding the maximum voltage.

Thanks, from what you say, it seems to me that there is a bit of leeway in the requirements for GTI...so i reckon a sinusoidal current source...being pumped into the mains, would be ok?

The voltage generally, will be set by the mains itself, so too the frequency.....and as you  suggest, power factor does not have to be unity......many deliberately wangle the output current to pump more current into the mains.

So that engineer who can design a sinusoidal current source, in phase with the mains, is a long way toward designing a GTI, i believe?

On page 11 of this, its seen that a sinusoidal current source is being designed for the GTI...
https://www.ti.com/lit/ug/tiduay6e/tiduay6e.pdf?ts=1633885097383

[Phoenix]

Thanks, from what you say, it seems to me that there is a bit of leeway in the requirements for GTI...so i reckon a sinusoidal current source...being pumped into the mains, would be ok?

The requirements for GTI are quite strict. In fact they keep getting tighter as more renewables and batteries are installed and grid authorities learn more about them.

The first step in a very complex controller design is the sinusoidal current regulation. Then you build and build on that until you meet the standards and meet the application.

[Circlotron]

The first step in a very complex controller design is the sinusoidal current regulation.

Seeing the mains voltage waveform is a bit flat topped in many locations, which would be the correct thing for the inverter to do? Feed a perfect sinusoidal current into the mains, or a current waveshape that exactly matches the distorted voltage waveshape?

[David Hess]

The first step in a very complex controller design is the sinusoidal current regulation.

Seeing the mains voltage waveform is a bit flat topped in many locations, which would be the correct thing for the inverter to do? Feed a perfect sinusoidal current into the mains, or a current waveshape that exactly matches the distorted voltage waveshape?

Or should it try to correct the mains waveform by injecting a current based on the difference between an ideal sine wave and the distorted mains?

[Phoenix]

Seeing the mains voltage waveform is a bit flat topped in many locations, which would be the correct thing for the inverter to do? Feed a perfect sinusoidal current into the mains, or a current waveshape that exactly matches the distorted voltage waveshape?

The "correct" thing is to obey the standards. I am most familiar with AS/NZS 4777.2 which has a current THD and harmonics requirement - but this test is performed with a low distortion grid simulator (with defined harmonic limits also). The other countries will have similar requirements.

What happens under a more distorted grid - well that's undefined .

I tend to let the inverter have whatever it's normal disturbance rejection is and distort the current. Disturbance rejection is really a function of the filter inductance and switching frequency/sampling rate. But you could clean up certain harmonics with cascaded resonant controllers (like in that TI document) quite easily.

[Faringdon]

The first step in a very complex controller design is the sinusoidal current regulation. Then you build and build on that until you meet the standards and meet the application.

Thanks, the sinusoidal current regulation is done in the atatched schematic.
I must confess this method doesnt need much software...just enough software to get the sinusoidal
reference...which needs to be variable in amplitude, as you know.
...So is this OK to take forward towards a Grid Tied inverter design?
LTspice sim and pdf schem attached.

[Phoenix]

...So is this OK to take forward towards a Grid Tied inverter design?

Not a chance. You might've be able to push power into the grid in an ugly fashion using this but you will never meet any performance or standards requirements.

Do you even know the grid connect inverter requirements?

[Faringdon]

Thanks, yes i  would agree its not a "completed" grid tie inverter, and not by any means....my apologies if i misled into thinking that it was.
But why isnt it possible to take this forward towards  being a grid tied inverter?
Its putting a sinusoidal current into a stiff mains source....isnt that a good starting point?

Do you even know the grid connect inverter requirements?

Thanks, yes, the completed product has to be able to synchronize to the mains, have OV, OC protection.
Not power mains when mains is off.
Not put too many harmonics into mains.
Sinusoidal current injected should have good power factor. (excepting the "special situations" said elsewhere in this post).
Not put power into mains when mains is outside 230V +/-10%

[Phoenix]

Thanks, yes i  would agree its not a "completed" grid tie inverter, and not by any means....my apologies if i misled into thinking that it was.
But why isnt it possible to take this forward towards  being a grid tied inverter?
Its putting a sinusoidal current into a stiff mains source....isnt that a good starting point?..

Not all current regulators and PWM schemes are equal. The one you have simulated won't meet performance requirements such as power factor and harmonics. There is a reason people use microcontrollers for this. It looks more like something you'd see on a rectifier (which usually have requirements of just a basic load).

Do you even know the grid connect inverter requirements?

Thanks, yes, the completed product has to be able to synchronize to the mains, have OV, OC protection.
Not power mains when mains is off.
Not put too many harmonics into mains.
Sinusoidal current injected should have good power factor. (excepting the "special situations" said elsewhere in this post).
Not put power into mains when mains is outside 230V +/-10%

Which country or region is this for? The specs are not accurate for US or EU or Australia (likely everywhere else too). It's far more complicated!