I Get a giggle every time someone says something has to be complex just to make it simple. I Laugh every time!
I make simple to be simple. Because I am simple. How simple is that?

As I have pointed out in several past posts there are countless ways to make one of these and as a simple man I came to the conclusion for my designs that the simple iron core low frequency method was the simplest, easiest, and cheapest way to feed the power back to the grid. And with the right Pi circuit it does give a clean sine wave that follows the grid wave form very cleanly and effectively.

I have never needed to monitor anything more than the instantaneous voltage of the line side wave form in order to get good clean returning power. All other line monitoring is just for safety, to make sure the line power is at the right voltage and frequency.

If the output amps get too high thats what fuses and circuit breakers are for. Same with the input side. If the input power exceeds the GTI capacity for too long the thermal protection limits shut it down and redirect the incoming power to a load dump circuit.
If the line voltage or frequency get to far off it again diverts the incoming power to the load dump device and then waits until the line side power returns to normal.

As far as using the HF switching to isolate the input from the output I see some gains and some losses. It does give a better and slightly more efficient use of the low end input power but is more complex and honestly does not have any overall great operating efficiency gains over a simple transformer. HF switching is much lighter physically but when you factor in the actual physical volume its actually bigger per watt than a standard transformer of the same capacity so nothing was gained size wise.

The LF iron core transformers are going to make the GTI heavier. But I figure, so what! The wind generators, solar panels or micro hydroelectric power source and the line connection are all stationary so what purpose does it serve or what gain do you get if the GTI is more portable? It cant actually go any place while its doing its job!
A lighter unit will save a customer a few dollars on shipping cost but a simpler and less complex unit is cheaper to build. And that savings will off set any added shipping costs.

From my personal experiences the standard voltage controlled switching device H-Bridge cant be beat! Its simple, easy to control, and protect.
It eliminates a need for multiple H-bridges or switching circuits to be used.
With the right control circuit design its capable of both current limiting and voltage limiting and PWM wave form shaping at 10's of KHz drive frequencies.

All other functions are done in the control side of the system. And once the control circuit is built and can do all of the input and output monitoring then its just a matter of changing voltage and current sensing
components in order to adapt it from a multi watt to a multi kilowatt control and monitoring capacity.

Just my opinion, but I do like the conceptual ideas and I am open to the possibilities that there could be other ways too.

I just think simple is simple and complex never makes anything actually simpler.

But efficiency bothers me. If it uses a iron 50Hz transformer then you need to disconnect it for the other 18-20 hours of the day when the solar panel is not working. That adds a lot more complexity, like a relay to connect it, ie more parts and more power wastage. And when operating, the efficiency of iron transofrmer square wave inverter is not gonna be much over 60-65%.

The actual control circuits can run a simple SSR or a mechanical relay to do the actual connecting and disconnecting as the input power goes up and down.
Its standard in every one of my systems.
I even included it in the 'Tiny GTI, maybe' thread. By unhooking the main power transformer every time the input power drops below the minimum feed back power point for more than a few seconds it automatically disconnects from the line.
So all that is powered from the line is the control circuit at that point and that has a power drain of less than 2 watts including the control transformer if one is used in the control circuit at all. For comparison my LED digital alarm clock has a input power rating of 10 watts.

I have posted several very basic voltage sensing circuits that are specifically designed just to handle that line connection function. Each one costs only a few dollars to make and can either run a mechanical relay or a SSR. I prefer SSR's because there can be a lot of actual cycling during a typical day.

As far as the actual operating efficiency the standard iron core 50- 60 cycle conversion is actually surprisingly efficient. I have done many different home made versions of the IGBT type H - bridges and I use the standard IR2110 IC's designed just for direct control of Mosfet or IGBT half bridge switching devices. They have the correct control functions built right in and automatically drive the gates at full saturation voltage with incredibly fast rise and fall times. Plus they use a standard logic level compatible input signal.
That IC gives a real life application the highest possible switching device efficiency they can run at so very little input energy is wasted by the actual switching itself. Its the same efficiency a HF type switcher will have but only running a much lower frequency.
That leaves the iron core transformer as the big efficiency loss device. But typical transformers operating efficiency is still 85 - 90 % over its full operating range. The smaller transformers do have some what less actual efficiency when compared to bigger transformers though. However again by using a small toroid core transformer that efficiency can be raised back up.
Anyone can check with any transformer manufacture and get information on the actual efficiencies of their transformers and then know exactly how efficient they are. The numbers are alway higher than everyone seems to think that a LF transformer is. I wish I knew why its such a common myth that LF transformers are inefficient. They are one of the most efficient voltage or current converting devices ever created.

On my actual designs I have measured and confirmed actual conversions efficiencies of around 85% at peak power levels.
The as calculated losses from the switching devices is less than 5% of the total input energy at its least efficient point. The other 10% loss is the transformer.
Not to bad for home made!
Anyone using a Mosfet or IGBT based H - Bridge that is driven off of IC's designed specifically for voltage controlled switching devices will likely be able to confirm this. It standard commercial inverter drive circuitry. I have even found some of the cheap 12 VDC to 120 VAC portable inverters use these IC's or other cousins of them for driving the output H - bridges that work on 170 volt DC to create the Ac output itself.
Most home made HF switching power supplies will not be any higher in efficiency either. Getting over that 85% hump requires more expensive parts and more complex control.
Its not that I am against HF based designs. I just have not seen any real life commercially made switching power supplies that had efficiencies much over 85% except for the HF fluorescent light ballasts used for the large 4 - 8 ft tube lights. They are commonly rated at 90 - 96 % peak efficiency.

I am glad a few people are now taking the home made GTI concept far more seriously! The more people doing it the better!

Not a problem.
If I remember my transformer theory correctly most standard low frequency iron core transformers are actually more efficient when operating with square waves.
I think it has to do with the wave form rising much faster and then staying at peak voltage longer. The magnetic flux stays moving at its maximum rate longer and thusly transfers more energy.

I could be wrong but as I recall a sine wave is not the most efficient wave form for transformers to work with. HF PWM is basically all square wave and I understand that is part of why its more efficient.

HF PWM is more efficient because it's done typically in switching supplies, where you want ON or OFF. Sine waves will cause a transistor to go through it's linear region which wastes power in the transistor as heat. Square waves help with maintaining an absolute on/off situation, with modern Mosfets being able to pass so much current and have such low on resistances the sharper the square wave you can deliver the better.

I think you might be right on the square waves being more efficient through a transformer though, but that's just my gut feeling. My knowledge of inductance, core material types, and saturation is almost negative =) I just know they exist.