How to grid-locked/tied synchronous inverters work in alternative energy setups?

If you connect up a synchronus motor (or generator) to the network, then the torque is proportional to the lead/lag from the mains to its rotational phase. It's moderately trivial to arrange for some means of synchronisation, so there is no net lead/lag at all from your inverter, you're just pushing amps in in phase with the mains.

Sure, but my question was not about how to synchronise but the 'blend' for want of a better word such that the surplus power goes to either the mains supply of the local load, I.e. in neither case is the locally generated power thrown away.

Search on the keywords 'asychronous alternator' or 'induction generator'.

It is an ordinary induction motor, driven by any source of mechanical power, usually intermittant in nature.

Any time the mechanical power drives the shaft of the induction motor at greater than synchronous speed, the motor is connected to the mains. Because it is running faster than sync speed it generates AC power, and always at the frequency of the mains.

It's irrelevant.

You feed all the power to the grid, and connect your appliances to the same point. You need some scheme to check that you've not been spectacularly unlucky, and the power you are using is exactly the power you are generating and the line has gone down just outside your house.

For example, don't feed the grid at 0-20V, and don't turn on your inverter unless the 0-20V slope looks right.

I am looking designs and explanatory notes for entirely electronic methods. Nothing using rotary mechanical parts.

its pretty simple really.

Firstly, the "blend":

the energy source connects to the grid (as does the house and its appliances), and attempts to pump in power (assuming V,f etc are at appropriate levels to do so). The impedance of the grid is low, and the impedance of the house is governed solely by the load it draws (IOW whats turned on).

The energy source pumps current into the connection. some of that current flows into the house, the rest into the grid. The division is governed by the three impedances - grid, house and inverter. Should the house load decrease (whilst power generated remains constant) then the portion of current flowing to the house decreases, and the portion flowing to the grid increases. A "T" network of three series R-L circuits is a pretty good model.

secondly, the box of tricks:

normally one has two inverters - one to connect to the grid (fixed voltage & frequency), one to the AC generator (variable voltage & frequency). In between the two is a DC bus, containing a sufficient amount of capacitance to smooth the input (and output) power [P(t) = Ppksin^2(wt)] to its average value.

the grid-tie interface is often done by increasing the voltage and/or frequency to push the desired power (IOW that being sucked out of the generator) into the grid (there are limits on the percentage change in V,f).

HTH

Cheers Terry