Hey!

Let's suppose I have several flow current turbines in a river. Those turbines are generating electricity through a PM-Sychronous generator (each has its own generator, which means each of them has a different rotational speed, thus different frequencies). I want to connect them all the grid. Is it possible to connect them in parallel and then get the final power then to the frequency inverter (which is on the riverside) which in turn leads it to the grid? Or do I have to install an inverter in each unit in the river in order to connect them all? What's the best way for wiring them up?
Thanks!

I have never hooked generators in parallel, but I do understand much of the theory involved.
1)First thing for power to be put back into the grid it has to operate at the 60 cycle frequency that the power company is using. You could imagine the mess that our grid would be if every generation facility didn't synchronize with the others?
2)if each generator is turning at a different speed and you can't control the speeds to match each other then I don't know of another way other than 3 separate inverters that are synched individually to the grid. The problem with trying to match speeds amongst the three individual generators could be difficult, for several reasons, flow rates, loads on each generator could be different thus slowing one down or speeding one up. I think it would be a mess.
3)current movement into the grid depends on voltage. An example of this is how the utilities move power from the west coast to the east and visa versa. In order for the power plants in the northwest to move power to the east on the grid, it has to slightly increase its voltage output (speed up the generator a little). If the northwest needs power from the east then the east has to up its voltage or the west needs to decrease its voltage, which can happen automatically because of loads, but the theory still stands.
For you to move your power into the grid you have to have your voltage slightly higher than the source you are feeding into. From my experience the best way I know is through the use of an inverter. There may be other ways, but grid inverters for the home producer require that the grid have power on it to work. The reasoning behind this is two fold. One, the grid provides the inverter with the sync needed for the inverter to match the 60 cycle grid voltage. It also gives the inverter the grid voltage reference so that the inverter can put out a voltage high enough to move power into the grid. When the grid goes down so do the inverters. This is primarily for safety purposes. Suppose the grid went down and the inverter kept running and the grid suddenly came on line, but the sine wave was 120 degrees out of phase. It might make a big smoke bomb of your inverter. I am retired now, and new technology is always coming out, but I see these elements as something that will always have to be dealt with. I hope this helps. If you have further questions I will try to answer them. I worked in industry for over 30 years and I saw a bunch of different inverter technology during that time. They are doing some pretty wild things with them and the development is continuing daily. I am sure there are companies that deal with your specific needs and have the designed equipment to fit your application without a lot of wizardry. "Keep It Simple Stupid" was always at the forefront of my thinking. If you find something more simple I would love to hear from you. I like to keep my mind active and involved in the trade and this site gives me a chance now and then to think. I appreciate your time.