Besides for protection purposes, if you install the SCIG to the grid through a transformer, the system automatically achieves the balance between the mechanical and electromagnetic torque right?
Besides it would with different rotor speeds right?
I've read that induction generators usually work on a fixed rotor speed but I don't know why? Can anyone explain please?
All power sources connected to a grid with alternating current must be precisely in phase. This is why traditional sources (hydro turbines, wind turbines, steam turbines, etc.) all rotate at a fixed frequency. However, wind turbine efficiency and mechanical design benefit from being able to rotate at a (slightly) variable frequency. By converting the resulting power to DC and then back to AC with the precisely correct frequency, the turbine can have some variability in rotation speed while the power remains compatible with the grid.
An induction motor/generator is a rotor spun at some speed wr inside a magnetic field rotating at some frequency wf. For wr < wf the device draws power from the magnetic field source (the net) and behaves as a motor; for wr=wf the motor is spinning synchronously transferring no power and for wr>wf the device transfers power to the magnetic field source (and hence the net). With any given system there is a frequency wc given by XL = wcL = R where L is the system inductance and R its resistance and the peak mechanical torque on the rotor is developed at wf-wc (motoring) or wf+wc (generating).
The torque curve ( a quartic) can be quite peaky and this is why many induction motors won't start easily under no load without some starter arrangement.
I understand what you say, but if you connect your induction generator directly to the grid, then the excitation needed to rotate at a higher speed than the synchronous one will be provided from the grid itself and that's a good thing!
If you install an AC/DC/AC converter then the initial excitation is more tricky to get (you need capacitors and a bidirectional flow), so I still don't see why a converter is installed :(
If you DO install an AC/DC/AC converter then you have a much greater speed range for your wind turbine as you can make the excitation frequency anything you like (within reason of course). But is it worth the complexity? Only I suspect in areas where the wind is very variable and your turbine struggles (by trying to work over a wide range of speeds, impossible with an induction generator) to match the 'impedance' of the wind and so transfer the most power possible.
The point is in the variation of the rotational speed in the generator side. To operate at the maxim power point, the generator rotational speed has to be varied and power converters are required to connect a variable frequency generator to the grid. Therefore most of the variable speed wind turbines require some sort of AC/DC/AC power converters.
There are other advantages of variable speed wind turbines. Less efforts in the mechanical structure when fast wind gusts are applied to the turbine blades, better power quality considering that part of the wind speed variations are absorb by speed changes in the rotating blades. etc.
In addition, to use power converters allows to fulfil the grid codes, particularly to provide Low Voltage Ride Through capability to the wind energy conversion system. In most of the countries this is now compulsory for large wind turbines.
Roberto Cardenas listed the reasons for AC/DC/AC very well. Up to about 2 MW power it is still possible to design directly on line generator mechanics that tolerate the mechanical stress from wind gust with the help of additional rotor resistance connected when necessary to increase the slip of the generator. This is the "Danish concept wind turbine" (google for it for details). Above 2 MW the slip is not enough and frequency converter is needed to extend the speed range by changing the stator frequency when needed. Naturally frequency converter allows the speed to be adapted according to the wind speed so that maximum power is always obtained.
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