How does the induction generator works? how can we synchronize with Grid supply. up to which rating it is suitable.

Sandeep, In it's simplest form an induction generator is simply an induction motor that is rotated above synchronous speed. Unlike a synchronous generator, the field in the rotor is created by drawing large amounts of reactive current from the line.

Synchronization is automatic because line frequency is applied to the stator once the prime mover has accelerated the rotor to near synchronous speed. Remember, unlike a synchronous generator, there is no rotating field on the rotor until it is attached to the grid so the normal matching of voltage, phase angle and speed is reduced to speed alone.

The fact that there is no field until grid connection (and hence no voltage produced) also means that an induction generator is generally incapable of isolated operation, though there are special designs involving capacitors at the terminals, but that is a totally separate topic.

The rating is limited by the size of the prime mover and the ability of the grid to supply the significant inductive current required for excitation without creating destroying the grid voltage profile and/or creating instability.

Dear Alan Thanks for your valuable information. I wants to know about the small steam back pressure turbines having induction generator of 200 to 400 kw power.

Sandeep, In this case size doesn't matter, induction generators follow the same operation regardless of their rating. Your main control is the governor, more fuel/steam gives you more kW, but you have no direct control over the terminal voltage/reactive power unless you have some form of switched capacitors near the generator terminals.

Another issue to be aware of: induction generators operate at variable speeds, function of the slip-torque characteristic of the machine. This means that the shaft speed of the induction generator will vary with the active power output of the generator and even with the voltage at its terminal.

Therefore, make sure that the steam turbine is capable of operating at off-nominal frequencies.

Granted, I don't know much about small steam turbines and I don't know what would be a typical range of speed for safe operation of this type of turbine.

Induction generator mostly used in wind energy generation,induction generator synchronising with gird by soft starter with help of plc.when induction generator start rotating its draw reactive power from grid to create magnetic field on its starter,its draw power from grid until machine goes in generation,when machine goes to generation,the plc match voltage and frequency to grid of alternator.if parameters match by pass contactor closed,these smooth operation done by soft starter.

I would say that a doubly-fed induction generator (DFIG) is more expensive than a synchronous machine of same rating, could anyone comment on that?

I see the DFIG requiring three sets of brushes for the field windings, a more complicated set of rotor windings, and the back-to-back converter. The synchronous generator would have an excitation system and probably a synchronoscope.

Why do I mention this cost comparison?

I understand using a DFIG machine for wind turbines, as the variable speed characteristic of the induction generator becomes a very desirable characteristic.

On the other hand, for a steam turbine there is no advantage on having a DFIG. It would be better to go for a synchronous generator, instead.

So, I was under the assumption that the induction generator he mentioned was the "conventional" induction machine, with a squirrel-cage rotor winding.

Sandeep, Lema offers very valuable advice, you really need to get the critical frequency information from the manufacturer, some turbine designs have critical points not too far above their rated speed.

You also need to consult the generator capability curves to ensure that you do not exceed the allowable reactive loads that are a result of low load and/or poor power factor operation. You can incur significant iron and end turn damage from heating due to the high flux density. Protective relaying also needs to be carefully reviewed