Example A 40 m three bladed wind turbine

Example : A 40 -m, three bladed wind turbine produces 600 k. W at a wind speed of 14 m/s. Air density is the standard 1. 225 kg/m 3. Under these conditions, a) At what rpm does the rotor turn when it operates with a TSR of 4. 0? b) What is the tip speed of the rotor? c) If the generator needs to turn at 1800 rpm, what gear ratio is needed to match the rotor speed to the generator speed? d) What is the efficiency of the complete wind turbine (blades, gear box, generator) under these conditions? Solution

• The answers derived in the above example are fairly typical for large wind turbines. • A large turbine will spin at about 20– 30 rpm; • The gear box will speed up by roughly a factor of 50– 70; • The overall efficiency of the machine is usually in the vicinity of 25– 30%.

WIND TURBINE GENERATORS : The following figure shows the basic system for a wind turbine with a synchronous generator. The generator and blades are connected through a gear box to match the speeds required of each.

The Induction Generator : Most of the wind turbines use induction generators rather than the synchronous machines because : a) They do not rotate at a fixed speed. b) They do not need exciter, brushes, and slip rings. c) They are less complicated and less expensive and require less maintenance. Operation: a) When the stator is provided with three-phase excitation current and the shaft is connected to a wind turbine and gearbox, the machine will start operation by motoring up toward its synchronous speed.

b) When the wind speed is sufficient to force the generator shaft to exceed synchronous speed, the induction machine automatically becomes a threephase generator delivering electrical power back to its stator windings. Types of excitation of Induction generator : a) When the induction generator is connected to grid, it takes the magnetizing current from the grid it self. b) When the induction generator is stand alone, it takes the magnetizing current using 3 -phase capacitors connected to its terminals. In this case the induction generator will be self excited.