Why speed of the wind turbine permanant magnet synchronous generator speed reduces when connected to the load
This is more an aerodynamics than electronics question.

When a turbine aerofoil is gliding through the air, it settles down to a speed where the 'propulsive' force, the lift * aerofoil angle, is equal to the total load, comprising aerodynamic drag, and frictional and load resistance from the output shaft. 

If the load is increased, then the excess of load over the propulsive force will slow the turbine down initially. What this does is, for constant wind velocity, and a constant mechanical blade angle, it increases the angle of attack, increases the aerodynamic blade angle, which increases the propulsive force to match the new load. This is the stable operating region of the turbine, where the wind power input can cope with the load, and small load increases result in small speed decreases.

If you increase the load too much, then the blades will slow down so much that the angle of attack increases too much, and the blades enter an aerodynamic stall. This is an unstable operating region. The lift now decreases with angle of attack. A small increase in load results in a decrease in speed and a decrease in lift, which results in the turbine stopping.