The problem isn't so much that the spinning components in the turbine aren't heavy enough, it's that they are connected to the grid through a frequency converter. In a normal turbine, as you said, the turbine and generator spins in sync with the grid, generating 50 Hz of AC current. Since the flow of water or steam is quite steady, and you basically have full control over the stream of the medium, this isn't a problem. Wind turbines on the other hand can't control the medium which is powering them, and as such their rotational speed fluctuates a lot. The trim and yaw controls of the rotor helps them a bit, but can't fix everything. Enter the frequency converter. It takes the varying output of the generator and rectifies it into DC current, and then runs that DC current through an inverter which creates a perfect sinusoidal 50 Hz wave that's compatible with the grid (actually, it also creates reactive power to compensate for local voltage variations in the grid, but that's a bit more complicated). It's basically an UPS with it's output facing the grid. The problem with this is that the DC stage inside the frequency converter essentially decouples the spinning mass of the turbine (which is actually quite heavy, since the rotor isn't light by any stretch of the imagination, especially when you include the long radius of it) from the grid, negating it's energy storage capabilities. <br><br>TL:DR; Blame the frequency converter!