Why Do Wind Turbines Have a Power Curve Limit?

[QwertyXP]

Hello,

As wind speed rises, the power output of wind turbine also rises. However, after it reaches a certain value (rated power), it levels off, i.e., it doesn't increase any further.

According to my teacher, there is a limit to the power generation capability of the generator and hence the output power does not go beyond a certain value. However, he also said that we have to use gear mechanisms to keep the output power at that value. I saw nearly the same description on this website:
http://www.wind-power-program.com/turbine_characteristics.htm

Problem is, I see a contradiction in this description. If the capability of the generator is already limited, then why do we also have to deliberately design the turbine (using gear mechanisms etc.) in such a way as to maintain the output at the rated value?

Thank you.

 

[NeuronsAtWork]

I'm not sure that the capability of the generator is necessarily fundamentally limited from an electrical output point of view, but may well be from a mechanical standpoint. Any generator--even a basic residential gas powered one--certainly has a maximum safe operating RPM, beyond which one runs the risk of damaging the unit both structurally and through excessive current/heat for the wire gauge used in its construction. So, the gas engine has a governor to limit its speed to the design speed of the generator. Same with a wind turbine--but instead of a governor they feather the blades to help maintain a safe RPM until even that becomes ineffective, at which point the unit is brought to a stop. The engineers who designed the unit did so with a specific voltage/current/frequency at a specific speed in mind. That becomes the 'sweet spot' for the generator. If the design RPM is 3500, obviously it was not then meant to run at 7500 without causing excess wear and damage--perhaps catastrophic. (It's also possible that speed limiting has something to do with the frequency output, but that's a guess.)