# Wind Turbine Startup Process

In this article, we discuss the startup process for a typical horizontal axis wind turbine.  These turbines may be shutdown for a variety of reasons: the wind may be too slow, too high or the turbine may have been shut down for maintenance.  The process below is followed to restart the machine.

• First, an event must initiate the startup.  Typically this is measured wind speed growing beyond a threshold, often denoted $$V_{in}$$ and called the cut-in wind speed.  (Wind turbines have an anemometer on their nacelle to measure wind speed.)  Alternatively, the wind turbine may be manually turned on if it had been manually shut down previously.
• Once startup is initiated, the rotor and yaw brakes are released.  In the case of an active yaw turbine, the nacelle is yawed to face into the wind.  Rotor brakes apply friction to the low and/or high-speed shaft(s) to hold the rotor in place when it is shut down.  Releasing these brakes, without adding generator torque, allows the rotor to essentially freewheel.
• As the rotor begins freewheeling the blades are feathered to achieve maximum rotor acceleration from the oncoming wind.  At this point a number of steps may be performed in the power electronics, but we will not go into that here.
• Once the rotor speed exceeds a threshold $$\Omega_{in}$$ the wind turbine begins normal operation and the startup procedure is complete.  The turbine stays in this state until shut down due to a fault, manual command or the rotor speed falling below a threshold.

When shutdown, a wind turbine's blades will feather to slow the rotor.  Once the rotor speed is below a threshold, the rotor brake will be applied to bring the rotational speed to 0 and keep it there.