A wind turbine converts the kinetic energy of wind into electrical energy which is then transmitted to a substation at a wind farm. Generally, in a wind turbine, a nacelle houses components along with a drive train for converting the mechanical energy into electricity. The drive train in a wind turbine is usually meant the assembly of a rotor, rotor shaft, gear box, generator shaft, coupling and generator. The wind turbines are designed to withstand several on field operating scenarios (normal operation, extreme gusts, grid loss, blocked blade, etc).
Nowadays, the impact of wind turbines on the grid is no longer negligible so network operators are making stricter the wind turbine grid connection requirements. Some of these requirements are defined in terms of the voltage dips that the wind turbines must be able to withstand without disconnecting from the grid, and the maximum time to recover the power production after the grid recovers from the grid.
Voltage dip scenario is one of the most difficult operating cases for a wind turbine. The occurrence of a voltage dip in the grid originates a transitory that affects not only to the electrical performance of the wind turbine but also to the mechanical one. In case of a disturbance on the network, when the voltage goes below a certain value, it is mandatory to decrease the wind turbine power production due to electrical restrictions. As the dip voltage dynamics are very fast, this reduction is required to be made in a very abrupt way. The only way to get such reduction in a short time is by decreasing the generator torque. The torque dip excites the drive train in its resonant frequency, resulting in drive train oscillations. Considering that the aerodynamic torque remains the same (same wind speed and pitch angle), the generator speed increases due to the difference between generator torque and rotor torque. If the over-speed protection system is triggered, the wind turbine is disconnected from the grid and stopped which default on the network operator requirements. As a consequence, the first objective is to limit the generator speed during the disturbance. This must be done without generating other kind of alarms and keeping aerodynamic torque enough to recover the previous dip voltage power production in the short time required by the network operator.
As said before the network operators requirements do not only define the voltage dips that the wind turbines must be able to withstand but also, the maximum time to recover the power production after the grid recovers from the grid. As in any close loop the wind turbine generator torque control is going to exhibit a maximum overshoot and damp oscillations before reaching the steady state. This maximum torque overshoot highly depends on the way the control provides the torque recovery in terms of the torque value applied but also the drive train oscillation phase at its natural frequency, that has been excited by the fast torque change along the voltage dip. So the transient respond of the system when recovering from the grid is going to depend on the control logics followed. Some control methods have been used in the past focused on changing the current drive train damper operation during voltage dip transients These logics do not improve the way the generator torque and pitch angles are controlled to fulfilled the settling time requirements minimizing the maximum overshoot, either the best way to integrate the drive train action to get the best damping of the drive train main frequency oscillation mode or the way to improve the converter controller and wind turbine controller cooperation. In another approach, the solution has been provided in terms of electrical design, the use of full converter with brake chopper can reduces the torque overshoot in the drive train. This method needs installation of new wind turbine generators and extra cost.
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