Influence of the transition from part load to full load operation on the mechanical subsystem of the drive train of a wind energy converter

Abstract

Considering control theory aspects, the operation of wind energy converters can basically be distinguished between part load and full load operation. At part load operation, the system is operated below nominal wind speed and control methods are generally designed to gain as much power as possible by the wind. For this purpose, generally MPPT tracking methods with underlying torque or speed control are used. At full load operation, the system is operated at nominal wind speed and beyond before it is disconnected when the wind speed becomes too strong. Full load operation is characterized by the fact that the power withdrawn from the wind must be limited to the nominal power of the system. For this purpose, a pitch system is used. The speed remains almost constant at this operating range, and the torque introduced from the rotor side is limited by an adjustment of the rotor blades. Due to the fact, that both of the control systems access the same mechanical system and are therefore coupled, interactions arise at the transition point around the nominal wind speed which involves extensive mechanical loads when both control methods are not adjusted to each other. The transition between the two operating ranges may lead to significant increases of the load spectrum of the drive train and to a mechanical loading of the wind power system. The special straining of the drive train is particularly emphasized within the scope of this paper. Of considerable influence are control methods of the respective operating ranges (MPPT and pitch control methods). The relatively "hard" transition between the operating ranges, may lead to stimulations of the eigen-frequencies of the drive train, particularly when the transition is crossed several times. In this case it is important to adjust the control methods to each other and to use control systems with "soft" transition ranges (e.g. SK controller).