Calculation method for torque transients in drive trains of variable speed wind turbines after grid failure

摘要

Both in constant as well as in variable speed wind turbines grid failure may cause dynamical torques in the drive trains. In general these dynamical loads require derating of the gearbox, which is expressed by the so called service factor. Variable speed wind turbines equipped with line commutated converters, offer the possibility to reduce this dynamical stress by proper control of the power electronic converter which is connecting the generator to the grid. In this report both a protection method as well as a method to calculate the current and torque transients after a grid short circuit are elaborated. The methods apply to variable speed wind turbines with a conversion system consisting of a synchronous machine, a machine commutated controlled rectifier, a DC-link and a line commutated converter on the grid side. With the calculation method a conservative approximation of the mechanical stress for both the controlled as well the uncontrolled case can be made. The calculation method is based on subsequent calculation of the DC-link current after a short circuit, the resulting electro magnetic torque and finally the resulting shaft torque. The electro magnetic torque is basically calculated from power angle, stator current and air-gap flux. The proposed protection method aims at reduction of the mechanical stress that occurs after a grid failure. The method is based on fast detection of a failure and subsequently advancing the rectified delay angle to 90(deg o). The theoretical base of the method is included in the calculation method mentioned above. As a case study, the method is applied to the IRFLET test rig. The methods have experimentally been verified. Results have been reported separately.