Derivation of a complete transfer function for a wind turbine generator system by experiments

In this paper, an effort is made to derive a complete transfer function of a variable-speed wind turbine generator (WTG) system. This transfer function is important for designing a sensorless speed controller and performing its stability. The proposed WTG system includes a wind turbine, a permanent magnet synchronous generator (PMSG), and a switch mode rectifier (SMR) for implementing a maximum power point tracking (MPPT) controller. The complete transfer function is obtained using a reduced-order model (ROM) method, which is verified by experimental data. Firstly, experiments are carried out with a WTG emulator to measure the real transfer function of a WTG system under duty cycle and load variations. Subsequently, a full-order model (FOM) of a WTG system is established (using the dynamic voltage equations of a PMSG in the d-q reference frame and a small-signal equivalent circuit for a SMR) and the errors between the simulation and experimental results are minimised. Based on the ROM of a WTG system, it is possible to investigate the range of speed control as well as the behaviour of the WTG system under its parameter, wind speed and load variations. It is concluded that the simulation results of the ROM are reliable and very close to the simulation (using a FOM) and experimental results.