Frequency Characterization of Type-IV Wind Turbine Systems

Paper in proceeding, 2016

The continuous need for renewable energy sources is a driving force for a fast development of wind turbine technologies. It is well known that control interactions can arise if the wind farms and the interconnecting system, for example ac collector system or high voltage direct current (HVDC)-link, are not properly integrated. One tool to assess the stability of the system is to analyse the input impedance of the wind farm together with the connecting grid impedance. In this regard, this paper investigates the impact of different system parameters in the input admittance of the generating unit. The admittance is analysed for a wide range of frequencies. Moreover, the passive and non-passive behavior of the admittance is highlighted and the risk of interaction between the wind turbine and other elements of the grid is discussed. The system under consideration consists of a multi megawatt type-IV wind turbine system and a fully-rated voltage source converter (VSC)-based testing equipment used as verification tool for frequency scanning. First, the mathematical model of the system and the scanning method are presented. The input admittance is calculated for a variety of operating conditions including variations of control settings such as phase-locked loop (PLL) and close-loop current control bandwidth and system parameters, such as the output filter configuration. Finally, the investigated methodology is verified using time-domain simulations and field test results.