Type IV Wind Turbine System Impedance Modelling for Harmonic Analysis: On the Use of a Double Synchronous Reference Frame and Notch Filter

Several efforts are being done nowadays to improve the modelling of Wind Turbine Systems (WTSs) for harmonic analysis in Offshore Wind Power Plants (OWPPs). Due to the high influence of the different control structures in the Power Electronic Converters (PECs) on the dynamic response of a WTS, each structure needs to be modelled specifically. Following these lines, this paper addresses the impedance part of the harmonic model in the case of a double Synchronous Reference Frame control structure. The main focus lies on the correct modelling of one of the main elements of this structure: the notch filter tuned at twice the fundamental frequency. The inclusion or disregard of this notch filter is very important because, as shown in works by other authors, this filter can have a big influence in the shaping of the WTS output impedance and such of the OWPP. However, the modelling procedure for the notch filter followed previously ignores the cross-couplings that this element creates in the αβ frame, which leads to a wrong calculation of the WTS impedance in the lower frequency range. The proper modelling of this notch filter and its implications are detailed in the paper first theoretically and then by numerical simulations.