Extraction of Transfer Functions for Decomposition of Primary and Secondary Current Harmonic Emission in a Wind Farm

Application of power electronic converters in wind turbines results in generation of harmonic and interharmonic. Also, the switching of converters results in turbines active output power distortions. Harmonic interaction of wind farms with external grid, harmonic propagations in it and amplification of these propagations due to resonance of underground transmission cables, can leads to increase in power quality problems. This paper presents a method for study and classification of conventional current harmonic emissions in a wind farm. This method uses the concept of transfer function in order to evaluate the harmonic contributions of each wind turbines at point of connection as well as the contribution of external grid background harmonic voltage on each turbine current harmonic distortion. In this regard, extraction of voltage and current transfer functions, transfer admittance and impedance is done based on circuit theory and harmonic modeling of wind farm components from medium voltage (MV) side of turbine transformer in frequency domain. Two classification of harmonic propagation called as “primary emission” and “secondary emission” is evaluated based on these functions. With use of this method, the contribution of each turbine and also the contribution of external grid at overall wind farm emission can be decomposed. Also, the impact of transfer function of conventional LCL filter used in wind turbines, on each those transfer functions, is studied. The important conclusion is that the presence of this filter causes that the amplitude of primary harmonic transfer function from one turbine to another turbine, in special frequency range, be more than the amplitude of primary harmonic transfer function from one turbine to the grid.