Offshore wind farm harmonic resonance analysis, part I: Converter harmonic model

Offshore wind power technology has matured significantly and now directly competes with conventional and onshore wind power generation. Thanks to continuous technological developments and significant cost reduction, offshore wind power is closing the gap with the onshore wind power about the levelized cost of electricity; however the technology challenges in the offshore environment still poses threats to the long-term operational reliability and profitability of offshore wind farm projects. This article focuses on one such threat: harmonic resonance in the offshore grid and presents an analytical solution to identify and mitigate it by applying advanced harmonic modelling of an offshore wind turbine. The analytical derivation of the frequency dependent positive and negative sequence impedance of a converter harmonic model of a 8MW generic wind turbine is presented. It concludes by comparing the frequency domain converter harmonic model of a generic 8MW offshore wind turbine with its time domain counterpart as well as the ideal current source model (conventional solution) under the distorted grid condition. The results obtained confirm that the converter harmonic model demonstrates superior accuracy when compared to the ideal current source model (conventional solution) for low order harmonics resonance interaction assessment. To conclude, the discrepancies in the results obtained from the time domain and the frequency domain models respectively deserves further investigation. Particularly the frequency dependent impedance as derived from the average model of the associated power electronics should be examined in detail as its validity depends strongly on the phenomena one is interested in (typically the average model provides sufficient accuracy when the frequency of interest under investigation is ten times smaller than the switching frequency).