1 Parametric Variation for Detailed Model of External Grid in Offshore Wind Farms Vladimir Myagkov * , Lennart Petersen † , Saioa Burutxaga Laza ‡ , Florin Iov § , Lukasz Hubert Kocewiak ¶ *†‡§ Department of Energy Technology, Aalborg University. ¶ DONG Energy. Denmark Email: * mjagkow@hotmail.com, † le.petersen@gmx.de, ‡ saioa.burutxaga.laza@gmail.com, § fi@et.aau.dk, ¶ lukko@dongenergy.dk Abstract—The representation of the external grid impedance is a key element in harmonic studies for offshore wind farms. The external grid impedance is here represented by two different approaches: by a simplified impedance model, based on values for short-circuit power and XR-ratio and by locus diagrams given for a range of harmonic orders. Harmonic studies are carried out with these two different representations and their results are compared subsequently, giving information about the specifity of data that is required for assessing the worst case resonances. This analysis provides the basis for defining a procedure for conducting harmonic studies in wind farms that can be used in commercial project developments. Index Terms—external grid, wind energy, offshore wind farm, wind turbine, harmonic analysis, grid codes. I. I NTRODUCTION O FFSHORE wind farms (OWFs) are being connected to national transmission grids. The integration of these renewable generation plants into present power systems poses challenges that must be tackled in order to preserve the correct operation of the system. A clear example of these challenges are the harmonic issues implied in the high voltage alternating current (HVAC) connection of large OWFs. In frequency domain studies resonances between capacitive elements of the OWF and inductive elements of the system (e.g. external grid, transformers) can be a problem. If a resonance point hits the frequency of a certain harmonic that is present in the system, the resonance impedance could effectively amplify the harmonic above acceptable limits [1]. These harmonic distortion limits are stated in local grid codes and are defined at the point of common coupling (PCC) between the OWF and the external grid (EG). Therefore, OWF operators should include necessary equip- ment (e.g. filters) to ensure the functioning of the system in accordance with the grid codes. For the proper design of this equipment harmonic analysis of the system is carried out by means of simulations [2]. In these harmonic studies the representation of the EG impedance is of great importance. Transmission system operators (TSO) and distribution network operators (DNO) provide different grid impedance models for harmonic studies. In most cases the EG impedance at a PCC is only estimated for the fundamental frequency based on the short-circuit ratio at the corresponding voltage level [3]. However, harmonic resonances are influenced by the frequency dependent grid impedance. In this way, different representations of the EG impedance may lead to different results regarding the impedance at the PCC and thus to different harmonic resonances where voltage distortions may occur. In this study the EG impedance is represented by two different approaches: by a simplified impedance model and by grid locus diagrams. By using the simulation tool PowerFactory, harmonic studies are carried out for a generic, but realistic, OWF with these two different representations and their results are compared. This analysis provides the basis for defining a methodology for conducting harmonic studies in OWFs in order to identify realistic conditions that lead to the worst case harmonic distortions and to investigate and propose feasible mitigation methods. The rest of this paper is organized as follows: Section II describes the composition of the applied benchmark OWF and the relevant grid code requirements for this study. Section III outlines the modelling approach for the OWF components, whereby Section IV focusses on the EG representation. The simulation results are presented in Section V, which leads to a proposed methodology for harmonic assessment in Section VI. II. SYSTEM DESCRIPTION AND REQUIREMENTS A. System Characterization of the Benchmark Wind Farm Network A benchmark OWF located in United Kingdom is used as a base case for this study. The OWF is designed taking into account the requirements for transmission systems in UK and general engineering rules for OWF topologies. The OWF topology is presented in Fig. 1. It comprises 50 wind turbine generators (WTGs) of variable speed, full-scale power converter and a rated power of 3.6 MW. The OWF power is transferred to the onshore grid by an export cable, where the PCC is defined. B. Grid Code Requirements The UK grid code [4] sets the principles for operating power plants, determining the relationship between the National Grid Transmission System (NGET) and all users of the National Electricity Transmission System (NETS). Regarding voltage waveform quality requirements, OWFs in UK should follow the planning criteria stated in the Engineering Recommendation G5/4 [5]. The power plant operator should fulfill the planning