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Abstract-This work reviews the state-of-the-art in the field of harmonic resonance problems in Wind Power Plants (WPPs). Firstly, a generic WPP is modeled according to the equivalent circuits of its passive and active components. Main focus is put on modeling active components, i.e. the ones based on power converters. Subsequently, pros and cons of frequency and time domain analysis methods are outlined. The next sections are devoted to mitigation methods implemented in the power electronics converters. From the wind turbine perspective, different techniques to enhance the robustness of the controller are analyzed. Subsequently, the suitability for active damping of harmonics using STATCOM devices is assessed, with focus both on control techniques and power converter technologies.

University in partial fulfilment of the requirements for the PhD degree in Electrical Engineering. The research has been carried out between the 1.09.2010 and the 15.07.2013 at the Department of Energy Technology for Energinet.dk by which I was hired as a PhD-student for the entire project period.The project has been followed full time by two supervisors: Professor Claus Leth Bak (Department of Energy Technology) and Dr. Unnur Stella Gudmundsdottir (Energinet.dk).Energinet.dk has fully funded the research leading to this thesis "Online Location of Faults on AC Cables in Underground Transmission Systems". This funding has been vital for this research project. Travelling to conferences, two visits at forging research institutions, renting of laboratory equipment and performance of and field measurements was made possible thanks to support from the company. During the projet period I supervised two master projects and thought one semester course in power system transients.The scientific papers written as a part of this PhD are included in the back of both the printed version of the thesis as well as in the PDF-file. The papers should not be considered a part of this monograph, but are enclosed if the reader is interested.This thesis has five parts and appendices. An overall literature reference list is presented at the end of the main report. A list of the authored publications is presented at the end of the thesis. Literature references are shown as [i], where i is the number of the literature in the reference list. Tables, figures and equations are shown as C.F, where C is the chapter number and F is a unique number for the figure or table. AcknowledgementsI owe gratitude to many people who helped me in different ways. I would like to thank:• My supervisors, Claus Leth Bak and Unnur Stella Gudmundsdottir for all their help, comments, and many technical discussions during the last three years.• Carl-Erik Madsen, Henrik Kastberg and Carl Willi Hansen for their help with carrying out the field measurements. In the same regards, a very special thanks is extended to Filipe Faria da Silva and Unnur Stella Gudmundsdottir for participating during all measurements.• Filipe Faria da Silva for numerous technical discussions, for reading large parts of the thesis and for many good times in the office.• Rasmus Schmidt Olsen, Unnur Stella Gudmundsdottir, Joachim Holbøll, Per Balle Holst, Poul Erik Pedersen, Thomas Kvarts, Carsten Rasmussen, Claus Leth Bak and Bjarne Søndergaard Bukh for their contributions and comments during the project period.• Everyone at the transmission department at Energinet.dk for their contributions.• Professor Ali Abur for his hospitality and comments to my work during my three month stay at North Eastern University in Boston 2012.• • A special thanks to Dr. Jeewantha Da Silva for numerous discussions, valuable advice and guidance throughout the entire project period.• To Prof. Athula Rajapakse and PhD-student Kasun Nanayakkara both from University of Manitoba, Winnipeg, Manitoba for valuable discuss...

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