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Literature references are shown as [i. References to igures and tables are shown as Egecal i. The feedback from the academia as well netter from the industry was especially fruitful in order to create a good cooperation between both units, which is re lected in this report. I would like to thank especially Prof. Neville Watson and Prof. Alan Wood for their signi icant contribution datign the research and numerous mettet discussions. Furthermore, I would like to express my appreciation and gratefully acknowledge the daitng of Dr. Without good planning and cooperation, the measurement campaigns would not be inished with such a big success.

My grateful thanks go to all colleagues at DONG Energy for their help and encouragement they have given in these three years. It is worth mentioning that the knowledge sharing within the Grid Analysis department signi icantly contributed to the project development. I also owe special thanks to all academic and administrative Erfaring med dating pa nettet egedal in the Efaring of Energy Technology at Aalborg University. Without their tremendous help on the scienti ic as well as organisational ield, the project would not be inalised without any delays and with a suf icient scienti ic contribution. Research is a long and complex process which sometimes requires more time and sacri ice than actually expected.

They are connected through a egdal MV cable network with practicably no consumption and connected to egedwl transmission system by long HV cables. This represents new challenges to the industry in relation to understanding the nature, propagation and effects of harmonics. Recently, the wind power sector is rapidly developing. This creates new challenges to the industry, and therefore more and more research projects, including harmonic analyses especially focused on wind power applications, are conducted and that is why the project was initiated and successfully developed.

Also experience from the past regarding offshore projects developed in the company and various harmonic aspects causes a need to carry out extensive harmonic research. The research project was initiated by the industry and carried out in cooperation with academia. In order to organize the project development process, the research development framework was suggested based on rationalistic tradition approach in order to provide knowledge and better understanding of different aspects e. Based on the framework, also the structure of the report was organized. This allows the reader to go through all of the stages in project development starting from measurements, through data processing and analysis, and inally ending up on modelling and models application.

Different aspects of validation in time domain, frequency domain, and by application of statistical methods are mentioned in relation to respective problems. Measurements constitute a core part in industry-oriented research. Due to this fact, the research project owes its uniqueness and contributes new insight to the academia. It is proven that an analysis of such systems as large offshore wind farms considers many aspects related to extended and accurate models, complex measurement campaigns and of course appropriate and more suitable data processing methods.

Before any of the above aspects could be seriously taken into consideration, a reliable and robust measurement system is needed. This is achieved by carefully designing the hardware and the software layers of the measurement system. It is explained in the report that it is of great importance to know the nature of generated harmonics in large offshore wind farms in order to apply the most suitable XI 13 data processing technique. Time-frequency analysis based on multiresolution wavelet transform is used in order to perform time-frequency domain analysis helpful to distinguish harmonic origin and observe short-term variation. Non-parametric spectrum estimation is successfully applied to interpolated signals adjusted according to the varying power system frequency.

Different data processing techniques are presented and applied depending on the signal i. Based on an in-depth investigation of measurements, it is observed that certain harmonic components generated by the grid-side converter in the wind turbine are affected by two driven frequencies, i. Therefore, harmonic assessment made by major part of commercial power quality meters is to some extent inappropriate, and their measurements interpretation can be misleading. Different statistical tools were used in order to analyse the origin and nature of various harmonic components. A comprehensive comparison of harmonic voltages and currents based on probability distribution estimation and appropriate statistics calculation mean, variance, probability density function, etc.

Such approach gives a better overview and comparison of harmonic components variation and occurrence frequency. Several frequency domain methods of describing wind farms comprising of various components such as wind turbines, transformers, cables, etc. It is explained that large offshore wind farms can introduce additional unwanted resonances within the low frequency range. This can signi icantly affect overall system stability. Therefore, the analysis and design optimization of large offshore wind farms are more complex than smaller onshore wind farms.

Nowadays, wind turbines are complex devices equipped with the newest technologies.

Therefore, also harmonic analysis of such devices is not a straightforward task. Harmonic studies, due to the complexity of the wind turbine structure, can be focused on several parts such as control strategy, modulation technique, converter structure, and hardware implementation. Various control strategies are taken into consideration and their impact on possible harmonic emission and overall system stability. An analysis is performed mainly in the frequency domain. One analyses how particular components in the control structure e. The in luence of control strategies on overall wind farm stability is also deeply investigated. Appropriate stability indices are suggested and applied in several study cases.

Carefully modelled and aggregated large wind farms in frequency domain together with the wind turbines frequency response give a good overview about large offshore wind XII 14 farm behaviour for different frequencies. Such approach is successfully used in studies of real-life existing wind farms. Since harmonics in wind turbines and wind farms are characterized by different origin and nature, comparison of them may be problematic. Therefore, sometimes selective validation of particular frequency components is more suitable. It was observed that comparison of results in frequency domain and time domain, as well as application of statistical methods, is the core part of results understanding.

Based on presented studies, we see that large offshore wind farms, in comparison to typical onshore wind farms, can affect more unwanted resonance scenarios. Unwanted resonances can cause overall wind farm stability and performance e. Therefore, it is of great importance to carefully analyse wind farms, especially large offshore wind farms, also from a harmonic perspective. This industrial PhD project is focused on investigating the best possible way to perform various harmonic studies of offshore wind farms including some conditions not taken into consideration before. Application of new methods and widening the range of models contributes to achieve the necessary higher reliability of offshore wind farms as large power generation units in electrical power systems.

Pa baggrund af disse rammer, blev rapportens opbygning fastlagt. Dette opna s gennem grundigt design af ma lesystemets hardware- og softwarelag. Ikke-parametrisk spektralanalyse anvendes pa interpolerede signaler tilpasset de varierende elsystemfrekvenser. En sa dan tilgang giver et bedre overblik og en bedre sammenligning af harmoniske komponenters variationer og forekomst. Dette kan have en betydelig ind lydelse pa systemets generelle stabilitet. Forskellige reguleringsstrategier er blevet overvejet sammen med deres ind lydelse pa dannelsen af harmoniske svingninger og generel systemstabilitet.

En analyse ga r ud pa at inde ud af, hvordan forskellige komponenter i reguleringskonceptet f. Egnede stabilitetsindeks er foresla et og anvendt i lere konkrete cases. Derfor er selektiv validering af speci ikke frekvenskomponenter til tider mere anvendelig. XV Table of Contents This creates new challenges to the industry. Therefore, more and more research projects, including harmonic analyses especially focused on wind power applications, are conducted. This chapter brie ly presents nowadays trends as well as a historical background of both wind power and the development of harmonic analyses.

At the end of the chapter, based on the brief introduction to the wind power and harmonic analysis, the project hypothesis is put forward. They are mainly connected through a widespread MV subsea cable network and long HV cables to the transmission system. At the same time, the interest in the power quality of wind farms has increased as renewable energy sources become more important to face the global environmental challenges. The new technology which is less tolerant to voltage quality disturbances as well as the widespread use of power electronic converters contribute to the relevance of the power quality [1. Furthermore, from the distribution system operator s and the transmission system operator s perspective it is of great importance to supply adequate power quality to consumers.

Therefore, wind farms should ful il power quality requirements. For the wind farm developer an optimal assessment means a cost-effective design ful illing the requirements [1. The interest in the power quality of wind farms has increased as renewable energy sources become more important to face the global environmental challenges, and the power industry grows with the trend of embedded and dispersed generation. The new technology which is less tolerant to voltage quality disturbances as well as the widespread use of power electronic converters contribute to the relevance of the power quality. It is of major importance to assess the possible impact that wind farms have on the power system also from a power quality perspective.

Additionally, from both the power system i. Therefore, wind farms should be designed with special care, taking many aspects into consideration. Some of them, including frequency domain, will be taken into account in this report. The process of the power quality assessment must be carried out not only on a wind turbine manufacturer level but also on a wind farm developer level. Thus, additional wind turbine analyses for speci ic applications i. Due to this fact and extensive analyses based on reallife measurements, advanced data processing and multidomain modelling were performed and compared with data provided by manufacturers.

The power electronic devices with advanced semiconductor technology and advanced control methods that are used in wind turbines for transferring power from the generator to the grid can meet the most demanding grid requirements seen today [1. However, there are issues with regard to the power quality, voltage stability, transmission losses and reliability that need to be addressed and improved [1. Such issues are obviously important elements in the efforts to reach renewable energy targets while maintaining a reliable and robust power system [1.

The report concerns full-scale back-to-back converter wind turbines which are, and are going to be, widely applied in already developed and future DONG Energy's wind farms. It is a variable-speed wind turbine utilizing full scale frequency converters. There is a transformer to step up the voltage installed in each wind turbine. The wind turbine transformer is connected with a vacuum circuit breaker to the MV network. The tendency in the industry shows that instead of using a partially rated converter such as in a doubly fed induction generator, fully rated converters can be applied to control active and reactive power injection into the system.

The power converted from wind energy to electrical energy can be injected by the power converter from any kind of rotating generator i. In general, the AC frequency that is developed at the stator terminals of the generator is directly proportional to the speed of the rotation of the rotor. The voltage at the generator terminals also varies as a function of speed, depending on the particular type of generator and the lux level. Such schemes are used with renewable energy systems and give the widest range of speed control but at the expense of a converter. It should be rated to cope with the full generator output power [1. Such a power conversion scheme is presented in Figure 1.

The rapid power electronic development contributes to the use of fully rated converter systems and will probably replace the partially rated converters and doubly fed induction generators. Current trends show that permanent magnet machines become more attractive in wind power applications. It is due to its ef iciency, since no magnetizing or ield current is necessary to provide the magnetic ield, and it also allows introducing new generator topologies applicable in speci ic conditions. The permanent magnet generator size is smaller than the synchronous wound ield and asynchronous squirrel cage induction generators, and it does not need an external power supply source to support excitation [1.

The full scale converter system allows full control of active and reactive power. Although few control schemes are possible, the generator-side converter is normally operated to control the generator torque loading while the grid-side converter is controlled to maintain constant voltage on the DC-link and control the power output. Both converters are normally current-controlled to achieve their control objectives. However, it should be emphasized that the pulse-width modulated converters can adjust the phase and magnitude of the injected phase voltage. If the charge of the capacitor decreases, the control loop will decrease the power transfer to increase the voltage and vice versa.

It should be realized that the grid inverter is also able to control the reactive power supplied to the system which becomes very important to satisfy contemporary grid codes. Although there is some cross-coupling between the terms, this can be taken into consideration in the control structure. Harmonics were described by Claudius Ptolemy who related musical harmonies to the properties of mathematical proportions derived from the production of sounds themselves. Ptolemy argued for basing musical intervals on mathematical ratios, in agreement with the followers of Pythagoras, backed up by empirical observation.

Ptolemy wrote about how musical notes could be translated into mathematical equations and vice versa [1. Those harmonies he considered to be distributed in all aspects of the physical universe. In particular, they were there in the phenomena of the planets and the human soul [1. Nowadays, harmonic analysis is the branch of mathematics that studies the representation of functions or signals as the superposition of basic waves. The basic waves are called harmonics, but the name harmonic in this context is generalized beyond its original meaning of integer frequency multiples.

It can be said that nowadays harmonic analysis reaches back to a French mathematician and physicist, Joseph Fourier, who originally de ined the Fourier series for real-valued functions of real arguments and used the sine and cosine functions as the basis set for the decomposition. Fourier series could be used only for analysis of periodic functions. Many different approaches of de ining and understanding the concept of Fourier series have been developed since that time. All are consistent with one another, but each of which emphasizes different aspects of the topic simultaneously extending signi icantly the basic concept of harmonic analysis.

Some of the more powerful and elegant approaches, based on mathematical ideas and tools not available at the time Fourier extended his original work, are successful in harmonic analysis at present. Many other Fourier-related transforms e. Laplace transform, Fourier transform, short-time Fourier transform, wavelet transform have since been de ined, extending the initial idea to other applications. This general area of inquiry is now called by physicists and engineers harmonic analysis. In the past the power system comprised mainly passive components with relatively linear operating range as well as synchronous generators.

Harmonic analysis of such systems is state-of-the art right now. Nowadays modern power systems include more and more power electronic components than in the past. Also renewable energy sources e. Power electronic equipment in modern power systems is obviously a source of additional harmonic components not seen previously. On the other hand, the application of advanced and fast control in grid-connected power converters introduces possibility of control higher than the fundamental frequency components. Appropriately used power electronics can de initely improve the quality of power. To show how harmonics are becoming more and more important in electrical engineering and power system studies, a popular electrical engineering database was investigated.

Institute of Electrical and Electronics Engineers IEEE is considered by many as the biggest professional association gathering electrical and electronics engineers from many ields. The association has an extended digital library comprising 6 30 more than 3 million technical and scienti ic documents. Unfortunately, access to the documents, if not open for all interested parties and therefore knowledge sharing, is limited especially for small research units all around the world.

Dating egedal nettet Erfaring med pa

Analyses of such a database from a harmonic and wind power research perspective can provide a good overview of how advancement of Erfating in both areas has developed in negtet space of the years. At the beginning, let us see how publications regarding harmonics have appeared. Number of publications Harmonics 0 Figure 1. The irst paper regarding harmonics in the IEEE database is frombut the most signi icant development within the area of harmonics started in late 80ies. Up until now, there are more than of publications only about harmonics. Obviously, harmonic analyses can be applied in many ields of electrical engineering. Let us additionally see how in the same period the interest in wind turbines and wind farms development has increased.

This tendency can be easily seen in Figure 1. This general interest in wind power can also be easily seen in the increasing number of commercial projects.

Thermal homo of electrical components with consequent homo of their useful life or homo. Possible errors in results obtained from each of stages can signi icantly homo other stages. Harmonic studies, due to the complexity of the wind homo homo, can be focused on several parts such as control homo, homo homo, homo structure, and hardware homo.

Therefore, harmonics analyses rgedal this particular ield of electrical engineering seem to be important also from a market tendencies perspective. It netyet be concluded that this tendency will be increasing within the next few years. Therefore, new research projects are required to extend knowledge and experience in the area of renewable energy sources. This also implies an increasing interest of harmonic research within this area of power industry. Erraring it can be seen also in Figure 1. It seems to be a natural way of research. Although it also indicates that there is much cating experience with wind farm analysis as a whole system comprising many wind turbines and other nettet.

The irst observed publication in the IEEE database regarding wind turbines is frombut regarding wind farms it is much later Even less experience is gained in offshore wind power solutions. This ield is new irst mention in in comparison with onshore installations. Anyway, nowadays knowledge sharing capability and technology Erfaring med dating pa nettet egedal allow deriving knowledge from other ields and continuous improvement. The voltage source converter as harmonic datlng Modelling and analysis of wind turbines and wind farm network elements in relation to harmonics i.

Operation of VSC with harmonic resonances near its characteristic frequency Engineering standards and power quality standards. Based on those assumptions, a project hypothesis was de ined. Is datnig possible to model offshore wind farms equipped with many voltage source converter wind turbines and connected to the power systems in time and frequency domain in a consistent way to analyse possible harmonic generation, harmonic medd and power system nsttet variation due to overall system changes? This is especially associated with the application of harmonic analyses in wind turbines and wind farms. Frequency domain analyses associated with eigenfrequencies are even more important in large offshore wind farms.

The project hypothesis was speci ied in this chapter in order to clearly specify the scope of the project. It is assumed that the project is focused on the development of various analysis methods and tools which can be successfully used in future commercial wind farm projects development. Additionally, extensive knowledge regarding harmonic phenomenon in wind farms was achieved. Bak, "Harmonic analysis of offshore wind farms with full converter wind turbines," in Proc. Akhmatov, "Experience with voltage control from large offshore windfarms: Nordic Wind Power Conference, Bornholm, [1.

Baggini, Handbook of Power Quality. Wiley and Sons, [1. Stiesdal, "Power Converters," U. Patent B2, May 13, 34 [1. John Wiley and Sons, [1. Cambridge University Press, [1. Brill Academic Publishers, [1. Barker, Scientific Method in Ptolemy's Harmonics. Also an introduction to harmonics in power systems and wind farms is presented. The research development framework is suggested based on rationalistic tradition approach in order to provide knowledge and better understanding of different aspects e. Some issues related with knowledge sharing in order to better utilize the project indings are also mentioned.

The term product can be understood broadly, starting from commercial products that can be sold on the market and ending with methods, tools, procedures or standards that are assumed to be used internally in order to accelerate other products development process, usually commercial products are inancially bene icial. Product design is part of a more comprehensive process called the product innovation process. In short, product innovation is the development of a new business activity relating to a new product. A research and development project is characterized by exploring new, immature technologies.

Often, a positive business case for the implementation cannot be established. Normally, the project is undertaken to discover or establish facts and principles. Since harmonic distortion of voltages and currents can have negative effects on power system components, the necessity of performing harmonic studies is justi ied. DONG Energy is one of Europe s leading wind power generators and has developed strong wind energy capabilities. The company has more than 19 years of experience in 13 37 constructing and operating offshore wind farms. Needless to say, the knowledge of power quality in wind farms as well other electrical and non-electrical aspects increases after every new wind project is commissioned.

It must be emphasized that every new project creates new challenges and sometimes unpredictable phenomena. Due to this fact, a need to extend and intense research area in the wind industry is seen and in many cases strongly desired. New research activities simultaneously with commercial wind projects are born and an unavoidable symbiosis starts between the industry, commercial and research and development units. This shows that research projects are not arti icial marvels, but stems from the industry and cannot exist without appropriate cooperation between industrial and academic units. The main expected project indings are the following: For is the modern love college essay contest writes a similar category as being samuel pepys' favourite watering hole.

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