The interest in the utilization of offshore wind power is increasing significantly. Due to the shortage of in-land locations for wind farm and the wind speed offshore is potentially higher than that of onshore, which leads to a much higher power production. In this thesis a large offshore wind farm is modeled using Matlab simulation package. In the simulations active stall regulated wind turbines driving fixed speed asynchronous generators are used. Two different types of interconnections are modeled and compared, one is the Voltage Source Converter (VSC) based HVDC link and the other one uses high voltage AC (HVAC) cable interconnection. Transmission faults are simulated in each system and the transient response are examined. Three phase fault and single line to ground fault are used to compare the performance of the VSC based HVDC interconnection system and HVAC interconnection. It is found that compared to the traditional HVAC transmission, the VSC based HVDC transmission would have better performance under various system disturbances.

Large off-shore wind farms raise the concern of widespread tripping of off-shore wind generator in the presence of system faults and corresponding voltage dips that could potentially cause system wide blackout. In this thesis an offshore wind farm and three different types of power transmission are modeled and studied using simulation software. Off-shore wind farm composed of fixed speed induction generators and HVAC interconnection, HVAC interconnection plus STATCOM and HVDC interconnections are studied. Onshore grid faults are simulated for each interconnection. Voltage tolerance curves are established to assess fault ride through capability of each interconnection and compared with different grid transmission ride through capacity required by grid operator.

The supply of energy is a key factor in modern societies. As the old fossil sources for energy are dwindling, conflicts arise between competing nations and regions. Fossil energy sources also contribute to the pollution of the environment and emission of greenhouse gases. With renewable energy sources many of these drawbacks with fossil fuels can be eliminated as the energy will be readily available for all without cost or environmental impact. Combining the renewable energy sources will be very effective, particularly in commercial areas where lake of electricity is high. The cost of combining onshore wind and solar power plant is affordable. Furthermore there is no power failure or load shedding situation at any times. When it is manufactured in a large scale, cost of this integrated natural resources power generation system is affordable. Moreover there is no power failure or load shedding situation at any times. Therefore, it is the most reliable renewable power or electricity resources with less spending and highly effective production. ref [1]. The thesis work would take planning of offshore renewable plant (Lillgrund) with considering the resources of renewable power. The study would take in account combining the Lillgrund wind farm with solar system and take close look into the advantage and disadvantage of combining the renewable resources together and figure out if such station can work in proper way and provide sufficient power production. The study would take in account the effect of each resource on other resource, also calculations would be done. The study site is Lillgrund in south of Sweden. The Lillgrund wind farm is the most important offshore wind power plant installed in Sweden with a total capacity of 110 MW, corresponding to 48 turbines. ref [2].

Grid codes are becoming more demanding on power generating units due to the factthat the complexity of the power grid is increasing. The penetration of wind powerhas grown over the last years and it is clear that wind farms need to be addressedwith the same type of grid codes as conventional generation units. There is howeveran undeniable difference between the technology in conventional synchronousgeneration units, and the asynchronous generation units in wind farms. This thesis has reviewed the current grid code in Sweden and compared it to the newcode proposed by ENTSO-E, “the Requirements for Generators”, in the aspect ofwind farms with an installed power of 30 MW or more. The comparison has beencomplemented by an analysis of how wind farms of two different technologies(Doubly fed induction generators and full power converters) can meet therequirements and technical proposals have been given on how to be able to meetcompliance with the new grid codes. The Requirements for Generators contains many non-exhaustive and optionalrequirements, because of this it has been difficult to, at this stage, exactly point outthe technical impact on the grid connection of future wind farms in Sweden. For manyof the requirements in the Requirements for Generators there is no equivalent in theSwedish Grid Code (SvK FS2005:2) but counterparts can thus be found in existingpractices and therefor does not imply any major differences for the industry. The requirements of frequency regulation, synthetic inertia and reactive powercapability are the main components of the RfG which will require additional softwareand hardware installations for future wind farms in Sweden.

Στην παρούσα διδακτορική διατριβή μελετήθηκε η διασύνδεση ενός υπεράκτιου Αιολικού Πάρκου (ΑΠ) επαγωγικών μηχανών ονομαστικής ισχύος 90MW με ένα απομακρυσμένο ασθενές δίκτυο ΕΡ. Η περίπτωση αυτή παρουσιάζει σημαντικές δυσκολίες λόγω του χαμηλού λόγου ισχύων βραχυκύκλωσης ανάμεσα στις δύο πλευρές και των έντονων διακυμάνσεων της ισχύος που απομαστεύεται από τον άνεμο. Με σκοπό να περιορισθούν οι δυσκολίες αυτές και δεδομένου ότι τα διασυνδεδεμένα άκρα είναι απομακρυσμένα (άρα επιβάλλεται η μεταφορά με καλώδιο ΣΡ), επιλέχθηκε η εφαρμογή της διασύνδεσης του ΑΠ στο δίκτυο ΕΡ μέσω της τεχνολογίας Εναλλασσομένου Ρεύματος/ Συνεχούς Ρεύματος/ Εναλλασσομένου Ρεύματος (ΕΡ/ΣΡ/ΕΡ) με Μετατροπείς Πηγής Τάσης (ΜΠΤ). Η τεχνολογία αυτή επιτρέπει τον ανεξάρτητο έλεγχο της πραγματικής από την άεργο ισχύ και έτσι καθιστά δυνατή τη διασύνδεση δικτύων με χαμηλό λόγο ισχύων βραχυκύκλωσης. Ένα επιπλέον πρόβλημα το οποίο επιλύεται μέσω της χρήσης της τεχνολογίας ΕΡ/ΣΡ/ΕΡ με ΜΠΤ είναι αυτό της δυναμικής αντιστάθμισης αέργου ισχύος των επαγωγικών γεννητριών. Ωστόσο, παρόλα τα οφέλη που αποκομίζονται εφαρμόζοντας την τεχνολογία ΕΡ/ΣΡ/ΕΡ με ΜΠΤ, ο έλεγχος της διασύνδεσης παραμένει μια απαιτητική διαδικασία. Ο σχεδιασμός του συστήματος ελέγχου απαιτεί λεπτομερή γνώση της συμπεριφοράς του συστήματος και ακριβή ρύθμιση ώστε να αποκτηθεί η επιθυμητή έξοδος. Η παρουσία έντονα μεταβαλλόμενων και με θόρυβο εισόδων μπορούν να καταστήσουν πολύ δύσκολη την επιλογή των παραμέτρων ελέγχου οι οποίες παρέχουν σωστή συμπεριφορά σε οποιεσδήποτε συνθήκες λειτουργίας. Σύμφωνα με τα παραπάνω κριτήρια ελέγχου, το σύστημα ελέγχου που επιλέχθηκε για την παραπάνω διασύνδεση σχεδιάστηκε σύμφωνα με την υπολογιστική νοημοσύνη, η οποία βασίζεται στην ποιοτική περιγραφή των ελεγχόμενων διαδικασιών και στη μίμηση του συλλογισμού ενός έμπειρου ανθρώπου-χειριστή της διαδικασίας. Οι κλάδοι της υπολογιστικής νοημοσύνης που εφαρμόσθηκαν είναι τα ασαφή συστήματα και τα υβριδικά νευρο-ασαφή συστήματα. Πιο συγκεκριμένα, για τον έλεγχο της διασύνδεσης του ΑΠ με το ασθενές δίκτυο ΕΡ, σχεδιάστηκαν και μελετήθηκαν τρεις διαφορετικές εκδοχές, οι οποίες περιελάμβαναν: 1. ένα ασαφές σύστημα ελέγχου, 2. ένα νευρο-ασαφές σύστημα ελέγχου, 3. ένα προσαρμοζόμενο ασαφές σύστημα ελέγχου, το οποίο ρυθμίζεται σε πραγματικό χρόνο. Προκειμένου να ελεγχθεί η αποτελεσματικότητα του συστήματος ελέγχου, αλλά και να συγκριθούν οι τρεις εκδοχές του ως προς την αποτελεσματικότητά τους σε διαφορετικές διαταραχές, η αξιολόγηση του συστήματος περιέλαβε τρία μέρη. Στο πρώτο μέρος παρουσιάσθηκε η απόκριση του συστήματος σε μια βηματική αύξηση της μέσης τιμής της ταχύτητας του ανέμου. Η μεταβολή που επιλέχθηκε είναι ιδιαίτερα απότομη με σκοπό να ελεγχθεί η απόκριση του συστήματος υπό ακραίες συνθήκες. Στο δεύτερο μέρος παρουσιάσθηκε η απόκριση του συστήματος σε μια βηματική μείωση της μέσης τιμής της ταχύτητας του ανέμου, εξίσου απότομη με την προηγούμενη, ώστε να σχηματισθεί μια ολοκληρωμένη εικόνα για τη δυναμική συμπεριφορά του συστήματος κατά τη μετάβασή του στις διάφορες περιοχές λειτουργίας. Στο τρίτο μέρος έγινε μια αξιολόγηση της αρμονικής παραμόρφωσης, η οποία προκαλείται από τη διακοπτική λειτουργία του αντιστροφέα, στις κυματομορφές του ρεύματος και της τάσης στο ΣΚΣΔ, σύμφωνα με τα προτεινόμενα όρια του κανονισμού ΙΕΕΕ 519. Μέσω εκτενών αποτελεσμάτων εξομοίωσης αποδείχθηκε ότι το προτεινόμενο σύστημα ελέγχου και στις τρεις περιπτώσεις αντιδρά γρήγορα στις μεταβολές ισχύος του ανέμου και η διασύνδεση ΕΡ/ΣΡ/ΕΡ με ΜΠΤ τροφοδοτεί ομαλά το σύστημα ΕΡ με πραγματική ισχύ υπό σχεδόν σταθερή εναλλασσόμενη τάση. Επιπρόσθετα, λόγω του ελέγχου μεταβλητών στροφών, το ΑΠ οδηγείται σε μέγιστη αεροδυναμική απόδοση, χωρίς να απαιτείται μέτρηση της ταχύτητας του ανέμου ή του ρότορα των μηχανών. / In this thesis is studied the connection of an offshore Wind Farm (WF) with induction generators to a weak ac grid. This case presents several difficulties, as a result of the low short circuit ratio between the two interconnected ends and the fluctuating and unstable nature of the wind power. In order to constrain these difficulties and taking into account the long distance between the two interconnected ends (which necessitates the use of a dc cable), it was chosen to use the technology of the High Voltage Direct Current (HVDC) link based on Voltage Sourced Converters(VSCs) for the connection of the WF to the weak ac grid. This technology presents the advantages of dc transmission and due to the high switching capability of the VSCs’ valves, it can instantly regulate the reactive power and consequently the ac voltage, independently of the real power flow. So, the short circuit ratio between the two ends of the link does not have to be high. Furthermore, through this link, the dynamic reactive power compensation of the induction generations can be achieved. However, despite the benefits which are obtained through the technology of the HVDC link based on VSCs, the control of the link is still a demanding procedure. The design of the control system demands a comprehensive knowledge of the system behavior and accurate tuning in order to achieve the desirable output. The presence of highly fluctuating and noisy input signals can make the selection of the control parameters which ensure proper behavior in any operating conditions very difficult. Taking into account the above requirements, the control theory that was implemented to the control system of the above mentioned link is that of computational intelligence. Computational intelligence-based controllers do not require precise mathematical modeling of the system nor complex computations. They rely on the human ability to understand the system behavior and are based on qualitative control rules. In addition, they have inherent abilities to deal with imprecise or noisy data. The categories of computational intelligence that were used in the control system design are fuzzy systems and hybrid neuro-fuzzy systems. More precisely, three alternative versions of the control system of the link between the WF and the weak ac grid were designed: 1. a simple, manually tuned fuzzy control system , 2. a hybrid neuro-fuzzy control System 3. an adaptive, on-line tuned fuzzy control system. In order to test the performance of the control system and compare its three versions, the test of the system includes three parts. In the fist part was studied the performance of the system under a step increase of the wind speed. The step increase was chosen to be very steep, in order to test the system under extreme conditions. In the second part was studied the response of the system under a respective step decrease of the wind speed, in order to test the system performance under any operating condition. In the third part was examined the harmonic content of waveforms of the current and voltage at the point of common coupling, according to the limits of the international standard IEEE 519. Through extended simulation results it was shown that the proposed control system quickly reacts to the step changes of the wind power and the HVDC link based on VSCs manages to feed the weak ac grid with the power from the wind under almost stable ac voltage. In addition, due to the ability of the control system to adjust the stator frequency of the induction generators in relation to the wind velocity, maximum power absorption of the WF is achieved, without monitoring the wind speed or the rotor speed.

This research proposes an efficient reliability modeling and simulation methodology in power systems to include photovoltaic units, wind farms and storage. Energy losses by wake effect in a wind farm are incorporated. Using the wake model, wind shade, shear effect and wind direction are also reflected. For solar modules with titled surface, more accurate hourly photovoltaic power in a specific location is calculated with the physical specifications. There exists a certain level of correlation between renewable energy and load. This work uses clustering algorithms to consider those correlated variables. Different approaches are presented and applied to the composite power system, and compared with different scenarios using reliability analysis and simulation. To verify the results, reliability indices are compared with those from original data. As the penetration of renewables increases, the reliability issues will become more important because of the intermittent and non-dispatchable nature of these sources of power. Storage can provide the ability to regulate these fluctuations. The use of storage is investigated in this research. To determine the operating states and transition times of all turbines, Monte Carlo is used for system simulation in the thesis. A conventional power system from IEEE Reliability Test Systems is used with transmission line capacity, and wind and solar data are from National Climatic Data Center and National Renewal Energy Laboratory. The results show that the proposed technique is effective and efficient in practical applications for reliability analysis.

Wind power, as a source of renewable energy, is growing very fast. Especially so, is consideration given to offshore wind farms where expansion is due in part to limited social and visual impact, reduced noise effect of turbines, and at the same time higher production of offshore wind turbines. Maintenance is always a considerable and costly part of the wind power investment, especially for offshore projects, but it could not improve as fast as the increasing wind industry in the world. The operation and maintenance management of wind farms should have always a reliable and structured planning to have an economical investment. At the beginning of the growth of wind industry, companies tried to transfer responsibilities of the failure and loss of production to the insurance companies, but it cannot be continued any more. These days even the insurance companies ask about regular inspection or condition monitoring. In other words, they ask for a reliable strategy for operation and maintenance. Both preventative and corrective maintenance are used in offshore wind farms. Preventive maintenance is usually performed at the first sign of failure, and in so doing it helps reduce costs associated from lost production. Having a perfect preventive maintenance program is not easy and it usually needs more inspection and online monitoring. To select a suitable strategy, data should be gathered from different stakeholders who are involved in the project. The stakeholders could be turbine designers, construction companies, transportation companies, operation staff, etc. The reason is that each group has the data which could help to define a reliable strategy of maintenance. Reliability includes measurements, e.g., failure rate, repair time, and availability. Reliability is the ability of components or system to perform their function under given operational condition and for a predicted period of time. However always preventive maintenance especially for offshore wind farm is faced with uncertainty due to bad weather, access difficulty, and logistic limitation. Reliability Centered Maintenance (RCM) is a systematic qualitative technique that balances preventive and corrective maintenance. It chooses the right preventive maintenance activities for the right component at the right time to reach the most cost efficient solution. This research points out that RCM, as an experienced methodology in other industries, could be a good method for scheduled preventive maintenance in offshore wind power for purposes of lowering cost while improving reliability and safety. RCM implementation is always facing with uncertainty. Engaged uncertainty to RCM is known as Reliability and Risk Centered Maintenance RRCM and it could lead maintenance process to an optimal preventive schedule with minimum uncertainty.

Operations and maintenance (O&M) activities represent a significant share of the expenses during the lifetime of offshore wind farms. When compared to onshore wind farms, O&M costs are increased for the offshore case, as specialized vessels, weather windows and rough conditions mean more failures, downtime (decreasing availability), spare parts, and man-hours.This study comprises an analysis of the available O&M data from a selected offshore wind farm. The results and conclusions from this investigation could then be used to evaluate possible reliability improvements and compare options for the maintenance strategies, as well as to ponder the convenience of warranty periods and O&M agreements between wind farm operators and wind turbine manufacturers or O&M service providers.The life-cycle cost (LCC) concept is utilized in the analysis of the wind farm survey for this thesis. LCC analysis could be the starting point to make decisions regarding specific wind turbine models, as selecting the turbines with the lowest initial cost may not be necessarily the scenario which also costs the least amount of money when taking into consideration the whole life cycle. It may also be a great tool to forecast future operational incomes and expenses of offshore wind farms.

Wind turbines connected to the distribution network can be the cause of voltage fluctuations and resulting fluctuations in the light intensity emitted by light bulbs. These fluctuations may cause people disturbance. A model has been developed to obtain a flicker prediction which is useful in the design process of a wind farm. The model is based exclusively in the frequency domain (FD). This new approach allows very fast and efficient evaluation. The impact of individual parameters is often easier to recognise and evaluate in a FD-representation. The following factors leading to flicker disturbances from a wind farm have been considered in detail: The wind spectrum: Effects of terrain and wind farm wakes on the wind turbulence spectrum have been considered and existing models have been expanded. The wind coherence: A new coherence model for large separation distances has been derived for use within a wind farm. Effects of the terrain on the coherence of power produced by turbines within a wind farm have been considered. The wind turbine: A simplified dynamic wind turbine model allows the prediction of turbine specific contributions to flicker for a variety of wind turbines using a minimal set of parameters. The flickermeter: Flicker measurements are found to sometimes neglect the impact of low frequency voltage variations. These are found to be very important for the correct flicker prediction. A new FD-flickermeter has been developed. The model has been validated against experimental data and a sensitivity analysis shows which parameters are most likely to influence the voltage flicker and which are best altered to minimise the flicker.

abstract: With a ground-based Doppler lidar on the upwind side of a wind farm in the Tehachapi Pass of California, radial wind velocity measurements were collected for repeating sector sweeps, scanning up to 10 kilometers away. This region consisted of complex terrain, with the scans made between mountains. The dataset was utilized for techniques being studied for short-term forecasting of wind power by correlating changes in energy content and of turbulence intensity by tracking spatial variance, in the wind ahead of a wind farm. A ramp event was also captured and its propagation was tracked. Orthogonal horizontal wind vectors were retrieved from the radial velocity using a sector Velocity Azimuth Display method. Streamlines were plotted to determine the potential sites for a correlation of upstream wind speed with wind speed at downstream locations near the wind farm. A "virtual wind turbine" was "placed" in locations along the streamline by using the time-series velocity data at the location as the input to a modeled wind turbine, to determine the extractable energy content at that location. The relationship between this time-dependent energy content upstream and near the wind farm was studied. By correlating the energy content with each upstream location based on a time shift estimated according to advection at the mean wind speed, several fits were evaluated. A prediction of the downstream energy content was produced by shifting the power output in time and applying the best-fit function. This method made predictions of the power near the wind farm several minutes in advance. Predictions were also made up to an hour in advance for a large ramp event. The Magnitude Absolute Error and Standard Deviation are presented for the predictions based on each selected upstream location. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2014