<p>The rapid development of wind power technology is reshaping conventional power grids in many countries across the world. As the installed capacity of wind power increases, its impact on power grids is becoming more important. To ensure the reliable operation of a power system which is significantly fed by wind power, the dynamics of the power system must be understood, and the purpose of this study is to develop suitable analytical tools for analysing the dynamic impact of large-scale wind parks on the stability of a power grid, and to investigate the possibility of improving the stabilisation and damping of the grid by smart control strategies for wind turbines.Many of the newer, larger turbines now being produced are variable speed turbines, which use doubly fed induction generators (DFIGs). These are induction generators which have their stator and rotor independently excited. When unconventional generators of this type are used in a power system, the system behaves differently under abnormal dynamic events.  For example, new types of generators cause different modes of oscillation in the power system, not only because of their dynamic characteristics, but also because they load the system differently.Very large power oscillations can occur in a power system as a result of internal disturbances.  Ordinarily these oscillations are slow and, in principle, it is possible to damp them with the help of wind power.  This leads to the idea of using a power system stabiliser (PSS) for a DFIG.  In order to damp oscillations in the system, it is necessary to understand the equipment causing these oscillations, and the methods to optimally damp the oscillations.Voltage stability is another important aspect of the safe operation of a power system. It has been shown that the voltage stability of a power system is affected by induction generators.  The voltage stability must therefore be carefully analysed in order to guard against a power system collapse.By using modal analysis and dynamic simulations, we show that the presence of a wind farm in the vicinity of a power system will improve the angular behaviour of the power system under small disturbances, but may decrease voltage stability under larger disturbances. We compare the performance of wind turbines to that of conventional synchronous generator power plants, and we show that a wind park consisting of DFIGs, which are equipped with PSSs, may be used as a positive contribution to power system damping.</p>

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<p>The rapid development of the global economics has made power systems allover the world become large-scale interconnected grids. This increases the capabilityof power grids to transfer power over the long distance to serve the desiredpower demand with the minimum cost of operation. Unfortunately, it alsoenables the propagation of local failures into global networks. In other words,if a blackout happens in a power system, the size and the damage may significantlyincrease.</p><p>One of the main ways in which blackouts become widespread is cascadingfailures. This type of failure originates after a critical component of the systemhas been removed fromthe service by protective relaying. As a consequence, theload handled by the failed component needs to be redistributed which mightcause an overloading on other components in the system.</p><p>On the other hand, the high power electronics controllable devices suchas Voltage Source Converters-based High Voltage Direct Current (VSC-HVDC)transmission are recently developed. These electronics devices have the potentialadvantages such as the ability to independently control active and reactivepower, and maintain voltage to be at acceptable level. Therefore, they are consideredto be the promising devices that with an appropriately designed controlstrategy, they can substantially improve the performance and reliability of thepower system.</p><p>This thesis presents the possibility to consider protection system status inthe control of VSC-HVDC link. A great deal of this research is development ofcoordination between this power electronic device and protection system which normally are considered separately. The derivation of protection system has been selected to determine the operation of VSC-HVDC. The methodology isbased on utilizing the signal created from a logical evaluation of relay and simplificationsof certain parameters. By introducing information from the relays tothe VSC-HVDC link via Central Control Unit (CCU), the modulation of transmitted power is devised in order to reduce the risk of system-wide failures. In turn,this means an avoided blackout.Furthermore, this thesis also includes the preliminary suggestion to selectthe location of VSC-HVDC. The methodology is based on predicting voltage instabilityusing voltage stability indices and related parameterswhich are derivedby using Singular ValueDecomposition method. The solutions indicate an effectivelocation for applying corrective action such as load shedding. This optimallocation is selected to reinforce the control strategy of VSC-HVDC in order toprevent cascading failures in the more encompassing systems.</p> / QC20100615

This is a study of potential improvements of diagnostic methods used on high-voltage generators and motors.  It considers offline electrical measurements on the main insulation of stator windings, where a sinusoidal voltage is applied between the winding and the stator-core, and the total current through the insulation (dielectric spectroscopy, DS) and the rapid current-pulses arising from discharges (partial discharge, PD) are measured. The proposed methods differ from existing practice in industrial DS and PD measurements in that the applied voltage is varied in amplitude and in frequency, harmonics of the voltage and current are measured, and the DS and PD measurements are made simultaneously, with comparison of results.  Based on literature, models and measurements, the problems and advantages of these methods are assessed in this work. Harmonics provide a way of separating linear and nonlinear sources of current, and reveal the waveform of the current.  Measurement of total PD charge by DS methods provides complementary information to the conventional PD measurement; the difference in results between these types of measurement is shown by literature and experimental results to be large.  Simultaneous measurement allows direct comparison of the relation between the DS and PD results, and saves time compared to separate measurements.  The varied frequency, down to the millihertz range, provides additional information about the insulation.  Much of the potential for DS methods on machine insulation is spoiled by the end-winding stress grading.  Models and measurements of the currents in this grading are presented, with discussion of how much effect the disturbance has and how well it can be predicted by modelling. / QC 20100906

The response of a cable can be used to analyze the variation of the material characteristics along its length. For diagnosis of possible ageing, it is necessary to know how cable design, material properties and cable insulation ageing affects the wave propagation. A cable model has therefore been worked out based upon the high frequency properties of the cable insulation and conductor systems. The high frequency characteristics of the semi-conducting screens, new and water-tree aged cross-linked polyethylene (XLPE) insulation have been determined by two measuring techniques. Semi-conducting screens have a great influence on the attenuation of cables at high frequencies. A dielectric function based on an empirical formula is fitted to the measured characteristics of the semi-conducting screens. The dielectric function is used in the cable model set-up to predict the propagation constant of the cables. The high frequency characteristics of the water-tree degraded XLPE insulation obtained from measurements are related to the water content of the water trees of the samples. An effective materials model is developed to estimate the water content of the water trees and the conductivity of the water. The propagation constant from the cable model is compared to the propagation constant obtained from measurements made on cables. Using approximate high frequency cable models, the relative contributions of the conductor and insulation systems to the attenuation and phase velocity of the cables are analyzed. / QC 20101018

At the Division of Electricity at UppsalaUniversitythere are ongoing researches, including a research concerning turbo generators. Turbo generators produce a large share of electricity in the world; however they are costly mainly due to the fact that the cooling and ventilation that are needed to eliminate the excess heat from the generators are expensive. Turbo generators work with high efficiency and in this project, the main task is to simulate a generator while taking into account that the generator must be designed against self-oscillation, centrifugal force, exhaustion and heat. The turbo generator must be simulated without being too expensive and complicated. In this project, the dimensions of the turbo generator depend on the efficiency of it, and where the losses are in the generator. Furthermore, the optimum values need to be chosen with the help of the ACE simulation program. The report begins with an introduction describing the project followed by a task description that is dealing with the project. The project aims to obtain an optimal design that is simple to construct, with good durability and preferably at a reasonable cost. The function of the turbo generator has been studied, as well as the FEM calculations that affect the durability and structural strength of the generator. In this report you can further read more in detail about the methods used and how solutions were reached to solve the problems that were encountered. The conclusion of this project was that the turbo-generator which were simulated is a turbo-generator that has a lower load angle compared with Björn Bolund turbo-generator. It also has a high efficiency which makes it more stable than Björn Bolund and the high efficiency makes it save energy and cooling equipment.

<p>The response of a cable can be used to analyze the variation of the material characteristics along its length. For diagnosis of possible ageing, it is necessary to know how cable design, material properties and cable insulation ageing affects the wave propagation. A cable model has therefore been worked out based upon the high frequency properties of the cable insulation and conductor systems. The high frequency characteristics of the semi-conducting screens, new and water-tree aged cross-linked polyethylene (XLPE) insulation have been determined by two measuring techniques. Semi-conducting screens have a great influence on the attenuation of cables at high frequencies.</p><p>A dielectric function based on an empirical formula is fitted to the measured characteristics of the semi-conducting screens. The dielectric function is used in the cable model set-up to predict the propagation constant of the cables.</p><p>The high frequency characteristics of the water-tree degraded XLPE insulation obtained from measurements are related to the water content of the water trees of the samples. An effective materials model is developed to estimate the water content of the water trees and the conductivity of the water.</p><p>The propagation constant from the cable model is compared to the propagation constant obtained from measurements made on cables. Using approximate high frequency cable models, the relative contributions of the conductor and insulation systems to the attenuation and phase velocity of the cables are analyzed.</p>

<p>Offshore wind farms have recently emerged as promising renewable energy sources. For increasing distances between offshore generation and onshore distribution grid, HVDC transmission systems based on voltage source converters can be a feasible and competitive solution. This thesis presents a comprehensive evaluation of a novel integrated wind farm topology that includes the generator drive system, the turbine interconnection and the HVDC transmission.</p><p>In the proposed concept, every wind turbine is connected to a single-phase medium-frequency collection grid via a distribution transformer and a cycloconverter, which allows the wind turbines to operate at variable speed. The collection grid is connected to an HVDC cable via a transmission transformer and a single-phase voltage source converter. This thesis evaluates in detail the principle of operation, which is also verified with system simulations in PSCAD.</p><p>The proposed concept promises several potential benefits. Converter switching losses and stress on the semiconductors for example can be considerably reduced by applying a soft-switched commutation scheme in all points of operation. Single-phase medium-frequency transformers have comparably low losses and their compact size and low weight implies an important benefit in an offshore environment. In addition, the voltage source converter is considerably simplified by the reduction to one phase leg, which implies a substantial cost saving.</p><p>Several technical challenges are identified and critically evaluated in order to guarantee the feasibility of the proposed concept. Especially the design of the medium-frequency collection grid is crucial as unwanted system resonances can cause dangerous overvoltages. Most of the technical challenges concern the specific characteristics of the proposed concept. The insulation of the single-phase medium-frequency transformers for example needs to withstand the high voltage derivatives. This thesis contains also considerations regarding the dimensioning and optimization of different system components.</p><p>A survey of different transmission systems for the grid connection of wind farms shows the potential of the proposed concept, which addresses several problems associated with electrical systems of wind farms. Both the requirements for variable-speed operation of the wind turbines and an interface for HVDC transmission are fulfilled in a cost-effective way. Compared to conventional voltage source converter based HVDC transmission systems, the initial costs are reduced and the expected annual energy production is increased. In addition, the proposed voltage source converter based HVDC transmission system can fully comply with recent requirements regarding the grid connection of wind farms.</p>

<p>There has been a pressure to operate power systems closer to their security limits. This has partially been due to financial imperatives following the deregulating of markets. Other practical difficulties have been obtaining authorization from regulatory bodies to build power plants and transmission lines.</p><p>In this situation it is essential to monitor the system and to have tools that can predict the distance to the point of collapse (PoC). Much effort has been put into research of the phenomenon voltage collapse, and many approaches have been explored. Both dynamic and steady-state behavior have been studied thoroughly, though very few protection and control schemes have been implemented. In this dissertation the possibility of an index based on local area measurements have been explored. Voltage stability can be classified as either a transient or a long-term stability problem, and the index proposed in this dissertation is based on long-term dynamics.</p>

There has been a pressure to operate power systems closer to their security limits. This has partially been due to financial imperatives following the deregulating of markets. Other practical difficulties have been obtaining authorization from regulatory bodies to build power plants and transmission lines. In this situation it is essential to monitor the system and to have tools that can predict the distance to the point of collapse (PoC). Much effort has been put into research of the phenomenon voltage collapse, and many approaches have been explored. Both dynamic and steady-state behavior have been studied thoroughly, though very few protection and control schemes have been implemented. In this dissertation the possibility of an index based on local area measurements have been explored. Voltage stability can be classified as either a transient or a long-term stability problem, and the index proposed in this dissertation is based on long-term dynamics.