<p>With the recent developments in semiconductors and control equipment, Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) has attracted the growing interest of researchers. The use of VSC technology and Pulse Width Modulation (PWM) has a number of potential advantages: short circuit current reduction; rapid and independent control of the active and reactive power, etc. With such highly favourable advantages, VSC-HVDC is definitely going to be a large part of future transmission and distribution systems. HVDC technology based on VSC technology has been an area of growing interest recently because of its suitability in forming a transmission link for transmitting bulk amount of wind power. This thesis deals with the control of VSC-HVDC. The objective of the work is to understand the control structure of the VSC-HVDC system, and establish the tuning criteria for the PI controllers of the converter controllers. A model of a VSC based dc link using PWM Technology is developed. A mathematical model of the control system based on the relationships between voltage and current is described for the VSC. A control system is developed combining an inner current loop controller and outer dc voltage controller. The vector control strategy is studied and corresponding dynamic performance under step changes and system fault is investigated in PSCAD/EMTDC simulation package. The simulation results verify that the model can fulfill bi-directional power transfers, fast response control and that the system has good steady state performance. The controller parameters tuned according to the developed tuning criteria is found to provide acceptable system performances.</p>

<p>The North Sea has a vast amount of wind energy with largest energy per area densities located about 100-300Km of distance from shore. Should this energy be tapped by offshore wind farms, HVDC transmission would be the more feasible solution at such long subsea distances. On the other hand Norwegian oil/gas platforms in the North Sea use electricity from gas fired turbines at offshore sites. These gas turbines have much less efficiency than onshore generation of electricity and also release large amounts of green house gases. Therefore supplying the platforms with power from onshore transmitted by HVDC will result in benefits both from economic and environmental protection perspectives. Given these two interests for HVDC in the Norwegian offshore, the use of Multiterminal HVDC (MTDC) is a potential solution for the integration of the wind farms and oil/gas platforms into the onshore grid system. Hence, this thesis focuses on the operation and control of MTDC systems. The MTDC system is desired to be capable of interfacing with all kinds of AC grids namely: stiff, weak and passive grid systems. Compared to the classical thyristor based converter, VSC has several features that make it the most suitable converter for making of MTDC, the most decisive being its ability of bidirectional power transfer for fixed voltage polarity. VSC-HVDC is also suitable for implementing control of active and reactive current in synchronously rotating d-q reference frame which in turn results in decoupled control of active and reactive power. In the first two chapters of the thesis literatures are reviewed to understand operation of VSC and its use in HVDC systems. Afterwards controllers are developed for different AC connections (stiff, weak and passive) and for different DC parameter (power, DC voltage) control modes. DC voltage and active power control are implemented by active current control and AC voltage and reactive power control are achieved by reactive power compensation. Tuning techniques for the PI controllers are discussed and used in the simulation models. Finally control techniques for reliable operation of MTDC are developed. In order to validate theoretical arguments, each of the control schemes was developed and simulated in PSCAD/EMTDC simulation software. Simulation results indicate that satisfactory performance of VSC-HVDC was obtained with the proposed active/reactive power controllers, AC/DC voltage controllers, frequency and DC overvoltage controllers. For coordinated multiterminal operation, voltage margin control method and DC voltage droop characteristic were used. These are control methods based upon realization of desired P-UDC characteristic curves of converter terminals. Four-terminal MTDC system with different AC grid connections was used to study the multiterminal operation. Simulations have shown that voltage margin control method results in reliable operation of MTDC during loss of a terminal connection without the need for communication between terminals. The use of DC voltage droop control along with voltage margin control enabled load sharing among VSC-HVDC terminals in DC voltage control mode according to predetermined participation factor.</p>

<p>Direct electrical heating (DEH) of pipelines is a flow assurance method that has proven to be a good and reliable solution for preventing the formation of hydrates and wax in multiphase flow lines. The technology is installed on several pipelines in the North Sea and has become StatoilHydros preferred method for flow assurance. Tyrihans is the newest installation with 10 MW DEH for a 43 km pipline. However, the pipeline represents a considerable single-phase load which makes the power system dependent on a balancing unit for providing symmetrical conditions. This limits the step out distance and is not suitable for subsea installation. Aker Solutions has proposed several specially connected transformers for subsea power supply of DEH systems, Scott-T being one of them. The Scott-T transformer is a three-to-two-phase transformer which provides balanced electrical power between the two systems when the two secondary one-phase loads are equal. By implementing this transformer, it can be possible to install the power supply subsea as there is no need for a balancing unit. In addition, the system may be applicable for long step out distances. This is because the pipeline is inductive and can use the reactive power produced by the long cable which also can increase the critical cable length. There are however some limitations on this system using the Scott-T transformer. There is a large variation in the magnetic permeability between individual joints of the pipeline. This can result in different load impedance of the two pipe sections connected to the Scott-T transformer. The result is unbalance in the power system. The method of symmetrical components is applied to investigate the behavior during unbalanced loading of the Scott-transformer. The relationship between the negative- and the positive sequence component of the current is used to express the degree of unsymmetry. For the simulations in SIMPOW, the Scott-T transformer is modelled by the use of Dynamic Simulation Language. The simulations on the DSL model give correct and reliable results for analysing the the degree of unsymmetry in the Scott-T transformer. When the load impedance of one pipe section is varied, simulation proves that it can change between 0.75 and 1.34 per unit of the other pipe impedance. The Scott-T transformer does still provide electrical power between the two systems which is below the limit for the degree of unsymmetry (15%). Case 1 and Case 2 introduce two possible configurations for a subsea DEH system with the Scott-T transformer implemented. The configurations include an onshore power supply which is connected to a subsea power system for direct electrical heating and a subsea load at the far end of the subsea cable. The pipeline in Case 1 is 100 km long and is divided into two pipe sections of 50 km which are connected to a Scott-T transformer. The pipeline in Case 2 is 200 km long and is divided into four pipe sections of 50 km each. There are two Scott-T transformers in Case 2. For normal operation of the subsea load (50 MW, cosfi=0.9) and heating the pipe content from the ambient sea emperature, the results indicate that tap changers are necessary to keep the Scott-T transformers secondary terminal voltage at 25 kV. This meets the requirement in both cases for heating the pipe content from 4 to 25 degrees celsius within 48 hours after a shutdown of the process. The degree of unsymmetry is zero for both cases when the system is operated as normal. However, all system simulations indicate that reactive power compensation has to be included for Case 1 as well as for Case 2 in order to have a power factor of unity at the onshore grid connection. The fault scenarios indicate that the degree of unsymmetry is dependent on both the type of fault and the power supply in the system. For Case 1, the relationship (I-/I+) is only of 3.3% in the subsea cable when there is a short-circuit at DEHBUS3, but as much as 87% at the grid connection. The degree of unsymmetry in the Scott-T transformer is then 67%. This is far beyond the limit for maximum negative sequence component of 15%. The significant unsymmetry in the line between the grid and BUS1 is most likely due to the large power delivered to the fault. During the fault, the reactive power delivered to the system increases from 10.6 Mvar to 131.9 Mvar after the fault, but the active power increases only from 75.2 MW to 87.1 MW. This means that it is most likely the reactive power that contributes to the consequent unsymmetry and negative sequence component of the current. There are two Scott-T transformers installed in Case 2. If the DEH system is only heating the pipe section closest to shore (at DEHBUS33), simulations show that the three-phase power system becomes unsymmetric which results in different phase currents. The degree of unsymmetry at the grid connection is 32% when only the pipe section at DEHBUS33 is heated. In addition, the unbalance in the three-phase system caused by SCOTT1 involves unbalance in the SCOTT2 transformer as well. The load voltages are not equal in magnitude and dephased of 90 degrees for this mode, but are 32 kV and 35 kV respectively and dephased of 88 degrees. This concludes a very important behavior of the Scott-T transformer. The simulations conclude that the Scott-T transformer provides symmetrical conditions for both configurations when the two load impedances are equal. However, Case 2 shows an important result when installing two Scott-T transformers in the same system. Unbalanced loading of one of the specially connected transformers gives unsymmetrical conditions in the three-phase system which results in unbalanced load voltages for the other Scott-T transformer. The analysis is limited to the configurations given for Case 1 and Case 2, but shows typical results when an alternative transformer connection is implemented in a DEH system.</p>

<p>There is an increasing amount of power processed through power electronics in the areas of generation interface, energy storage and loads. This increment enables possibilities for improved solutions for efficient generation and use of electric power. Traditionally loads in AC distribution systems have been seen as passive and individual elements rather than active components of the system. An AC distribution system with high percentage of power electronic loads can be susceptible for instability under abnormal operation conditions if the converter controls are not designed for such conditions. This thesis introduces a solution of reactive current control for a constant power load (CPL) in an AC distribution system. A CPL is a load that draws a constant amount of active power without regard to any drops in system voltage. The resistance seen from the AC distribution system to the CPL is known as a negative input resistance which is characteristic for this kind of load. A drop in AC system voltage will result in increased current and vice versa. Hence the resulting resistance will be negative. Negative resistance behaviour is not favourable for system stability and the work has focused on a control of the converter that reduces the instability effect of the load. By controlling the reactive current component to inject current for support of the AC distribution system voltage during faults and other interferences, the load becomes an active participant in the AC distribution system. An AC distribution system has been built in PSCAD/EMTDC. An asynchronous generator and a fixed capacitor are used as distributed generation. The CPL consists of an AC/DC converter and a voltage dependent current source in order to keep constant DC power at the load. Focus has been on the converter control implementing a voltage oriented vector control based on a two phase rotational reference frame. Measurements of current and voltage with different voltage drop levels have been done for comparison of a CPL with and without reactive current control. The simulation example has verified the negative resistance phenomenon of the CPL. Results shows that the AC system voltage is less vulnerable in case of faults if the converter control is designed to inject reactive current into the AC distribution system.</p>

<p>Offshore wind power has proven to be a renewable energy source with a high potential, especially in the North Sea, where an important development is going on. The location of the wind farms tends to move far from the coast to benefit stronger and more constant wind. In the same time, the power output of the wind farm is increasing to several hundreds of MW up to 1 GW. In the European liberalized electricity market, the interconnection of the countries become very important to facilitate the cross-border trade of electricity but also to improve the reliability of the grid. Combining this both aspects into one, a big offshore HVDC grid connecting countries and large wind farms spread allover the North Sea is currently being studied and developed. So in addition of the challenge given by a high penetration of the wind power production in the European power production scheme, new challenges are opened especially for the offshore transmission. This master thesis presents the integration to grid of a single 1 GW or a cluster of wind farms connected to an oil rig with different connection scheme based on HVDC transmission using the Voltage Source Converter (VSC) technology. The connection of the offshore wind farms is done either with a single HVDC transmission or two HVDC transmissions connected to the main grid at two different Points of Common Connection situated in the south-west of Norway. The wind farms are not represented in detail but by a single generator. They are equipped for the simulation with Double Fed Induction Generators (DFIG) to be representative of the reality, almost half of the wind turbines are today equipped with DFIG technology. Two disturbances are used to test the electrical stability of the system: a classical 150ms three fault phase in agreement with the grid code requirements on the ride fault through requirements and 100ms fault leading to the tripping of a line. The impact of using different types of generator is also investigated with the simulation of cluster wind farm where a wind farm is equipped with Fixed Speed Generator (FIG). The emphasis is put on the response of the VSC-converter and to a lesser extent on the behaviour of the wind turbine generator. It is demonstrated the capacity of the VSC-converter to stabilize a small grid alone and to “isolate” a disturbance. The voltage and the frequency offshore are practically unaffected by a fault onshore and vice versa. As expected, it is demonstrated that the multiplication of the VSC-HVDC converter in a grid improves the stability of the system. Finally, it has been noticed that there maybe some interactions if several different types of generators are used. The replacement of a generator by another type inside the wind farm cluster may change completely the dynamic behaviour after a disturbance. Simulations are performed with PSS/E.</p>

<p>Problem description: The objective of this project was to create a simple model of the European power system and to investigate the effect an increasing amount of on- and offshore wind power will have on the North European power market in general and Norway in particular. The scenarios contain increasing amounts of installed wind power capacity, both on- and offshore. Emphasis was to be on the area surrounding the North Sea. The project covers the following issues: - Simulations of simplified power system scenarios set in the years 2005, 2020 and 2030. - Study how an increasing amount of installed wind power will affect energy prices, power production distribution, and power transmission flows. - Investigate how an offshore grid consisting of interconnections between offshore wind farms will affect the system. The task: The simulations in this project were performed using simple power market model. The model included 6 price areas: Denmark West, Denmark East, Norway, Sweden/Finland, Germany and UCTE/Others. The existing market model was modified in the following manner: - Split Norway into three price areas: Norway North, Middle and South - Add the Netherlands - Add the United Kingdom - Add corresponding offshore price areas for areas neighbouring the North Sea. Wind series were generated for each wind generator using reanalysis data. Scenarios were created for the years 2005, 2020 and 2030. In these scenarios, wind power capacities are increasing as time progresses. The 2020 and 2030 scenarios have been simulated with two alternative grid configurations: one where the offshore areas are connected only to their respective onshore areas and one where the offshore areas are also interconnected in an offshore grid. In total 7 different scenarios were simulated. Results: Wind power is able to supplant a large share of energy originally produced by con-ventional thermal generators. The presence of an offshore grid does not have any dramatic effects on energy production for the system, though it is possible to conclude that the presence of an offshore grid may contribute to slightly shift the power system in favour of renewable energy sources. Wind power will cause a significant reduction in energy prices in all areas, resulting in reduced energy costs for the entire system. Analysis of lost wind and hydro power reveals the importance of sufficient transmission capacity when large quantities of wind power are added to the system. Scenario 4 features enormous quantities of lost hydro power in the North and Middle of Norway due to transmission limitations. Analyses of power transmissions reveal that the offshore grid is over-dimensioned. Rationalizing the grid by reducing transmission capacities to more realistic levels will give a more cost-effective solution. This was demonstrated by performing a quick simulation and analysis of a scenario featuring such a rationalized grid. Wind power will cause more frequent variations in hydro power generation, due to balancing needs. Parts of the increased variability in the hydro generators can be explained by the increasing amount of wind power in the system, while other parts are most likely caused by limitations in the simulation model itself. Conclusion: Given the number of assumptions made in the grid, in cost calculations and in the model at large, it is more important to focus on general trends than on concrete numerical values. However, it is clear that increasing the amount of on- and offshore wind power in the European power system will have a beneficial impact to society's energy costs. It is also clear that wind power has the potential to dramatically reduce CO2-emissions caused by power generation. The offshore grid seems to be more beneficial to the power producers than to consumers since it causes slightly higher energy prices and providing a measure of flexibility as to where offshore wind power production is sent. Wind power will present challenges, especially regarding transmission grid development. A sufficiently dimensioned grid will be essential to the successful implementation of such amounts of wind power, both with respect to profitability and in order to avoid waste of potential wind or hydro energy.</p>

<p>Reducing grid harmonics and increasing grid stability are both major issues for the operation of rail vehicles. For stability investigation of complex power systems, simplified system models are in need in order to reduce the model complexity and the simulation time. In this thesis work the effect of modelling a voltage source converter (VSC) for traction power system with and without the detailed pulse width modulated (PWM)-switch model is modeled. Effect of different operating conditions for the switching model on the harmonic content of the system is also analyzed. The same disturbance is imposed for the two models and the low frequency oscillation of the DC- link voltage response is compared and analyzed. The effect of semiconductor switching on the stability limit of the system is also investigated. Furthermore, the performance of a PWM time delay compensation technique during transient is analyzed. The result shows that in the model including the switching the DC- link voltage oscillation is damped and has a better stability margin compared to the average model. In the detailed switching model a converter loss is included while in the average model a no loss ideal case scenario is considered. As far as the switching harmonic is considered, the switching model with an operational condition of a high switching frequency and a switching frequency with an integer multiple of the fundamental frequency has a low harmonic content on the system compared to the operating condition of a low switching frequency which is not integer multiple of the fundamental frequency. A unipolar voltage switching technique has also a tremendous advantage over the bipolar voltage switching technique as far as this harmonic content in the system is concerned. Using a unipolar voltage switching technique reduces the harmonic content in the overall railway system. For triangular carrier modulators, an average time delay from the reference voltage to the actuated PWM terminal voltage of half the switching frequency is assumed .The delay in DC- link voltage control loop caused by the switching dead-time effect was improved by compensation of dead-time in the inverse-park transformation block of the control loop. The comparison of the compensated and non-compensated model proves that the compensated model is better in terms of the overshoot of amplitude of transient.</p>

<p>In order to comply with governmentally announced greenhouse gas emission reductions goals and to consolidate an independent and stable electric power and energy supply, Norway must increase its installed renewable energy based power generation capacity. Profitability estimations, today’s available technical solutions and regulations concerning preservation of natural resources leave construction of new small hydro power plants behind as the most plausible alternative together with construction of wind farms. Global trends such as technologic development and progress and the public opinion indicate that future wind farms in Norway will be located offshore. The assumption is supported by the recent handing out of a concession to an offshore wind farm project for the first time in Norwegian history. The projects name is Havsul 1 and the licence involves construction of 350 MW offshore wind power. Havsul 1 will be located in Mid-Norway, which is the region in Norway where the Norwegian Transmission System Operator (TSO) Statnett is most concerned about their ability to execute their task of assuring safety of power supply in the future. The concern owes to lack of generation capacity and transmission constraints. Experience show that commissioning of large offshore wind farms will impose power balance associated challenges on the TSO. By applying a slightly modified model developed by Sintef of the Nordel synchronous system in the power simulation tool DIgSILENT PowerFactory, grid connection of 350 MW and 1000 MW offshore wind farms to a bus bar representing Mid-Norway were investigated, targeting reduction of frequency excursions. To execute the reduction task, a so-called centralised Load Frequency Control (LFC) scheme was implemented and four hydro power plants were designated to provide regulating power pursuant to a priority key that used their response times as input. To simulate power fluctuations in the time span of hours, real time wind data acquired from the Danish offshore wind farm Horns Rev 1 was used as input in the offshore wind farm model. These data were kindly provided by the Swedish power company Vattenfall. The power fluctuations simulations showed that LFC is a well-fitted tool for bridling frequency excursions in the Nordel synchronous system caused by fluctuating power generation in an offshore wind farm. During the power fluctuations, which were of a particularly challenging kind, the system frequency complied with Statnett’s normal operation requirements of 50 ±0.1 Hz. The results weren’t too surprising since LFC has been used successfully in Europe for many years. They did however show that the amounts of the so-called frequency controlled normal operation reserves in Nordel may need to be expanded in case of a massive expansion in wind power in Norway. Fault Ride Through (FRT) investigations were also conducted by introducing 3-phase short circuit faults at selected bus bars. The simulations showed that the FRT requirements in Norway were not violated even in the worst case simulations. Some choices regarding the setup of the model may have exalted the simulation results.</p>

<p>The work consists in developing a power conversion unit for solar panel connected to the grid. This unit will be a single phase inverter in the low power range (24/48 V - 100/200 W), with digital control and without a DC-DC stage. The final system should be suitable for laboratory work. This master project will be complete, starting with a detailed specification of the project and finishing with an experimental validation.</p>

<p>Characterization of Epoxy (diglycidyl ether of Bis-phenol A cured with Tri ethylene Tetra amine) without fillers was done. The Water absorption test at 95°C shows that at saturation the epoxy contains a water concentration of 2.089%. The diffusion coefficient of absorption is calculated as 0.021 cm2/s. The diffusion coefficient of desorption is calculated as 0.0987 cm2/s. The diffusion is almost 5 times faster than absorption. Also the material looses weight as the hydrothermal aging progresses. The water in the sample leads to chain scission which leads to the weight loss. The weight loss is more incase of absorption followed by desorption than only absorption. The chain scission leads to decrease in the mechanical strength by around 45%. The diffusion of water from the samples doesn’t affect the mechanical strength of the materials. The glass transition temperature reduces by 20°C with water inside the sample. The diffusion of water out of the sample only increases by around 10°C. The Dielectric response of the material shows that after the water absorption the sample shows high losses at lower frequencies. Also the increase in the real part of the permittivity increases with low frequency. The rapid increase in the real art of the permittivity of the material at lower frequencies can be attributed to a polarization at the electrode due both to accumulation of the charge carriers and to chain migrations. The breakdown test of the samples shows that with water in the sample the breakdown strength of the material decreases by 10 KV, but the material regains its dielectric strength when the water is diffused out. This shows that the chain scission and weight loss of the samples has no or minimum effect on the dielectric strength of the sample</p>