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  • 51.
    Johansson, Lars
    KTH, Superseded Departments, Electric Power Systems.
    A context for requirements engineering1998Licentiate thesis, monograph (Other scientific)
  • 52.
    Johnsson, Anders
    KTH, Superseded Departments, Electric Power Systems.
    On human and computer communication in power system control: the consequences of new interaction patterns1998Licentiate thesis, comprehensive summary (Other scientific)
  • 53.
    Jones, Lawrence E.
    KTH, Superseded Departments, Electric Power Systems.
    On zero dynamics and robust control of large AC and DC power systeems1999Doctoral thesis, monograph (Other scientific)
  • 54.
    Jäderström, Anna
    KTH, School of Electrical Engineering (EES), Electric Power Systems. KTH, Superseded Departments, Electric Power Systems.
    Coordinated regulation of wind power and hydro power with separate ownership2004Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    bad format, retyped

    Both wind power and hydro power must be built in immediate proximity to the source. Almost all hydro power is concentrated in the north of Sweden, since the largest rivers can be found here. The northern parts of Sweden also have good wind conditions and are therefore interesting for wind power projects. The transmission system in such areas may not have the sufficient capacity for additional power production, which calls for some kind of coordinated regulation between the hydro power producer and the wind power producer, in order to not produce more than can be transferred. A strategy for how to coordinate both power productions will avoid an expensive reinforcement of the net. In this thesis different strategies for coordinated regulation are studied and simulated using a linear programming model (LP). The major planning objective is to keep production goals and physically constraints of the power system. A model including objective function and constraints are presented for each strategy. Computational results are reported showing that both sides benefit from the coordination strategies and a better utilization of the transmission net. 

  • 55.
    Kaiser, Franz
    KTH, Superseded Departments, Electric Power Systems.
    Begränsningar och möjligheter för vindkraft på norra Gotland1999Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 56.
    Khayat, Andre
    KTH, Superseded Departments, Electric Power Systems.
    En förenklad kraftbalansmodell för det nordicska kraftsystemet2000Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 57.
    Kisitu, Robley
    KTH, Superseded Departments, Electric Power Systems.
    Identification of Lightly Damped Eigenvalue Clusters in Power Systems and Other Large Dynamical Systems2002Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 58.
    Kolessar, Rémy
    KTH, Superseded Departments, Electric Power Systems.
    Studies and circuit-oriented modeling of the power diode turn-on process1999Licentiate thesis, monograph (Other scientific)
  • 59.
    Kostina, Natalja
    KTH, Superseded Departments, Electric Power Systems.
    Dynamiska ekvivalenter för studier av Powerformer2000Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 60.
    Kovac, Tanja
    KTH, Superseded Departments, Electric Power Systems.
    Aerodynamisk optimering av Pitchreglerade vindturbiner2002Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 61.
    Krah, Julius Hartwig
    KTH, Superseded Departments, Electric Power Systems.
    A Study of Unusual Parameter Dependent Eigenvalues Movements1999Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 62.
    Kronman, Mikael
    KTH, Superseded Departments, Electric Power Systems.
    Styrning av stora effektflöden i ett expanderande storstadsnät2004Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 63.
    Kvarngren, Mats
    KTH, Superseded Departments, Electric Power Systems.
    Screening of sharp points in compact transmission towers1997Licentiate thesis, monograph (Other scientific)
  • 64.
    Larsson, Tomas
    KTH, Superseded Departments, Electric Power Systems.
    Voltage source converters for mitigation of flicker caused by arc furnaces1998Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    The thesis deals with flicker and how to mitigate it bymeans of a Voltage Source Converter (VSC). Flicker, orvariations in illumination intensity, is above a certain levelvery annoying. The background to flicker is voltage amplitudefluctuating in certain frequency intervals caused bytime-fluctuating loads. The thesis focuses on the electric arcfurnace which has been identified as one of the majorcontributors to flicker.

    State of the art- technology for flicker mitigation is theStatic Var Compensator (SVC) used for reactive power control.Also, a saturable reactor series connected with an arc furnacecuts load current peaks and thereby reduces voltage variationscausing flicker. Investigations have however shown that theperformance of this technology is in some cases not enough tomitigate the flicker to acceptable levels.

    A quasi steady-state approach has been used to find thatvoltage variations in the grid are due to both reactive andactive power variations of the load. Implementing apparatus forflicker mitigation, also the reactance of a transformer betweenthe connection point and the flicker sensitive point in thegrid has to be considered. It was shown that reactive powerovercompensation can be used to counter balante the voltagevariations caused by the active power. It was also shown thatvoltage components contributing to flicker are limited tosidebands of the fundamental voltage.

    Studing the voltage variations using instantaneous values ofcurrents and voltages created a new insight in the flickerprocess. Except for the instantaneous real and imaginarycurrents, also variations in the lime derivative of the realload current give rise to voltage amplitude modulation causingflicker.

    A suitable technology for flicker mitigation is the VSCoperated with Pulse Width Modulation (PWM). By using aswitching frequency in the order of 1- 2 kHz, an efficientmeans for flicker mitigation is obtained. Digital simulationswere performed with a VSC used for flicker mitigation in amodel of an art fumace installation to verify the flickermitigation algorithms. Investigations on measurement delay,switching frequency and compensator bandwidth were performedwith respect to flicker level. Studies in a real-time analoglow power model including an arc fumace and a VSC indicatedthat the earlier results were reliable.

    Keywords:flicker, mitigation, volmge source converter,pulse width modulation, PWM, am furnace, control, powerelectronics, analog model, saturable reactor, static varcompensator, SVC

  • 65.
    Latorre Sanchez, Hector Fabio
    KTH, Superseded Departments, Electric Power Systems.
    Thyrister controlled series capacitor and its application in the interconnection Colombia - Ecuador 230 kV2002Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 66.
    Lee, Denis Hau Aik
    KTH, Superseded Departments, Electric Power Systems.
    Voltage and power stability of HVDC systems1998Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    This research work had been motivated by the emergence of multiple HVDC links in close proximity within a power system area, such as those in Southem Scandinavia and Northem Europe. Such system tonfigurations, generally known as multi-infeed HVDC systems, are expected to exhibit voltage and power stability problems when one or more of the constit uent AUDC interconnections are weak. The primary objective of this work is therefore to study the voltage and power stability of HVDC systems under these situations, with an emphasis on multi-infeed configurations.

    In the tirst part of this work, the voltage and power stability of multi-infeed HVDC systems are studied from a quasi-static approach. For this purpose, existing analytical concepts and methods for weak single ACfDC interconnections have to be extended or new ones developed. A mathematital technique known as modal analysis is the basic analytical tool employed. Modal anal ysis has been successfully applied to the voltage stability analysis of AC power systems but its use for analysis of multi-infeed HVDC systems is unprecedented. In the tontext of multi-infeed HVDC systems, there are salient differentes such as the incorporation of voltage sensitivity and maximum power concepts which are pertinent to HVDC applications. Here, the application techniques of modal analysis are fully developed into a comprehensive tool suitable for the voltage/power stability analysis of multi-infreed HVDC systems. These include determining the voltage/power stability margins, criticality of system locations, and effectiveness of remedial measures with respect to multi-infeed HVDC systems. The developed techniques are applied to a practical power system and has shown to be feasible and suitable for use by the HVDC industry. Sinte the developed concepts and methods take root from those for the single ACIDC interconnection, the study also tompares them to show their fundamen tal similarities and differentes.

    Voltage stability and loads of power systems are closely intertwined. In the tontext of AUDC interconnections, a recent study had also shown the influence of static load charactetistics on the power stability of a specific HVDC system model. An original intent was to show these similar influences for multi-infeed HVDC systems but it was also essential to consider other system contigurations to cast the investigations into a more general and fundamental tontext. Thus HVDC system models representa tive of typical system tonfigurations are used, viz. a single-infeed, single-infeed with a parallel AC line, multi-infeed system configuration. It is shown in this work that static load characteristics have a consistent pattem of influence on the power/voltage stability of these system tonfigurations. It is thus possible to introduce general concepts of Active Load Characteristics Index (ALCI) and Load Characteristics Sensitivity (133) to estimate a bound for and characterize these influences. An important ob jective of these investigations relating to static load characteristics is to show that their effects do not invalidate the analytical concepts and methods developed for the multi-infeed HVDC system.

    In the second part of this work, voltage and power stability of HVDC systems are studied from a dynamit approach. For power stability, the impact of dynamit system modelling on the power stability limit of ACIDC interconnections is examined. Com mon industry practice assumes certain quasi-static conditions pertaining to the voltage control in the receiving AC system. It is shown that voltage dynamits has a significant impact on the power stability limit based on the quasi-static assumptions. Quasi static analysis also ascribes voltage instability of AUDC interconnections to the singularny of the powerflow Jacobian. This may be viewed from a nonlinear dynamital system perspective, as the system having undergone a saddle-node bifurcation. Consequently, an aim is to establish the relationship between the quasi-static and dynamit conditions for voltage instability via saddle-node bifurcation. Within the framework of nonlinear dynamital system theory, another possible mechanism of voltage instability is the Hopf bifurcation. This oscillatory voltage collapse mechanism is shown to occur in ACIDC systems. In these tontexts, fundamental conditions for existence of these nonlinear phenomena are derived analytically for the single-infeed HVDC system. The ultimate aim is to extend these results to investigate the nonlinear phenomena in multi-infeed HVDC sys tems.

  • 67.
    Lefevre, Louis
    KTH, Superseded Departments, Electric Power Systems.
    Design of line-start permanent magnet synchronous motors using analytical and finite element analysis2000Licentiate thesis, monograph (Other scientific)
  • 68. Lindgren, E.
    et al.
    Söder, Lennart
    KTH, Superseded Departments, Electric Power Systems.
    Minimizing regulation costs in multi-area markets2005In: 15th Power Systems Computation Conference, PSCC 2005, Power Systems Computation Conference , 2005Conference paper (Refereed)
    Abstract [en]

    In a power system with many fast and/or large changes in the generation or the consumption, for example in a system with large amounts of wind power, the frequency control is more complicated to handle efficiently. Minimizing regulation costs for the system operator in such a system requires the possibility to simulate the frequency control, but the existing methods for calculations are neither efficient enough with respect to computation time nor accurate enough. This paper shows how the frequency control can be optimized, thereby reducing the regulation costs, using a multi-area model combined with a DC load flow. The multi-area modeling is described and the optimization is applied to a numerical example.

  • 69.
    Lundberg, Petra
    KTH, Superseded Departments, Electric Power Systems.
    Belastningsfördelning i maskade lågspänningsnät1997Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    missing

  • 70.
    Lundgren, Anders
    KTH, Superseded Departments, Electric Power Systems.
    On measurement and modelling of 2D magnetization and magnetostriction of SiFe sheets1999Doctoral thesis, monograph (Other scientific)
  • 71.
    Löfvenberg, Per
    KTH, Superseded Departments, Electric Power Systems.
    Prognosering av elenergianvändning1999Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 72.
    Ma, Qiang
    KTH, Superseded Departments, Electric Power Systems.
    Dynamisk analys av vindkraftverk anslutna till stamnät2004Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 73.
    Madueno Rios, Tomas
    KTH, Superseded Departments, Electric Power Systems.
    En studie över effekten av att använda distribuerad generering i ett landsortsnät2002Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 74.
    Magalhães de Oliveira, Marcio
    KTH, Superseded Departments, Electric Power Systems.
    Power Electronics for Mitigation of Voltage Sags and Improved Control of AC Power Systems2000Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    The thesis deals with the application of compensators andswitches based on power electronics in AC transmission anddistribution systems. The objective of the studieddevices/equipment is the power flow and voltage control intransmission systems and the mitigation of voltage sags andmomentary interruptions to critical loads in distributionsystems.

    For validating the power electronics based devices/equipmentdescribed in the thesis, scaled models at a real-time simulatorhave been built. Simulation results of these models arepresented and discussed in the thesis.

    The equipment studied in the thesis exploit the fast controlactions that can be taken by power electronics devices, whichare much faster than the speed of conventional equipment andprotection systems, based on electromechanical devices. In thisway, the power quality of distribution systems is improved,regarding duration and magnitude of voltage sags (dips) andmomentary interruptions, which are the most relevant types ofdisturbances in distribution systems.

    The thesis presents some compensators based onforced-commutation voltage-source converters for correctingvoltage sags and swells to critical loads. The seriesconverter, usually denoted Dynamic Voltage Restorer (DVR), hasbeen proved suitable for the task of compensating voltage sagsin the supply network. The use of solid-state devices ascircuit breakers in distribution systems has also been studiedwith the objective of achieving fast interruption or limitationof fault currents. The location and practical aspects for theinstallation of these solid-state breakers are presented. Ithas beenshown that a configuration based on shunt and seriesconnected solid-state devices with controllable turn-offcapability can also provide voltage sag mitigation, without theneed of transformers and large energy storage elements.

    The operation and control of two Flexible AC TransmissionSystem (FACTS) devices for voltage and power flow control intransmission systems, namely the Static Synchronous Compensator(STATCOM) and the Unified Power Flow Controller (UPFC),respectively, are also studied. A faster response compared totraditional equipment consisting of mechanically based/switchedelements is then achieved. This allows a more flexible controlof power flow and a secure loading of transmission lines tolevels nearer to their thermal limits. The behaviour of thesedevices during faults in the transmission system is alsopresented. Keywords: power electronics, power quality, voltagesags, voltage-source converters, Custom Power, FACTS, real-timesimulations, solid-state devices.

  • 75.
    Magnusson, Niklas
    KTH, Superseded Departments, Electric Power Systems.
    AC losses in high-tempreture superconductiong tapes2000Doctoral thesis, comprehensive summary (Other scientific)
  • 76.
    Magnusson, Niklas
    et al.
    KTH, Superseded Departments, Electric Power Systems.
    Schönborg, Niclas
    KTH, Superseded Departments, Electrical Systems.
    Wolfbrandt, Anna
    KTH, Superseded Departments, Electrical Systems.
    Hörnfeldt, Sven
    KTH, Superseded Departments, Electrical Systems.
    Improved experimental set-up for calorimetric AC loss measurements on HTSs carrying transport currents in applied magnetic fields at variable temperatures2001In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 354, no 1-4, p. 197-201Article in journal (Refereed)
  • 77.
    Magnusson, Niklas
    et al.
    KTH, Superseded Departments, Electric Power Systems.
    Wolfbrandt, Anna
    KTH, Superseded Departments, Electrical Systems.
    AC losses in high-temperature superconducting tapes exposed to longitudinal magnetic fields2001In: Cryogenics (Guildford), ISSN 0011-2275, E-ISSN 1879-2235, Vol. 41, no 10, p. 721-724Article in journal (Refereed)
    Abstract [en]

    The AC losses in a high-temperature superconducting (HTS) tape were measured calorimetrically in longitudinal magnetic fields while carrying transport currents. The experimental data were compared to relevant loss equations based on the critical state model. Also, with no transport current, the longitudinal losses were compared to losses due to perpendicular and parallel magnetic fields. At high magnetic fields the AC losses in longitudinal fields were one-third of those in parallel field. These relatively low losses make the HTS tapes available today competitive to conventional conductors in special applications where the conductor is exposed to longitudinal magnetic fields.

  • 78.
    Maliti, Kelvin C.
    KTH, Superseded Departments, Electric Power Systems.
    Modeling and analysis of magnetic noise in squirre-cage induction motors2000Doctoral thesis, monograph (Other scientific)
  • 79.
    Maliti, Kelvin C.
    KTH, Superseded Departments, Electric Power Systems.
    On modelling of magnetic noise in inductionmotors1997Licentiate thesis, monograph (Other scientific)
  • 80.
    Malmquist, Anders
    KTH, Superseded Departments, Electric Power Systems.
    Analysis of a gas turbine driven hybrid drive system for heavy vehicles1999Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    The goal of this thesis has been to analyze the performanceand behavior of a gas turbine driven hybrid drive train. Thethesis covers both computer simulations and experimental tests.In two case studies, a number of measurements have been made ongas turbine driven hybrid vehicles that are developed by Volvoand ABB.

    In the recent years, much effort is currently put into thedesign and analysis of hybrid drive trains. Many studiesinvolve computer simulations, but they are often made on ageneral level. This thesis concentrate on gas turbine drivenhybrids for heavy vehicles, a field that has previously notbeen covered to a large extent in academic studies.

    A major contribution to the field of hybrid drive traindesign is the development of detailed simulation models thathave a close connection to hybrids that are actually built andtested. The access to detailed gas turbine data has furtherenhanced the possibility to design a dynamic model of the gasturbine driven and the electric circuits. The combination ofsimulations and extensive field experience gains new knowledgeon the properties of gas turbines in hybrid drive trains.

    Two simulation models have been developed in Matlab andSimulink. One is a quasi-steady state model that can be usedfor drive cycle simulations, e.g. a complete bus line. Theother is a transient model that combines the thermodynamicproperties of the gas turbine, the mechanical properties of thecombined turbine-generator shaft, the electric power circuitand the control system. The transient model has been used tosimulate the power response during accelerations andretardation.

    An analysis of the internal energy flows and the systemefficiency of a hybrid drive train contributes to theunderstanding of the properties of series hybrid drive trains.An important part of the topology is that the system is basedon a DC/DC-converter that is connected between the battery andthe DC-bus. It controls the DC-bus voltage and by this,indirectly, the speed of the gas turbine. With thisarrangements, an uncontrolled diode rectifier can rectify thegenerator output.

    The control of the output power requires increased knowledgeon the behavior of this apparently simple circuit: thepermanent magnet synchronous generator that is feeding a dioderectifier with additional shunt capacitors at the generatorterminals for reduction of generator current harmonics. Thiscircuit is highly non-linear and a circuit model hasestablished in Matlab and Power System Blockset for theevaluations of the circuit. The results of the circuitsimulations are important inputs to the transient model.

    Improved system efficiency is one of the key factors for acommercial success of the gas turbine series hybrids. In thelight of this, design alternatives with a flywheel or a supercapacitor energy storage have been studied. A conclusion isthat a straightforward approach with a controlled rectifier isthe best compromise from technological point of view.

    Keywords:battery, bus, diode, dc-converter,distribution truck, efficiency, emissions, EV, flywheel, gasturbine, generator, HEV, high-speed, hybrid, inverter, matrixconverter, optimization, permanent magnet, Power SystemBlockset, rectifier, simulation, super capacitor, ultracapacitor, vehicle model.

  • 81.
    Matevosyan, Julija
    KTH, Superseded Departments, Electric Power Systems.
    Automatisk spänningsreglering. Prestanda och utrustning för reaktiv effektgererering och jordning i olika elnät på Lidingö.2001Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 82.
    Maupas, Florent
    KTH, Superseded Departments, Electric Power Systems.
    Inpact of Voltage Control Equipment on the Network Intergrration of Large Offshore Wind Farms interconnected with HVAC2004Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 83.
    Montango G, Raul Enrique
    KTH, Superseded Departments, Electric Power Systems.
    Vindkraftens påverkan på elnätsförluster på Öland2001Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 84.
    Mugala, Gavita
    et al.
    KTH, Superseded Departments, Electrical Systems.
    Eriksson, Roland
    KTH, Superseded Departments, Electric Power Systems.
    Gäfvert, Uno
    ABB.
    High frequency characterization of the semi-conducting screens of medium voltage XLPE cables2002In: Electrical Insulation and Dielectric Phenomena, 2002 Annual Report Conference on, Cancun, 2002, p. 887-890Conference paper (Refereed)
    Abstract [en]

    In this study, a high frequency measurement technique to characterize the semi-conducting materials of the medium voltage XLPE cables in the frequency range of 30 kHz to 500 Mhz has been developed. The influence of the experimental set-up and the sample preparation methods are investigated. A dielectric response model is then developed for the semiconducting materials and this is incorporated into a model for the whole cable. The propagation characteristics obtained from the cable model are then compared with those obtained from measurements carried out on the actual XLPE cables.

  • 85.
    Mugala, Gavita
    et al.
    KTH, Superseded Departments, Electrical Systems.
    Eriksson, Roland
    KTH, Superseded Departments, Electric Power Systems.
    Gäfvert, Uno
    ABB.
    Pettersson, Per
    Vattenfall.
    Measurement technique for high frequency characterization of semi-conducting materials in extruded cables2004In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 11, no 3, p. 471-480Article in journal (Refereed)
    Abstract [en]

    Knowledge on the dependence of wave propagation characteristics on material properties and cable design is important in establishing diagnostic methods for cable insulation. In this study, a high frequency measurement technique to characterize the semi-conducting screens in medium voltage cross-linked polyethylene (XLPE) cables has been developed. The frequency ranges from 30 kHz to 500 MHz. The influence of the experimental set-up, sample preparation methods, pressure and temperature are investigated. A dielectric function is developed for the semiconducting screens and this is incorporated into a high frequency model for the cable. The propagation characteristics obtained from the high frequency cable model are compared with those obtained from measurements made on the same cables.

  • 86.
    Mugala, Gavita
    et al.
    KTH, Superseded Departments, Electrical Systems.
    Pettesson, Per
    Vattenfall.
    Eriksson, Roland
    KTH, Superseded Departments, Electric Power Systems.
    Development of a measurement technique for high frequency characterization of insulation materials2004In: 2004 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, NEW YORK: IEEE , 2004, p. 206-209Conference paper (Refereed)
    Abstract [en]

    A technique to measure the high frequency characteristics of insulation materials has been developed. Using this technique, high frequency characteristics of known insulation materials have been obtained to check its accuracy. The test cell, measurement technique and the extraction of the relevant parameters from the measurements are discussed.

  • 87. Neimanis, R.
    et al.
    Eriksson, Roland
    KTH, Superseded Departments, Electric Power Systems.
    Diagnosis of moisture in oil/paper distribution cables-part I: Estimation of moisture content using frequency-domain spectroscopy2004In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 19, no 1, p. 9-14Article in journal (Refereed)
    Abstract [en]

    Dielectric spectroscopy (dielectric response measurements) has been applied for nondestructive estimation of humidity in oil-paper cable insulation. The experiments have been based upon two field-aged cables, 20 and 50 years old. Paper samples from these cables have been characterized and subjected to environments with different relative humidity. Dielectric loss and capacitance have been measured in a frequency range 1 MHz to 1 kHz and related to the moisture content determined by Karl Fisher titration. A method has been verified where the moisture content is correlated to the minimum value of loss tangent (tan 6). A number of field measurements have been performed where the method has been applied to estimate the moisture content in the distribution cables.

  • 88.
    Nilsberth, Anders
    KTH, Superseded Departments, Electric Power Systems.
    Evaluation of electricity contracts on a deregulated market1999Licentiate thesis, monograph (Other scientific)
  • 89.
    Nilsson, Olof
    KTH, Superseded Departments, Electric Power Systems.
    Short term scheduling of hydrothermal power systems with integer hydro constraints1997Doctoral thesis, monograph (Other scientific)
  • 90.
    Nilsson, Oskar
    et al.
    KTH, Superseded Departments, Electric Power Systems.
    Söder, Lennart
    KTH, Superseded Departments.
    Integer modelling of spinning reserve requirements in short term scheduling of hydro systems: An application to power trading1998In: Proceedings of the 1997 4th International Conference on Advances in Power System Control, Operation and Management. Part 1 (of 2), 1998, no 450, p. 139-144Conference paper (Refereed)
    Abstract [en]

    This paper investigates consequences for the power system operation, if links are installed between Sweden and thermal dominated systems. We have applied a short term planning method to high, medium and low load scenarios. For each scenario we calculate the consequences for different combinations of extra power exchange and provision of spinning reserve. For the different cases we compare the operation cost, the possibility of using the links as intended and the number of start-ups of hydro units.

  • 91.
    Nilsson, Oskar
    et al.
    KTH, Superseded Departments, Electric Power Systems.
    Söder, Lennart
    KTH, Superseded Departments, Electric Power Systems.
    Sjelvgren, D.
    Integer modeling of spinning reserve requirements in short term scheduling of hydro systems1997In: IEEE Power Engineering Review, ISSN 0272-1724, E-ISSN 1558-1705, Vol. 17, no 9, p. 36-Article in journal (Refereed)
    Abstract [en]

    In short term power system scheduling, reserve margins are kept in order to maintain the security of the system if a nonforecasted event occurs. The security is maintained by redispatch of the generators in the system. During the first seconds of the disturbance it is only possible to redispatch the online units. The maximal increase of the generation during these first seconds of the disturbance is called the spinning reserve. In hydro dominated power systems the spinning reserve is kept in the hydro system, since hydro units are easy to redispatch. For an online hydro unit the spinning reserve is the marginal between maximum generation and the actual generation. If a hydro plant has several units, only the online units will contribute to the spinning reserve. This means that we need to introduce integer variables to model the spinning reserve contribution from a hydro plant with several units. In the literature there are several examples of integer models for representation of the generation of hydro plants. The contribution of this paper is that it incorporates the integer representation of the spinning reserve into an integer model of generation scheduling. To this model we apply variable splitting and Lagrange relaxation to decompose the problem into one subproblem for the hydrological constraints and one subproblem for each hydro plant. In order to find feasible solutions we use a combination of network programming and heuristic search. The dual problem constructed by the decomposition is solved by a subgradient method. In the paper we apply the proposed method to a numerical example.

  • 92.
    Nipp, Eckart
    KTH, Superseded Departments, Electric Power Systems.
    Permanent magnet motor drives with switched stator windings1999Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    Permanent magnet (PM) motors aretoday regarded as aninteresting solution for a wide range ofinverter-fedvariable-speed drives. The generally increasedinterest in these motors hasled to many investigations on theirfeasibility for vehicle propulsion.Consequently, they are alsoof interest for traction applications which led tothe researchproject that is presented in this thesis. The mostimportantadvantages that are expected in comparison to thestate of the artasynchronous motors are lower losses and ahigher torque density. Often thefield weakening speed range isimportant, but difficult to obtain withPM machines because theinductance in the direction of the magnetization tendsto below. An alternative can be to switch different coils groups ofthestator winding into different configurations. This is thecentral topic ofthis thesis.

    Various aspects of the design of PM motor drives areconsidered withspecial attention to the requirements for theapplication of the switchedwinding concept. The studies werethereby limited to the inner rotor, radialflux topology. It wasfound that two winding parts per phase, implying fourdifferentwinding connections, is the only interesting solution. Anadvantageof switched windings is that the internal voltage ofthe machine will neverexceed the maximum inverter output, whichincreases the operation safety.Furthermore the machine designcan uncompromisingly be optimized for operationbelow basespeed, which means low inductances implying a large airgaplengthand thick magnets. A problem with switched windings isthat circulatingcurrents can occur. To diminish them, a 2/3magnet covering of the polesurface must be chosen incombination with a non-salient rotor. Moreover itwas found thatthe eddy current losses in the magnets can reachnon-negligiblelevels and must be considered when designing adrive system.

    The major drawback of switched stator windings is probablythe occurrenceof torque interruptions when changing theconfiguration. The durations ofthese zero torque periods andtheir avoidance have been examined thoroughly inthis thesis bymeans of simulations and experiments on a specially designedandbuilt 50kW test machine. It was found that semi-conductorwinding switchesshould be used to avoid noticeable jerks. Anovel method is suggested on howto perform windingreconfigurations so that torque interruptions are avoided.Theprinciple is based on temporary operation of the machine withits windingsin asymmetric configurations which is analyzedanalytically and validated byexperiments. It is proven thatsuccessive winding switching is an appropriatemeans to entirelyomit torque interruptions.

    Finally, the application of PM motors with switched statorwindings in asubway propulsion drive is briefly considered.Different aspects of theirfeasibility are taken intoconsideration and the great potential of suchdrives is pointedout.

    Keywords:Permanent magnet synchronous motors, tractionpropulsion, inverter supply, circulating currents, variablespeed, torque interruptions, switched stator windings,successive winding reconfigurations, fieldweakening, contactorsvs. thyristor switches, permanent magnet motor design, windingreconfiguration tests, magnet losses

  • 93.
    Norgren, Elisabet
    KTH, Superseded Departments, Electric Power Systems.
    Modelling av nätvärn i belastningsfördelningsberäkningar1996Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Missing

  • 94. Noroozian, M.
    et al.
    Ghandhari, Mehrdad
    KTH, Superseded Departments, Electric Power Systems.
    Andersson, Göran
    KTH, Superseded Departments, Electric Power Systems.
    Grönquist, J.
    Hiskens, I.
    A robust control strategy for shunt and series reactive compensators to damp electromechanical oscillations2001In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 16, no 4, p. 812-817Article in journal (Refereed)
    Abstract [en]

    This paper examines the enhancement of power system stability properties by use of thyristor controlled series capacitors (TCSCs) and static var systems (SVCs). Models suitable for incorporation in dynamic simulation programs used to study angle stability are analyzed. A control strategy for damping of electromechanical power oscillations using an energy function method is derived. Using this control strategy each device (TCSC and SVC) will contribute to the damping of power swings without deteriorating the effect of the other power oscillation damping (POD) devices. The damping effect is robust with respect to loading condition, fault location and network structure. Furthermore, the control inputs are based on local signals. The effectiveness of the controls are demonstrated for model power systems.

  • 95. Noroozian, M.
    et al.
    Ängquist, L.
    Ghandhari, Mehrdad
    KTH, Superseded Departments, Electric Power Systems.
    Andersson, Göran
    KTH, Superseded Departments, Electric Power Systems.
    Use of UPFC for optimal power flow control1997In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 12, no 4, p. 1629-1634Article in journal (Refereed)
    Abstract [en]

    This paper deals with optimal power flow control in electric power systems by use of unified power flow controller (UPFC). Models suitable for incorporation in power flow programs are developed and analysed. The application of UPFC for optimal power flow control is demonstrated through numerical examples. It is shown that a UPFC has the capability of regulating the power flow and minimising the power losses simultaneously. An algorithm is proposed for determining the optimum size of UPFC for power flow applications. The performance of UPFC is compared with that of a phase shifting transformer (PST).

  • 96.
    Nyberg, Marie
    KTH, Superseded Departments, Electric Power Systems.
    Analys av tillförlitligheten hos stamnätets objekt2004Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 97.
    Olofsson, Magnus
    KTH, Superseded Departments, Electric Power Systems.
    Optimal Operation of the Swedish Railway Electrical system: An Application of Optimal power Flow1996Doctoral thesis, monograph (Other academic)
  • 98.
    Olofsson, Magnus
    et al.
    KTH, Superseded Departments, Electric Power Systems.
    Andersson, Göran
    KTH, Superseded Departments, Electric Power Systems.
    Söder, Lennart
    KTH, Superseded Departments, Electric Power Systems.
    Optimal operation of the Swedish railway electrical system1995In: Proceedings of the International Conference on Electric Railways in a United Europe, 1995, no 405, p. 64-68Conference paper (Refereed)
    Abstract [en]

    Along some railway lines in Sweden, 130 kV transmission lines have been built and connected to the contact lines through transformers. With the present characteristic of the frequency converters, injected active power cannot be controlled. To overcome this drawback, control of the static converter active power is studied. With this control, the active power injections can be adjusted to minimize the operation cost of the system. To calculate the optimal active power settings, an extended linear programming based optimal power flow algorithm has been tested. In this article, a brief description of the problem of optimizing the operation of the Swedish system is given. To demonstrate the different parts of the problem, examples are given.

  • 99.
    Olofsson, Magnus
    et al.
    KTH, Superseded Departments, Electric Power Systems.
    Andersson, Göran
    KTH, Superseded Departments, Electric Power Systems.
    Söder, Lennart H.
    KTH, Superseded Departments, Electric Power Systems.
    Linear programming based optimal power flow using second order sensitivities1995In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 10, no 3, p. 1691-1697Article in journal (Refereed)
    Abstract [en]

    For the electric railway traction in Sweden, a system of 130 kV transmission lines connected parallel to the contact lines is being build. Within this system, the generation costs differs only slightly between different nodes with power injections. In order to optimize the power flow in the system, an extended optimal power flow algorithm based on successive linear programming, has been developed. To improve the convergence, a method based on a second order sensitivity approximation of the active power losses of the total system is applied. Numerical examples are given to show the enhanced convergence properties.

  • 100. Olsson, M.
    et al.
    Söder, Lennart
    KTH, Superseded Departments, Electric Power Systems.
    Optimal regulating market bidding strategies in hydropower systems2005In: 15th Power Systems Computation Conference, PSCC 2005, Power Systems Computation Conference , 2005Conference paper (Refereed)
    Abstract [en]

    The transmission system operator (TSO) is responsible for keeping the balance between production and consumption in the power system. In order to do so, the TSO continuously trades power on the so-called regulating market. Hydropower is a flexible power source well suited for trading on the regulating market because it allows fast changes in generation output. This paper describes an optimization model for creating optimal bidding strategies for the regulating market from a hydropower producer perspective. The model is based on nonlinear stochastic programming, where the regulating market prices are considered stochastic variables. The model also includes generation of regulating power price scenarios and scenario tree construction.

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