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  • 51.
    Ayddan, Jonas
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Özbek, Emin
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Transformer test bench -implementation and usability2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The task of this master's thesis was to design a transformer test bench while focusing on safety and usability. There are several safety risks when working with high voltages, therefore the high demand for safety precautions.

    A bench was worked upon, and attachments, such as module frames

    and banana connectors were integrated into the bench. Further, a list of

    components were modied and installed on the new bench. A new design,

    using a magnetic non-contact safety interlock switch and an easy-to-apply circuitry, was proposed and implemented. In addition to the implementation,

    a code in LabVIEW was written in order to handle the measured data

    from the tests that can be performed on a transformer. Several tests were

    performed on a single-phase transformer and a three-phase transformer in

    order to verify the function of the transformer test bench, with respect to

    the design. Complications emerged due to random additive noise. After some

    work, the noise was suppressed and the results showed that the equipment

    worked well with the new design.

    Finally, a user's manual was written in order to guide the user through the

    wire connections, tests and the developed software in LabVIEW.

  • 52.
    Babu, Sajeesh
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Reliability Evaluation of Distribution ArchitecturesConsidering Failure Modes and Correlated Events2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Power systems are in a phase of automation where intelligent components and sub-systems are employed to monitor, control and manage the grid. Quantifying the complex consequences on system reliability, from the integration of such automated and semi-automated equipment into the existing grid is important for maintenance optimisation and fault mitigation. This thesis identifies the  advanced approaches in power system reliability analysis with the potential to capture the complications and correlations in modern power grid after reviewing the traditional reliability evaluation methods.

    A method for modelling the different modes of failures, possible in a substation and feeder architecture along with the probable false tripping scenarios was developed. An improved Reliability Block Diagram based approach was designed to count in the traditionally unaccounted failure cases affecting both the primary grid and the protectionand control equipment. The effect and corresponding trend of additional feeder lines in a radial distribution system on the net interruption rate experienced at load ends of feeders are derived and modelled. Such real-world substation architectures are analysed and the aforementioned trends are compared with those from the practical grid. Thus, the analysis was able to identify and measure the complex hidden failure probabilities due to both unwanted operation of breakers and functional failure of protection systems.

    The measured probabilities were used to calculate the impact of protection and control equipment on system  reliability. The obtained results were verified by comparing it with the observations by energy researchers on ten years of protection system failure statistics. The application of the model and results, in optimal maintenance planning and power network optimisation are identified as the next step.

  • 53.
    Babu, Sajeesh
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Reliability Evaluation of Distribution Systems Considering Failure Modes and Network Configuration2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Power distribution networks are recognized as the constituent part of power systems with the highest concentration of failure events. Even though the faults in distribution networks have a local effect when compared to the generation and transmission sides, major contingency escalation events are being more frequently reported from this section. The various aspects regarding the reliability and performance of distribution networks are identified as an important topic. Integration of new technologies, automation and increased penetration of distributed generation is expected to make improving and even sustaining high reliability standards a complex task.

     

    This thesis presents developed approaches to quantify and analyze the complex correlated failure probabilities of different failure modes in distribution networks. A theoretical simulation model that relates to real world data to measure false tripping probabilities is developed and tested. More simplified approaches that utilities can exercise with readily available data in fault registers are also established.  Optimal configurations that could improve system performance and respective investment costs are analyzed and savings in system reliability at the cost of grid investments are modelled. The optimization helps in prioritizing the most critical investments by considering the system impact of reconfigurations focusing on meeting customer demands and respecting transfer capacities of weak links. The value of existing networks and willingness of the grid owner in investing can be integrated into suggestive alterations to assist decision making in planning and maintenance allocation.

     

    The thesis makes both system specific and generalizable observations from detailed data collection from power utilities. The observations and results have potential in aiding future research by giving important understanding of the reliability impacts of network structures and of control and protection equipment.

  • 54.
    Babu, Sajeesh
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    System Reliability CentredDistribution Network Reconfiguration Considering Investment and Outage CostManuscript (preprint) (Other academic)
  • 55.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Reliability and Sensitivity Analysis of Substation and Feeder DesignConsidering Correlated Failures2017Conference paper (Refereed)
  • 56.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jürgensen, Jan Henning
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the status of reliability studies involving primary and secondary equipment applied to power system2014In: 2014 International Conference on Probabilistic Methods Applied to Power Systems, PMAPS 2014 - Conference Proceedings, IEEE , 2014, p. 6960653-Conference paper (Refereed)
    Abstract [en]

    Growth in infrastructure and energy utilization consistently put forward the demand for added quality and quantity of electric power. Reliability concerns over power systems are widespread within its different associated divisions like 'primary' power system structure, protection system, control equipment, ICT (Information and Communication Technologies) etc. This paper is a review of the present status of practices regarding reliability analysis in these divisions and works towards collectively assessing some of the studies in the respective areas. The idea of integrating reliability analysis from the above areas is introduced along with pointing out the major challenges associated. A set of tools for operators to make use in these reliability evaluations and modelling are mentioned. The earlier attempts towards combined overall system reliability analysis are discussed and the approach in this regard with the help of 'control functions' is emphasised. The paper includes works dealing with theory, different methodologies and data associated with power system reliability.

  • 57.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Shayesteh, Ebrahim
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Enarsson, Lars Einar
    Reliability Evaluation of Distribution Structures Considering the Presence of False Trips2018In: IEEE Transactions on Smart Grid, ISSN 1949-3053, E-ISSN 1949-3061Article in journal (Refereed)
    Abstract [en]

    This paper presents a method for modelling the different modes of failures in a substation and feeder architecture along with updating the possible false tripping scenarios in it. A traditional approach to collectively assess the failure modes using reliability block diagram is reviewed, and the method is updated to count in the unaccounted false tripping scenarios. A generalizable radial feeder branching structure is adopted and the effect of total feeder length and number of feeders from each busbar is examined and modelled with the help of the updated reliability block diagram. The modelled trends are also studied from real-world substation architectures. Thus, the analysis attains an improved estimation of the complex hidden failure probabilities combining theoretical and practical models.

  • 58.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jürgensen, Jan Henning
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Tjernberg, Lina Bertling
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Analyses of Smart Grid Technologies and Solutions from a System Perspective2015In: Smart Grid Technologies - Asia (ISGT ASIA), 2015 IEEE Innovative, IEEE conference proceedings, 2015, p. 1-5, article id 7387089Conference paper (Refereed)
    Abstract [en]

    This paper consolidates the data, analysis andobservations from a case study conducted in cooperation withthe Smart Grid Gotland project. The analysis identifies howelectrical power consumption interacts with distributedelectricity generation such as wind and solar power andpresents how it correlates to weather data and smart gridsolutions. The analysis model developed based on the Gotlandnetwork is generic and hence can be functional in investigatingother power networks of different size, voltage level andstructures. The key observations from the study of smart gridsolutions such as dynamic load capacity and energy storagesolutions are specified. Based on the project, an overview offuture risks and opportunities of smart grid systems is presented.

  • 59.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Shayesteh, Ebrahim
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Analysing Correlated Events in Power System Using Fault Statistics2016In: Probabilistic Methods Applied to Power Systems (PMAPS), 2016 International Conference on, IEEE, 2016Conference paper (Other academic)
    Abstract [en]

    Power system automation requires logical presumptions made on practical grids to correctly comprehend and manage complex and correlated faults occurring in real world systems. Traditional grid fault analysis methods lack in-depth understanding of these complex events and demand development of approaches that make use of available data to address this problem. Here, the traditional classification approach and challenges relating control equipment in power system are reviewed and a method observing the affected customers during faults along with grid design is discussed based on Swedish case study data. Various contrasting observations are made on the data recorded over two time periods to understand the trend developing over years. Moreover, it will be shown that the classification method also has potential in identifying weak spots in the grid when it comes to the reliability of control equipment.

  • 60. Bach, V.
    et al.
    Frohlich, J.
    Jonsson, B. Lars G.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bogolubov-Hartree-Fock mean field theory for neutron stars and other systems with attractive interactions2009In: Journal of Mathematical Physics, ISSN 0022-2488, E-ISSN 1089-7658, Vol. 50, no 10Article in journal (Refereed)
    Abstract [en]

    A simplification of the Bogolubov-Hartree-Fock theory, which is a natural generalization of the traditional Hartree-Fock theory, is derived. This simplification allows to express the pairing interaction in terms of the one-particle density matrix for systems interacting by attractive pair potentials, such as the Newtonian gravitational potential.

  • 61. Bahramirad, Shay
    et al.
    Khodaei, Amin
    Matevson, Julia
    Li, Zuyi
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Passo, Essa Aleksi
    Fotuhi-Firuzabad, Mahmud
    Special Section on Asset Management in Smart Grid2015In: IEEE Transactions on Smart Grid, ISSN 1949-3053, E-ISSN 1949-3061, Vol. 6, no 2, p. 953-954Article in journal (Other academic)
  • 62. Balram, Pavam
    et al.
    Tuan, Le Anh
    Bertling Tjernberg, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Centralized charging control of plug-in electric vehicles and effects on day-ahead electricity market price2015In: Plug In Electric Vehicles in Smart Grids, Springer, 2015, p. 267-299Chapter in book (Refereed)
    Abstract [en]

    Global policy targets to reduce greenhouse gas emissions have led to increased interest in plug-in electric vehicles (PEV) and their integration into the electricity network. Existing electricity markets, however, are not well suited to encourage direct participation of flexible demand from small consumers such as PEV owners. The introduction of an aggregator agent with the functions of gathering, aggregating, controlling and representing the energy needs of PEV owners in the electricity market could prove useful in this regard. In this chapter, a mathematical model of PEV aggregator for participation in the day-ahead electricity market is described. The modeling is done by treating each of the individual vehicle batteries as a single large battery. The centralized charging and discharging of this battery is then scheduled based on the traveling needs of the PEV owners determined by an aggregated driving profile and the cumulative electrical energy needs of vehicles over the optimization horizon. Two methods for scheduling PEV demand named as joint scheduling method (JSM) and aggregator scheduling method (ASM) are presented. The two methods are subsequently used to observe the effects of introducing flexible scheduling of PEVs on the day-ahead market price in an IEEE test system and a Nordic test system. Results from the IEEE test system case studies will indicate that the scheduling of PEV energy through direct centralized control at high PEV penetration levels of 50 % or greater could lead to potential lowering of day-ahead market prices as compared to an indirect control method such as the use of fixed period charging. Results from the Nordic test system case study shows that controlled scheduling of PEV demand could lead to only a small increase in day-ahead market price of electricity.

  • 63.
    Bangalore, Pramod
    et al.
    Chalmers.
    Bertling Tjernberg, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Condition Monitoring and Asset Management in the Smart Grid2016In: Condition Monitoring and Asset Management in the Smart Grid, John Wiley & Sons, 2016, p. 1-13Chapter in book (Other academic)
    Abstract [en]

    One of the main characteristics of a smart grid is the availability of large volumes of data, for example, gained from sensors. This data can be utilized as a tool to estimate the state of the system as a whole or any component within the system. In order to achieve actionable information from the variety of data that is available from smart grids, it is important to use the correct mathematical and signal processing tools. Furthermore, the future smart grid is expected to have high levels of reliability. This can be achieved by integrating the condition monitoring systems with maintenance management, wherein the focus is shifted from corrective maintenance to predictive condition-based maintenance.

    This chapter introduces the concept of reliability-centered asset management (RCAM). The RCAM approach provides the possibility of both qualitative and quantitative analysis toward optimal maintenance strategy. Furthermore, various issues with condition monitoring in smart grids have been discussed along with some literature that suggest possible solutions for these issues. Finally, a detailed case study of a data-based condition monitoring method based on artificial neural network is presented to demonstrate one of the many possibilities to use data from various measurement systems to reach actionable decisions.

  • 64.
    Bangalore, Pramod
    et al.
    Chalmers.
    Bertling Tjernberg, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Self Evolving Neural Network Based Algorithm for Fault Prognosis in Wind Turbines: A Case Study2014In: International Conference on Probabilistic Methods Applied to Power Systems (PMAPS), 2014Conference paper (Refereed)
    Abstract [en]

    Asset management of wind turbines has gained increased importance in recent years. High maintenance cost and longer downtimes of wind turbines have led to research in methods to optimize maintenance activities. Condition monitoring systems have proven to be a useful tool towards aiding maintenance management of wind turbines. Methods using Supervisory Control and Data Acquisition (SCADA) system along with artificial intelligence (AI) methods have been developed to monitor the condition of wind turbine components. Various researchers have presented different artificial neural network (ANN) based models for condition monitoring of components in a wind turbine. This paper presents an application of the approach to decide and update the training data set needed to create an accurate ANN model. A case study with SCADA data from a real wind turbine has been presented. The results show that due to a major maintenance activity, like replacement of component, the ANN model has to be re-trained. The results show that application of the proposed approach makes it possible to update and re-train the ANN model. 

  • 65. Bangalore, Pramod
    et al.
    Tjernberg, Lina Bertling
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    An Artificial Neural Network Approach for Early Fault Detection of Gearbox Bearings2015In: IEEE Transactions on Smart Grid, ISSN 1949-3053, E-ISSN 1949-3061, Vol. 6, no 2, p. 980-987Article in journal (Refereed)
    Abstract [en]

    Gearbox has proven to be a major contributor toward downtime in wind turbines. The majority of failures in the gearbox originate from the gearbox bearings. An early indication of possible wear and tear in the gearbox bearings may be used for effective predictive maintenance, thereby reducing the overall cost of maintenance. This paper introduces a self-evolving maintenance scheduler framework for maintenance management of wind turbines. Furthermore, an artificial neural network (ANN)-based condition monitoring approach using data from supervisory control and data acquisition system is proposed. The ANN-based condition monitoring approach is applied to gearbox bearings with real data from onshore wind turbines, rated 2 MW, and located in the south of Sweden. The results demonstrate that the proposed ANN-based condition monitoring approach is capable of indicating severe damage in the components being monitored in advance.

  • 66. Bantavis, P. I.
    et al.
    Kolitsidas, Christos
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jonsson, B. Lars G.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Empliouk, T.
    Kyriacou, G. A.
    A Wideband Switched Beam Antenna System for 5G Femtocell Applications2017In: 2017 IEEE Antennas and Propagation Society International Symposium, Proceedings, Institute of Electrical and Electronics Engineers (IEEE), 2017, p. 929-930Conference paper (Refereed)
    Abstract [en]

    This work introduces a wideband switched beam system for femtocell 5G base stations. The system consists of a 4 x 1 Vivaldi linear array and a 4 x 4 Butler matrix able to operate from 1.9-5.1 GHz. A soft surface is introduced along the outer edges of the vivaldi elements of the array for side lobes and back radiation suppression.

  • 67. Bao, Fanglin
    et al.
    Evans, Julian S.
    Fang, Maodong
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. Zhejiang University, China; South China Normal University, China .
    Inhomogeneity-related cutoff dependence of the Casimir energy and stress2016In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 93, no 1, article id 013824Article in journal (Refereed)
    Abstract [en]

    The Casimir stress and energy density are widely used to study the Casimir force, but they diverge in inhomogeneous systems, making the force seem to be infinite as well. Here we characterize the asymptotic behavior of the Casimir stress and energy density in inhomogeneous systems. We show an unambiguous map of all cutoff-dependent terms in the asymptotic expansion to the pressure and the surface tension through the insertion of multiple boundary layers. This result reveals an elegant subtraction to retrieve a finite stress that yields a cutoff-independent force.

  • 68. Bao, Fanglin
    et al.
    Luo, Bin
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. KTH, School of Information and Communication Technology (ICT), Centres, Zhejiang-KTH Joint Research Center of Photonics, JORCEP.
    First-order correction to the Casimir force within an inhomogeneous medium2015In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 91, no 6, article id 063810Article in journal (Refereed)
    Abstract [en]

    For the Casimir piston filled with an inhomogeneous medium, we regularized and expressed the Casimir energy with cylinder kernel coefficients by using the first-order perturbation theory. When the refractive index of the medium is smoothly inhomogeneous (i.e., derivatives of all orders exist), a logarithmically cutoff-dependent term and a quadratically cutoff-dependent term in the Casimir energy are found. We show that in the piston model these terms vanish in the force and thus the Casimir force is always cutoff independent, but these terms will remain in the force in the half-space model and must be removed by additional regularizations. We give explicit benchmark solutions to the first-order corrections of both Casimir energy and Casimir force for an exponentially decaying profile. The present method can be extended to other inhomogeneous profiles. Our results should be useful for future relevant calculations and experimental studies.

  • 69.
    Bayle, Valerian
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Definition of New Critical Distances for Transformers Re-energization during a Black Start2017Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This study aims to find new critical distances for transformer re-energization during network

    restoration after a blackout.

    Energizing a power transformer, in a grid that is lightly loaded due to the restoration process, involves

    a risk of temporary harmonic overvoltage. The overvoltages can, if they exceed a certain limit for a

    certain time, be harmful for the components of the grid. The rule which already exists at RTE states

    that there is need of a simulation study if the distance between the transformer to be energized and

    the closest load exceeds a certain length, depending on the transformer type.

    However, the critical distances already used are not satisfying, because in most cases the overvoltages

    are far from being dangerous, even for longer distances. This fact is confirmed in the study of a real

    restoration configuration in the thesis. Moreover, the analysis of the studies that have led to the critical

    distances shows that some of their modelling assumptions are questionable. The auxiliaries of the

    nuclear unit, in particular, have to be modelled to obtain correct results.

    Taking into account these remarks, new critical distances have then been determined. For this

    determination the models are chosen to be as conservative as possible. The new critical distances now

    depend on the type of nuclear unit and on the rating of the transformers to be energized.

  • 70.
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Glow corona discharges and their effect on lightning attachment: Revisited2012In: Lightning Protection (ICLP), 2012 International Conference on, IEEE , 2012, p. 6344402-Conference paper (Refereed)
    Abstract [en]

    Previous studies in the literature have suggested that glow corona discharges could be potentially used to control the frequency of lightning flashes to grounded objects. Such studies use simplified one-dimensional corona drift models or basic empirical equations derived from high voltage experiments to assess the effect of glow corona on the initiation of both streamers and upward connecting leaders under the influence of a descending lightning leader. In order to revisit the theoretical basis of these studies, a two-dimensional glow corona drift model has been implemented together with a self-consistent upward leader inception and propagation model -SLIM-. A 60 m tall lightning rod is used as a study case. It is found that the shielding effect of the glow corona space charge has been strongly overestimated in the literature. Furthermore, it is shown that streamers under the influence of a descending leader are initiated significantly earlier from the cylindrical body rather than from the corona-emitting area of the rod. Considering the effective shielding potential of glow corona, it is also shown that the presence of glow corona reduces the downward lightning attractiveness of 60 m tall lightning rods by less than 15%. This result shows that the efficiency of lightning rods is not strongly influenced by the generation of glow corona as opposed to the suggestions of previous studies.

  • 71.
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Self-consistent leader inception and propagation model —SLIM—: Response to some criticism2012In: Proceedings of the International Conference on Lightning Protection (ICLP), 2012, IEEE conference proceedings, 2012, p. 1-7Conference paper (Refereed)
    Abstract [en]

    The self-consistent leader inception and propagation model -SLIM- has been increasingly used to study the attachment of lightning flashes to grounded structures. However, criticism about some assumptions of SLIM has been raised in a recent publication. This paper intends to reply to this criticism and to openly discuss the validity of the model. Moreover, solid scientific evidence from theory and experiments is presented to support the validity of the assumptions considered by SLIM. It is shown that the streamer corona charge calculation and the leader inception condition used by SLIM give proper estimates which are completely justified. In addition, it is shown that the predictions of this state-of-the-art model based on the latest developments in the physics of leader discharges has been found in agreement with results available from laboratory, rocket triggered lightning experiments and field observations.

  • 72.
    Becerra Garcia, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Vazquez, Pedro A.
    Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane2017In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 29, no 12, article id 123605Article in journal (Refereed)
    Abstract [en]

    This paper presents a self-consistent model of electrohydrodynamic (EHD) laminar plumes produced by electron injection from ultra-sharp needle tips in cyclohexane. Since the density of electrons injected into the liquid is well described by the Fowler-Nordheim field emission theory, the injection law is not assumed. Furthermore, the generation of electrons in cyclohexane and their conversion into negative ions is included in the analysis. Detailed steady-state characteristics of EHD plumes under weak injection and space-charge limited injection are studied. It is found that the plume characteristics far from both electrodes and under weak injection can be accurately described with an asymptotic simplified solution proposed by Vazquez et al. ["Dynamics of electrohydrodynamic laminar plumes: Scaling analysis and integral model," Phys. Fluids 12, 2809 (2000)] when the correct longitudinal electric field distribution and liquid velocity radial profile are used as input. However, this asymptotic solution deviates from the self-consistently calculated plume parameters under space-charge limited injection since it neglects the radial variations of the electric field produced by a high-density charged core. In addition, no significant differences in the model estimates of the plume are found when the simulations are obtained either with the finite element method or with a diffusion-free particle method. It is shown that the model also enables the calculation of the current-voltage characteristic of EHD laminar plumes produced by electron field emission, with good agreement with measured values reported in the literature.

  • 73.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB Corporate Research, Power Technologies, Electrical Apparatus and Diagnostics group, Sweden .
    Corona discharges and their effect on lightning attachment revisited: Upward leader initiation and downward leader interception2014In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 149, p. 316-323Article in journal (Refereed)
    Abstract [en]

    Previous studies have suggested the possibility of using glow corona discharges to control the frequency of lightning flashes to grounded objects. In order to revisit the theoretical basis of this proposal, the self-consistent leader inception and propagation model - SLIM - is used together with a two-dimensional glow corona drift model. The analysis is performed to quantify the effect of glow corona generated at the tip of ground-based objects on the initiation and propagation of upward positive connecting leaders under the influence of downward lightning leaders. It is found that the presence of glow corona does not influence the performance of Franklin lightning rods shorter than 15 m, while it slightly reduces the lateral distance of rods up to 60 m tall by a maximum of 10%. Furthermore, the results indicate that it is not possible to suppress the initiation of upward connecting leaders by means of glow corona. It is found instead that unconventional lightning protection systems based on the generation of glow corona attract downward lightning flashes in a similar way as a standard lightning rod with the same height.

  • 74.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Glow corona generation and streamer inception at the tip of grounded objects during thunderstorms: revisited2013In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 46, no 13, p. 135205-Article in journal (Refereed)
    Abstract [en]

    The initiation of streamers prior to a lightning strike can be reportedly inhibited by glow corona discharges generated from tall objects. In contrast to previous studies based on a simplified one-dimensional model of glow corona, a two-dimensional evaluation of the corona ion drift from tall objects is used here to analyse this effect quantitatively. Proper estimates for the corona space charge distribution generated during both the charging process of a thundercloud and the descent of the downward stepped leader are thus calculated. It is found that the shielding effect of the corona space charge on the streamer inception is not as severe as previously reported. Estimations of the effective height of the downward leader tip at which streamer inception takes place are presented and discussed for lightning rods and dissipation array systems.

  • 75.
    Becerra, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the Estimation of the Charge of Positive Streamers Propagating in Air2014In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 21, no 2, p. 627-634Article in journal (Refereed)
    Abstract [en]

    Streamer discharges are an important breakdown mechanism in air-based electrical insulation systems. This paper introduces a method to estimate the spatial distribution of the charge density of positive streamers in air, based on the solution of a Poisson inverse problem by optimization. In contrast to other methods, it does not require tuning parameters and can also be used in configurations including dielectric interfaces or preexisting space charge. Three different experimental datasets reported in the literature are used to validate the method. Good agreement between the measurements and the predictions of the method is found.

  • 76.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Friberg, Andreas
    ABB Corporate Research, Västerås.
    Arc jets blown by outgassing polymers in air2014In: Proceedings of the 20th International Conference on Gas Discharges and Their Applications GD2014, GD2014 , 2014, p. 1-4Conference paper (Refereed)
    Abstract [en]

    This paper describes experimental results about the behaviour of arc jets transversely blown in the presence of outgassing polymers (POM –CH2O– or PMMA –C5H8O2–). The arc jets are ignited in air between copper electrodes under a 2 kA, 50 Hz AC current. High speed photography and optical emission spectroscopy are used to study the mechanism leading to the increase of the arc voltage when polymers are used instead of non-ablating materials (e.g. quartz). It is found that the transversal blowing flow caused by the injection of ablation vapours have a weak effect on the arc voltage build-up. Instead, the chemical changes in the plasma environment appear to better explain the observed increase in the arc voltage when polymers are used. 

  • 77.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Electrohydrodynamic motion due to space-charge limited injection of charges in cyclohexane2014In: Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids, ICDL 2014, 2014Conference paper (Refereed)
    Abstract [en]

    A self-consistent numerical model is presented in order to study the electrohydrodynamic (EHD) motion generated by a stationary, space-charge limited injection of charge in the point-plane geometry. In this multiphysics model, the continuity equations for charge carriers and Poisson's equation are coupled with Navier-Stokes equations and the heat equation. This model is used to study the EHD motion of cyclohexane in the negative point-plane geometry for sharp points with tip radius of 0.2 μm. It is shown that the injection of charges from a very sharp point electrode results in the formation of a thin plume with high liquid velocity. The results show large differences in the liquid velocity close to the point electrode compared to the average velocity estimated by the well-known electrohydrodynamic mobility. The difference between the width of the charged core and the hydrodynamic plume is analyzed and presented. It is shown that the local heating of the liquid is strongly reduced by the convective losses generated by EHD motion. Finally, it is found that the liquid temperature in cyclohexane in the vicinitiy of sharp points under space charge limited injection can reach temperatures slightly above boiling temperature, without generating bubbles.

  • 78.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. Division for Electricity, Uppsala University.
    Frid, Henrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the modeling of the production and drift of carriers in cyclohexane2013In: Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP, IEEE , 2013, p. 905-908Conference paper (Refereed)
    Abstract [en]

    The modeling of the mechanisms of generation, loss, multiplication and transport of charge carriers is vital for the simulation of the prebreakdown process in dielectric liquids. Unfortunately, there is a lack of suitable coefficients to describe the electron generation and transport of carriers in liquids, which hinders the development of numerical models with sufficient predictive power. In this paper, the drift-dominated continuity equations for electrons and ions are coupled with Poisson's equation in order to simulate the carrier production and drift in the liquid phase under positive and negative voltages in cyclohexane. The estimations of the model are compared with measurements of current-voltage characteristics and Trichel current pulses reported in the literature for needle-plane configurations. In the analysis, the electron generation mechanisms suggested for dielectric liquids are analyzed and discussed. It is found that estimations based on the Zener equation for field-dependent molecular ionization do not agree with measurements for negative sharp points. It is also shown that the proper estimation of the electric current in the liquid phase should consider a field-dependent attachment term as well as the electrohydrodynamic movement of the liquid.

  • 79.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB Corporate Research.
    Gati, R.
    Time-resolved ablation of polymers exposed to ac arcs in air2013In: 20th Symposium on Physics of Switching Arc 2013, FSO 2013, Brno University of Technology, Physics , 2013, p. 84-87Conference paper (Refereed)
    Abstract [en]

    Polymers exposed to the radiation emitted by electric arcs generate vapours that are used to improve the interruption performance of electric switching apparatus. However, the basic physics of this process (known as ablation) under the broadband radiation spectra of electric arcs is not well understood. This paper presents measurements of time-resolved, ablation depth profiles of polymers (PA6-C6H11ON- and PMMA-C5H8O2-) exposed to the radiation of a single wall-stabilized arc ignited in a 5 mm air gap under AC currents with a peak of 2.5 kA. The profiles are obtained by exposing the surface of polymers moving with a velocity of 1 m/s, to the arc radiation through a 0.8 mm thick slit on one of the constricting glass walls. The measured profiles are correlated with the electrical and radiated power. It is found that there is a significant shift between the time-variation of the radiated power and the ablation depth, indicating that thermal decomposition is the main mechanism involved in the experiment.

  • 80.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Long, Mengni
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Schulz, Wolfgang
    Austrian Electrotechnical Association (OVE), Department of Austrian Lightning Detection and Information (ALDIS).
    Thottappillil, Rajeev
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the Estimation of the Lightning Incidence to Offshore Wind FarmsManuscript (preprint) (Other academic)
    Abstract [en]

    Field observations have shown that the frequency of dangerous lightning events to wind turbines, calculated according to the IEC standard 61400-24:2010, is grossly underestimated.This paper intends to critically revisit the evaluation of the incidence of downward lightning as well as self-initiated and other-triggered upward flashes to off shore wind power plants. Three different farms are used as case studies. The conditions for interception of stepped leaders in downward lightning and the initiation of upward lightning is evaluated with the Self-consistent Leader Inception and Propagation Model (SLIM). The analysis show that only a small fraction of damages observed in the analysed parks can be attributed to downward lightning. It is also estimated that only a small fraction (less than 19%) of all active thunderstorms in the area of the analysed parks can generate sufficiently high thundercloud fields to self-initiate upward lightning. Furthermore, it is shown that upward flashes can betriggered even under low thundercloud fields once a sufficiently high electric field change is generated by a nearby lightning event. Despite of the uncertainties in the incidence evaluation, it is shown that upward flashes triggered by nearby positive cloud-to-ground flashes produce most of the dangerous lightning events to the casestudies.

  • 81.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Pettersson, Jonas
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bianchetti, Romeo
    ABB Corporate Research, Switzerland.
    On the Radiation Losses Introduced by the Vapour Layer Formed By Arc-Induced Ablation of Polymers in Air2016Conference paper (Refereed)
  • 82.
    Becerra, Marley
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB Corporate Research, Sweden.
    Piva, Daniele
    ABB Low Voltage Products Division, Italy.
    Gati, Rudolf
    ABB Corporate Research,Switzerland.
    Dominguez, Gustavo
    On the optical radiation of ablation dominated arcs in air2011In: 19th Symposium on Physics of Switching Arc 2011, FSO 2011, Brno University of Technology , 2011, p. 113-116Conference paper (Refereed)
    Abstract [en]

    The optical emission of high current electric arcs confined between polymeric (PA6 - C6H11ON- and PMMA -C5H8O2-) and ceramic (Al2O 3) walls has been investigated experimentally in air. Arcs standing between copper electrodes with a separation of 8 mm under AC currents with peak values of 2 kA are studied here. The arc optical emission in the range between 250 and 700 nm is measured with a fast spectrometer at the mid-section of the arc column, while the spatial variations of the radiation is recorded with high speed photography. It is found that the ablation of the polymeric walls strongly reduce the intensity of emitted copper peaks as well as the arc column radius. Furthermore, a strong hydrogen Balmer-alpha peak is detected, which indicates the strong injection of hydrogen in the arc atmosphere by the ablation of the polymeric walls.

  • 83.
    Beckman, Claes
    et al.
    University of Gävle.
    Eklund, Lars
    Karlsson, Bo
    Lindmark, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Ribbenfjärd, David
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wirdemark, Per
    Verifying 3G licence requirements when every dB is worth a billion2006In: First European Conference on Antennas & Propagation: EuCAP 2006 / [ed] H. Lacoste & L. Ouwehand, 2006, p. 1-4Conference paper (Refereed)
    Abstract [en]

    In the year 2000, the Swedish Telecom regulator: “Post&Telestyrelsen”, PTS, granted in a “beauty contest” four licenses for operations of 3G systems. To verify the coverage and the license requirements, PTS, has developed a test procedure where the field strength of the primary Common Pilot Channel, CPICH, is measured in a drive test. Designing such a test constitutes a number of challenges mainly due to the fact that in 3G the accuracy in the measurement needs to be extremely high since even a small systematic error of ~1dB could in Sweden have the consequence that each operator would have to build an extra +1000 sites at a staggering cost of ~1billion SEK!

    The present paper gives an overview of the considerations behind the design of the test method used for verification of the 3G licence requirements in Sweden.

  • 84. Benbouzid, M. E. H.
    et al.
    Reyne, G.
    Meunier, G.
    Kvarnsjo, L.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dynamic modelling of giant magnetostriction in Terfenol-D rods by the finite element method1995In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 31, p. 1821-1824Article in journal (Refereed)
    Abstract [en]

    As a contribution to the development of methods for the design and the analysis of devices based on giant magnetostrictive materials, a model for the simulation of the dynamic behaviour of the nonlinear magnetoelastic medium is presented. The coupled magnetic, magnetoelastic and mechanical equations that describe the magnetostrictive problem are solved by means of the finite element method. The thin sheets bending principle (surface splines) is used to introduce in the simulation the nonlinear properties of giant magnetostrictive materials, obtained by static characterizations

  • 85.
    Bengtsson, Mats
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Lilliesköld, Joakim
    KTH, School of Electrical Engineering (EES), Industrial Information and Control Systems.
    Norgren, Martin
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Skog, Isaac
    KTH, School of Electrical Engineering (EES), Signal Processing.
    Sohlström, Hans
    KTH, School of Electrical Engineering (EES), Micro and Nanosystems.
    Developing and Implementing a Program Interfacing Project Course in Electrical Engineering2012Conference paper (Refereed)
    Abstract [en]

    In this paper, we describe the ideas behind a second-year Design-Build course in Electrical Engineering. Electrical Engineering is a theoretical subject, and in such it is difficult to maintain the theoretical level in project courses introduced too early in the program, especially when core subjects like electromagnetic field theory are involved. This issue is addressed and we also describe our approach for the assessment of the students. We also discuss the different goals that were set up prior to the course from a program perspective; how we reasoned when designing the course, the assessment structure, and the output once the course was implemented

  • 86.
    Bergerland, Sune
    et al.
    Karlstads El- och Stadsnät.
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Summary of the Swedish tariff regulation and impact of changes on investment strategies2015Conference paper (Refereed)
    Abstract [en]

    This paper evaluates how upcoming changes in the Swedish tariff regulation could affect distribution system operators (DSOs), with focus on reinvestment planning. This is done by general analyses as well as by authentic calculation examples of a real power distribution system. The paper describes the Swedish tariff regulation with expected changes, provides a summary of changes in Swedish laws and regulation affecting DSOs between 1996 and 2016, describes how a DSO at local distribution level conduct their reinvestments, illustrates economic calculation examples and finally presents analyses and conclude the results. Analysis results presented show that the outcome from the regulation is sensitive towards relatively small changes in WACC and age structure. The tariff cap allowed will however be significantly reduced for all tested scenarios. A reinvestment rate of in average~10 % regarding meters and IT and ~2.5 % regarding all other categories could be a rough guideline to meet the new incentives, but that could differ depending on the actual age structure of the DSO.

  • 87.
    Bergman, David
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    The Effect of Partial Discharge Aging on the Dielectric Response of Polymers2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This work investigated the effect of partial discharges (PD) on the complex permittivity

    of PVC, PC, PTFE, PE and PA6. The material samples were subjected to prolonged

    PD activity and the complex permittivity of the material was measured by dielectric

    spectroscopy (DS) before and after the sample was aged. The results showed a change

    in the loss factor for the tested materials, some were more affected than others and

    showed different behaviors when subjected to the PD aging. Hence it was not possible

    to find any general trend in the complex permittivity of the tested materials. Several

    of the materials exhibited a loss factor with -1/2 slope at low frequencies corresponding

    to diffusion. The -1/2 slope implies that the loss factor and the dynamic component of

    the real permittivity should be equal, which could not be seen. Therefore it is uncertain

    whether what was observed is a diffusion process or not. The results could suggest that

    PD aging causes a change in the trapping characteristics of the material. Furthermore,

    space charges deposited on the cavity wall from the PD activity could diffuse into the

    bulk of the material where some of the charges are trapped. If a DS measurement

    is performed before the trapped charges have had time to recombine or be conducted

    away, it could affect the complex permittivity. This was demonstrated by performing

    another DS measurement on an aged PC sample which had been left to rest for 35 days.

    The measurement showed that the complex permittivity had returned to almost the

    same state as before aging. Measurement problems were encountered which affected the

    measurements in this work. The problems are believed to be caused by the measurement

    electrodes used for the DS measurements not being heavy enough to eliminate small

    unwanted air-gaps between the electrodes and sample. Furthermore, incomplete results

    were obtained for some measurements which was believed to be due to the capacitance of

    the sample being at the limit of what the measurement instrument was able to measure.

  • 88. Bergqvist, A.
    et al.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A phenomenological differential #x2010;relation #x2010;based vector hysteresis model1994In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 75, p. 5484-5486Article in journal (Refereed)
    Abstract [en]

    A phenomenological inherently vector hysteresis model employing simple differentials as a means of keeping track of the past history of the magnetic field is proposed. This results in a simple and computationally fairly efficient formulation for vector hysteresis of any dimension. An expression for determining model parameters from experimental data or from the Preisach function is given. The model exhibits rotational hysteresis and reduction of remanent magnetization by an orthogonal field and reduces to the classical Preisach model in one dimension. Details concerning the numerical implementation are discussed and computational examples demonstrating model properties are presented.

  • 89. Bergqvist, A.
    et al.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A phenomenological magnetomechanical hysteresis model1994In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 75, no 10, p. 5496-5498Article in journal (Refereed)
    Abstract [en]

    To obtain accurate design tools for applications involving giant magnetostrictive materials, magnetomechanical hysteresis effects should be taken into account. The problem consists of determining the magnetization and mechanical strain from the combined past history of magnetic field and mechanical stress. In this work, coupled magnetomechanical hysteresis has been modeled by using simple path #x2010;dependent differentials to accumulate the past history in functions related to the magnetization and strain through material #x2010;dependent parameters. By using anhysteretic curves and a few additional parameters to characterize a material, major, and minor loops with respect to both field and stress have been calculated for Terfenol #x2010;D and have shown good agreement with experiments.

  • 90. Bergqvist, A.
    et al.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A stress-dependent magnetic Preisach hysteresis model1991In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 27, p. 4796-4798Article in journal (Refereed)
    Abstract [en]

    The authors present a generalization of the classical Preisach model which handles coupled magnetic and mechanical hysteresis. Magnetostrictive materials are known to have hysteresis with respect to both magnetic field H and mechanical stress lambda;. To test the validity of the model, experiments where the two components H and lambda; have been verified in many different ways have been performed on Terfenol-D and compared to results computed from the model. Some of these results are presented. This stress-dependent model is found to have an accuracy comparable to that of the classical Preisach model

  • 91. Bergqvist, A.
    et al.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A thermodynamic representation of pseudoparticles with hysteresis1995In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 31, p. 3539-3541Article in journal (Refereed)
    Abstract [en]

    A method of expressing pseudoparticles with hysteresis within a context of irreversible thermodynamics is investigated. The state of a pseudoparticle is uniquely determined by its magnetization and its evolution is governed by entropy maximization. Hysteresis appears if the free energy is a nonconvex function of magnetization. The vectorial nature of magnetization and dependence on rate are accounted for in a systematic manner. Some basic properties are derived for quasistatic processes. In particular, it is found that in the scalar case, the magnetization is a monotonically increasing functional with respect to field and that for quasistatic processes, this implies the wiping-out property

  • 92. Bergqvist, A.
    et al.
    Tiberg, H.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Application of a vector Preisach model in a magnetic circuit1993In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 73, p. 5839-5841Article in journal (Refereed)
    Abstract [en]

    Magnetic hysteresis effects have been included in a finite #x2010;element description of a magnetic circuit by using the classical vector Preisach model for the constitutive relation between H and B. The influence of an external electric circuit is taken into account by adding equations derived from Faraday #x2019;s law. Computational results are presented for a magnetic circuit used in a magnetostrictive device.

  • 93. Bergqvist, Anders
    et al.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A model for Magnetomechanical Hysteresis and Losses in Magnetostrictive Materials1996In: J. Appl. Phys., Vol. 79, no 8Article in journal (Refereed)
  • 94. Bergqvist, Anders
    et al.
    Lundgren, Anders
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Experimental testing of an anisotropic vector hysteresis model1997In: IEEE Transactions on Magnetics, Vol. 33, no 5, p. 4152-54Article in journal (Refereed)
  • 95. Bergqvist, Anders
    et al.
    Lundgren, Anders
    Stillesjö, Fredrik
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Statistical modelling of magnetic and magnetostrictive properties due to domain wall motion1999In: Journal of Applied Physics, p. 5190-5192Article in journal (Refereed)
  • 96. Bergqvist, J.
    et al.
    Lundgren, A.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Computationally efficient vector hysterisis model using flux density as known variable1998In: Non-linear electromagnetic systems, p. 463-Article in journal (Refereed)
  • 97.
    Bergsåker, Claudia
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Impact of transformer core size on the reactive power requirement of power transformers due to GIC2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Geomagnetically induced currents (GIC) are a natural phenomenon which arises due to solar

    storms. During a solar storm, large amounts of magnetized plasma are ejected from the surface

    of the sun. When this plasma reaches earth, it causes fluctuations in the geomagnetic field. Such

     fluctuations may induce DC over-currents in long transmission lines. These currents affect the

    transmission system several dierent ways; In particular high voltage transformers are sensitive to

    GIC. When the over-current flows through the transformer windings the reactive power absorption

    of the transformer increases, which may lead to voltage instability in the power system. For this

    project, the main issue has been to determine whether or not an increase in the size of the transformer

    core leads to the reactive power absorption being less sensitive to GIC. In order to investigate

    this issue a recently developed transformer model has been used; the Hybrid transformer model.

    This model combines the principle of duality with a matrix representation of the transformer. The

    Hybrid transformer model, which has recently been implemented in the power system simulations

    software PSCAD, has been used to simulate GIC events in transformers of varying core sizes. The

    results from these simulations indicate that a larger transformer core is associated with a smaller

    increase in reactive power absorption during a GIC event. It is also clear that the reactive power

    absorption as a function of GIC magnitude is a non-linear function when the Hybrid transformer

    model is applied. This function has previously been considered a linear function.

  • 98.
    Berrier, Audrey
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Shi, Yaocheng
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Siegert, Jörg
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Srinivasan, Anand
    KTH, School of Information and Communication Technology (ICT), Material Physics, Semiconductor Materials, HMA.
    Impact of dry-etching induced damage in InP-based photonic crystals2008In: PHOTONIC CRYSTAL MATERIALS AND DEVICES VIII, 2008, Vol. 6989, p. U9890-U9890Conference paper (Refereed)
    Abstract [en]

    In this work variations of the carrier lifetime in a GaInAsP/InP quantum well in two-dimensional PhC structures etched by Ar/Cl-2 chemically assisted ion beam etching as a function of the processing parameters is investigated. It is shown that the deposition conditions of the SiO2 mask material and its coverage as well as other process steps such as annealing affect the carrier lifetimes. However the impact of patterning the semiconductor on the carrier lifetime is dominant, showing over an order of magnitude reduction. For given PhC lattice parameters, the sidewall damage is shown to be directly related to the measured carrier lifetimes. A simple qualitative model based on sputtering theory and assuming a conical hole-shape development during etching is used to explain the experimental results.

  • 99.
    Bertling, Lina
    et al.
    Chalmers.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jensen, Jolanta
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Setreus, Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    RADPOW development and documentation2008Report (Other (popular science, discussion, etc.))
    Abstract [en]

    This report summarizes the status of the computer program RADPOW. RADPOW is a program for Reliability Assessment of electrical Distribution POWer systems. It was developed at KTH School of Electrical Engineering, within the research program EKC and the research project on reliability of new electrical distribution systems. Further on, RADPOW has been used and further developed within the RCAM research group at KTH School of Electrical Engineering.

    This status report contains a brief description of the RADPOW_2006 version, the Loadflow module from the RADPOW_1999_PH version and a description of the work done in the resulting RADPOW_2007 version. This version now includes a tested load flow module and the ability to calculate the latest component importance indices developed within the RCAM research group. The source code for the program has also been restructured and commented in a more detailed level than before.

  • 100.
    Bertling, Lina
    et al.
    Chalmers.
    Larsson, Mats B-O
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Evaluation of the Customer Value of Component Redundancy in Electrical Distribution Systems2005In: Power Tech, 2005 IEEE Russia, IEEE , 2005, p. 1-8Conference paper (Refereed)
    Abstract [en]

    A new regulatory model for the Swedish electrical distribution system operators has been proposed, and is now being implemented. The model is referred to as the Network Performance Assessment Model (NPAM). The NPAM is based on fictive reference networks. These emanate from a radial network that is reinforced with redundant components if it increases the customer value more than required investment cost for higher system reliability. The NPAM involves a paradigm shift for the Swedish DSO from a system based on compensation for costs to one based on performance. Evidently, it is of great importance to both the regulator and the different DSOs to evaluate how well the NPAM calculates the performance i.e. the customer values. The aim for this paper is to take a first step in this process; by putting light on how these redundant reference networks are created and used for the NPAM. In order to do this a comparative study has been made for a small test system where two different approaches are used for identifying the resulting redundant reference network for a system. The NPAM approach uses a Monte Carlo simulation technique, and the comparative approach uses an analytical based reliability assessment tool RADPOW, developed at KTH.

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