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  • 1.
    Ackermann, Thomas
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Distributed resources and re-regulated electricity markets2007In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 77, no 9, p. 1148-1159Article in journal (Refereed)
  • 2.
    Ackermann, Thomas
    et al.
    KTH, Superseded Departments.
    Andersson, G.
    Söder, Lennart
    KTH, Superseded Departments.
    Distributed generation: a definition2001In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 57, no 3, p. 195-204Article in journal (Refereed)
  • 3.
    Almas, Muhammad Shoaib
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Baudette, Maxime
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems. Statnett.
    Utilizing synchrophasor-based supplementary damping control signals in conventional generator excitation systems2018In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 157, p. 157-167Article in journal (Refereed)
    Abstract [en]

    A supplementary function of Excitation Control Systems (ECSs) for synchronous generators is that of a Power System Stabilizer (PSS). The PSS implementation in these ECSs only allows the use of a limited type of pre-defined local input measurements and built-in PSS algorithms. To adapt existing ECSs to take advantage of synchrophasors technology, this paper proposes and implements a prototype wide-area damping controller (WADC) that provides synchrophasor-based damping input signals to existing ECSs. The developed WADC comprise (i) a real-time mode estimation module, (ii) synchrophasor’s communication latency computation module, and (iii) phasor-based oscillation damping algorithm executing in a real-time hardware prototype controller.

    Through Real-Time Hardware-in-the-Loop (RT-HIL) simulations, it is demonstrated that synchrophasor-based damping signals from the WADC can be utilized together with a commercial ECS, thus providing new options for selection of the best feedback signal for oscillation damping.

  • 4.
    Armendariz, Mikel
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Paridari, Kaveh
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Wallin, Edel
    Vattenfall R&D.
    Comparative Study of Optimal Controller Placement Considering Uncertainty in PV Growth and Distribution Grid Expansion2018In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 155C, p. 48-57Article in journal (Refereed)
    Abstract [en]

    Distributed generation (DG) and especially grid-connected residential photovoltaic (PV) systems areemerging and high penetration levels of these can have an adverse impact on several low voltage (LV)distribution grids in terms of power quality and reliability. In order to reduce that effect in a cost-effectivemanner, the traditional distribution grid planning process is being reengineered by incorporating the gridcontrol operations and considering the uncertainties e.g., DG power, demand and urban/rural expansionplans. One of the challenges is to determine if the required technology deployment to operate the gridscan provide a better solution in terms of quality and cost than the traditional approach, which is prin-cipally based on cable reinforcement and change of transformers. In addition, if controllers were to bedeployed, it would be important to determine where they should be placed and at what stage of theexpansion planning, especially when the planning is assumed to be non-deterministic.Therefore, following this situation, in this paper we propose an optimal way to deploy and to operateutility’s controllable resources at the distribution grid and additionally we consider the uncertaintiesrelated to PV growth and distribution grid expansion. Thus, we include the non-deterministic multistageperspective to the controller placement problem. Furthermore, we perform a techno-economic analysis ofthe results and we show that an optimal controller placement allows removing the overvoltage problemsarising in the LV grid in a more cost-effective way compared to a typical traditional grid reinforcementapproach.

  • 5.
    Babazadeh, Davood
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Muthukrishnan, Arvind
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Mitra, Pinaki
    HVDC, ABB Sweden, Ludvika.
    Larsson, Tomas
    HVDC, ABB Sweden, Ludvika.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Selection of DC Voltage Controlling Station in an HVDC Grid2016In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 144, p. 224-232Article in journal (Refereed)
    Abstract [en]

    This paper proposes a real-time quantitative evaluation of HVDC converters’ in an HVDC grid to select the suitable DC slack converter. This real-time evaluation considers the strength of connecting AC grid and the converter's on-line capacity margin as selection metrics. The strength of AC grid is evaluated in real-time by the estimation of grid short circuit capacity using recursive lease square algorithm. Given these selection metrics, the credibility of HVDC stations in controlling the DC voltage can be offered to the system operator in real-time for further operational decisions. This paper also studies the practical aspect of the estimation algorithm regarding selection of the operating points. As major contribution, it suggests to intelligently use a naturally occurring droop response in HVDC grids as a second operating point in the estimation algorithm to calculate the short circuit capacity. The method has been tested through set of scenarios using a real-time co-simulation platform. This platform includes real-time power system simulator to model AC/DC grid, industrial HVDC controllers and corresponding ICT systems. The results show that the proper selection of DC slack station can improve the AC system response and DC voltage drops during disturbances.

  • 6.
    Babazadeh, Davood
    et al.
    KTH, School of Electrical Engineering (EES), Industrial Information and Control Systems.
    Van Hertem, Dirk
    ELECTA Research Group, KU Leuven, 3001 Leuven-Heverlee, Belgium.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Industrial Information and Control Systems.
    Study of Centralized and Distributed Coordination of Power Injection in Multi-TSO HVDC Grid with Large Off-shore Wind Integration2016In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 136, p. 281-288Article in journal (Refereed)
    Abstract [en]

    This paper studies centralized and distributed schemes for the coordinationof power injection in an HVDC grid connected to large wind generation.This coordination of power injection aims to keep the power balanced withinthe HVDC grid especially during large wind disturbances. Furthermore, thecoordination tries to follow the converters’ schedules set by the connectingAC-TSOs every 15−minutes. This schedule comes from an overall combinedAC/DC economic dispatch calculation with lower resolution compared to thiscoordination. In this paper, the coordination of power injection has been formulatedas a non-linear constrained optimization problem for the centralizedarchitecture. Next, the centralized optimization problem is decomposed tosub-problems using the Auxiliary Problem Principle (APP) method for thedistributed architecture. This distributed optimization problem is solved byexchanging the required information between the AC TSOs. These two differentapproaches have been evaluated for a 5-terminal HVDC grid. Furthermore,a sensitivity analysis has been carried out to find the optimal updatingrate of power injection set-points. The result shows that the more frequentupdating of power injection coordination in the centralized architecture improvesthe generation reserves in each AC area.

  • 7. Díez-Maroto, L.
    et al.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Almas, Muhammad Shoaib
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Jónsdóttir, G. M.
    Rouco, L.
    A WACS exploiting generator Excitation Boosters for power system transient stability enhancement2017In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 148, p. 245-253Article in journal (Refereed)
    Abstract [en]

    Excitation Boosters (EB) are designed to improve transient stability of synchronous generators equipped with bus fed static excitation systems. They can be controlled using either local or remote signals following a disturbance. This paper explores how critical clearing times (CCT) can be improved by EBs controlled using remote signals. Particularly, Pseudo Center of Inertia (PCOI) and Dominant Interarea Path (DIP) signals derived from Phasor Measurement Units (PMU) within a Wide Area Control System (WACS) are used. Prototype controllers are tested by means of a Real Time (RT) Hardware-in-the-Loop (HIL) experimental setup.

  • 8. Edimu, M.
    et al.
    Alvehag, Karin
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Gaunt, C. T.
    Herman, R.
    Analyzing the performance of a time-dependent probabilistic approach for bulk network reliability assessment2013In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 104, p. 156-163Article in journal (Refereed)
    Abstract [en]

    The conventional sequential Monte Carlo Simulation (MCS) considers states in which a component is both in and out of service. Sequential MCS has been applied in different analyses whilst considering both symmetrical and asymmetrical probability distributions. The Beta distribution is however not one of the commonly recommended distributions for use in sequential MCS due to the complexity in deriving its inverse transform. A new sequential MCS technique that applies the Beta distribution is proposed in this paper. The technique is a time-dependent probabilistic approach (TDPA) that uses probability density functions (PDFs) to characterize stochastic network parameters in terms of their season- and time-dependency and simulates the component down (failure) states. The effect of this simulation approach on reliability calculations is analyzed using a published test network. The impact of dispersion and skewness in PDF based input models on a reliability analysis is also investigated. The results show that the TDPA can replicate the conventional sequential MCS analysis. The TDPA computation is also significantly faster. The simulation results of the TDPA also show that dispersion and skewness of component failure rate PDFs significantly influence a reliability analysis.

  • 9. Elkington, Katherine
    et al.
    Knazkins, Valerijs
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Ghandhari, Mehrdad
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    On the stability of power systems containing doubly fed induction generator-based generation2008In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 78, no 9, p. 1477-1484Article in journal (Refereed)
    Abstract [en]

    This article is concerned with the impact of large-scale wind farms utilising doubly fed induction generators on the stability of a general power system. Inspection of the eigenstructure of the power system provides a foundation for assessing the impact, which is then quantified by means of detailed numerical simulations. Simplified state-space models are used to describe the dynamics of the generators in a very simple system, whose network is described by algebraic relations. A third order model is derived for a doubly fed induction generator. Mathematical models are then used to identify the behavioural patterns of the system when it is subject to disturbances. Eigenvalue analysis reveals some interesting properties of the system for small disturbances, and shows that the addition to a power system of doubly fed generators, such as those in wind farms, improves the response of the system to small disturbances. However, numerical simulations show that it can have an adverse impact after larger disturbances.

  • 10.
    Eriksson, Robert
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Knazkins, Valerijs
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Coordinated control of multiple HVDC links using input-output exact linearization2010In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 80, no 12, p. 1406-1412Article in journal (Refereed)
    Abstract [en]

    This paper is concerned with the investigation of a new control technique for the conventional High Voltage Direct Current (HVDC) link. The proposed technique relies upon nonlinear state feedback linearization of the AC/DC power system. The idea in input-output exact feedback linearization is to algebraically transform nonlinear systems dynamics into a linear control problem using a nonlinear pre-feedback loop, and then for the linearized power system one can design another feedback loop using a well established technique such as a linear-quadratic regulator. The primary goal of the controller presented in this paper is to contribute to the enhancement of both the transient and the small-signal stability of the power system. Since the proposed state feedback linearization does not rely on the assumption that there is only small deviation of the states from an equilibrium, the enhancement of both is feasible. The simulation results obtained in the framework of the study show that the proposed controller is capable of stabilizing the system in various system operating conditions.

  • 11.
    Farrokhabadi, Mostafa
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    An efficient automated topology processor for state estimation of power transmission networks2014In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 106, p. 188-202Article in journal (Refereed)
    Abstract [en]

    A robust network topology processor that can be utilized in both traditional and PMU-based state estima- tors is developed. Previous works in the field of topology processing are scrutinized and their drawbacks are identified. Building on top of the state of the art, an algorithm covering the limitations of available topology processing approaches and including new features is proposed. The presented algorithm was implemented in MATLAB and tested using two different power networks with detailed substation config- urations (bus/breaker models) including a modified version of the IEEE Reliability Test System 1996. As the topology processor is intended to supply network topologies to a PMU-based Sate Estimator, the IEEE Reliability Test System 1996 is simulated in real-time using the eMegaSim Opal-RT real-time simulator which is part of “SmarTS Lab” at KTH Royal Institute of Technology. Testing is carried out through several test scenarios and computation times are calculated. It is shown that the computation times are adequate for supporting a PMU-only state estimator. 

  • 12.
    Firouzi, Seyed Reza
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems. SmarTS Lab.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems. SmarTS Lab.
    Ruiz-Alvarez, Albert
    Catalonia Institute for Energy Research (IREC), Barcelona, Spain.
    Hooshyar, Hossein
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems. SmarTS Lab.
    Mahmood, Farhan
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems. SmarTS Lab.
    Interpreting and Implementing IEC 61850-90-5 Routed-Sampled Value and Routed-GOOSE Protocols for IEEE C37.118.2 Compliant Wide-Area Synchrophasor Data Transfer2016In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 144, p. 255-267Article in journal (Refereed)
    Abstract [en]

    Flexibility and adaptability requirements of future electric power grids for integrating DERs call for the developmentof Wide-Area Monitoring, Protection And Control (WAMPAC) applications, utilizing synchrophasor measurementsprovided by the Phasor Measurement Units (PMUs).IEEE C37.118 is the most utilized protocol for real-time exchange of synchronized phasor measurement data. In orderto full some gaps not addressed in IEEE C37.118, and also to harmonize with the IEC 61850 power utility automationstandard, the IEC 61850-90-5 technical report has been developed. IEC TR 61850-90-5 introduces a mechanism fortransfer of digital states and time synchronized phasor measurement data over wide-area networks between PhasorMeasurement Units (PMUs), Phasor Data Concentrators (PDCs) and WAMPAC applications in the context of IEC61850.This work interprets the IEEE C37.118.2 and IEC 61850-90-5 Routed-Sampled Value and Routed-GOOSE protocolsand describes the design and implementation of a library named Khorjin with the functionality of (1) an IEEE C37.118.2to IEC 61850-90-5 gateway and protocol converter and, (2) an IEC 61850-90-5 subscriber and trac parser.The main contribution of this work is the development of Khorjin library using only standard C libraries (i.e.independent from any operating system). This is allowing the use of the library in dierent platforms.The design requirements and functionality of the Khorjin library has been tested in the KTH SmarTS Lab Real-TimeHardware-in-the-Loop (HIL) simulation environment to assess its conformance to the functional requirements of IEEEC37.118.2 and IEC 61850-90-5 standards.

  • 13.
    Hamon, Camille
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Perninge, Magnus
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    A computational framework for risk-based power system operations under uncertainty. Part II: Case studies2015In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 119, p. 66-75Article in journal (Refereed)
    Abstract [en]

    With larger penetrations of wind power, the uncertainty increases in power systems operations. The wind power forecast errors must be accounted for by adapting existing operating tools or designing new ones. A switch from the deterministic framework used today to a probabilistic one has been advocated. This two-part paper presents a framework for risk-based operations of power systems. This framework builds on the operating risk defined as the probability of the system to be outside the stable operation domain, given probabilistic forecasts for the uncertainty, load and wind power generation levels. This operating risk can be seen as a probabilistic formulation of the N - 1 criterion. In Part I, the definition of the operating risk and a method to estimate it were presented. A new way of modeling the uncertain wind power injections was presented. In Part II of the paper, the method's accuracy and computational requirements are assessed for both models. It is shown that the new model for wind power introduced in Part I significantly decreases the computation time of the method, which allows for the use of later and more accurate forecasts. The method developed in this paper is able to tackle the two challenges associated with risk-based real-time operations: accurately estimating very low operating risks and doing so in a very limited amount of time.

  • 14.
    Hamon, Camille
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Perninge, Magnus
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    A computational framework for risk-based power systems operations under uncertainty. Part I: Theory2015In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 119, p. 45-53Article in journal (Refereed)
    Abstract [en]

    With larger penetrations of wind power, the uncertainty increases in power systems operations. The wind power forecast errors must be accounted for by adapting existing operating tools or designing new ones. A switch from the deterministic framework used today to a probabilistic one has been advocated. This two-part paper presents a framework for risk-based operations of power systems. This framework builds on the operating risk defined as the probability of the system to be outside the stable operation domain, given probabilistic forecasts for the uncertainty (load and wind power generation levels) and outage rates of chosen elements of the system (generators and transmission lines). This operating risk can be seen as a probabilistic formulation of the N - 1 criterion. The stable operation domain is defined by voltage-stability limits, small-signal stability limits, thermal stability limits and other operating limits. In Part I of the paper, a previous method for estimating the operating risk is extended by using a new model for the joint distribution of the uncertainty. This new model allows for a decrease in computation time of the method, which allows for the use of later and more up-to-date forecasts. In Part II, the accuracy and the computation requirements of the method using this new model will be analyzed and compared to the previously used model for the uncertainty. The method developed in this paper is able to tackle the two challenges associated with risk-based real-time operations: accurately estimating very low operating risks and doing so in a very limited amount of time.

  • 15.
    Hamon, Camille
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Perninge, Magnus
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    An Importance Sampling Technique for Probabilistic Security Assessment In Power Systems with Large Amounts of Wind Power2016In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 131, p. 11-18Article in journal (Refereed)
    Abstract [en]

    Larger amounts of variable renewable energy sources bring about larger amounts of uncertainty in the form of forecast errors. When taking operational and planning decisions under uncertainty, a trade-off between risk and costs must be made. Today's deterministic operational tools, such as N-1-based methods, cannot directly account for the underlying risk due to uncertainties. Instead, several definitions of operating risks, which are probabilistic indicators, have been proposed in the literature. Estimating these risks require estimating very low probabilities of violations of operating constraints. Crude Monte-Carlo simulations are very computationally demanding for estimating very low probabilities. In this paper, an importance sampling technique from mathematical finance is adapted to estimate very low operating risks in power systems given probabilistic forecasts for the wind power and the load. Case studies in the IEEE 39 and 118 bus systems show a decrease in computational demand of two to three orders of magnitude.

  • 16.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Electric power and energy systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Individual failure rates for transformers within a population based on diagnostic measures2016In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 141, p. 354-362Article in journal (Refereed)
    Abstract [en]

    The high monetary value of a transformer has placed the transformer life-time optimization into the focus of asset management. The average failure rate has created reasonable results within reliability modeling, however, it cannot reflect the probability of failure for an individual transformer. In this paper, a method is introduced to calculate individual failure rates for a transformer population based on failure statistics and diagnostic measurements such as dissolved gas, and 2-furfuraldehyde analysis. The method is applicable to all types of components and the comprehensibility makes it effective for practical implementation. The results are evaluated against two health indices based on a weight factor and fuzzy logic. It can be observed that the presented individual failure rates are plausible representatives of the transformer's probability of failure. Therefore, the results can also be utilized for asset management decision-making.

  • 17.
    Kotb, Omar
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Ghandhari, Mehrdad
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Eriksson, R.
    Leelaruji, R.
    Sood, V. K.
    Stability enhancement of an interconnected AC/DC power system through VSC-MTDC operating point adjustment2017In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 151, p. 308-318Article in journal (Refereed)
    Abstract [en]

    This paper investigates the improvement of dynamic and transient stability in interconnected AC/DC power systems through operating point adjustment in an embedded VSC-MTDC network. Hopf Bifurcation (HB) is known to result in oscillatory instability in the power system, therefore a sufficient margin to HB has to be maintained to enhance the dynamic stability and ensure a secure operation of the system. The enhancement of dynamic stability is based on the adjustment of the MTDC converters’ controller setpoints to drive the system away from HB point and increase the damping of oscillatory modes. The variation of each setpoint is calculated in proportion to the eigenvalue sensitivity to the respective setpoint. On the other hand, transient stability enhancement is based on operating point adjustment according to Power Transfer Distribution Factors (PTDFs) with the aim to reduce inter-area power flow over AC lines. The proposed method is applied to a VSC-MTDC network interconnecting two IEEE 14-bus sections. The results show an improvement in the system's stability margin in case of various disturbances such as load increase and short circuit fault. Time-domain simulations in DIgSILENT are used to corroborate the results obtained through small signal stability analysis.

  • 18.
    Kotb, Omar
    et al.
    KTH, School of Electrical Engineering (EES), Electric power and energy systems. KTH.
    Ghandhari, Mehrdad
    KTH, School of Electrical Engineering (EES), Electric power and energy systems.
    Eriksson, Robert
    Svenska Kraftnät.
    Sood, Vijay
    UOIT.
    On small signal stability of an AC/DC power system with a hybrid MTDC network2016In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 136Article in journal (Refereed)
    Abstract [en]

    Multi-terminal HVDC (MTDC) networks are being contemplated for large scale integration of renewable energy sources, as well as for the interconnection of asynchronous AC systems. A hybrid MTDC network that combines line commutated converters (LCCs) and voltage source converters (VSCs) can combine the benefits of both technologies. This paper presents a mathematical model of an AC/DC power system with an embedded hybrid MTDC network interconnection. Small signal stability analysis of the power system is conducted based on the linearization of the model. The impact of VSC controller gains on the dominant modes in the system is investigated. The contributions of the converters and generators to different modes of the system are investigated based on the participation matrix analysis. Auxiliary controllers are applied at the converters for the purpose of damping power oscillations in case of disturbances. The results of small signal stability analysis are validated by time-domain simulations in MATLAB.

  • 19.
    Latorre, Hector
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Ghandhari, Mehrdad
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Active and reactive power control of a VSC-HVdc2008In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 78, no 10, p. 1756-1763Article in journal (Refereed)
    Abstract [en]

    Voltage source converter-based HVdc(VSC-HVdc) systems have the ability to rapidly control the transmitted active power, and also to independently exchange reactive power with transmissions systems. Due to these characteristics, VSC-HVdcs with a suitable control scheme can offer an alternative means to enhance transient stability, to improve power oscillations damping, and to provide voltage support. In this paper, a VSC-HVdc is represented by a simple model, referred to as the injection model. Based on this model, an energy function is developed for a multi-machine power system including VSC-HVdcs. Furthermore, based on Lyapunov theory (control Lyapunov function) and small signal analysis (modal analysis), various control strategies for transient stability and damping of low-frequency power oscillations are derived.

  • 20.
    Long, Mengni
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Thottappillil, Rajeev
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the Attachment of Dart Lightning Leaders to Wind TurbinesIn: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046Article in journal (Other academic)
    Abstract [en]

    Wind turbines are prone to damages due to lightning strikes and the blades are one of the most vulnerable components. Even though the blade tip is usually protected in standard designs,lightning damages several meters away from it have also beenobserved in field studies. However, these damages inboard fromthe tip cannot be explained by the attachment of downwardstepped leaders or the initiation of upward lightning alone. In this paper, the attachment of dart leaders in an upward lightning flashis investigated as a mechanism of strikes to inboard sections of the blade and the nacelle. Dart leaders in an upward lightning flashuse the channel previously ionized by the preceding stroke or thecontinuous current. The analysis is performed with the self-consistent leader inception and propagation model SLIM. A commercial wind turbine with 45 m long blades and hub height of 80 m is analysed as a case study. The impact of the prospective return stroke peak current, the rotation angle of the blade and the wind on the location of lightning strikes on this mechanism is analysed. The probability of lightning attachment of dart leaders along the blade for the case study is also calculated. It is shown that the dart leader attachment is a mechanism that can explain lightning strikes to the nacelle and to the inboard region several meters away from the blade tip. However, this mechanism cannot explain the lightning strikes observed in the close vicinity of theblade tip (in the region between 1.5 and 6 m from it). The modelling study here also shows that for the turbine under consideration, nacelle receptors intercept most of dart leaders,around 73%, even if the initial continuous currents flow to the blade tip. Overall, it is estimated that around 80% of strikes inupward lightning flashes attach to the tip receptor of the blade.

  • 21.
    Long, Mengni
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra Garcia, Marley
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Thottappillil, Rajeev
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the attachment of dart lightning leaders to wind turbines2017In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 151, p. 432-439Article in journal (Refereed)
    Abstract [en]

    Wind turbines are prone to damages due to lightning strikes and the blades are one of the most vulnerable components. Even though the blade tip is usually protected in standard designs, lightning damages several meters away from it have also been observed in some field studies. However, these damages inboard from the tip cannot be explained by the attachment of downward stepped leaders or the initiation of upward lightning alone. In this paper, the attachment of dart leaders in an upward lightning flash is investigated as a mechanism of strikes to inboard sections of the blade and the nacelle of large wind turbines. Dart leaders in an upward lightning flash use the channel previously ionized by the preceding stroke or the continuous current. The analysis is performed with the self-consistent leader inception and propagation model (SLIM). A commercial large wind turbine with 45 m long blades and hub height of 80 m is analysed as a case study. The impact of the prospective return stroke peak current, the rotation angle of the blade and the wind on the location of lightning strikes on this mechanism is analysed. The probability of lightning attachment of dart leaders along the blade for the case study is also calculated. It is shown that this damage mechanism could create a new strike point only when the blade of a wind turbine rotates sufficiently from its initial position (at the inception of the initial upward leader) until the start of the dart leader approach. Thus, dart leader attachment is a mechanism that can explain lightning strikes to the nacelle and to the inboard region several meters away from the blade tip in large wind turbines. However, dart leader attachment cannot explain the lightning strikes observed in the close vicinity of the blade tip (in the region between 1.5 and 6 m from it).

  • 22.
    Matevosyan, Julija
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Olsson, Magnus
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Hydropower planning coordinated with wind power in areas with congestion problems for trading on the spot and the regulating market2009In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 79, no 1, p. 39-48Article in journal (Refereed)
    Abstract [en]

    In this paper a day-ahead planning algorithm for a multi-reservoir hydropower system coordinated with wind power is developed. Coordination applies to real situations, where wind power and hydropower are owned by different utilities, sharing the same transmission lines, though hydropower has priority for transmission capacity. Coordination is thus necessary to minimize wind energy curtailments during congestion situations. The planning algorithm accounts for the uncertainty of wind power forecasts and power market price uncertainty. Planning for the spot market and the regulating market is considered in the algorithm. The planning algorithm is applied to a case study and the results are summarized in the paper.

  • 23.
    Mazidi, Peyman
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Sanz Bobi, M. A.
    Strategic maintenance scheduling in an islanded microgrid with distributed energy resources2017In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 148, p. 171-182Article in journal (Refereed)
    Abstract [en]

    This paper addresses passive and active preventive maintenance scheduling in an islanded microgrid with storage and renewable energy sources. At first, under a centralized framework, a single-level cost-minimization formulation for passive maintenance scheduling is developed and used as a benchmark in the operation. An independent microgrid operator is responsible for the operation in this framework. Then, through a bi-level formulation, the active maintenance scheduling and operation is carried out with profit-maximization objective. These two developed frameworks provide the houses with opportunity to earn profit and the regulator and the operator to analyze the performance of the system. The bi-level formulation is transformed into a single-level problem through Karush–Kuhn–Tucker conditions. Furthermore, the proposed model provides the capability of incorporating condition monitoring data into the operation. The model is validated through a test system and the outcomes demonstrate the advantages, applicability and challenges of utilizing the proposed model.

  • 24.
    Nasri, Amin
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Eriksson, Robert
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Ghandhari, Mehrdad
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Using trajectory sensitivity analysis to find suitable locations of series compensators for improving rotor angle stability2014In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 111, p. 1-8Article in journal (Refereed)
    Abstract [en]

    This paper proposes an approach based on trajectory sensitivity analysis (TSA) to find most suitable placement of series compensators in the power system. The main objective is to maximize the benefit of these devices in order to enhance the rotor angle stability. This approach is formulated as a two-stage problem, whose first-stage describes prior to fault occurrence and whose second-stage represents the power system behavior involving a set of severe faults. The first-stage focuses on small signal stability, while the second-stage deals with transient stability of power system. In this vein, the trajectory sensitivities of the rotor angles of generators with respect to the reactances of transmission lines are calculated. Two equivalent rotor angles are introduced to find stability indices corresponding to the first- and the second-stage of the proposed approach. Numerical results from IEEE 10-machine 39-bus test system demonstrate the usefulness of the proposed method.

  • 25. Negra, N. B.
    et al.
    Todorovic, J.
    Ackermann, Thomas
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Loss evaluation of HVAC and HVDC transmission solutions for large offshore wind farms2006In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 76, no 11, p. 916-927Article in journal (Refereed)
  • 26.
    Olsson, Magnus
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Perninge, Magnus
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Modeling real-time balancing power demands in wind power systems using stochastic differential equations2010In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 80, no 8, p. 966-974Article in journal (Refereed)
    Abstract [en]

    The inclusion of wind power into power systems has a significant impact on the demand for real-rime balancing power due to the stochastic nature of wind power production The overall aim of this paper is to present probabilistic models of the impact of large-scale integration of wind power on the continuous demand in MW for real-time balancing power This is important not only for system operators, but also for producers and consumers since they in most systems through various market solutions provide balancing power.

    Since there can occur situations where the wind power variations cancel out other types of deviations in the system, models on an hourly basis are not sufficient Therefore the developed model is in continuous time and is based on stochastic differential equations (SDE) The model can be used within an analytical framework or in Monte Carlo simulations.

  • 27.
    Olsson, Magnus
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Perninge, Magnus
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Simulation of real-time balancing power demands in power systems with wind power2010In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 80, p. 966-974Article in journal (Refereed)
  • 28.
    Perić, Vedran S.
    et al.
    KTH, School of Electrical Engineering (EES).
    Sarić, A. T.
    Grabež, D. I.
    Coordinated tuning of power system stabilizers based on Fourier Transform and neural networks2012In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 88, p. 78-88Article in journal (Refereed)
    Abstract [en]

    This paper analyzes optimal tuning of power system stabilizers (PSSs) as the main resource for small-signal stability enhancement in power systems. The procedure is based on dynamic power system response and its frequency amplitude spectrum. Since the optimization model is very complex, there are difficulties in defining the algebraic relation between optimization criteria and PSS parameters and the authors concluded that classical optimization techniques are inappropriate for application in practice. To avoid these problems, application of artificial neural networks (ANNs) as efficient functional approximators is proposed. Optimal PSS parameters are determined by trust region based optimization, where the ANN represents an input function. Robustness of the optimization is ensured with the proposed ANN structure which considers an arbitrary number of different power system operating conditions (including single contingencies). For verification of the proposed methodology, two test systems are used: the New England-New York 68-node, 16-machine test system and the 75-machine dynamic model of the Serbian power system. Poorly damped modes of oscillation are identified and damped by installation of PSSs at appropriate locations with ANN-based optimally tuned parameters.

  • 29.
    Perninge, Magnus
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Approximating the loadability surface in the presence of SNB-SLL corner points2013In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 96, p. 64-74Article in journal (Refereed)
    Abstract [en]

    Power system voltage security assessment is generally applied by considering the power system loadability surface. For a large power system, the loadability surface is a complicated hyper-surface in parameter space, and local approximations are a necessity for any analysis. Unfortunately, inequality constraints due to for example generator overexitation limiters, and higher codimension bifurcations, make the loadability surface non-smooth. One situation that is particularly difficult to handle is when a saddle-node bifurcation surface intersects a switching loadability limit surface. In this article we intend to investigate how several local approximations can be combined to obtain an adequate approximation of the loadability surface near such intersections.

  • 30.
    Perninge, Magnus
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Optimal activation of regulating bids to handle bottlenecks in power system operation2012In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 83, no 1, p. 151-159Article in journal (Refereed)
    Abstract [en]

    In this article we investigate how to optimally activate regulating bids to handle bottlenecks inpower system operation. This will lead to an optimal stopping problem, and activation of aregulating bid is to be performed when the transfer through a specific system bottleneck reachesa certain value. Compared to previous research in the area the work presented in this articleincludes a more detailed model of the structure of the regulating market, and reaction times ofactors on the regulating market is taken into consideration. The emphasis of the presentation willbe application to a two area test system. The method is compared to Monte Carlo simulation ina numerical example. The example shows a promising result for the suggested method.

  • 31.
    Perninge, Magnus
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Optimal distribution of primary control participation with respect to voltage stability2010In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 80, no 11, p. 1357-1363Article in journal (Refereed)
    Abstract [en]

    In competitive electricity markets the transmission system will at times be heavily loaded. At these occasions prevention of voltage instability is an important objective that the system operator has to meet. In this paper a method for finding the primary control participation that maximizes the margin from an operating point to the saddle-node bifurcation surface is proposed. The arising optimization problem is solved using a steepest descent method. The proposed method can find its applications both in generation planning and in real-time operation of electric power systems.

  • 32.
    Perninge, Magnus
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Risk estimation of the distance to voltage instability using a second order approximation of the saddle-node bifurcation surface2011In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 81, no 2, p. 625-635Article in journal (Refereed)
    Abstract [en]

    Prevention of voltage instability in electric power systems is an important objective that the systemoperators have to meet. Under certain circumstances the operating point of the power system may startdrifting towards the set of voltage unstable operating points. If no preventive measures are taken, aftersome time, the operating point may eventually become voltage unstable. It will thus be preferable to havea measure of the risk of voltage collapse in future loading states. This paper presents a novel method forestimation of the probability distribution of the load-space distance to the point where voltage instabilityinduced by saddle-node bifurcation occurs. Another result of the method is an estimate of the probabilitydistribution of the time to voltage instability for a power system with uncertain future loading scenarios.The method uses a second order approximation of the saddle-node bifurcation surface. The proposedmethod can be used in power system security assessments.

  • 33.
    Picciariello, Angela
    et al.
    KTH, School of Electrical Engineering (EES).
    Reneses, Javier
    KTH, School of Electrical Engineering (EES).
    Frias, P.
    KTH, School of Electrical Engineering (EES).
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Distributed generation and distribution pricing: Why do we need new tariff design methodologies?2015In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 119, p. 370-376Article, review/survey (Refereed)
    Abstract [en]

    Due to the increasing amount of DG (distributed generation) in distribution grids, new challenges are arising in the distribution sector in many countries. Depending on the DG penetration, location, concentration, size and generation technology, the DG impact on network costs can be either negative or positive. These additional costs or benefits can be allocated to the DG owners through network tariffs. New cost allocation methodologies, based on a cost causation principle, are therefore required. This paper addresses several issues arising within network tariff design due to the integration of DG. Furthermore, it reviews the methodologies proposed so far to tackle those issues. Recommendations for setting up a new, cost causation-based, methodology are finally drawn.

  • 34.
    Song, Meng
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Alvehag, Karin
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Widén, Joakim
    Parisio, Alessandra
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Estimating the impacts of demand response by simulating household behaviours under price and CO2 signals2014In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 111, p. 103-114Article in journal (Refereed)
    Abstract [en]

    To facilitate the implementation of demand response (DR), it is necessary to establish proper methods to estimate and verify the load impacts of it. This paper develops a simulation model to investigate the joint influence of price and CO2 signals in a DR program in the ex ante evaluation. It consists of a Markov-chain load model for forecasting the power demands of residential consumers and a scheduling program for providing optimal schedules for smart appliances. A case study of the Stockholm Royal Seaport project is analysed to demonstrate how to apply the simulation model to assess a DR program by simulating consumers' behaviour change in response to the DR signals. The results show that consumers' attitude to the signals and willingness to change (expressed by weight), and time preference) largely affect the load shift, bill saving and emission reduction. Moreover, by observing the load shifts over different lengths of the testing period, the model could also provide suggestions on the required testing period to get sufficient load data to distinguish the load patterns between consumers in different testing groups.

  • 35.
    Theethayi, N.
    et al.
    Uppsala University.
    Liu, Y. Q.
    Uppsala University.
    Montano, R.
    Uppsala University.
    Thottappillil, Rajeev
    Uppsala University.
    On the influence of conductor heights and lossy ground in multi-conductor transmission lines for lightning interaction studies in railway overhead traction systems2004In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 71, no 2, p. 186-193Article in journal (Refereed)
    Abstract [en]

    Railway overhead traction system is a classic example of scattered conductor configuration, where tracks and other wires form multi-conductor transmission lines (MTLs) with large variation in conductor heights above ground and they are spread across regions having different soil conditions. Lightning transient analysis in such systems has not received much attention earlier. Here we analyze the influence of conductor heights and lossy ground on the induced voltages in a two conductor MTLs for the case of a direct lightning strike. For transient analysis, modified time domain transient ground impedance expressions having better early and late time behavior was used. The dependence of transient ground impedance on conductor heights and ground resistivity are presented and discussed. The early time transient ground impedances are unaffected by ground resistivity but their decaying nature is highly dominated by ground resistivity. It is found, if one of the conductors is close to ground (a rail) and if it is at large vertical distance from struck conductor (an auxiliary power line), then with increasing ground resistivity the peak induced voltages in the conductor close to ground initially increase, then decrease and finally tend to remain constant (within 100-10,000 Omega m). This phenomenon is opposite to that compared to conductors that are close to each other with minimum vertical separation (two auxiliary power lines), where the peak induced voltages increase with increasing ground resistivity. The study focuses mainly to access when a mutual coupling due to system geometry or due to ground losses becomes dominant in determining induced effects from lightning in MTLs, which could be an important contribution to the lightning interaction studies for electrified railway systems.

  • 36.
    Theethayi, N.
    et al.
    Uppsala University.
    Liu, Y. Q.
    Uppsala University.
    Montano, R
    Uppsala University.
    Thottappillil, Rajeev
    Uppsala University.
    Zitnik, M.
    Uppsala University.
    Cooray, V.
    Uppsala University.
    Scuka, V.
    Uppsala University.
    A theoretical study on the consequence of a direct lightning strike to electrified railway system in Sweden2005In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 74, no 2, p. 267-280Article in journal (Refereed)
    Abstract [en]

    Direct lightning strike to a single-track electrified railway system in Sweden is modeled in this paper. Using this model, the induced voltages in each of the nine conductors at heights varying from 0.5 m (tracks) to 10 m above the ground are estimated. The effect of the finitely conducting ground is included using a time domain expression for the transient ground impedance that has better early time and late time behavior. The main interconnection between the conductors and the flashover strength of the supporting insulators is included in the simulations. A simple model for the arc channel during flashover of the insulators and the ionization of the soil around the pole foundations is also included in the model to assess the possible realistic surge voltage distribution in the system. It is shown in the paper that finite ground conductivity, interconnections between the conductors, arcing phenomena of insulation flashover and grounding of the poles decide the voltage/current distribution in the conductors. Simulations have been also carried out to determine the voltages on the lines and across the rails as function of distance from the point of strike as it could be a necessary data for deciding the possible future protection schemes. It was found that for a lightning stroke of 31 kA peak, large common mode and differential mode surges exist on the lines which could create excessive voltages between the line and neutral of the transformer and might pose a threat to the various low voltage equipments used for telecommunication, signaling and control.

  • 37.
    Theethayi, Nelson
    et al.
    Uppsala University.
    Thottappillil, Rajeev
    Uppsala University.
    Liu, Yaqing
    Uppsala University.
    Montano, Raul
    Uppsala University.
    Important parameters that influence crosstalk in multiconductor transmission lines2007In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 77, no 8, p. 896-909Article in journal (Refereed)
    Abstract [en]

    Transient surges in one of the overhead conductors, due to direct lightning strikes, causes crosstalk [C.R. Paul, Analysis of Multiconductor Transmission Lines, John Wiley & Sons, Inc., 1994; C.R. Paul, Introduction to Electromagnetic Compatibility, John Wiley & Sons, Inc., 1992] in other adjacent conductors. It is a common electromagnetic interference (EMI) phenomenon observed in power lines, communication lines and electrified railway lines. In this paper we investigate the crosstalk in multiconductor transmission lines (MTLs) above finitely conducting ground as a function of ground conductivity, heights of the receptor conductor and the terminal loads. For receptor conductor close to the ground, compared to the emitter conductor [C.R. Paul, Analysis of Multiconductor Transmission Lines, John Wiley & Sons, Inc., 1994; C.R. Paul, Introduction to Electromagnetic Compatibility, John Wiley & Sons, Inc., 19921, the decrease in ground conductivity increases the crosstalk peak currents at near end (end near to the source in the emitter conductor) of the receptor conductor, but at the far end it could either increase or decrease depending upon the line height and ground conductivity. It is found that the ground impedance [J.R. Carson, Wave propagation in overhead wires with ground return, Bell. Sys. Tech. J. 5 (1926) 539-554; Y.J. Wang, S.J. Liu, A review of methods for calculation of frequency dependant impedance of overhead power transmission lines, Proc. Natl. Sci. Cone. ROC (A), 25 (6), (2001) 329-338; E.D. Sunde, Earth conduction effects in transmission systems, 1st ed., Dover Publications Inc., New York, 1968; A. Deri, G. Tevan, A. Semlyen, A. Castanheira, The complex ground return plane a simplified model for homogenous & multilayer earth return, IEEE Trans. PAS 100 (8) (1981) 3686-3693; K.C. Chen, K.M. Damrau, Accuracy of approximate transmission line formulas for overhead wires, IEEE Trans. EMC 31 (4) (1989) 396-397; A. Semlyen, Ground return parameters of transmission lines an asymptotic analysis for very high frequencies, IEEE Trans. PAS 100 (3) (1981) 1031-1038; E.F. Vance, Coupling to Cable Shields, Wiley Interscience, New York, 1978; J.R. Wait, Theory of wave propagation along a thin wire parallel to an interface, Radio Sci. 7 (6) (1972) 675-679; R.G. Olsen, J.L. Young, D.C. Chang, Electromagnetic wave propagation on a thin wire above earth, IEEE Trans. Amen. Propag. 48 (9) (2000) 1413-1418; M. D’Amore, M.S. Sarto, Simulation models of a dissipative transmission line above a lossy ground for a wide-frequency range. I. Single conductor configuration, IEEE Trans. EMC 38 (2) (1996) 127-138; M. D’Amore, M.S. Sarto, Simulation models of a dissipative transmission line above a lossy ground for a wide-frequency range. II. Multiconductor configuration, IEEE Trans. EMC 38 (2) (1996) 139-149; F. Rachidi, C.A. Nucci, M. Ianoz, C. Mazzetti, Influence of lossy ground on lightning induced voltages on overhead lines, IEEE Trans. EMC 38 (3) (1996) 250-264; F. Rachidi, C.A. Nucci, M. lanoz, Transient analysis of multiconductor lines above a lossy ground, IEEE Trans. Power Deliv. 14 (1) (1999) 294-302; FM. Tesche, M.V. Ianoz, T. Karisson, EMC Analysis Methods and Computational Models, John Wiley and Sons Inc., 1997; A.K. Agrawal, H.J. Price, S.H. Gurbaxani, Transient response of multiconductor transmission lines excited by a nonuniform electromagnetic field, IEEE Trans. EMC 22 (2) (1980) 119-129] has profound influence in all the crosstalk cases studied here. Hence, a brief review and comparison of different closed form ground impedance expressions under the limits of transmission line approximation [EM. Tesche, M.V. lanoz, T Karlsson, EMC Analysis Methods and Computational Models, John Wiley and Sons Inc., 1997] and its behavior at both high and low frequencies is presented. It is shown that low frequency approximation of ground impedance is not sufficient for lightning transient studies involving ground conductivities lower than 10 mS/m. The observations presented in the paper have important implications in EMI studies of large distributed outdoor systems, such as the railway network, subjected to lightning strikes.

  • 38. Tohidi, Yaser
    et al.
    Aminifar, Farrokh
    Fotuhi-Firuzabad, Mahmoud
    Generation Expansion and Retirement Planning Based on the Stochastic Programming2013In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 104, p. 138-145Article in journal (Refereed)
  • 39.
    Tohidi, Yaser
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Hesamzadeh, Mohammad Reza
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Baldick, R.
    Biggar, D. R.
    Transmission network switching for reducing market power cost in generation sector: A Nash-equilibrium approach2017In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 146, p. 71-79Article in journal (Refereed)
    Abstract [en]

    The transmission network switching is proposed in the literature as a way to improve social welfare in liberalized power markets. 〈7〉 Moreover, exercise of market power by strategic generating companies (Gencos) causes some extra cost in electricity market which can be alleviated by implementing appropriate switching policies. This paper contributes to the existing literature by developing a mathematical model that explores, from an economic perspective, the transmission network switching in the context of market power. The strategic Gecnos are modelled based on the Cournot game. The Nash equilibrium of the game between Gencos is formulated as an equilibrium problem with equilibrium constraint (EPEC). The EPEC problem is transformed to a mixed-integer linear feasibility problem. To handle the multiple-Nash-equilibria situations, the solution concept of the extremal-Nash equilibrium (ENE) is introduced. A mixed-integer linear program (MILP) is derived for finding ENE. The transmission switching decisions are modelled as binary variables controlled by the system operator (TSO). The TSO minimizes the system dispatch cost calculated at ENE and network reconfiguration cost. The TSO minimizes the cost using its transmission switching decisions. The problem faced by the TSO is a mixed-integer bilevel linear program (MIBLP) with binary variables in both upper and lower levels. The upper level models the TSO's action and the lower level the oligopolistic Gencos (competing in Cournot game). A (parallel) branch-and-bound technique is used to solve the developed MIBLP model. An illustrative 3-bus example system and the IEEE-RTS96 are modelled and carefully studied. 〈8〉The numerical results demonstrate that: 1 – the (parallel) branch-and-bound technique can effectively solve the developed MIBLP, 2 – using the developed model, the system operator can change topology of the network by switching the lines in order to reduce the adverse effect of the strategic behaviour of Gecnos.

  • 40.
    Zhou, Helin
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Thottappillil, Rajeev
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Diendorfer, Gerhard
    A new approach to calculate electric fields and charge density distribution when lightning strikes a tall object2014In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 113, no SI, p. 15-24Article in journal (Refereed)
    Abstract [en]

    We derive electric field expressions, associated with lightning strikes to a tall object, using the monopole (Continuity Equation) technique which is distinctly different from the traditional dipole (Lorentz Condition) technique. Expressions to calculate the charge density along the tall object and lightning channel based on the assumptions of the transmission line model of the lightning strikes to a tall object and a series point current source placed at the object top, are also derived. These expressions are used to calculate the very close-range electric fields in the monopole (Continuity Equation) technique in terms of the retarded current and charge density along the tower and lightning channel and their results are compared with those calculated from the traditional dipole (Lorentz Condition) technique in terms of the retarded current along the tower and lightning channel. Alternative explanations are provided to the inversion of polarity of the vertical electric field at very close range based on distribution of charge density along the tower and lightning channel.

  • 41. Zhu, K.
    et al.
    Rahimi, S.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Industrial Information and Control Systems.
    Zhang, B.
    Design phasor data concentrator as adaptive delay buffer for wide-area damping control2015In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 127, p. 22-31Article in journal (Refereed)
    Abstract [en]

    Phasor Data Concentrators (PDCs) are employed to aggregate data frames from multiple Phasor Measurement Units (PMUs) with the same time-stamp into data packets. This function is an essential step in the utilization of PMU data in Wide-Area Monitoring and Control (WAMC) system. This is facilitated by assigning a fixed and heuristically decided wait time to the PDC buffer. The processed PMU data packets are sent to WAMC applications or similarly PDCs at higher hierarchies in the WAMC system when either a PDC buffer is full or its wait time has passed. In this paper, two methods to configure adaptive PDC wait time based on recent PMU traffic delay patterns are proposed. The purpose is to reduce the frequency of performing delay compensations in wide-area damping control system. With the adaptive PDC delay buffer in place, the wide-area damping controller only switches its gain once every five seconds, given the studied PMU traffic delay scenarios, instead of the current practice which requires the control gain to be adapted on a per PMU data frame basis, e.g., 50 or 60 times every second. The proposed methods offer a perspective to efficiently utilize the supporting Information and Communication Technology infrastructure with the purpose to simplify the design and implementation of wide-area damping control system.

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