# [comp] Sähkötekniikan korkeakoulu / ELEC

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Item Circuit paradigm in the 21(Helsinki University of Technology, 2001) Porra, Veikko; Valtonen, Martti; Hartimo, Iiro; Ilmonen, Matti; Simula, Olli; Veijola, Timo; Department of Electrical and Communications Engineering; Sähkö- ja tietoliikennetekniikan osastoItem Digital implementation of full-order flux observers for induction motors(European Power Electronics and Drives Association, 2002) Hinkkanen, Marko; Luomi, Jorma; Electric Drives; Sähkötekniikan ja automaation laitos; Department of Electrical Engineering and Automation; Sähkötekniikan korkeakoulu; School of Electrical EngineeringThis paper deals with flux estimation for induction motor drives by using a full-order flux observer. A problem of full-order flux observers is their need for computationally demanding discretization methods in order to work stably and accurately at high speeds. An implementation of the full-order flux observer using the stator and rotor fluxes as state variables in the stator reference frame and in the rotor reference frame, respectively, was recently proposed. This paper describes how an observer gain can be included in this structure. It is shown that discretization errors of the proposed implementation are small and that there is more freedom to choose an observer gain, even if the simple forward Euler discretization is used.Item Influence of magnetic saturation on induction motor model selection(International Conference on Electrical Machines (ICEM), 2006) Hinkkanen, Marko; Repo, Anna-Kaisa; Luomi, Jorma; Sähkötekniikan ja automaation laitos; Department of Electrical Engineering and Automation; Electric Drives; Sähkötekniikan korkeakoulu; School of Electrical EngineeringEffects of magnetic saturation on various equivalent circuit models of an induction motor are studied. The parameters of 2.2-kW and 37-kW motors with closed rotor slots are analyzed using finite element computations. Both skewed and unskewed rotor slots are studied. The magnetizing inductance not only depends on the main flux, but is also dependent on the rotor current, especially if the rotor slots are skewed or closed. The stator leakage inductance is essentially constant, while the rotor leakage inductance depends significantly on the rotor current. The performance of vector-controlled drives can be improved by using a T or Γ-equivalent circuit model whose inductances also depend on the rotor current.Item Small-signal model for saturated deep-bar induction machines(Institute of Electrical & Electronics Engineers (IEEE), 2007) Hinkkanen, Marko; Repo, Anna-Kaisa; Cederholm, Mikaela; Luomi, Jorma; Sähkötekniikan ja automaation laitos; Department of Electrical Engineering and Automation; Electric Drives; Sähkötekniikan korkeakoulu; School of Electrical EngineeringA small-signal model is presented for saturated deep-bar induction machines. Inductances are allowed to saturate as a function of their own current (or flux), and the mutual saturation effect originating mainly from closed or skewed rotor slots is also included in the model. The model fulfills the reciprocity conditions, and it can applied to parameter estimation and to the analysis and development of flux angle estimation methods. The model is applied to estimating the parameters of a 37-kW deep-bar cage-induction machine, using the data from time-stepping finite-element analysis (FEA). The proposed model fits very well to the FEA data in a wide frequency range.Item Understanding of tuning techniques of converter controllers for VSC-HVDC(Helsinki University of Technology, 2008) Bajracharya, Chandra; Molinas, Marta; Suul, Jon Are; Undeland, Tore M.A mathematical model of a voltage source converter is presented in the synchronous reference frame for investigating VSC-HVDC for transferring wind power through a long distance. This model is used to analyze voltage and current control loops for the VSC and study their dynamics. Vector control is used for decoupled control of active and reactive power and the transfer functions are derived for the control loops. In investigating the operating conditions for HVDC systems, the tuning of controllers is one of the critical stages of the design of control loops. Three tuning techniques are discussed in the paper and analytical expressions are derived for calculating the parameters of the current and voltage controllers. The tuning criteria are discussed and simulations are used to test the performance of such tuning techniques.Item Designing maximum length sequence signal for frequency response measurement of switched mode converters(Helsinki University of Technology, 2008) Vilkko, Matti; Roinila, TomiSwitched mode power converters are extensively used in powering consumer products. Requirements of short time-to-market and low bill of materials set new challenges to designers. The designers should efficiently create stable and robust designs using low-cost components with high parameter variations. Methods to test statistically the dynamical properties of final design would enable the designers to evaluate control design before production ramp-up. However, currently available testing methods require intervention into internal circuitry or are time consuming and too laborious. This paper describes how Maximum Length Binary Sequences can be used to obtain a high through-put frequency response measurement method to analyze the dynamics of switched mode converters. Some well known computational methods for the frequency response calculation are listed and the design of an appropriate maximum length excitation signal is presented. The proposed signal design procedure takes time-aliasing effect, noise reduction, and the disturbing harmonics generated by pulse width modulation of the converter controller into consideration. The results are verified by experimental measurements from a high-frequency switched mode converter.Item StatCom with capacitive energy storage for compensation of cyclic loads(Helsinki University of Technology, 2008) Xie, Hailian; Ängquist, Lennart; Nee, Hans-PeterPulsating power consumption of a cyclic load may cause considerable disturbances in the voltage at the connection point. A StatCom can be employed to maintain the bus voltage level. With the integration of energy storage to the StatCom, the power quality at the connection point can be further improved by the additional active power compensation capability. This paper studies the control strategy for a StatCom with capacitive energy storage for compensation of a cyclic load. Simulations verify the proposed control scheme and show that both the phase jump and magnitude deviation of the bus voltage can be reduced by means of reactive and active power compensation.Item Current-sourced buck converter(Helsinki University of Technology, 2008) Leppäaho, Jari; Karppanen, Matti; Suntio, TeuvoSolar and magnetic energy harvesting may benefit from the use of current-sourced converters for interfacing those sources into the practical usage due to their current-source nature. The paper investigates the implementation and dynamics of such converters by means of a current-sourced buck converter. Duality concepts are applied to obtain the converter from the corresponding voltage-sourced buck converter. The dynamic analysis is carried out in continuous capacitor-voltage-mode under direct-duty-ratio control. The theoretical findings are verified by extracting the transfer functions from the Matlab™-Simulink-based switching models.Item Selecting the accurate solar panel simulation model(Helsinki University of Technology, 2008) Leban, Krisztina; Ritchie, EwenThis paper considers selection of the suitable simulation model for solar panels [1]. Two models for silicon polycrystalline photovoltaic solar cells [2], [3] were analysed. The selection is made by comparing the Power vs. Voltage and Current vs. Voltage curves obtained from the simulation to those obtained experimentally. The goal is to identify if a simulation model gives correct information about the behaviour of one single solar panel for varying temperature and irradiance. Final validation is done by comparing experimental and simulation results. The single panel model was integrated into array of 16 elements. This can be integrated in the simulation of an isolated micro grid or in a hybrid energy system [4], [5]. By comparing simulation results from both models with experimental results we can conclude that model 2 is the most reliable one.Item Grid connected converters for photovoltaic, state of the art, ideas for improvement of transformerless inverters(Helsinki University of Technology, 2008) Schimpf, Fritz; Norum, Lars E.The paper presents a short overview of the state of the art for grid tied PV inverters at low and medium power level (1..100 kW), mainly intended for rooftop applications. The inverters are categorized according to the configuration of the PV system, the configuration of the conversion stages within the inverter and whether they use transformers or not [1], [3]. After the introduction of the state of the art of inverters for PV systems with and without transformers, the paper focuses on some known problems and challenges for transformerless inverters. Topologies without transformers have big advantages like low weight, volume and cost. In addition they often reach higher efficiencies than topologies with transformers. Therefore they are important for future developments.Item A discussion about a start-up procedure of a doubly-fed induction generator system(Helsinki University of Technology, 2008) Da Silva, J. L.; Oliveira, R. G. de; Silva, S. R.; Rabelo, B.; Hofmann, W.The present work proposes a start-up procedure for a doubly-fed induction generator (DFIG) driven by a wind turbine. In the DFIG topology the stator is connected direct to the grid while the rotor is connected to a back-to-back converter. This structure requires some care during grid synchronization to avoid undesired overloads. The main goal of a start-up procedure is to reduce the stresses on the electrical and mechanical components during synchronization. In this work, it is achieved by equalization among the induced stator voltage, the voltage over the filter capacitor and the grid voltage. A phase-locked-loop (PLL) computes the grid voltage phase displacement required for the system control orientation and synchronization procedures. Previous to the starting-up, the output currents set-points of the grid-side converter and rotor-side converter are computed in order to produce the same grid voltage over the filter capacitors and on the stator terminals. The whole procedure is summarized in seven different steps: turbine acceleration by the pitch control, DC-link pre-charge; initialization of the filter current controllers; filter synchronization; DC-Link voltage adjust; rotor current controllers' initialization and stator synchronization. The proposed start-up procedure was tested in a 4 kW test bench and the description of each step as well as simulation and experimental results are presented throughout the paper.Item Power collection and integration on the electric grid from offshore wind parks(Helsinki University of Technology, 2008) Mogstad, Anne Berit; Molinas, MartaThere is a lot of potential in offshore wind parks due to the amount of available area. The parks get bigger in size and will continue to grow in the future. In this article a new converter topology for offshore wind parks is proposed. This topology is meant to be for large offshore wind parks sited far from shore and it is based on DC collection and transmission. All the converters are located in the nacelle of the wind turbines and the turbines are connected in series directly connected to shore without any transformation stages. The electrical system, from the generator to the grid connection of the turbine, is modeled in PSCAD. The model consists of an induction machine, a 3-phase AC to 1-phase AC converter, a high frequency high power transformer and a full-bridge converter. The AC-AC converter has a new type of reverse-blocking IGBTs and the switches are controlled with a dedicated switching pattern. The simulations show that the switching pattern gives the expected square wave voltage from the AC-AC converter. The new converter topology reduces the converter losses due to fewer converter stages, the architecture of the reverse-blocking IGBT and the new switching pattern.Item Insertion loss and network parameters in the analysis of power filters(Helsinki University of Technology, 2008) Kostov, Konstantin S.; Kyyrä, Jorma J.; Department of Electrical Engineering; Sähkötekniikan laitos; Faculty of Electronics, Communications and Automation; Elektroniikan, tietoliikenteen ja automaation tiedekuntaThe insertion loss (IL) is regarded as the best interference suppression characteristic of power filters or suppression components. The IL definitions are considered and as an alternative the paper suggests the use of network parameters. It is a known fact that the standard IL measurements do not provide reliable information about the operational performance of a suppressor. This is largely due to the source and load mismatch, which is typical in power lines. Arguments are presented, showing that network parameters allow for more complete and reliable characterization of power filters and components. The IL would not be abandoned, because the network parameters provide enough information to obtain not only the standard IL, but also the IL in a non 50 Ω system. A new treatment of "worst case" or minimum IL is proposed, which is also based on network parameters. Furthermore, input, output, or transfer impedances, simulation models, and other characteristics, can be obtained from the network parameters, but not from the currently published standard IL data.Item Inductance identification of an induction machine taking load-dependent saturation into account(Institute of Electrical & Electronics Engineers (IEEE), 2008) Ranta, Mikaela; Hinkkanen, Marko; Luomi, Jorma; Sähkötekniikan ja automaation laitos; Department of Electrical Engineering and Automation; Electric Drives; Sähkötekniikan korkeakoulu; School of Electrical EngineeringThe paper proposes an identification method for the inductances of induction machines, based on signal injection. Due to magnetic saturation, a saturation-induced saliency appears in the induction motor, and the total leakage inductance estimate depends on the angle of the excitation signal. The proposed identification method is based on a small-signal model that includes the saturation-induced saliency. Because of the saturation, the load also affects the estimate, and measurements are needed in different operating points. Using the identified total leakage inductance, an estimate of the stator inductance can be obtained. The identification method is applied to computer simulations and laboratory experiments of a 2.2-kW induction motor.Item Modeling of Mutual Saturation in Induction Machines(Institute of Electrical & Electronics Engineers (IEEE), 2008) Tuovinen, Toni; Hinkkanen, Marko; Luomi, Jorma; Sähkötekniikan ja automaation laitos; Department of Electrical Engineering and Automation; Electric Drives; Sähkötekniikan korkeakoulu; School of Electrical EngineeringMutual saturation between the main flux and the rotor leakage flux appears in induction machines, especially if the rotor slots are skewed or closed. Conventional saturation models used in connection with dynamic equivalent-circuit models do not take this phenomenon into account. In this paper, explicit functions for modeling the mutual saturation are proposed. The proposed functions are physically reasonable, they are easy to fit, and the number of their parameters is small. The proposed functions can be used in real-time applications and in computer simulations where high accuracy and physical consistency are preferable. The model fits well to the data obtained from finite element analysis or experimental data of a 2.2-kW motor.Item Variable speed pumped storage hydropower for integration of wind energy in isolated grids : case description and control strategies(Helsinki University of Technology, 2008) Suul, Jon Are; Uhlen, Kjetil; Undeland, ToreThis paper presents the use of variable speed pumped storage hydropower plants for balancing power fluctuations from wind power in an isolated grid. A topology based on a synchronous machine and a full scale back-to-back voltage source converter is suggested for obtaining variable speed operation of a pump-turbine unit. This topology has not been previously investigated for variable speed pumped storage power plants, but can now be considered relevant for small and medium sized pumped storage units because of the development of voltage source converter drives for higher voltage levels. A possible case for implementation of such a system is described based on the situation on the Faroe Islands, where controllable energy storage can help to allow for a higher share of renewable energy in the power system and by that to reduce the dependency on fossil fuels. Power control of the pumped storage unit by load following for direct compensation of the fluctuations in power output from a wind farm will limit the influence on the operation of the rest of the grid. By utilizing the pumped storage to take part in the primary frequency control of the power system, the frequency response to other changes in production or load will also be improved.Item Keynote presentation: Recent trends in variable speed wind turbine drives(Helsinki University of Technology, 2008) Niiranen, JoukoItem SCEE 2008 book of abstracts : the 7th International Conference on Scientific Computing in Electrical Engineering (SCEE 2008), September 28 – October 3, 2008, Helsinki University of Technology, Espoo, Finland(Helsinki University of Technology, 2008) Roos, Janne; Costa, Luis R. J.; Department of Radio Science and Engineering; Radiotieteen ja -tekniikan laitos; Faculty of Electronics, Communications and Automation; Elektroniikan, tietoliikenteen ja automaation tiedekuntaThis report contains abstracts of presentations given at the SCEE 2008 conference.Item Control principles for blackstart and island operation of microgrid(Helsinki University of Technology, 2008) Laaksonen, Hannu; Kauhaniemi, KimmoIn some unexpected situations a microgrid may become unstable after transition to islanded mode and all DG units must be disconnected from microgrid. In case of these events a restoration strategy for microgrid blackstart is needed. Also if the islanded microgrid is divided into different protection zones in case of a fault, fault management strategy with capability of very fast operation is needed to maintain stability in the healthy section of the islanded microgrid. The control of microgrid voltage and frequency during the microgrid blackstart is not possible without energy storage unit. In this paper sequence of actions for the microgrid blackstart operation as well as control principles of some DG units during blackstart are defined and simulated with two different microgrid configurations. Also one simulation case considering fault management strategy and control principles during fault in islanded microgrid is presented. Based on the simulations dimensioning principles for the needed energy storage and size of simultaneously controlled loads can be drawn.Item An LVDC distribution system concept(Helsinki University of Technology, 2008) Salonen, Pasi; Kaipia, Tero; Nuutinen, Pasi; Peltoniemi, Pasi; Partanen, JarmoAn LVDC distribution system is a new innovation in a field of electricity distribution. This paper presents the basic principles to use power electronics devices in electricity distribution systems. Recent technological and economical developments in power electronic components enable to apply power electronics in LV network. This development makes an LVDC distribution system concept establishment possible. In this respect the occurring progression connects power electronics and distribution system development strongly together. The LVDC distribution system basic concept concentrates at DC/AC interface focusing on implementation of customer-end inverter. The previously made analyses have shown that presented LVDC distribution system has existing techno-economical potential in electricity distribution networks.