515 research outputs found

    A Review on the Need of HVDC Transmission System

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    With the growing demand of electricity on a daily basis, we cannot rely on conventional electric authority systems like long-haul distributed power stations as well as complicated and heavy load / distribution networks. High voltage direct current (HVDC) transmission systems include an extremely imperative role in authority systems. Without the appropriate study of the HVDC system, it is unfeasible to obtain an accurate mathematical model of the system and in the absence of proper modeling the influence transmitted in the HVDC system cannot be considered. The power transmitted through the HVDC system depends upon the competence of the controller and the converter station.Conservatively, the PID controller was used for the polar current control of the rectifier and the excitation control on the inverter side. This paper is an indication of the HVDC system and covers the essential part of the foundation of the HVDC system. Due to enlarged demand for power at the load center and concentration to distributed power generation, a lot of high capacity long distance HVDC systems are requisite and are intended to achieve various advantages. As growth in the power electronics field advances, HVDC systems are more consistent

    Synchronous Machine Emulation of Vsc for Interconnection of Renewable Energy Sources through Hvdc Transmission

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    The majority of the energy demand over the past years has been fulfilled by centralized generating stations. However, with a continuously increasing energy demand, the integration of decentralized renewable energy sources (RES) into the power system network becomes inevitable even though these sources affect the stability of the grid due to their intermittency and use of various power converters. The transmission of power over long distances from RES is usually accomplished either by AC or DC transmission. High voltage DC transmission (HVDC) is preferred over high voltage AC transmission (HVAC) due to numerous and complex reasons, such as its lower investment cost for long transmission cables, lower losses, controllability, and limited short circuit currents. Several control methods for grid-connected voltage source converters (VSCs), such as power-angle and vector-current controls, are being adopted in RES interconnections. However, these methods face several issues when used for a weak grid interconnection. This thesis develops a control strategy for a VSC–HVDC transmission system by referring to the synchronverter concept. In the proposed method, the sending-end rectifier controls emulate a synchronous motor (SM), whereas the receiving end inverter emulates a synchronous generator (SG) to transmit power from one grid to another. The two converters connected by a DC line provide a synchronverter HVDC (SHVDC) link. Given the high demand for sustainable energy, integrating RES—which can be extended to wind-based resources—into the long-haul HVDC link becomes essential. Therefore, in this thesis, a windfarm with a type 4 permanent magnet SG is integrated into the HVDC link through a rectifier. Depending on the wind speed, the proposed control strategy automatically shares and manages the wind generator power on the DC side by using a battery energy storage system (BESS) connected to the HVDC link to stabilize the power fluctuations generated by the intermittency of the wind farm. The performance of the synchronverter-based HVDC transmission was verified by using a MATLAB Simulink model. Results show that the controller can effectively control the power flow from one grid to another and that the effect of wind fluctuation on the grid can be mitigated by introducing a BESS at the DC link. Therefore, by properly controlling the SHVDC, BESS, and RES connected to the HVDC system, the power from remote RES can be connected to a weak AC grid in a stable manner

    Pregled različitih tehnologija upravljanja naprednim mrežama za povećanje fleksibilnosti elektroenergetskih sustava i omogućavanje masovne integracije obnovljivih izvora energije

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    Over the last 15 years, major changes have taken place in the electricity sector. A significant increase in the share of renewable energy sources (RES) with variable generation, followed by the decommissioning of conventional power plants based on fossil fuels, has dramatically changed the way of the power system (EPS) operation. During this time, there has been inadequate and untimely investment in the transmission infrastructure. This occurred partly due to the lack of funding, and partly due to the climate change and the rising environmental awareness, as well as the influence of green activists making it difficult to obtain permits to build electrical grid facilities. Additionally, electricity consumption is steadily increasing due to population growth in the undeveloped and developing countries, and due to the rising living standard in the developed countries. Therefore, global electricity consumption is expected to triple by 2050. To meet the new demands, Transmission System Operators (TSOs) are deploying advanced transmission technologies based on a comprehensive application of information and communication solutions. These technologies increase the capacity, efficiency, and reliability of both the existing and new elements of the transmission system. These solutions applied vary from system to system and depend on many influencing factors. The application of these advanced technologies is particularly important for congestion management, as the power system operates closer and closer to stability limits, increasing the risk of collapse. The paper describes the technologies that transform the existing network into smart grids, primarily from the point of view of increasing the capacity of the existing infrastructure through different smart grid investments.U posljednjih 15 godina u elektroenergetskom sektoru dogodile su se velike promjene. Značajno povećanje udjela obnovljivih izvora energije (OIE) s varijabilnom proizvodnjom, praćeno gašenjem konvencionalnih elektrana na fosilna goriva, dramatično je promijenilo način rada elektroenergetskog sustava (EES). Tijekom tog vremena bilo je neodgovarajućih i nepravovremenih ulaganja u prijenosnu infrastrukturu. To se dogodilo dijelom zbog nedostatka financijskih sredstava, a dijelom zbog klimatskih promjena i porasta ekološke svijesti, kao i utjecaja zelenih aktivista koji su otežali dobivanje dozvola za izgradnju energetskih objekata. Osim toga, potrošnja električne energije u stalnom je porastu zbog rasta stanovništva u nerazvijenim zemljama i zemljama u razvoju te zbog povećanja životnog standarda u razvijenim zemljama. Stoga se očekuje da će se globalna potrošnja električne energije utrostručiti do 2050. Kako bi zadovoljili nove zahtjeve, operatori prijenosnih sustava (TSO) uvode napredne tehnologije prijenosa temeljene na sveobuhvatnoj primjeni informacijskih i komunikacijskih rješenja. Ove tehnologije povećavaju kapacitet, učinkovitost i pouzdanost postojećih i novih elemenata prijenosnog sustava. Ova primijenjena rješenja razlikuju se od sustava do sustava i ovise o mnogim utjecajnim čimbenicima. Primjena ovih naprednih tehnologija posebno je važna za upravljanje zagušenjima jer elektroenergetski sustav radi sve bliže i bliže granicama stabilnosti, povećavajući rizik od njegovog sloma. U radu su opisane tehnologije koje transformiraju postojeću mrežu u napredne elektroenergetske mreže, prvenstveno sa stajališta povećanja kapaciteta postojeće infrastrukture kroz različite investicije u napredne tehnologije

    Series connection of power semiconductors for medium voltage applications

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    The series connection of power semiconductor devices allows the operation at voltage levels higher than the levels allowed by one single semiconductor. However, due to individual parameter differences of the series connected devices it is difficult to ensure a proper voltage balance between the series connected power devices and if any semiconductor exceeds its maximum blocking voltage it will fail. Because of its gate controllability and its low gate energy requirements, the IGBT is the preferred choice when high number of switching devices must be connected in series. In this research work an IGBT gate driver has been developed which will control the behaviour of the IGBT during the switching process. In consequence, this gate driver should ensure a proper voltage balance between the series connected IGBT devices. Basically, this PhD research work deals with the analysis and the modelling of the behaviour of the IGBT / Diode, proposes an active gate control and shows its validity for the series connection of IGBT / Diode devices. Finally, voltage source converter topologies are briefly compared for reactive power compensation applications at Medium Voltage utility grids. The required blocking voltage capacity is achieved by means of the series connection of power semiconductor devices.La conexión en serie de semiconductores de potencia permite trabajar a tensiones de trabajo superiores a las que podría soportar un único semiconductor. Sin embargo, debido a diferencias en las características de los propios semiconductores es difícil garantizar el equilibrado adecuado de las tensiones de trabajo entre los distintos semiconductores conectados en serie. Si algún semiconductor supera su máxima tensión de trabajo este fallará. Debido a su controlabilidad y bajo requerimiento energético por puerta el IGBT es la opción preferida cuando se requiere la conexión en serie de gran cantidad de semiconductores. En este trabajo de investigación se ha desarrollado un driver para IGBT que permita el control del proceso de conmutación del IGBT y garantice el equilibrado de las tensiones entre los IGBTs conectados en serie. Básicamente, en este trabajo de investigación se presenta el análisis y modelado del comportamiento del IGBT/Diodo, el control activo empleado para controlar el proceso de conmutación del IGBT y su validez para la conextión en serie. Finalmente, se presenta una pequeña comparación de convertidores de fuente de tensión para aplicaciones de compensación de energía reactiva conectados directamente a redes de Media Tensión. La capacidad de bloqueo requerida se obtiene mediante la conexión en serie de semiconductores de potencia.Potentzi erdi eroaleen serie elkarketak, erdi eroale batek jasan dezakeena baino tentsio maila altuagoan lan egitea ahalbideratzen du. Hala ere, erdi eroaleen arteko ezaugarri ezberditasunak direla eta, zaila egiten da tentsio banaketa egokia zihurtatzea. Erdi eroaleetariko batek bere gehienezko tentsio maila gainditzen badu honek huts egingo du. Erdi eroale asko seriean elkartu behar direnean IGBT-a izaten da aukerarik hobetsiena bere kontrolagarritasuna eta behar duen ateko energia maila baxua dela eta. Ikerketa lan honetan IGBT baten ateko “driver”-a garatu da. Honek IGBT-aren konmutazio prozesua kontrolatu behar du eta aldi berean, seriean elkarturiko IGBT-en artean, tentsio banaketa egokia lortu. Funtsean, ikerketa lan honetan IGBT eta Diodo-aren analisia eta modelatua erakusten dira. Modu berean “driver”-ean erabilitako kontrola eta bere baliozkotasuna IGBT/Diodo-en serie elkarketarako erakusten dira. Azkenik, Tentsio Ertaineko sarera zuzenean konektaturiko Tentsio Iturri Bihurgailuen arteko konparaketa bat egin da. Sareko tentsio maila altua dela eta erdi eroaleen serie elkarketa derrigorrezkoa da

    Power and frequency control of an offshore wind farm connected to grid through an HVDC link with LCC-based rectifier

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    Mención Internacional en el título de doctorThere is an increasing interest in the use of line-commutated converter (LCC) technology to connect large offshore wind farms (OWFs) placed far from the coast by means of a high voltage direct current (HVDC) link. This is due to the better features of LCCs compared to voltage-source converters in terms of cost, reliability and efficiency. However, this technology requires a frequency control in the OWF to allow the operation of both the wind turbine generator systems (WTGSs) and the LCC rectifier. Therefore, this Thesis presents two frequency control proposals. First, a centralized voltage and frequency control for an OWF connected through LCC-rectifier-based HVDC link is proposed. It is derived from an enhanced LCC-rectifier station average-value model which indicates that the active power balance at the point of common coupling drives the OWF voltage while the corresponding reactive power balance drives the OWF frequency. Even though voltage control cannot be applied in case of using a diode rectifier, the voltage magnitude variation is clamped between acceptable values. As a second proposal, a decentralized frequency control for the diode-rectifier-based HVDC link connection of OWFs is also presented. This control is based on a reactive power / frequency droop which allows the WTGSs to reach synchronous operation and equally share the reactive power without the need of communications among the WTGSs. Moreover, the control proposals do not rely on a phase-locked loop, so controls are not subject to grid disturbances or measurement noise. Another important specification of the proposed control strategies is that they do not modify the active power control channel of the WTGSs. Finally, the stability and the simulation results to assess the performance of both control proposals are studied.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: Alireza Nami.- Secretario: Oriol Gomis Bellmunt.- Vocal: Ana Belén Morales Martíne

    HVDC transmission : technology review, market trends and future outlook

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    HVDC systems are playing an increasingly significant role in energy transmission due to their technical and economic superiority over HVAC systems for long distance transmission. HVDC is preferable beyond 300–800 km for overhead point-to-point transmission projects and for the cable based interconnection or the grid integration of remote offshore wind farms beyond 50–100 km. Several HVDC review papers exist in literature but often focus on specific geographic locations or system components. In contrast, this paper presents a detailed, up-to-date, analysis and assessment of HVDC transmission systems on a global scale, targeting expert and general audience alike. The paper covers the following aspects: technical and economic comparison of HVAC and HVDC systems; investigation of international HVDC market size, conditions, geographic sparsity of the technology adoption, as well as the main suppliers landscape; and high-level comparisons and analysis of HVDC system components such as Voltage Source Converters (VSCs) and Line Commutated Converters (LCCs), etc. The presented analysis are supported by practical case studies from existing projects in an effort to reveal the complex technical and economic considerations, factors and rationale involved in the evaluation and selection of transmission system technology for a given project. The contemporary operational challenges such as the ownership of Multi-Terminal DC (MTDC) networks are also discussed. Subsequently, the required development factors, both technically and regulatory, for proper MTDC networks operation are highlighted, including a future outlook of different HVDC system components. Collectively, the role of HVDC transmission in achieving national renewable energy targets in light of the Paris agreement commitments is highlighted with relevant examples of potential HVDC corridors

    Hybrid AC-High Voltage DC Grid Stability and Controls

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    abstract: The growth of energy demands in recent years has been increasing faster than the expansion of transmission facility construction. This tendency cooperating with the continuous investing on the renewable energy resources drives the research, development, and construction of HVDC projects to create a more reliable, affordable, and environmentally friendly power grid. Constructing the hybrid AC-HVDC grid is a significant move in the development of the HVDC techniques; the form of dc system is evolving from the point-to-point stand-alone dc links to the embedded HVDC system and the multi-terminal HVDC (MTDC) system. The MTDC is a solution for the renewable energy interconnections, and the MTDC grids can improve the power system reliability, flexibility in economic dispatches, and converter/cable utilizing efficiencies. The dissertation reviews the HVDC technologies, discusses the stability issues regarding the ac and HVDC connections, proposes a novel power oscillation control strategy to improve system stability, and develops a nonlinear voltage droop control strategy for the MTDC grid. To verify the effectiveness the proposed power oscillation control strategy, a long distance paralleled AC-HVDC transmission test system is employed. Based on the PSCAD/EMTDC platform simulation results, the proposed power oscillation control strategy can improve the system dynamic performance and attenuate the power oscillations effectively. To validate the nonlinear voltage droop control strategy, three droop controls schemes are designed according to the proposed nonlinear voltage droop control design procedures. These control schemes are tested in a hybrid AC-MTDC system. The hybrid AC-MTDC system, which is first proposed in this dissertation, consists of two ac grids, two wind farms and a five-terminal HVDC grid connecting them. Simulation studies are performed in the PSCAD/EMTDC platform. According to the simulation results, all the three design schemes have their unique salient features.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

    Control of Voltage-Source Converters Considering Virtual Inertia Dynamics

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    Controlling power-electronic converters in power systems has significantly gained more attention due to the rapid penetration of alternative energy sources. This growth in the depth of penetration also poses a threat to the frequency stability of modern power systems. Photovoltaic and wind power systems utilizing power-electronic converters without physical rotating masses, unlike traditional power generations, provide low inertia, resulting in frequency instability. Different research has developed the control aspects of power-electronic converters, offering many control strategies for different operation modes and enhancing the inertia of converter-based systems. The precise control algorithm that can improve the inertial response of converter-based systems in the power grid is called virtual inertia. This thesis employs a control methodology that mimics synchronous generators characteristics based on the swing equation of rotor dynamics to create virtual inertia. The models are also built under different cases, including grid-connected and islanded situations, using the swing equation with inner current and voltage outer loops. Analysis of the simulation results in MATLAB/Simulink demonstrates that active and reactive power are independently controlled under the grid-imposed mode, voltage and frequency are controlled under the islanded mode, and frequency stability of the system is enhanced by the virtual inertia emulation using swing equation. On this basis, it is recommended that the swing equation-based approach is incorporated with the current and voltage control loops to achieve better protection under over-current conditions. Further works are required to discover other factors that could improve the effectiveness of the models
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