• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 93
  • 21
  • 19
  • 15
  • 11
  • 8
  • 5
  • 5
  • 3
  • 3
  • 1
  • 1
  • 1
  • Tagged with
  • 226
  • 68
  • 64
  • 62
  • 54
  • 49
  • 44
  • 40
  • 38
  • 37
  • 33
  • 32
  • 32
  • 30
  • 28
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Factors affecting the reliability of VSC-HVDC for the connection of offshore windfarms

Beddard, Antony James January 2014 (has links)
The UK Government has identified that nearly 15% of the UK’s electricity generation must come from offshore wind by 2020. The reliability of the offshore windfarms and their electrical transmission systems is critical for their feasibility. Offshore windfarms located more than 50-100km from shore, including most Round 3 offshore windfarms, are likely to employ Voltage Source Converter (VSC) High Voltage Direct Current (HVDC) transmission schemes. This thesis studies factors which affect the reliability of VSC-HVDC transmission schemes, in respect to availability, protection, and system modelling. The expected availability of VSC-HVDC systems is a key factor in determining if Round 3 offshore windfarms are technically and economically viable. Due to the lack of publications in this area, this thesis analyses the energy availability of a radial and a Multi-Terminal (MT) VSC-HVDC system, using component reliability indices derived from academic and industrial documentation, and examining the influence of each component on the system’s energy availability. An economic assessment of different VSC-HVDC schemes is undertaken, highlighting the overall potential cost savings of HVDC grids. The connection of offshore windfarms to a MT HVDC system offers other potential benefits, in comparison to an equivalent radial system, including a reduction in the volume of assets and enhanced operational flexibility. However, without suitable HVDC circuit breakers, a large MT HVDC system would be unviable. In this thesis, a review of potential HVDC circuit breaker topologies and HVDC protection strategies is conducted. A HVDC circuit breaker topology, which addresses some of the limitations of the existing designs, was developed in this thesis, for which a UK patent application was filed. Accurate simulation models are required to give a high degree of confidence in the expected system behaviour. Modular Multi-level Converters (MMCs) are the preferred HVDC converter topology, however modelling MMCs in Electromagnetic Transient (EMT) simulation programs has presented a number of challenges. This has resulted in the development of new modelling techniques, for which the published validating literature is limited. In this thesis these techniques are compared in terms of accuracy and simulation speed and a set of modelling recommendations are presented. Cable models are the other main DC component which, upon analysis, is found to have a significant impact on the overall model’s simulation results and simulation time. A set of modelling recommendations are also presented for the leading cable models. Using the modelling recommendations to select suitable MMC models, radial and MT EMT MMC-HVDC models for the connection of typical Round 3 windfarms are developed in this thesis. These models are used to analyse the steady-state and transient performance of the connections, including their compliance to the GB grid code for AC disturbances and reactive power requirements. Furthermore, the MT model is used to investigate the effect of MT control strategies on the internal MMC quantities.
42

Humidity dependent conductivity of air in HVDC applications and the role of surfaces

Svensson, Erik January 2023 (has links)
High Voltage Direct Current (HVDC) bushings are insulated devices designed to facilitate the safe passage of electric current across an earthed barrier. Understanding air conductivity is of utmost importance in the design of HVDC components, which is determined by ion density and ion mobility. Previous research has indicated a correlation between air conductivity, air humidity and phenomena on HVDC surfaces. This thesis aims to investigate and elucidate how adsorbed water may contribute to air conductivity. To accomplish this objective, an experimental study is conducted employing ellipsometry to measure the water layer thickness as a function of relative humidity and applied voltage. The results exhibit a thickness consistent with prior studies in the absence of applied voltage, but reveal an altered thickness when voltage is applied. While definitive conclusions cannot be drawn from this thesis alone, the findings provide some support for the hypothesis from earlier research, suggesting the involvement of surface phenomena in air conductivity. The experimental procedure presented in this thesis can be further expanded and refined to gain a more comprehensive understanding of the surface reactions at play. This could further develop the understanding of the factors influencing air conductivity in HVDC systems and contribute to the advancement of design methodologies in this field.
43

Harmonic State-Space Modelling of an HVdc Converter with Closed-Loop Control

Hwang, Sheng-Pu January 2014 (has links)
Frequency domain models for power electronic circuits are either based on iterative techniques such as Newton's method or linearised around an operating point. Iterative frequency domain models provide great accuracy as they are capable of calculating the exact switching instants of the device. On the other hand, the accuracy of a linearised frequency domain model relies on the magnitude of input waveform to be small so that the circuit's operating point does not vary or varies very little. However, an important advantage of a linearised model is its ability to provide insight into waveform distortion interaction, more specifically, the frequency cross-coupling around a power electronic circuit. In general, a linearised model for harmonic analysis would not normally include the description of feedback control. Likewise a linearised model for control analysis would usually disregard frequency interactions above the fundamental (or the most significant component); that is assuming the cross-coupling between harmonic frequencies does not affect the dynamics of control. However, this thesis proposes that a linearised model for control analysis shall also include the complete description of frequency cross-coupling between harmonics to produce the correct dynamic response. This thesis presents a harmonic state-space (HSS) model of an HVdc converter that incorporates the full effect of varying switching instants, both through control and commutation period dynamics, while remaining within the constraints of a linear time-invariant (LTI) system. An example is given using the HSS model to explain how a close to fifth harmonic resonance contributes to the dominant system response through the frequency cross-coupling of the converter and the controller feedback loop. The response of the system is validated against a time domain model built in PSCAD/EMTDC, and more importantly, the correct response cannot be produced without including the harmonic interactions beyond the fundamental frequency component.
44

Estimación de campos eléctricos y magnéticos en líneas aéreas en HVDC de cuatro polos

Cabezas Campos, Sebastián Andrés January 2012 (has links)
La incorporación de sistemas de transmisión de energía en corriente continua en un nivel de tensión de 500 kV, hace indispensable conocer cuáles serán sus posibles efectos en el entorno que circundará las líneas aéreas que transmitirán la energía eléctrica hasta los centros de distribución y consumo de energía. En esta memoria se plantea la creación de un modelo teórico que permita estimar de manera aproximada cuales serán algunos de los efectos que la operación de una línea aérea de cuatro polos de un sistema de transmisión en HVDC (500 kV) puede generar en su entorno. Primero, se recopilan los procedimientos teóricos existentes, en lo relativo al cálculo de los campos eléctricos y magnéticos estáticos que se generan en torno a las líneas y la ocurrencia del fenómeno corona, lo que permite estimar y predecir, con cierto grado de exactitud, el nivel de contaminación electromagnética que introducen en el ambiente. Otro efecto que se estima es el nivel de ruido audible producido en torno a las líneas aéreas en HDCV. Posteriormente, se realiza una adaptación de los modelos matemáticos al caso de una línea aérea de cuatro polos en un sistema HVDC de 500 kV, realizando los cálculos correspondientes para el caso en estudio, y luego, finalmente, poder construir un modelo que permita estimar la magnitud de los efectos que se mencionaron previamente. Finalmente, se comparan los resultados obtenidos con el caso de una línea de transmisión en HVDC bipolar de un circuito, para mostrar, en lo referente al campo eléctrico generado en el entorno de la línea, que, en el caso estudiado en este trabajo, es dos órdenes de magnitud mayor que en el caso de un solo circuito.
45

Modeling of modular multilevel converters using extended-frequency dynamics phasors

Rajesvaran, Shailajah 08 September 2016 (has links)
This thesis investigates modeling of modular multilevel converters (MMCs) using an averaging method known as extended-frequency dynamic phasors. An MMC can be used as an inverter or a rectifier in high voltage direct current (HVDC) system. This research develops a dynamic phasor model for an MMC operated as an inverter. Extended-frequency dynamic phasors are used to model a system with only interested harmonics present. The developed model is capable of capturing both the low and high-frequency dynamic behavior of the converter depending on the requirements of the study to be performed. The selected MMC model has 5 submodules per arm (6-level converter), nearest level control, capacitor voltage balancing, direct control and phase-locked loop (PLL) synchronization. With the above features, the developed dynamic phasor model is validated with electromagnetic transient model is developed using PSCAD simulation software. The results are compared at transient and steady state with disturbances. The main computational advantage of this modeling is achieving less simulation time with inclusion of harmonics of interest. / October 2016
46

Modulation of Modular Multilevel Converters for HVDC Transmission

Hassanpoor, Arman January 2016 (has links)
The outstanding features of modular multilevel converters (MMC) have recently gained much attention in the high-voltage direct-current (HVDC) transmission field. Power quality, converter cost and system performance are three crucial aspects of HVDC MMCs which are directly linked to the converter modulation and switching schemes. High power quality and performance require high switching frequency and large cell capacitor whereas low switching frequency and small cell capacitor are needed to reduce the converter cost. The main objective of this thesis is to propose a practical switching method for HVDC MMCs which balances the aforementioned contradictory requirements. A mathematical analysis of the converter switching pattern, against the power quality and converter cost, has been conducted to formulate an optimization problem for MMCs. Different objective functions are studied for the formulated optimization problem such as converter loss minimization, voltage imbalance minimization and computational burden minimization. This thesis proposes three methods to address different objective functions. Ultimately, a real-time simulator has been built to practically verify and investigate the performance of the proposed methods in a realistic point-to-point HVDC link. The most significant outcome of this thesis is the tolerance band-based switching scheme which offers a direct control of the cell capacitor voltage, low power losses, and robust dynamic performance. As a result, the converter switching frequency can reach frequencies as low as 70 Hz (with the proposed cell tolerance band (CTB) method). A modified optimized CTB method is proposed to minimize the converter switching losses and it could reduce the converter switching losses by 60% in comparison to the conventional phase shifted carrier modulation method. It is concluded intelligent utilization of sorting algorithm can enable efficient HVDC station operation by reducing the converter cost. / <p>QC 20160916</p>
47

Soluções de controlo para redes HVDC multi-terminal

Luís, Sérgio Manuel de Araújo January 2012 (has links)
Tese de mestrado integrado. Engenharia Electrotécnica e de Computadores (Energia). Faculdade de Engenharia. Universidade do Porto. 2012
48

Modeling of voltage source converter based HVDC transmission system in EMTP-RV

Hiteshkumar, Patel 01 August 2010 (has links)
Voltage Source Converter (VSC) applications include but are not limited to HVDC, Flexible AC Transmission System (FACTS) devices such as STATCOM, SSSC, UPFC and Wind generators and active filters. The VSC based HVDC system is a feasible option for bulk power transmission over long or short distances and the grid integration of renewable energy sources in existing transmission and distribution systems. The main requirement in a power transmission system is the precise control of active and reactive power flow to maintain the system voltage stability. The VSC operating with the specified vector control strategy can perform independent control of active/reactive power at both ends. This ability of VSC makes it suitable for connection to weak AC networks or even dead networks i.e. without local voltage sources. For power reversal, the DC voltage polarity remains the same for VSC based transmission system and the power transfer depends only on the direction of the DC current. This is advantageous when compared to the conventional Current Source Converter (CSC) based HVDC system. Furthermore, in case of VSC, the reactive power flow can be bi-directional depending on the AC network operating conditions. In this thesis, a 3-phase, 2-level, 6-switch VSC connected to an active but weak AC system at both ends of the HVDC link is developed using EMTP-RV. The VSC-HVDC transmission system model is developed using both direct control and vector control techniques. The direct control method is an approximate method in which the active power, AC voltages at both ends of HVDC link and DC link voltage are controlled directly by using PI-controllers. In vector control method, closed loop feed-forward control system is used to control the active power, reactive power at both ends and DC voltage. By comparing the simulation results, it is concluded that the vector control method is superior to the direct control because of the removal of the coupling between control variables to achieve the independent control of active and reactive powers at both ends of the HVDC link. / UOIT
49

Damping subsynchronous resonance oscillations using a VSC HVDC back-to-back system

Tang, Guosheng 06 July 2006
A problem of interest in the power industry is the mitigation of severe torsional oscillations induced in turbine-generator shaft systems due to Subsynchronous Resonance (SSR). SSR occurs when a natural frequency of a series compensated transmission system coincides with the complement of one of the torsional modes of the turbine-generator shaft system. Under such circumstances, the turbine-generator shaft system oscillates at a frequency corresponding to the torsional mode frequency and unless corrective action is taken, the torsional oscillations can grow and may result in shaft damage in a few seconds. <p> This thesis reports the use of a supplementary controller along with the Voltage Source Converter (VSC) HVDC back-to-back active power controller to damp all SSR torsional oscillations. In this context, investigations are conducted on a typical HVAC/DC system incorporating a large turbine-generator and a VSC HVDC back-to-back system. The generator speed deviation is used as the stabilizing signal for the supplementary controller. <p>The results of the investigations conducted in this thesis show that the achieved control design is effective in damping all the shaft torsional torques over a wide range of compensation levels. The results and discussion presented in this thesis should provide valuable information to electric power utilities engaged in planning and operating series capacitor compensated transmission lines and VSC HVDC back-to-back systems.
50

Grid Code Compliance – Wind farm HVDC connection

Västermark, Martin January 2013 (has links)
A rapid development of offshore wind power is planned in GB as a part to fulfil the EU2020 targets. 25 GW wind power capacity has been awarded to developers in nine different offshore zones outside the coast of UK. VSC-HVDC transmission is expected to be a both technical and economical favourable solution for transmitting the power into the main grid. This study investigates if such a transmission solution could comply with the regulatory framework in UK. Vattenfall and Scottish Energy Renewable will be part of this development and have been awarded the rights to develop 7200 MW of wind capacity outside the cost of East Anglia as a part of the offshore expansion plans in UK. The zone is broken down to several projects. The first project is called East Anglia ONE and this project is used as a reference case in this study. The GB Grid Code has been broken down into four areas, voltage and frequency variations; fault ride through requirements, active power control and reactive power control. Load flow calculations and dynamic simulations are designed to investigate compliance of each area. Further, simulations to investigate the interaction between the wind turbines and the offshore converter stations where done. A model representing East Anglia ONE was built in PSS/E and used to investigate grid codes compliance by load flow calculations and dynamic simulations. Data from earlier studies at Vattenfall was used to get a good representation of the wind park. A model representing a HVDC-transmission solution was provided by ABB. The results from load flow calculations and simulations show that a HVDC-solution can comply with the investigated parts of the grid codes. The limiting factor seems to be the capability to inject enough reactive power to the gird at small voltage dips during normal operation. This capability can, however, be enhanced with the right tap-changer settings at the onshore converter transformer.

Page generated in 0.022 seconds