A Modular IGBT Based Current Flow Controller for Multi-terminal HVDC Grids

Offshore wind turbines are preferred rather than onshore ones for their numerous advantages, such as land saving, higher wind speeds and higher power generation. However, AC power transmission would fail to deliver the generated power economically over distances longer than 80 kilometres using submarine cables. The more feasible option is to use High Voltage DC (HVDC) power transmission for offshore wind generation. Unlike AC transmission systems that have established power and current flow control methods, DC power transmission systems have only reliable power flow control techniques for point to point systems, which makes it one of the challenges preventing realisation of Multi Terminal HVDC grids (MT-HVDC) as cables may be subjected to higher currents causing overloading and thermal problems. Different HVDC power flow control schemes are suggested by controlling the AC/DC converters such as voltage droop control and voltage margin control. Other methods of power and current flow control based on the connection of new power electronic equipment to the grid have been also proposed. This thesis presents operation and control of an IGBT based Current Flow Controller (CFC) for MT-HVDC grid applications. The CFC is studied in its preliminary two-port configuration and possible modes of operation and dynamic models are produced. An extended topology is proposed to allow the CFC to be connected to more than two cables at a time. Although the proposed extended CFC topology is simple in construction and gave acceptable results in most case studies, it has shown some drawbacks in certain case studies where controlled currents have significant differences in magnitudes. To resolve this problem, a generalized Modular CFC (MCFC) topology is proposed which allows each current to be controlled independently and overcome the extended topology’s drawback. Moreover, a reduced count switch count topology is proposed which reduces the MCFC cost by half in cases of unidirectional current flow control. All proposed control strategies and topologies are validated using both computer simulation through MATLAB/SIMULINK and PSCAD/EMTDC software packages and experimental validation through Rapid Control Prototyping (RCP) with the aid of Opal RT real time simulator. Studies carried throughout this thesis show that the proposed MCFC may play an important role in current flow control applications in MT-HVDC grids due to its low cost, small footprint and accurate performance

Desenvolvimento de um condicionador unificado de qualidade de energia monofásico com controlo invertido

Dissertação do mestrado em Engenharia Eletrónica Industrial e ComputadoresA Qualidade de Energia Elétrica (QEE) é um tema cada vez mais importante no ramo da engenharia, pois os problemas na rede elétrica podem levar a que os equipamentos não funcionem corretamente. Isso, faz com que muitos clientes, no sector industrial serviços tenham prejuízos económicos muito elevados. Porém, muitos desses problemas ocorrem devido às cargas utilizadas pelos clientes que “poluem” a rede elétrica com potência reativa e harmónicas. Existem equipamentos de eletrónica de potência que mitigam estes problemas de Qualidade de Energia Elétrica, entre eles o Condicionador Unificado de Qualidade de Energia (Unified Power Quality Conditioner - UPQC). Esta dissertação aborda uma nova topologia deste equipamento chamado Condicionador Unificado de Qualidade de Energia com Controlo Invertido (iUPQC) Monofásico com a adição de baterias, para permitir o modo de alimentação ininterrupta da instalação (Uninterruptible Power Supply – UPS) denominando-se assim iUPQC-UPS. Um UPQC é um equipamento constituído por um condicionador ativo série (Conversor CC-CA) e um condicionador ativo paralelo (Conversor CC-CA) que ligam a um barramento CC comum. Este barramento irá ligar às baterias a partir de um conversor CC-CC bidirecional. O objetivo do UPQC com controlo invertido é que o condicionador ativo série absorva da rede a potência ativa necessária impondo uma corrente sinusoidal do lado da rede, enquanto que o condicionador ativo paralelo fornece às cargas uma tensão sinusoidal com frequência e valor eficaz normalizado. Desta forma, todas a potência reativa e harmónicas serão fornecidos pelo condicionador ativo paralelo. Esta dissertação apresenta um estudo bibliográfico dos condicionadores ativos de potência que mitigam os problemas de QEE, das topologias dos conversores de potência e dos seus algoritmos de controlo. Com base neste estudo, foi usada a ferramenta PSIM para fazer simulações dos modos de funcionamento do iUPQC-UPS. Posteriormente, foi desenvolvido o protótipo do iUPQC-UPS, constituído pelo sistema de controlo, sistema de comando e sistema de potência. Por ultimo, o protótipo foi validado ao retirar os resultados experimentais de todos os modos de funcionamento.Electrical Power Quality is an increasingly important topic in the engineering field, as the problems created in the electrical network lead to the equipment not working properly. This causes many customers in the industrial services sector to have very high economic losses. However, many of these problems occur due to the loads used by customers that “pollute” the Electric Grid with reactive power and harmonics. There are power electronics equipment that mitigate these Power Quality problems, among them the Unified Power Quality Conditioner (UPQC). This dissertation approaches a new topology of this equipment called Unified Power Quality Conditioner with Inverted Control (iUPQC) with the addition of batteries, to allow the uninterruptible power supply of the installation (Uninterruptible Power Supply - UPS), thus called iUPQC- UPS. This equipment consists of a series active conditioner (DC-AC converter) and a parallel active conditioner (DC-AC converter) that connect to a common DC bus. This bus will connect to the batteries from a bidirectional DC-DC converter. The objective is that the series active conditioner absorbs the necessary power from the power grid while imposing a sinusoidal current on the power grid side, while the shunt active conditioner provides the loads with a sinusoidal voltage with normalized frequency and effective value. In this way, all reactive power and harmonics will be provided by the shunt active conditioner The batteries will charge or discharge as needed by the network. That is, if the power of the network is greater than the power of the loads, the batteries will use power from the network to charge. If the power of the network is less than the power of the loads, the batteries will discharge when supplying power to the loads.Este trabalho de dissertação está enquadrado no projeto IC&DT “Quality4Power – Enhancing the Power Quality for Industry 4.0 in the era of Microgrids”, financiado pela Fundação para a Ciência e Tecnologia, com a referência PTDC/EEI-EEE/28813/201