Recent developments in HVDC transmission systems to support renewable energy integration

The demands for massive renewable energy integration, passive network power supply, and global energy interconnection have all gradually increased, posing new challenges for high voltage direct current (HVDC) power transmission systems, including more complex topology and increased diversity of bipolar HVDC transmission. This study proposes that these two factors have led to new requirements for HVDC control strategies. Moreover, due to the diverse applications of HVDC transmission technology, each station in the system has different requirements. Furthermore, the topology of the AC-DC converter is being continuously developed, revealing a trend towards hybrid converter stations. Keywords: Direct current transmission system, Topology, Control strategy, AC-DC converte

Control and Protection of MMC-Based HVDC Systems: A Review

The voltage source converter (VSC) based HVDC (high voltage direct current system) offers the possibility to integrate other renewable energy sources (RES) into the electrical grid, and allows power flow reversal capability. These appealing features of VSC technology led to the further development of multi-terminal direct current (MTDC) systems. MTDC grids provide the possibility of interconnection between conventional power systems and other large-scale offshore sources like wind and solar systems. The modular multilevel converter (MMC) has become a popular technology in the development of the VSC-MTDC system due to its salient features such as modularity and scalability. Although, the employment of MMC converter in the MTDC system improves the overall system performance. However, there are some technical challenges related to its operation, control, modeling and protection that need to be addressed. This paper mainly provides a comprehensive review and investigation of the control and protection of the MMC-based MTDC system. In addition, the issues and challenges associated with the development of the MMC-MTDC system have been discussed in this paper. It majorly covers the control schemes that provide the AC system support and state-of-the-art relaying algorithm/ dc fault detection and location algorithms. Different types of dc fault detection and location algorithms presented in the literature have been reviewed, such as local measurement-based, communication-based, traveling wave-based and artificial intelligence-based. Characteristics of the protection techniques are compared and analyzed in terms of various scenarios such as implementation in CBs, system configuration, selectivity, and robustness. Finally, future challenges and issues regarding the development of the MTDC system have been discussed in detail

Hybrid AC-High Voltage DC Grid Stability and Controls

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

Controle em espaço de estados utilizando lqr de conversores para aplicação em microrrede

Studies related to microgrids have grown considerably in recent years, becoming one of the most famous research topics in the field of electrical engineering, in Brazil and abroad. In this context, this work presents the modeling and control of an alternating current microgrid fed directly by a photovoltaic generation unit and an energy storage system, operating in the connected and isolated mode from the electrical grid. The Perturb and Observe algorithm is used to track the maximum power point of the fixed set of photovoltaic solar panels. The energy storage system is used due to the intermittency of photovoltaic generation and is used during all operation of the microgrid ensuring DC link voltage control. The topology of the converter, which interfaces between the direct current bus and the electrical grid, is a single stage, and its main feature is its multifunctionality: when connected to the electrical grid, acting in the grid supplying mode, operating as current source, controlling the current injected into the point of common coupling. However, in the grid forming mode, isolated from the electrical grid, the converter operates as a voltage source, controlling the amplitude and phase of the voltage at the common coupling point. The structure of the converter used is the same in both operating modes, changing only the control strategies. The transition from the operation and control mode is carried out with the help of the islanding detection technique based on the negative sequence current injection, with the opening of the switch, which connects to the electrical grid, performed when the level of negative sequence of the voltage after islanding exceeds the rated operating limit. The modeling of the proposed system is performed in state spaces and its control based on full state feedback, whose gains were found using the linear quadratic regulator. Simulation tests were carried out to validate the system, which proved the efficiency of the multifunctional converter and also the energy storage system. In addition, experimental results of the converter in the network forming mode have also shown satisfactory results.Estudos relacionados a microrredes vêm crescendo consideravelmente nos últimos anos, se tornando um dos tópicos de pesquisa mais recentes no campo da engenharia elétrica, tanto no Brasil como no exterior. Neste contexto, este trabalho apresenta a modelagem e o controle de uma microrrede em corrente alternada alimentada diretamente por uma unidade de geração fotovoltaica e um sistema de armazenamento de energia, operando no modo conectado e ilhado da rede elétrica. O algoritmo Perturba e Observa é utilizado para rastrear o ponto de máxima potência do conjunto fixo de painéis solares fotovoltaicos. O sistema de armazenamento de energia é utilizado devido à intermitência da geração fotovoltaica e está presente durante todo o funcionamento da microrrede, garantindo o controle de tensão no barramento de corrente contínua. A topologia do conversor, que faz interface entre o barramento de corrente contínua com a rede elétrica, é de um único estágio, e este apresenta como principal característica a sua multifuncionalidade: quando conectado à rede elétrica, atuando no modo supridor de rede, operando como fonte de corrente, controlando a corrente injetada no ponto de acoplamento comum. Entretanto, no modo formador de rede, ilhado da rede elétrica, o conversor opera como fonte de tensão, controlando amplitude e fase da tensão no ponto de acoplamento comum. A estrutura do conversor utilizada é a mesma em ambos os modos de operação, alterando apenas as estrategias de controle. A transição do modo de operação e controle é realizado com o auxílio da técnica de detecção de ilhamento baseada na injeção de corrente de sequencia negativa, sendo a abertura da chave, que faz conexão com a rede elétrica, realizada quando o nível de sequencia negativa da tensão após o ilhamento ultrapassa o limite de operação nominal. A modelagem do sistema proposto é feita em espaços de estados e o seu controle em realimentação de estados, cujos ganhos foram encontrados utilizando o regulador quadrático linear. Foram realizados testes de simulação para a validação do sistema, que comprovaram a eficacia do conversor multifuncional e também do sistema de armazenamento de energia. Além disso, resultados experimentais do conversor no modo formador de rede também demonstraram resultados satisfatórios

Power Flow Studies of HVDC Grids with DC Power Flow Controllers

High-Voltage Direct Current (HVDC) transmission, especially based on voltage source converters (VSCs), have attracted significant research interests due to renewable energy sources integration in power grids, notably offshore wind farms. Despite recent research contributions in the literature on HVDC systems, a number of challenges remain unsolved, such as lack of a comprehensive study regarding power-electronics-based devices in HVDC systems, suitable modelling approaches for sophisticated DC power flow controllers, power loss modeling of DC power flow controllers, powerful and practical DC power flow solvers, and the highly-meshed test structure of HVDC grids for power flow. To address these research gaps, in this thesis, a comprehensive literature review has been conducted on power electronics devices in HVDC systems in Chapter 2. These devices are divided into three categories in the review: 1) power converters; 2) DC/DC converters; and 3) DC power flow controllers (DCPFCs). As an emerging power electronics device being introduced less than a decade ago, DCPFCs are the main focus of this thesis. A novel unified Newton-Raphson (NR)-based DC power flow solver (DCPFS) is presented in Chapter 3 to solve the DC power flow problem in multi-terminal HVDC (MT-HVDC) grids by employing a novel DCPFC, the multi-port interline DC power flow controller (MIDCPFC). The proposed DCPFS modifies physical and control state variables of the whole system (MIDCPFC and the MT-HVDC grid) simultaneously to control power flow in HVDC lines, especially overloaded lines. The static model and the power injection model of the MIDCPFC are obtained and their equations are embedded within the designed DCPFS. The absence of the fictitious bus preserves the original conductance matrix of the system and its symmetry, and thus, the original system's Jacobin matrix only needs minor modifications in the developed unified NR-based DCPFS. Additionally, the proposed DCPFS is straightforward for implementation since the voltage of the intermediate capacitor of MIDCPFC is treated as an independent variable, as a result, there is no need to use external processes to control its value. The shunt conductance of HVDC lines is also considered. The comprehensive models have been proposed to model power losses of MIDCPFC and VSCs for the first time. Finally, a new modified 15-bus MT-HVDC grid is proposed and implemented for verification purposes. The obtained results verify the accuracy and efficacy of the proposed concepts, models, and formulations of this study. A novel sequential NR-based DCPFS is proposed in Chapter 4 to solve the DC power flow problem in MT-HVDC grids by employing MIDCPFC and decoupling the power flow equations of the MIDCPFC and the MT-HVDC grid. In the proposed sequential NR-based DCPFS, there is no trace of fictitious buses, the original conductance matrix of the system and its symmetry are preserved, and the shunt conductance of HVDC lines is considered for precise modeling. The structure of the proposed DCPFS is sequential, which decouples the MIDCPFC and grid related power flow equations. A prominent feature of the DCPFS is that it fully preserves the system's original Jacobin matrix and does not require any modification to that matrix, which reduces the computational burden. In addition, power losses of the MIDCPFC and VSCs are embedded in DC power flow equations. The proposed sequential NR-based DCPFS is straightforward to implement as the voltage of the MIDCPFC is treated as an independent variable, and consequently, no external process is needed to control it. Various scenarios are tested on a modified 15-bus MT-HVDC grid to verify the proposed sequential NR-based DCPFS. The accuracy and efficacy of the proposed approach is validated through these case studies

Advanced control of multi-microgrids for grid integration

Thanks to tremendous growing interest, the significant number of microgrids form a system called Multi-Microgrid, where multiple microgrids are interconnected to support local loads and exchange power to or from grid. Industry demands for advanced control and optimal coordination among microgrids with consideration of high penetration of renewable energy and complex system architectures. This thesis focuses on different key aspects of power systems and microgrids to develop novel approaches targeting the problem. Firstly, different topologies of microgrids are studied from the literature review and most popular system architectures are considered in the study for proposing advanced control techniques. Distributed control systems with nested formation in the microgrids are proposed for improved power sharing strategy. The distributed control is designed to achieve self-healing capability of multi-microgrids during any contingency event. Local controllers of the inverters in each microgrid are interconnected through the nested formation. A nested optimization algorithm is designed to achieve power exchange between different microgrids. Multi-terminal HVDC network based multi-microgrids have been proposed for advanced control strategy due to its widespread application in power system. Adaptive droop control has been proposed based on consensus algorithm and matrix-based solutions to provide frequency support and power sharing between AC microgrids through the HVDC network. The proposed adaptive droop algorithm is featured to maintain frequency and voltage during contingency events and ensure efficient power sharing. Distributed hierarchical control system is proposed as well for multi-microgrids with nested formation-based optimization techniques to ensure proper power sharing in four-level based multi-microgrid topologies. The algorithm features energy management within the multi-microgrid through virtual controllers of primary and secondary frequency control. In addition, to the energy management issue, low system strength of grid has been considered to offer a wide range of areas under the advanced control of multi-microgrid. In that regard, single machine infinity bus model has been considered to implement control of grid forming inverters for integration with weak grid. Novel grid resynchronization and virtual synchronous generator control has been proposed to achieve multi-microgrids integration with weak grids. Then, various simulation studies are performed to test the effectiveness of the proposed controls. The time domain simulations are performed on EMT power system tool PSCAD under different operating conditions, such as loading variations, N-1 contingency events, grid frequency change disturbance, islanding conditions etc. In addition to the time domain simulation studies, stability analysis of the proposed control has been carried out. In the stability analysis, pole-zero map, Nyquist plots and Bode plots have been demonstrated to analyse the stable conditions of the proposed control. The optimization algorithms results are also included in the simulation studies to reflect the performance of the control. Finally, the advanced control solutions outcomes through time domain and stability results are compared with conventional control. It has been demonstrated that all proposed solutions perform better than conventional approaches and reflect significant improvement on the multi-microgrids. Furthermore, industry standards have been considered in the weak grid integration study and case studies are carried out based on power industry practices, including industry regulatory grid codes according to the power industry in Australia. The results indicate that the proposed controls are able to satisfy industry grid codes

Study of the offshore wind farm based multi-terminal DC system and the application of high power DC-DC converters

Wind power is a type of renewable energy source which has vast potential for further development as it is featured as pollution-free and plentiful. However, wind power turbines also have negative consequences for the environment (in particular visual impact). Therefore, wind power bases should be built away from centres of population, and in particular offshore where the wind speeds are higher and more consistent. As is known, most of the electric energy is consumed in cities, so one needs to find a way to transfer the energy long distances efficiently. HVDC transmission lines/cables are often appropriate when wind farms are built more than about 80km from the shore. This thesis is concentrated on the research of offshore wind farm based Multi-Terminal HVDC systems where the Multi-Terminal HVDC system can extract and deliver power from and to more than one connection point. In the thesis, the study of a full system PLECS+ Simulink simulation model with single or dual branch, including wind model, wind turbine, PMSG, PWM rectifier, SAB DC-DC converter, IPOS DC-DC converter, HVDC cables, simplified onshore system, are presented, focusing on the investigation of the output ripple of multiple DC-DC converters on DC cables with different voltage and power ratings. THD and cable AC losses on the DC cables are also monitored and studied in the thesis. In the hardware part, two different down-scaled DC-DC converter models are built to verify the simulation results