With the increased complexity of modern power systems, it may be required more than one platform to do an intended study efficiently and accurately. This research was carried out to investigate the use of co-simulation in an application of power system. A back-to-back Voltage Source Converter (VSC) transmission system was modeled in PSCAD/EMTDC which is an Electro-Magnetic Type (EMT) software. Results were analyzed for some operating points of the system. Then the control system of the above system was modelled in MATLAB/SIMULINK while the rest of the system was modeled in PSCAD/EMTDC. Both of these systems were interfaced to obtain the complete system and results were analyzed under same operating points as the original PSCAD case.

This thesis deals with modeling, simulation and operating limits of high-voltage direct-current (HVDC) transmission systems that employ voltage-source converters (VSCs) as their building blocks. This scheme is commonly known as the VSC-HVDC transmission. A simulation-based study is undertaken in which detailed electromagnetic transient (EMT) models are developed for a back-to-back VSC-HVDC transmission system. Different control strategies are implemented and their dynamic performances are investigated in the PSCAD/EMTDC EMT simulator. The research presented in this thesis firstly specifies the factors that limit the operating points of a VSC-HVDC system with particular emphasis on the strength of the terminating ac system. Although the EMT model shows these limits it provides little analytical reason for their presence and extent. A phasor-based quasi-steady state model of the system including the phase-locked loop firing control mechanism is proposed to determine and characterize the factors contributing to these operating limits. Stability margins and limits on the maximum available power are calculated, taking into consideration the maximum voltage rating of the VSC. The variations of ac system short-circuit ratio (SCR) and transformer impedance are proven to significantly impact the operating limits of the VSC-HVDC system. The results show how the power transfer capability reduces as the SCR decreases. The analysis shows that VSC-HVDC converters can operate into much weaker networks, and with less sensitivity, than the conventional line commutated converters (LCC-HVDC). Also for a given SCR the VSC-HVDC system has a significantly larger maximum available power in comparison with LCC-HVDC. A second research thrust of the thesis is introduction of a simplified converter model to reduce the computational intensity of its simulation. This is associated with the admittance matrix inversions required to simulate high-frequency switching of the converter valves. This simplified model is based on the concept of dynamic average-value modelling and provides the ability to generate either the full spectrum or the fundamental-frequency component of the VSC voltage. The model is validated against the detailed VSC-HVDC circuit and shows accurate matching during steady state and transient operation. Major reductions of 50-70% in CPU-time in repetitive simulation studies such as multiple runs and optimization-based controller tuning are achieved.

A major issue with respect to VSC based systems is the propagation of harmonics to DC side loads due to AC voltage source unbalance. Standard dq-frame control techniques currently utilized offer little mitigation of these unwanted harmonics. Recently, resonant controllers have emerged as an alternative to dq-frame controllers for regulation of grid connected converters for distributed resources. Although these control systems behave somewhat similar to dq-frame controllers under balanced operating conditions, their behaviour under unbalanced operation is superior. Currently, there are no linearized state space models of resonant controlled VSC systems. This work will develop a linearized small signal state space model of a VSC system, where resonant current controllers are used for regulation of the grid currents. It will also investigate the stability of resonant controlled VSC based systems using eigenvalue analysis for HVDC applications.

A major issue with respect to VSC based systems is the propagation of harmonics to DC side loads due to AC voltage source unbalance. Standard dq-frame control techniques currently utilized offer little mitigation of these unwanted harmonics. Recently, resonant controllers have emerged as an alternative to dq-frame controllers for regulation of grid connected converters for distributed resources. Although these control systems behave somewhat similar to dq-frame controllers under balanced operating conditions, their behaviour under unbalanced operation is superior. Currently, there are no linearized state space models of resonant controlled VSC systems. This work will develop a linearized small signal state space model of a VSC system, where resonant current controllers are used for regulation of the grid currents. It will also investigate the stability of resonant controlled VSC based systems using eigenvalue analysis for HVDC applications.

abstract: An embedded HVDC system is a dc link with at least two ends being physically connected within a single synchronous ac network. The thesis reviews previous works on embedded HVDC, proposes a dynamic embedded HVDC model by PSCAD program, and compares the transient stability performance among AC, DC and embedded HVDC. The test results indicate that by installing the embedded HVDC, AC network transient stability performance has been largely improved. Therefore the thesis designs a novel frequency control topology for embedded HVDC. According to the dynamic performance test results, when the embedded HVDC system equipped with a frequency control, the system transient stability will be improved further. / Dissertation/Thesis / M.S.Tech Electrical Engineering 2013

Voltage source converter high voltage direct current (VSC-HVDC) technology has become increasingly cost-effective and technically feasible in recent years. It is likely to play a vital role in integrating remotely-located renewable generation and reinforcing existing power systems. Multi-terminal VSC-HVDC (MTDC) systems, with superior reliability, redundancy and flexibility over the conventional point-to-point HVDC, have attracted a great deal of attention globally. MTDC however remains an area where little standardisation has taken place, and a series of challenges need to be fully understood and tackled before moving towards more complex DC grids. This thesis investigates modelling, control and stability of MTDC systems. DC voltage, which indicates power balance and stability of DC systems, is of paramount importance in MTDC control. Further investigation is required to understand the dynamic and steady-state behaviours of various DC voltage and active power control schemes in previous literature. This work provides a detailed comparative study of modelling and control methodologies of MTDC systems, with a key focus on the control of grid side converters and DC voltage coordination. A generalised algorithm is proposed to enable MTDC power flow calculations when complex DC voltage control characteristics are employed. Analysis based upon linearised power flow equations and equivalent circuit of droop control is performed to provide further intuitive understanding of the steady-state behaviours of MTDC systems. Information of key constraints on the stability and robustness of MTDC control systems has been limited. A main focus of this thesis is to examine these potential stability limitations and to increase the understanding of MTDC dynamics. In order to perform comprehensive open-loop and closed-loop stability studies, a systematic procedure is developed for mathematical modelling of MTDC systems. The resulting analytical models and frequency domain tools are employed in this thesis to assess the stability, dynamic performance and robustness of active power and DC voltage control of VSC-HVDC. Limitations imposed by weak AC systems, DC system parameters, converter operating point, controller structure, and controller bandwidth on the closed-loop MTDC stability are identified and investigated in detail. Large DC reactors, which are required by DC breaker systems, are identified in this research to have detrimental effects on the controllability, stability and robustness of MTDC voltage control. This could impose a serious challenge for existing control designs. A DC voltage damping controller is proposed to cope with the transient performance issues caused by the DC reactors. Furthermore, two active stabilising controllers are developed to enhance the controllability and robust stability of DC voltage control in a DC grid.

The goal with this exam project is to take as much knowledge as possible from earlier courses I’ve taken at the webdevelopment program at Mid Sweden University and use this knowledge for a project for a company working with webdevelopment. I’ve chosen to do my exam project for David Törner Web agency who’s main goals are to work with Content Management Systems, (CMS). I’ve developed a custom theme and a webstore in the CMS WordPress with the plugin WooCommerce, a plugin that connects webstores with WordPress sites for easier administration and management of products. The webstore is selling bikes in the categories mountain bikes, enduro and downhill bikes, specially black bikes with high quality and it works as a demonstration of a webstore to show other companies what we are able to offer if they choose us to do a webstore in WordPress. The webstore also offers service on the bikes that the customer can book by themselves through the contact form. The design and foundation for the webstore have been designed and developed in the program Adobe XD in the form of moodboards, wireframes and design sketches and then coded in the program Visual Studio Code, (VSC) with the program language HyperText Preprocessor, (PHP), HyperText Markup Language, (HTML), Cascading Style Sheets, (CSS) and JavaScript. The design and the foundation of the webpages have been connected to WordPress and the webstore plugin WooCommerce and the webstore has been published online through a public webhosting service. / Syftet med detta examensarbete är att ta så mycket som möjligt utav kunskapen från tidigare kurser i webbutvecklingsprogrammet på Mittuniversitetet och applicera denna kunskap i ett arbete åt ett företag som inriktar sig inom webbutveckling. Jag har valt att göra mitt examensarbete åt David Törners Webbyrå som inriktar sig specifikt på webbsidor skapade med Content Management Systems, (CMS). Jag har utvecklat ett tema samt en webbutik i CMS:et WordPress med tillägget WooCommerce, som är ett tillägg som kopplar samman webbutiker med WordPressidor för enklare administrering samt hantering utav webbutiken och varorna. Webbutiken inriktar sig på försäljning utav cyklar inom kategorierna mountainbikes, enduro- samt downhill-cyklar, specifikt svarta och högkvalitativa cyklar samt fungerar som en demonstrations-webbutik att visa upp för företag som exempel på vad de skulle kunna få om de väljer en webbutik gjord i WordPress. Webbutiken erbjuder även service utav cyklar och att kunden via kontaktformulär kan boka sin service. Design och grund för webbutiken har utvecklats först i programmet Adobe XD i form utav moodboard, wireframes samt designskisser och sedan utvecklats och kodats i programmet Visual Studio Code, (VSC), med hjälp utav programspråket Hypertext Preprocessor, (PHP), HyperText Markup Language, (HTML), Cascading Style Sheets, (CSS) samt JavaScript. Designen och grunden på webbsidorna har sedan kopplats samman med WordPress samt webbutik-tillägget WooCommerce och publicerats på ett publikt webbhotell.

The Power Electronic Building Block (PEBB) concept leads to multifunctional converter systems, which provide robustness and flexibility in heavily power electronics based power systems. Systems comprised of flexible modular converters may have multiple possible operation conditions with respect to individual converters that meet the overall system goals. In this thesis, an optimization method for such flexible online power electronic systems is developed to minimize power losses of the overall group of converters in the system. Here the objective is to allocate sharing such that compensation objectives are met while the power loss of the entire parallel group of compensators is minimized. Considering optimization of an online power electronic system, convergence time and running in the feasible region should be taken into account. This thesis is

Com o aumento de uso de fontes renováveis para geração de energia e o aprimoramento da tecnologia de conversores fonte de tensão (VSC – Voltage Source Converter), sistemas baseados em corrente contínua não só se tornaram viáveis como também se tornaram uma alternativa vantajosa em diversas situações. O uso de diversos conversores conectados em uma rede, isto é uma rede VSC-HVDC multiterminal, se apresenta como um caminho para a conexão de diversas fontes, como geradores eólicos e solares de maneira eficiente e economicamente vantajosa. No entanto, este tipo de rede possui algumas limitações, por exemplo, é mais sensível a faltas que um sistema de corrente contínua baseado em conversores do tipofonte de corrente (CSC – Current Source Converter). Desta forma, o estudo de faltas em sistemas VSC-HVDC é necessário para o desenvolvimento de mecanismos de proteção para estes sistemas. Neste contexto é importante o desenvolvimento de metodologias para a localização de faltas em sistemas MTDC, uma vez que este tipo de informação pode ajudar as equipes de manutenção a encontrarem as falhas e resolverem o problema da maneira mais rápida possível. Embora existam diversos estudos e metodologias desenvolvidas para sistemas de corrente alternada, ou para sistemas de corrente contínua de dois terminais, os estudos em redes de corrente contínua multiterminais ainda são poucos e em geral atendem a apenas um tipo de rede, mostrando assim a necessidade de mais estudos na área. Desta forma, o objetivo desta dissertação de mestrado é o desenvolvimento de uma metodologia localização de faltas em um sistema HVDC multiterminal, que contará também com técnicas já propostas na literatura para a detecção das falhas.Neste documento são mostrados o embasamento teórico, os estudos realizados para o desenvolvimento do tema, bem como os resultados obtidos para a localização de faltas em um sistema teste de simulação. / The increase of the use of renewable sources for power generation and the enhancement of Voltage Source Converter (VSC) technology, DC-based systems have become viable and an advantageous alternative in many situations. The use of several converters connected in a network, ie a multi-terminal VSC-HVDC network, is presented as a way to connect several sources, such as wind and solar generators in an efficient and economically advantageous way. However, this type of network has some limitations, for example, it is more sensitive to faults than a DC current system based on Current Source Converter (CSC). Therefore, the study of faults in VSC-HVDC systems is necessary for the development of protections for these systems. In this context it is important to develop fault location methodologies in MTDC systems since this type of information can help maintenance teams to find fault and solve the problem as quickly as possible. Although there are several studies and methodologies developed for alternating current systems or for two-terminal DC systems, there are few studies on multiterminal DC networks and this studies generally address only one type of network, thus showing the need for more studies in the area. Thus, the objective of this master\'s thesis is the development of a fault localization methodology in a multi-terminal HVDC system, which will also have techniques already proposed in the literature for the detection of faults. In this document are shown the theoretical background, the studies carried out for the development of the theme, as well as the results obtained for fault localization in a simulation test system.

Tecnologias HVDC que utilizam conversores do tipo fonte de tensão, o VSC-HVDC, ainda não são completamente difundidas e aplicadas no Brasil, em contraste com outros países que começaram a estudar e empregar este tipo de transmissão. Comparado com o HVDC tradicional, o VSC-HVDC é uma tecnologia de transmissão mais eficiente e pode superar deficiências encontradas na transmissão em corrente contínua convencional. O VSC-HVDC pode ser utilizado de maneira mais eficiente nas novas redes de energia, para alimentar ilhas, integração de geração eólica, renovação das linhas em centros urbanos, aplicações multiterminais e conexão com sistemas fracos. Por se tratar de uma tecnologia recente, o VSC-HVDC ainda não é amplamente adotado e uma das principais limitações da utilização destes sistemas é a sua fragilidade diante faltas na linha de corrente contínua. Neste contexto, limitadores de corrente de falta (LCF) podem ser utilizados para minimizar o impacto das faltas. A ação dos limitadores é benéfica ao sistema durante condições de falta, contudo, ainda assim é necessária a atuação do sistema de proteção para extinguir a condição faltosa. Portanto, este trabalho visa propor e avaliar um novo esquema de proteção que opere de maneira seletiva e confiável para sistemas VSC-HVDC na presença de LCF baseados em materiais supercondutores ou LCF indutivos. Para tanto, foram implementadas quatro funções de proteção tradicionais das linhas em CC, a saber: direcional de corrente, diferencial, sobrecorrente com restrição de tensão e ondas viajantes, e ainda, foi proposta uma nova função de proteção, a de condutância, a qual apresentou o menor tempo de identificação de falta, considerando as faltas mais severas. Adicionalmente, foi avaliado o comportamento destas funções quando o sistema apresenta os LCF em série com a linha. Foi demonstrado que é possível extrair os benefícios dos LCF sem deteriorar a qualidade dos resultados das funções de proteção, o que aumenta a segurança e confiabilidade dos sistemas VSC-HVDC, uma vez que os impactos das faltas são minimizados e as mesmas são identificadas em um curto intervalo de tempo. / HVDC technology based on voltage source converters, VSC-HVDC, are not yet fully disseminated and applied in Brazil, in contrast to other countries that have begun to study and widely employ this type of transmission technology. When compared with traditional HVDC systems, VSC-HVDC systems are more efficient and can overcome the challenges encountered in the conventional direct current transmission. The VSC-HVDC can be used more efficiently in the new energy networks to feed islands, integration of wind generation, renewal of lines in urban centers, multiterminal applications and connection with weak systems. Because it is a recent technology, VSC-HVDC is not yet widely adopted and a major limitation of using these systems is their weakness against faults in the DC line. In this context, fault current limiters (FCL) can be used to minimize the impact of faults. The action of the limiters is beneficial to the system during fault conditions, however, the use of protection schemes to detect the fault and extinguish the faulty condition is still required. Therefore, this work aims to propose and evaluate a new protection scheme operating in selective and reliable way for VSC-HVDC systems in the presence of FCL based in superconductor materials or inductive FCL. To reach such a goal, in this work it has been implemented four traditional DC lines protection functions, namely: directional current, differential, overcurrent with voltage restraint and traveling waves, and also proposed a new protection function, which is based on the conductance. This last protection function has presented the lowest detection time, when considering the most severe faults. In addition, it was evaluated the behavior of these functions when the system presents the FCL in series with the DC line. It has been shown that it is possible to extract the benefits of FCL without deteriorating the quality of the results of the protection functions, which increases the safety and reliability of the VSC-HVDC systems, since the impact of faults is minimized and they are identified in a short time.