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

This thesis develops the dynamic phasor model of a line-commutated converter (LCC) high-voltage direct current (HVDC) transmission system. The mathematical definition and properties of dynamic phasors are utilized to model both the dc-side and the ac-side of a LCC-HVDC transmission system as well as 6-pulse Graetz bridge, which is the building block of such a system. The developed model includes low-frequency dynamics of the systems, i.e., fundamental frequency component (50 Hz) at the ac-side and dc component at the dc-side, and removes high-frequency transients. The developed model, however, is capable of accommodating higher harmonics if necessary. The model is also able to simulate the system during abnormal modes of operations such as unbalanced operation and commutation failure. In order to develop the dynamic phasor model of a line-commutated converter, the concept of switching functions is utilized. The developed model is capable of capturing large-signal transients of the system as well as steady state operating conditions. The model can be used in order to decrease the computational intensity of LCC-HVDC simulations. The developed model in this thesis enables the user to consider each harmonic component individually; this selective view of the components of the system response is not possible to achieve in conventional electromagnetic transient simulations. / October 2016

The recent introduction of a new converter topology, the modular multilevel converter (MMC) is a major step forward in voltage sourced converter (VSC) technology for high voltage, high power applications. To obtain a multilevel ac output waveform, a large number of semiconductor switches has to be used in the converter. The number of switches in the MMC for HVDC transmission is typically two orders of magnitudes larger than that in a two or three level VSC used in earlier generation. This large device count creates a computational challenge for electromagnetic transients (EMT) simulation programs, as it significantly increases the simulation time. The purpose of this research is to investigate whether the simulation can be speeded up. This research develops an efficient, time-varying Thévenin's equivalent model for the MMC converter based on partitioning the system’s admittance matrix. EMT simulation results show that the proposed equivalent model can drastically reduce the computational time without loss of accuracy. The use of the proposed equivalent method is demonstrated by simulating a point to point MMC based HVDC transmission system successfully with more than 100 levels. This approach enables what was hitherto not practical; the modeling of large MMC based HVDC systems on personal computers. With the assumption of ideal switch operation and using an equivalent average capacitor value based approach, an average valued model of MMC is also proposed in this thesis. The average model can be accurately used in most of the system level studies. The control algorithms and other modeling aspects of MMC applications are also presented in this thesis. One of the advantages of multilevel converters is the low operating losses as the smaller switching frequency of each individual power electronics switch and the low voltage step change during each switching. Using a recently developed, time domain simulation approach, the operating losses of the MMC converter are estimated in this thesis. When comparing the MMC operating losses against the losses of two-level VSC, the power loss for the two-level VSC is found to be significantly higher than the power loss of the MMC.

Power systems simulation tools enable to study and evaluate the performance of electrical power systems in different scenarios. This allows the development and implementation of new solutions to the challenges electrical grids face nowadays. In this sense, electromagnetic transient (EMT) simulation provides detailed information on the behaviour of the different components involved in the system. Moreover, among the wide range of existing tools, those based in Modelica language present certain advantages for power system simulation, such as equation- based modeling and the possibility of working in open- source environments. This project presents the development of components and reference grid in EMT formalism in the open- source environment OpenModelica, based on Modelica language. With the purpose of power system simulation, electrical components have been modeled in OpenModelica and gathered in a library for EMT simulation The performance of the different components has been validated by comparing the results of the EMT simulation of a 3buses reference grid in different case studies in OpenModelica and other EMT- based software. Furthermore, the comparison has been also established with phasor simulation in OpenModelica, enabling the evaluation of the differences between phasor and EMT simulation. The results show the main advantages and drawbacks of working with OpenModelica regarding other simulation tools and the lack of information provided by the phasor simulation, particularly in the case of a fault event. Additionally, certain difficulties encountered when working with OpenModelica have also been identified. / Simulering av kraftsystem gör det möjligt att studera och utvärdera prestandan i olika scenarion. Genom detta kan utveckling och implementering av nya lösningar på de utmaningar som elnäten står inför framöver ske. Elektromagnetisk transient (EMT)simulering ger detaljerad information om beteendet hos de olika komponenterna i systemet. Bland de många befintliga verktygen innehåller de som är baserade på Modelica- språket dessutom vissa fördelar för kraftsystemsimulering, såsom ekvationsbaserad modellering och möjligheten att arbeta i miljöer med öppen källkod. Den här uppsatsen presenterar en utveckling av komponenter och testelnät i EMT- formalism i öppen källkodsmiljö OpenModelica, baserat på programmeringsspråket Modelica. Elektriska komponenter har modellerats i OpenModelica och samlats i ett bibliotek för EMT- simulering. Målet är en detaljerad simulering av elkraftsystem. Komponenternas prestanda har validerats genom att jämföra resultatet av EMT- simuleringen av ett 3bussreferensnät i olika fallstudier i OpenModelica och annan EMT- baserad programvara. Sedan har jämförelsen även utförts med simuleringar i fasorformalism i OpenModelica. Den här jämförelsen har också möjliggjort utvärderingen av skillnaderna mellan fasor och EMT- simulering. Resultaten visar de största fördelarna och nackdelarna med att arbeta med OpenModelica njämfört med andra simuleringsverktyg. De visar också bristen på information om fasorsimuleringen, särskilt i fallet med ett elektriskt fel. Dessutom har vissa svårigheter identifierats med att arbeta med OpenModelica.

Les réseaux de transport d’électricité multi-terminaux à courant continu se révèlent être la solution adéquate pour une intégration massive d’énergie renouvelable dans les réseaux alternatifs existants. En effet, les réseaux en courant continu sont capables de transmettre de manière efficace des niveaux de puissance élevés sur de très longues distances par rapport aux réseaux alternatifs car, à partir d'une certaine puissance à transmettre, il existe une distance limite à partir de laquelle la transmission d’énergie en courant alternatif perd sa compétitivité face à la transmission en courant continu. L'un des principaux défis liés au développement de ces réseaux de transport d’électricité à courant continu ou Supergrid, concerne leur protection contre des défauts de type court-circuit sur des liaisons en courant continu. . Dans ce travail de thèse, un nouveau concept de stratégie de protection des réseaux en courant continu à haute tension en cas de défaut court-circuit est proposé. La stratégie repose sur une philosophie de protection ayant comme priorité la suppression du courant de défaut avant l’isolation de la liaison en défaut. Elle est basée sur l’utilisation de disjoncteurs mécaniques à courant continu sans avoir recours à des limiteurs de courant de défaut. Une séquence de protection primaire ainsi que deux séquences de sauvegarde en cas de défaillance de disjoncteurs ont été développées, testées et validées à l’aide de simulations de transitoires électromagnétiques et de simulations temps-réel. En outre, les algorithmes des relais de protection ont été implémentés avec l'aide de l’outil d’analyse fonctionnelle descendante SADT (Structured Analysis and Design System). Cette thèse a été effectuée dans le cadre du SuperGrid Institute, une plate-forme de recherche collaborative visant à développer des technologies pour les futurs réseaux de transport d'électricité et regroupant l'expertise d'industries telles que GE Grid Solutions et les laboratoires de recherche publique comme le laboratoire de génie électrique de Grenoble (G2Elab). / Multi-terminal High Voltage Direct Current (MTDC) grids,have been proven to be an adequate solution for massive integration of renewable energy power to existing High Voltage Alternating Current (HVAC) grids. Indeed, HVDC grids are capable of transmitting efficiently high level of power over very long distances compared to HVAC grids since, from a certain power to be transmitted, there is a limited distance from which the AC power transmission loses its efficiency and becomes very costly compared to DC power transmission. One of the main challenges related to the development of theses multi-terminal HVDC grids, or Supergrids, concerns their protection against DC short-circuit faults. In this thesis, a new concept of protection strategy for MTDC grids in case of permanent short-circuit fault on a DC cable has been proposed. The strategy is based on the non-selective fault clearing philosophy where the priority is given to the suppression of the fault current before isolating the faulty transmission line. The strategy is based on mechanical DC breakers and no fault current limiting devices are used. A primary protection sequence as well as two back-up sequences in case of breakers operation failure have been developed, tested and validated through Electromagnetic Transient (EMT) and Real-Time (RT) simulations. Also, algorithms to be implemented on protective relays have been designed with the help of the Structured Analysis and Design System (SADT). This PhD thesis has been performed in the frame of the SuperGrid Institute, a collaborative research platform aiming to develop technologies for the future electricity transmission network and bringing together the expertise of industries such as GE grid solutions and public research laboratories as the Grenoble Electrical Engineering Laboratory (G2Elab).