Individual information

Energy management in modular type converters constitutes a key aspect of their operational stability. This paper introduces a full energy management structure for the Extended Overlap-Alternate Arm Converter (EO-AAC) ensuring both equal energy distribution across all six stacks and the maintaining of ripple-free DC current during steady state. The performance of the control structure against active power step events is validated by detailed simulations using EMTP-RV software. Moreover, the full energy management allows the EO-AAC to have an equivalent controllability to Modular Multilevel Converter (MMC) through only two conducting stacks in overlap mode ensuring the power balance. From this observation, it is demonstrated that the use of control strategy like the virtual capacitor to support HVDC system, originally designed for MMC, is possible. Gathering all these controllers leads to a general conclusion which is the dynamic equivalence between EO-AAC and MMC.

Modular Multilevel Converter (MMC) is an established technology for HVDC or Multi-Terminal DC (MTDC) systems, due to its advantages over classical Voltage Source Converters (VSCs) such as two or three level VSCs. To achieve a full control of all state variables, it is essential to implement energy-based method in which a cascade control loop is employed to regulate all state variables including ac and differential currents, and stored energy within MMC arms. In addition to normal operation control, dc Fault Ride Through (DC-FRT) capability of the MMC is a crucial and challenging control issue especially for overhead line HVDC system where non-permanent dc fault occurrence is statistically more probable. Furthermore, the main problematic technical obstacle to develop HVDC/MTDC grids is the lack of mature dc fault protection. Since conventional control in normal operation cannot be employed in case of dc fault, an efficient control strategy is indispensable. The principal objectives of the novel control methodology are to (i) obtain DC-FRT capability, (ii) decay short circuit current to zero, (iii) secure the MMC through leg and arm energy balancing, (iv) support ac grid as a Static Synchronous Compensator (STATCOM) and (v) resume normal operation after dc fault clearance. The simulation results verify the validity of proposed control strategy to fulfill the abovementioned objectives in dc fault operation of the hybrid MMC.

The Modular Multilevel DC Converter is an attractive non-isolated topology to interconnect High Voltage DC Links. This paper presents the interaction among control, component design and efficiency of this converter. The impact of the two degrees of freedom on the design and the efficiency is analyzed.

This work focuses on the steady-state analysis of three types of MMC based STATCOM. For a given STATCOM rating, Double-Star Full Bridge, Single-Star Full Bridge and Single-Delta Full Bridge have been compared in terms of design and losses. In this approach, the number of submodules is chosen according to the voltage and current ratings of semiconductor devices while the submodule capacitor value is obtained by following an energy storage criterion to maintain the submodule voltages within an acceptable voltage range.

The MMC is the solution today to connect HVDC grids to the current HVAC grid. This paper proposes to evaluate the impact of Zero Sequence Voltage Injection variants, which until now, have not been extensively studied. Such techniques can involve, for example, a reduction of the SM capacitor value, the number of SM requirement and converter losses. The paper presents MMC current model, control and highlights the implication of the zero-sequence voltage. Grid current control structure with the introduction of the zero-sequence voltage is presented in different techniques. These modulation schemes are compared through two main quantities in MMC, the energy requirement defining the SM capacitance value and the power losses.

Over the last 15 years, VSC-based HVDC (High Voltage Direct Current) has become a mature technology for HVDC transmission schemes. The Modular Multilevel Converter (MMC) represents the recent development among the diverse available topologies of VSC and is allegedly the most promising solution today. In fact, the MMC topology offers significant benefits compared to the traditional two-level VSC (Voltage Source Converter), such as lower losses, distributed storage of capacitive energy, improved scalability to higher voltage ratings, a modular design, low total harmonic distortion and, hence, the potential lack of passive fillers on the AC-side of the converter. For the control design, some simplifying assumptions are made to derive an energy based Average Arm Model (AAM) that takes into account the internal dynamics (i.e. the total energy stored in the converter) which do not exist in the 2-level VSC, as well as the AC and DC side dynamics. This additional internal dynamics implies that the control system of the converter must possess additional control loops that govern the DC current and the total amount of stored energy in the SM capacitors of the MMC. The total stored energy in the MMC is then decoupled from the DC bus, but can also be potentially shared depending on the reference signal of the energy control loop. So, the Energy-based controller strategy is introduced, where extra control loops in cascade are added to regulate the dynamics of interest. In this work, an energy based control method is developed for the OPAL-RT’s 10 levels MMC prototype model (5 kW – 400 V). Only the high-level control is proposed and implemented thanks to real time simulation model on the CPU and ARTEMiS solver based on State-Space Nodal Solver. As mentioned above, the energy-based control is based on the possibility to control the AC and DC power separately. Thus, if the power that flowing through the converter is controlled by the AC (resp. DC) power reference, it will be possible to drive the energy level thanks to the other power reference DC (resp. AC). Therefore, to control globally the MMC inner dynamics and state space variables, additional controllers should be added. Thereby, energy based controls have been developed where all the state variables are controlled. In this configuration, the high-level control is composed of outer and inner closed loop controls, which allow controlling the power and the internal MMC energy. Then, based on the designed energy-based controller, the OPAL-RT’s MMC prototype model has been simulated under different operation conditions arising a good performance in steady state as well as during transients. Moreover, the advantages of the Per-unit approach such as the control design as well as the power converter sizing have been carried out across this work. Thus, the per-unit approach is performed on one hand for high voltage characteristic of MMC where the per-unit parameters are derived from EMTP library (based on INELFE project) and on the other hand for the low scale OPAL-RT mockup. As conclusion, the per-unit approach can be used for designing the control as previously said but it can be also very useful for the sizing process of the converter. While the per-unit approach has been used for many years for the classical power system elements (transformer, synchronous machines), it can be extended also to the sizing of power converter. So, in this work, the per-unit quantities of the main elements (capacitor, inductances) will be highlighted. Then, the high voltage characteristic will be considered as a reference and the low voltage characteristics will be compared and some conclusions will be underlined.

The modular multilevel converter (MMC) is the most accepted solution to connect a HVDC grid to an AC transmission grid. The Alternate Arm Converter (AAC) is another promising structure since it allows a DC short-circuits blocking capability similarly to the Full Bridge MMC while having a small impact on the power losses. Its footprint is smaller to the MMC since the needed number of modules is closer to 50% and the SM Capacitors are about three times smaller. The AAC is a hybrid structure between a 2 level VSC converter and an MMC one. Elements hampering the development of the AAC are its complexity to model and control, in particular the opening procedure of the director switches (DS) since these DS are directly connected in series to the arm inductance. This paper proposes a fast method to control the opening of the DS at zero current. The first part is focused on the instantaneous model and current control of the converter and AAC. The second part is focused on the opening method of the DS without generating overvoltage in the converter and taking into account the technical parameters of the various elements of the AAC. Finally, simulation results validate the DS opening control of the AAC converter.

The modular multilevel converter (MMC) is one of the available solution to connect a HVDC grid to an AC transmission one. The Alternate Arm Converter (AAC) is a promising structure since it allows a DC short-circuit blocking capability as the Full Bridge MMC while having equivalent losses to the half-bridge MMC. Its footprint is smaller than the MMC since the needed number of modules is closer to 50% and the SM Capacitors are about three times smaller. The AAC is a hybrid structure between a 2 level VSC converter and an MMC one. The main drawback of the AAC is the complexity of its control. This paper presents first, the instantaneous model and current control of the AAC. The second part is focused on the energetic model of the AAC and its control. Finally, simulation results validate the quality of the proposed control.

Le convertisseur DC-DC Modulaire Multi-niveaux (M2DC) est une topologie attrayante de convertisseur DC-DC non isolée pour les réseaux haute tension à courant continu (HVDC). Cet article présente dans un premier temps, le modèle et la commande du M2DC. Une étude de l’utilisation des degrés de liberté phares est menée dans un second temps pour réaliser une analyse de leurs impacts sur le dimensionnement des éléments du convertisseur tels que les condensateurs de sous modules ainsi que sur les pertes liées aux semi-conducteurs

La future intégration massive des sources renouvelables dans le réseau électrique crée un besoin fort en interfaces d’électronique de puissance à haute tension. Depuis la création du convertisseur modulaire multiniveau (MMC) au début des années 2000, l’intérêt pour les convertisseurs de type modulaire n’a cessé de croître, au point que de nouvelles structures dérivées du MMC ont été développées. Le sujet de la thèse est le Convertisseur à Demi-Bras Alternés (AAC), une structure de type hybride-modulaire adaptée aux applications haute tension tel que les systèmes HVDC. Parmi les différentes topologies de convertisseurs AAC, l’Extended Overlap-AAC (EO-AAC) a été choisie et analysée. Le travail de thèse est organisé selon trois axes principaux comprenant l’analyse en régime permanent, la synthétisation de la structure de commande ainsi que l’intégration au système, en particulier dans le système HVDC. Le premier chapitre aborde quelques considérations fondamentales sur la conversion AC/DC pour introduire, à travers une approche globale et systématique, cette nouvelle famille de convertisseurs hybrides modulaires dont l’EO-AAC fait partie. Le deuxième chapitre est consacré au dimensionnement de cette topologie. L’estimation du nombre de sous-modules, le dimensionnement des interrupteurs directeurs, l’évaluation des pertes ainsi qu’une analyse approfondie du besoin énergétique afin de dimensionner les condensateurs des sous-modules sont proposés. Ensuite, le troisième chapitre décrit une structure de contrôle pour chaque variable d’état. L’accent est mis sur la gestion de l’énergie en assurant une distribution égale de l’énergie interne du convertisseur entre les piles de SMs. Elle est réalisée par le biais de trois fonctions principales : le contrôle Total, les contrôles d’équilibrage Horizontal et Vertical. À partir de cette structure de gestion de l’énergie, il est mis en évidence que l’EO-AAC peut fournir une dynamique et des services aux systèmes similaires à ceux du MMC. Cette équivalence est prouvée dans la dernier chapitre par l’existence d’un modèle réduit commun dans le repère dq0 suggérant la possibilité d’inclure les EO-AAC dans l’analyse de stabilité des grands systèmes. Enfin, tous les modèles et contrôleurs sont rassemblés pour réaliser l’étude des EO-AAC connectés au réseau et ses performances sont comparées au MMC en termes de réponse dynamique.