A New MMC Topology Which Decreases the Sub Module Voltage Fluctuations at Lower Switching Frequencies and Improves Converter Efficiency

Modular Multi-level inverters (MMCs) are becoming more common because of their suitability for applications in smart grids and multi-terminal HVDC transmission networks. The comparative study between the two classic topologies of MMC (AC side cascaded and DC side cascaded topologies) indicates some disadvantages which can affect their performance. The sub module voltage ripple and switching losses are one of the main issues and the reason for the appearance of the circulating current is sub module capacitor voltage ripple. Hence, the sub module capacitor needs to be large enough to constrain the voltage ripple when operating at lower switching frequencies. However, this is prohibitively uneconomical for the high voltage applications. There is always a trade off in MMC design between the switching frequency and sub module voltage ripple

Performance evaluation and control of an MMC active rectifier with half-bridge and full-bridge submodules for HVDC applications

Dissertation (MEng (Electrical Engineering))--University of Pretoria, 2021.The modular multilevel active rectifier was designed and evaluated, whereby the half bridge and the full bridge DC-DC converters as its submodules for the high voltage direct current transmission were compared. It was found that, by taking advantage of the unipolar modulation scheme in the full bridge converter, the switching losses in the two converters are equal when they are both operated in the linear modulation region. Furthermore, operating the full bridge converter in the overmodulation region does not give it a pronounced advantage over the half bridge converter. The conduction losses in the full bridge converter are two times higher than those in the half bridge converter, due to double the number of semiconductor devices. However, using the half bridge converter in the high voltage direct current modular multilevel converter requires an expensive DC-side breaker, while use of the full bridge converter eliminates the need for such a breaker due to the intrinsic DC-side fault current blocking capability. The clear choice between the two requires industry cost data. A design methodology for the submodule capacitor average voltage loop controllers for phase-shifted carrier modulated modular multilevel converters was carried out from first principles. The methodology enables design of such controllers to be carried out in a step by step and straightforward manner without resorting to simulation or guesswork. A simple but effective submodule capacitor sizing method was proposed. The resulting submodule capacitor size was shown to be smaller than those resulting from other sizing methods proposed in the literature while achieving the submodule capacitor voltage ripple specifications. A robust DC bus voltage controller design for modular multilevel rectifiers was presented, whereby a design method for multilevel voltage source converters with DC link capacitors was adopted for modular multilevel rectifiers. Since the modular multilevel converters for HVDC application are designed without the DC-link capacitor to mitigate the effects of a possible DC-side fault current, the submodule capacitors in the modular multilevel converter acted as an equivalent DC link capacitor to accomplish the design.Electrical, Electronic and Computer EngineeringMEng (Electrical Engineering)Unrestricte

Study and evaluation of distributed power electronic converters in photovoltaic generation applications

This research project has proposed a new modulation technique called “Local Carrier Pulse Width Modulation” (LC-PWM) for MMCs with different cell voltages, taking into account the measured cell voltages to generate switching sequences with more accurate timing. It also adapts the modulator sampling period to improve the transitions from level to level, an important issue to reduce noise at the internal circulating currents. As a result, the new modulation LC-PWM technique reduces the output distortion in a wider range of voltage situations. Furthermore, it effectively eliminates unnecessary AC components of circulating currents, resulting in lower power losses and higher MMC efficiency.Departamento de Tecnología ElectrónicaDoctorado en Ingeniería Industria

A hybrid modular multilevel converter with reduced full-bridge submodules

A hybrid modular multilevel converter (MMC) with reduced full-bridge (FB) submodules (SMs) is proposed, where a high voltage rating half-bridge (HB) based MMC is connected in series with a low voltage rating FB-MMC in parallel with a fault breaking circuit on its DC side. Unlike conventional hybrid MMCs with mixed HB and FB SMs, the proposed topology uses the DC capacitor in the fault breaking circuit to block DC faults, while the FB-MMC only commutates the fault current from the FB-MMC to the fault breaking circuit. Thus, the proposed converter only requires around 10%-20% FB SMs, leading to reduced capital cost and losses compared to typical hybrid MMC. The optimal ratio of the FB-MMC and HB-MMC is assessed and comparative studies show superiority of the proposed topology over other alternatives. A case study with 10% FB SMs demonstrates the validity of the proposed hybrid MMC for DC fault blocking and post-fault system restart

A hybrid modular multilevel converter with reduced full-bridge submodules

A hybrid modular multilevel converter (MMC) with reduced full-bridge (FB) submodules (SMs) is proposed, where a high voltage rating half-bridge (HB) based MMC is connected in series with a low voltage rating FB-MMC in parallel with a fault breaking circuit on its DC side. Unlike conventional hybrid MMCs with mixed HB and FB SMs, the proposed topology uses the DC capacitor in the fault breaking circuit to block DC faults, while the FB-MMC only commutates the fault current from the FB-MMC to the fault breaking circuit. Thus, the proposed converter only requires around 10%-20% FB SMs, leading to reduced capital cost and losses compared to typical hybrid MMC. The optimal ratio of the FB-MMC and HB-MMC is assessed and comparative studies show superiority of the proposed topology over other alternatives. A case study with 10% FB SMs demonstrates the validity of the proposed hybrid MMC for DC fault blocking and post-fault system restart

Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters

The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients. In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems. This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems. HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study. Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications. One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations. This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage

Potential of Bipolar Full-Bridge MMC-HVdc Transmission for Link and Overlay Grid Applications

Bipolare HGÜ Systeme in Multi-Level-Ausführung stellen ein attraktives Lösungskonzept zur Bewältigung einer Vielzahl von Herausforderungen im Kontext heutiger Energiesysteme dar. Da dies jedoch auf Kosten einer deutlich erhöhten Systemkomplexität geschieht, ist ein tiefgreifendes Verständnis des transienten Verhaltens sowie der dynamischen Charakteristik von enormer Wichtigkeit. Diese Doktorarbeit beinhaltet eine detaillierte Analyse von grundlegenden Zusammenhängen bezogen auf bipolare HGÜ Systeme in Multi-Level-Ausführung und stellt ein generisches Regelungs-, Bilanzierungs- und Schutzkonzept vor. Die generelle Leistungsfähigkeit des Konzepts wird durch elektro-magnetische Transientensimulationen nachgewiesen

Energy based virtual damping control of FB-MMCs for HVDC grid

Full-bridge submodule based modular multilevel converters (FB-MMCs) have attracted wide attention due to the DC fault blocking capability. However, the blocking of the FB-MMC can only suppress its DC terminal current while the fault currents may still circulate along the meshed DC network. To address this issue, an energy based virtual damping control is proposed, where the DC terminal current of the FB-MMC is regulated to follow the DC voltage in the event of a DC fault. The FB-MMC is thus controlled as a virtual damping resistor to actively absorb the residual energy in the DC network and quickly suppress the potential circulating DC fault currents. This enables fast fault isolation using DC switches and thereby fast fault recovery after fault isolation. The fault isolation time is significantly reduced from around 120 ms with the FB-MMC simply being blocked and 50ms with the conventional FB-MMC fault control method to around 15 ms. The validity of the proposed control is verified in a three-terminal meshed DC network

CAPACITOR VOLTAGE BALANCING, FAULT DETECTION, AND FAULT TOLERANT CONTROL TECHNIQUES OF MODULAR MULTILEVEL CONVERTERS

Modular Multilevel Converters (MMCs) are distinguished by their modular nature that makes them suitable for wide range of high power and high voltage applications. However, they are vulnerable to internal faults because of the large number of series connected Sub-Modules. Additionally, it is highly recommended not to block the converter even if it is subjected to internal faults to secure the supply, to increase the reliability of the system and prevent unscheduled maintenance. This thesis introduces a fault tolerant control system for controlling the MMC in normal as well as abnormal operating conditions. This is done through developing a new adaptive voltage balancing strategy based on capacitor voltage estimation utilizing ADAptive LInear NEuron (ADALINE) and Recursive Least Squares (RLS) algorithms. The capacitor voltage balancing techniques that have been proposed in literature are based on measuring the capacitor voltage of each sub-module. On contrary, the proposed strategy eliminates the need of these measurements and associated communication links with the central controller. Furthermore, the thesis presents a novel fault diagnosis algorithm using the estimated capacitor voltages which are utilized to detect and localize different types of sub-module faults. The proposed fault diagnosis algorithm surpasses the methods presented in literature by its fast fault detection capability without the need of any extra sensing elements or special power circuit. Finally, a new Fault Tolerant Control Unit (FTCU) is proposed to tolerate the faults located inside the MMC submodules. The proposed FTCU is based on a sorting algorithm which modifies the parameters of the voltage balancing technique in an adaptive manner to overcome the reduction of the active submodules and secure the MMC operation without the need of full shut-down. Most of fault tolerant strategies that have been proposed by other researchers are based on using redundant components, while the proposed FTCU does not need any extra components. The dynamic performance of the proposed strategy is investigated, using PSCAD/EMTDC simulations and hardware in the loop (HIL) real-time simulations, under different normal and faulty operating conditions. The accuracy and the time response of the proposed fault detection and tolerant control units result in stabilizing the operation of the MMC under different types of faults. Consequently, the proposed integrated control strategy improves the reliability of the MMC

Control of modular multilevel converters in high voltage direct current power systems

This thesis focuses on a comprehensive analysis of Modular Multilevel Converters (MMC) in High Voltage Direct Current (HVDC) applications from the viewpoint of presenting new mathematical dynamic models and designing novel control strategies. In the first step, two new mathematical dynamic models using differential flatness theory (DFT) and circulating currents components are introduced. Moreover, detailed step-by-step analysis-based relationships are achieved for accurate control of MMCs in both inverter and rectifier operating modes. After presenting these new mathematical equations-based descriptions of MMCs, suitable control techniques are designed in the next step. Because of the nonlinearity features of MMCs, two nonlinear control strategies based on direct Lyapunov method (DLM) and passivity theory-based controller combined with sliding mode surface are designed by the use of circulating currents componentsbased dynamic model to provide a stable operation of MMCs in HVDC applications under various operating conditions. The negative effects of the input disturbance, model errors and system uncertainties are suppressed by defining a Lyapunov control function to reach the integralproportional terms of the flat output errors that should be finally added to the initial inputs. Simulation results in MATLAB/SIMULINK environment verify the positive effects of the proposed dynamic models and control strategies in all operating conditions of the MMCs in inverter mode, rectifier mode and HVDC applications.Esta tese visa proceder a uma análise abrangente de conversores multinível modulares (MMC) para transmissão a alta tensão em corrente contínua (HVDC), almejando apresentar novos modelos matemáticos em sistemas dinâmicos e projetar novas estratégias de controlo. Na primeira etapa são introduzidos dois novos modelos matemáticos dinâmicos que usam differential flatness theory e as componentes de correntes circulantes. Ainda, é estabelecida uma modelação matemática para o controlo preciso dos MMCs, operando em modo inversor ou modo retificador. Depois de apresentar as novas equações matemáticas, as técnicas de controlo mais adequadas são delineadas. Devido às características não lineares dos MMCs, são projetadas duas estratégias de controlo não-lineares baseadas no método direto de Lyapunov e no controlo do tipo passivity theory-based combinado com controlo por modo de deslizamento através do uso de modelos dinâmicos baseados em correntes circulantes para fornecer uma operação estável aos MMCs em aplicações de HVDC sob várias condições de operação. Os efeitos negativos das perturbações de entrada, erros de modelação e incertezas do sistema são suprimidos através da definição da função de controlo de Lyapunov para alcançar os termos de integraçãoproporcionalidade dos erros de saída para que possam finalmente ser adicionados às entradas iniciais. Os resultados da simulação computacional realizados em ambiente MATLAB/SIMULINK verificam os efeitos positivos dos modelos dinâmicos propostos e das novas estratégias de controlo em todas as condições de operação dos MMCs no modo inversor, retificador e em aplicações HVDC