Modified half-bridge modular multilevel converter for HVDC systems with DC fault ride-through capability

One of the main challenges of voltage source converter based HVDC systems is DC faults. In this paper, two different modified half-bridge modular multilevel converter topologies are proposed. The proposed converters offer a fault tolerant against the most severe pole-to-pole DC faults. The converter comprises three switches or two switches and 4 diodes in each cell, which can result in less cost and losses compared to the full-bridge modular multilevel converter. Converter structure and controls are presented including the converter modulation and capacitors balancing. MATLAB/SIMULINK simulations are carried out to verify converter operation in normal and faulty conditions

Controlled transition full-bridge hybrid multilevel converter with chain-links of full-bridge cells

This paper proposes a controlled transition full-bridge (CTFB) hybrid multilevel converter (HMC) for medium and high voltage applications. It employs a full-bridge cell chain-link (FB-CL) between the two legs in each phase to generate multilevel bipolar output voltage. The CTFB-HMC has twice dc voltage utilization or power density of conventional converters due to the bipolar capability of its full-bridge configuration. Hence, for the same power rating and same voltage level number, its total cells per phase are quarter that in modular multilevel converter (MMC), which reduces the hardware installation volume. Also, in the proposed converter, the total device number in the conduction paths is the same as in the half-bridge MMC, leading to low conduction losses. The FB-CL current of the CTFB converter has no dc component, which offers the potential to enhance the transient response. Comparative studies between the CTFB and other multilevel topologies are carried out to clarify its main features. The modulation strategies and parameter sizing of the proposed converter are investigated using a generic case. Simulation and experimental results are used to verify the effectiveness of the proposed approach

Design and implementation of 30kW 200/900V LCL modular multilevel based DC/DC converter for high power applications

This paper presents the design, development and testing of a 30kW, 200V/900V modular multilevel converter (MMC) based DC/DC converter prototype. An internal LCL circuit is used to provide voltage stepping and fault tolerance property. The converter comprises two five level MMC based on insulated gate bipolar transistors (IGBTs) and metal oxide semiconductor field effect transistor (MOSFET). Due to low number of levels, selective harmonic elimination modulation (SHE) is used, which determines the switching angles in such a way that third harmonic is minimized whereas the fundamental component is a linear function of the modulation index. In addition, instead of using an expensive control board, three commercial control boards are embedded. This is required to implement the sophisticated DC/DC converter control algorithm. Simulation and experimental results are presented to demonstrate the converter performance in step up and down modes

Control of Modular Multilevel Converters for Grid Integration of Full-Scale Wind Energy Conversion Systems

The growing demand for wind power generation has pushed the capacity of wind turbines towards MW power levels. Higher capacity of the wind turbines necessitates operation of the generators and power electronic conversion systems at higher voltage/power levels. The power electronic conversion system of a wind energy conversion system (WECS) needs to meet the stringent requirements in terms of reliability, efficiency, scalability and ease of maintenance, power quality, and dv/dt stress on the generator/transformer. Although the multilevel converters including the neutral point clamped (NPC) converter and the active NPC converter meet most of the requirements, they fall short in reliability and scalability. Motivated by modularity/scalability feature of the modular multilevel converter (MMC), this research is to enable the MMC to meet all of the stringent requirements of the WECS by addressing their unique control challenges. This research presents systematic modeling and control of the MMC to enable it to be a potential converter topology for grid integration of full-scale WECSs. Based on the developed models, appropriate control systems for control of circulating current and capacitor voltages under fixed- and variable-frequency operations are proposed. Using the developed MMC models, a gradient-based cosimulation algorithm to optimize the gains of the developed control systems, is proposed. Performance/effectiveness of the developed models and the proposed control systems for the back-to-back MMC-based WECS are evaluated/verified based on simulations studies in the PSCAD/EMTDC software environment and experimental case studies on a laboratory-scale hardware prototype

Design and Simulation of Modular Multilevel Converter Fed Induction Motor Drive

Traditional modular multilevel converter (MMC) applications in medium voltage induction motor drive are difficult, particularly at low speeds because of the higher magnitude of the voltage ripple of the sub-module capacitor. This paper uses a hybrid MMC, particularly at low frequencies, to achieve a lower peak-to-peak voltage ripple of the sub-module capacitor. The vector control strategy with the closed-loop speed control indicates an accurate and wide-speed range. MATLAB / Simulink is used to simulate and obtain the simulation results of hybrid and traditional MMC with induction motor drive and compare from the standpoint of capacitor voltage ripple. The results are shown the reduction of peak-to-peak voltage ripple of the sub-module capacitor as the hybrid MMC is operated

Optimized Modulation and Thermal Management for Modular Power Converters

The transition to a more and more decentralized power generation based on renewable energy generation is accompanied by high challenges. Modular power converters play a central role in facing these challenges, not only for grid integration but also to provide flexible services, highly efficient power transmission and safe storage integration. These goals are the key elements in becoming independent from fossil and nuclear power plants in near future. Even if the costs for renewable energy power plants like wind or photovoltaic systems are already competitive to conventional solutions, more flexible operation and further reduction in costs are required for faster global transformation towards sustainable energy systems. The further optimization of modular power converters can be seen as an ideal way to achieve these ambitious goals. It is therefore chosen as the focus of this work

Power quality improvement with a pulse width modulation control method in modular multilevel converters under varying nonlinear loads

UIDB/00667/2020 POCI-01-0145-FEDER-029803 (02/SAICT/2017) POCI-01-0145-FEDER-006961 (UID/EEA/50014/2019)In order to reach better results for pulse width modulation (PWM)-based methods, the reference waveforms known as control laws have to be achieved with good accuracy. In this paper, three control laws are created by considering the harmonic components of modular multilevel converter (MMC) state variables to suppress the circulating currents under nonlinear load variation. The first control law consists of only the harmonic components of the MMC's output currents and voltages. Then, the second-order harmonic of circulating currents is also involved with both upper and lower arm currents in order to attain the second control law. Since circulating current suppression is the main aim of this work, the third control law is formed by measuring all harmonic components of circulating currents which impact on the arm currents as well. By making a comparison between the switching signals generated by the three proposed control laws, it is verified that the second-order harmonic of circulating currents can increase the switching losses. In addition, the existence of all circulating current harmonics causes distributed switching patterns, which is not suitable for the switches' lifetime. Each upper and lower arm has changeable capacitors, named "equivalent submodule (SM) capacitors" in this paper. To further assess these capacitors, eliminating the harmonic components of circulating currents provides fluctuations with smaller magnitudes, as well as a smaller average value for the equivalent capacitors. Moreover, the second-order harmonic has a dominant role that leads to values higher than 3 F for equivalent capacitors. In comparison with the first and second control laws, the use of the third control-law-based method will result in very small circulating currents, since it is trying to control and eliminate all harmonic components of the circulating currents. This result leads to very small magnitudes for both the upper and lower arm currents, noticeably decreasing the total MMC losses. All simulation results are verified using MATLAB software in the SIMULINK environment.publishersversionpublishe

Cost-Effective Model Predictive Control Techniques for Modular Multilevel Converters

In this thesis, model predictive control (MPC) techniques are investigated with their applications to modular multilevel converters (MMCs). Since normally a large number of submodule (SM) capacitor voltages and gate signals need to be handled in an MMC, the MPC schemes studied in this thesis are employed for determining only the voltage levels of converter arms, while gate signals are subsequently generated by the conventional sorting method. Emphasis is given to inner-loop current control in terms of phase current and circulating current, aiming at performance enhancement and computation reduction. A variable rounding level control (VRLC) approach is developed in this thesis, which is based on a modification of the conventional nearest level control (NLC) scheme: instead of the conventional nearest integer function, a proper rounding function is selected for each arm of the MMC employing the MPC method. As a result, the simplicity of the NLC is maintained while the current regulating ability is improved. The VRLC technique can also be generalized from an MPC perspective. Different current controllers can be considered to generate the arm voltage references as input of the VRLC block, thus refining the control sets of the MPC. Based on the decoupled current models, the accumulated effect of SM capacitor voltage ripples is investigated, revealing that the VRLC strategy may not achieve a proper performance if the accumulated ripple is nontrivial compared to the voltage per level. Two indexes are also proposed for quantifying the current controllability of the VRLC. Benefiting from this analysis, A SM-grouping solution is put forward to apply such MPC techniques to an MMC with a large number of SMs, leading to an equivalent operation of an MMC with much reduced number of SMs, which significantly increases the current regulating capability with reduced complexity. As an example, the SM-grouping VRLC proposal is analyzed and its system design principles are described. This thesis also develops another MPC technique which directly optimizes the cost function using quadratic programming technique. Both a rigorous and a simplified procedure are provided to solve the optimization problem. Compared with the conventional finite control set (FCS)-MPC method which evaluates all voltage level combinations, the proposed scheme presents apparent advantage in terms of calculation cost while achieving similar performance