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Browsing by Subject "MMC"

Now showing 1 - 3 of 3
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    Control strategies of MMC-HVDC connected to large offshore wind farms for improving fault ride-through capability
    (2020-07) Choi, Woojung
    This paper proposes strategies to improve fault ride-through (FRT) capability of the modular multi-level converter (MMC) - high voltage direct current (HVDC) system connected to large offshore wind farms and performs simulations. In offshore wind power plants, HVDC system is indispensable for long-distance high-capacity transmission. The voltage rise of HVDC-link happens inevitably due to energy accumulation to satisfy low voltage ride-through (LVRT) regulation when a main grid fault occurs. This paper presents strategies for controlling HVDC-link voltages while minimizing the application of DC choppers and the mechanical and electrical stress of wind turbines through fast fault detection and current limit control of the master controller and wind turbine converter. PSCAD/EMTDC simulation is performed to verify the control strategies, and the results show that the FRT capability is enhanced by controlling HVDC-link voltage properly.
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    A Highly Modular Grid Interface For Utility Scale Renewables: Mmc With Isolated High Frequency Link Sub-Modules
    (2017-05) Otero-De-Leon, Ruben
    A novel isolated Modular Multilevel Converter that allows low voltage DC (LVDC) to medium voltage ac (MVAC) power transmission is proposed in this dissertation. The proposed topology is composed of a primary inverter and a collection of sub-modules. Each sub-module is composed of a high-frequency transformer, a diode bridge rectifier, a capacitor, and a half-bridge cell. The sub-modules are arrange in a three phase MMC configuration where the output of each sub-module is the half-bridge cell. The input of all sub-modules are connected to the primary inverter via a high-frequency bus bar. Each sub-module is capable of generating its own isolated DC voltage by tapping into the high-frequency bus bar. The galvanic isolation between the MMC side and the input of the inverter, and the unidirectional power flow, is ideal for use with PV panels. The proposed solution allows for scalability with the possibility of reaching MVAC, making it an ideal converter for large scale PV power plants. With the use of high frequency transformers the proposed topology can be built small and lightweight, allowing it for other applications. One example is as a converter for wind energy systems where the reduce size and weight can allow the converter to be located in the hub of smaller wind turbines. This strategy allows the turbine to transmit high voltage power at lower current levels, hence reducing the cable thickness and the conduction losses associated with it. In this dissertation, a detail analysis of the proposed topology, simulations and experimental results are shown. Simulations were created using the PLECS tool set in Matlab/Simulink. The hardware prototype is a proof of concept design to operate at 1kW, with an input of 100 V, and a 9-level output voltage with a ± 200 V limit. A total of 24 sub-modules were created, 8 for each phase of this three phase converter. Two alternatives for the MMC cell are proposed, a half-bridge solutions and a full-bridge solution, although only the first is developed in hardware. It is shown through simulation and experimentation that the average values of the capacitor voltages are self-balanced, and no additional balancing algorithm is needed. The simulation and experimental results confirm the overall intended operation of the proposed topology.
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    Modulation, Control and Performance Analysis of Asymmetrical Modular Multilevel Converters (A-MMCs)
    (2017-02) Srivastav, Kundan
    Modular multilevel converters (MMCs) are preferred converters for implementing high-power multilevel systems. The penalty they impose is the high number of devices needed to build them. To offset this challenge, this thesis introduces Asymmetrical Modular Multilevel Converters (A-MMCs). Unlike MMCs, each A-MMC module comprises of two half-bridge submodules, rated at asymmetric voltages. This system offers benefits like: (a) generation of four distinct voltage levels using one module; (b) 33% lesser semiconductor and gate drive requirement; (c) higher system efficiency; (d) reduction in overall cost and size. Hybrid Pulse-Width Modulation (Hybrid PWM) has been deployed to generate the drive pulses. To maintain asymmetric voltages, a novel voltage balancing algorithm has been proposed. Circulating current controller has been designed as well. The operation and performance validation was done using MATLAB Simulink and PLECS Blockset. A comparison, vis-`a-vis the MMC, was also performed, based on thermal performance and essential circuit voltages and currents.