| The intermittency and instability of renewable energy will impact the operation of the power grid.Energy storage systems can help stabilize the voltage and frequency of the power grid,allowing the renewable energy generation to meet grid requirements.Energy storage systems achieve the decoupling between power generation and power consumption,playing an indispensable role in the power regulation and frequency regulation of the grid.Due to its modularity,lower voltage harmonics,and lower voltage stress of switches,cascaded multilevel converters are widely used in grid-connected energy storage systems.In this thesis,based on a star-connected cascade multilevel battery energy storage system,the fault ride-through capability under battery faults and grid faults is researched.Firstly,by analyzing the operating principle and common modulation strategies of cascade multilevel energy storage system.The unipolar dual-frequency carrier phase-shifted modulation used to produce more voltage levels with the same amount of converter cells.Its lower output harmonics are verified by Fourier analysis and simulation.Considering the requirements of the project,the main parameters of the circuit are analyzed and calculated.Secondly,the modeling of the cascaded multilevel energy storage system with battery faults is accomplished.Faulty batteries are switch off to form a hybrid energy storage system,and vector analysis is used to obtain the operating range where the system can maintain its output voltage level.Based on the equivalent model,the individual phase current control based power distribution strategy is proposed to coordinate the battery converter cells and capacitor converter cells,allowing the system to maintain the operating capability within a reduced four quadrants area.The feasibility of the proposed method and analyzed operating range is verified by single-phase and three-phase simulations.Thirdly,the operation of the hybrid system under unbalanced grid is analyzed and the power flow generated by unbalanced grid is derived.The battery cells and capacitor cells are controlled individually.Based on existing power redistribution methods,battery cells are ordered to track the changing grid voltage,and capacitor cells are used to inject the zero sequence voltage or negative sequence current.In such a case,the power distribution among different cells and three phases can be coordinated.The feasibility of the proposed method is verified by simulations under various imbalance degrees.Finally,proposed methods are tested on the FPGA based hardware-in-the-loop platform.The converter circuit is established in the hardware-in-the-loop platform,and the control algorithm is embedded in the DSP controller.The correctness of control methods is verified and analyzed under battery faults and grid faults. |
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