| Energy storage modular multilevel converter (MMC) is a new topology structure based on MMC. The battery energy storage unit is connected to the sub module of MMC by bidirectional DC/DC converter, thus the energy is stored dispersively and the fault ride-through capability is improved. By adjusting the number of sub-modules in series, it is flexible to change the voltage and power. As a result, any kind of output voltage level can be reached easily, which reduces the harmonic content of output voltage and the electromagnetic interference. Energy storage MMC can be applied in industrial fields such as renewable energy generation, large-scale energy storage and electric traction. The paper has made research about its key technologies from several aspects as follows:(1) Several traditional modulation strategies are introduced and analyzed, then two new modulation strategies suitable for energy storage MMC have been put forward including constant frequency integration approaching modulation and fixed-frequency current hysteresis modulation. While constant frequency integration approaching modulation decreases narrow pulse, switching frequency and switching losses, fixed-frequency current hysteresis modulation features fast dynamic response speed as well as good robustness. Thus, the former is suitable for common applications while the latter is suitable for applications requiring fast dynamic response.(2) Balancing control strategy for submodule capacitor voltage of energy storage MMC has been proposed. With current direction of bridge arm current taken into consideration, the circulating mapping relations of virtual and practical submodules are changed continuously after the virtual submodules are built and the real-time practical submodules corresponding to the maximum and minimum of voltage are found. Consequently, the dynamic balance of submodule capacitor voltage is realized. Besides, corresponding control strategy for imbalance between bridge arms is also presented. The simulation result has proved the correctness and validity of the balancing control strategies.(3) The generation mechanism of bridge arm circulating current in energy storage MMC is introduced. The circulating current equations of energy storage MMC are deduced in the situation of n+1 level modulation which prove that bridge arm circulating current in energy storage MMC contains double frequency component mainly. Combined with traditional bridge arm circulating current suppression strategies, a general compensative suppression strategy for bridge arm circulating current is proposed which suppresses bridge arm circulating current effectively and easily. The simulation result has proved the correctness and validity of the circulating current suppression strategy.(4) A comprehensive energy control strategy is presented in the overall view of the system to ensure its long-term stable operation, which is based on the energy view. The strategy is put forward to lengthen the battery life by avoiding the over charge and discharge of battery. Besides, the coordinating work of converters of two parts is taken into account, the energy management is presented in the overall view of the system to ensure its long-term stable operation. The simulation result has verified the correctness and validity of the comprehensive energy control strategy.(5) The mathematic model of MMC under dq coordinate system is deduced and the grid-connect control strategy is proposed. Besides, the model of bidirectional DC/DC converter, which runs on two modes, is also deduced. Then, the control strategy of bidirectional DC/DC converter is proposed in the paper.(6) Reasons and concerning harm of energy storage MMC submodule faults are introduced. A non-redundant submodule fault-tolerant control strategy is put forward. The strategy removes the break submodules and changes the true Value Table. The mapping relations of virtual and practical submodules are changed selectively so that energy storage MMC submodules are able to operate with failure until the system recovers to normal state. The simulation result has verified the correctness and validity of the non-redundant submodule fault-tolerant control strategy.(7) The three-phase five-level MMC experiment platform has been constructed based on Dspace1103, on which experimental verification of both modulation strategy and control strategy proposed by this paper is conducted. The experimental results coincide with the simulation results basically, proving the correctness and validity of the strategies in the paper further. |
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