| Wind power,photovoltaic power generation and other clean energy in the power system accounted for an increasing proportion,the power grid on the wind power grid operation put forward strict requirements,one of which is the fault ride-through capacity.Energy storage is commonly used to improve the fault traversal capability of systems.Modular multilevel converters have been widely used worldwide due to their advantages such as good calibre of power output and strong fault crossing skill.Energy storage MMC,which combines energy storage technology with MMC,in the last several years has been a research field in engineering.In this paper,the battery is selected as the energy storage unit based on control system complexity,economy and constancy,and the working principle of MMC,control strategy and SOC equalization control principle of the energy storage battery are explained.The MMC with DC-side energy storage studied in this paper is an extension mode based on the traditional MMC to improve the system fault ride-through capability when new energy is connected,and the topology and modulation strategy are given along with the structural design of each module.In addition,the reasonableness of this topology is verified by building a model of the energy storage MMC-HVDC system and simulation results.Then the fault ride-through in the case of temporary power surplus at the receiving end of the energy storage type MMC-HVDC system is studied on the basis of the proposed topology.Taking the two-end energy storage MMC-HVDC as the research object,the energy storage SOC system control scheme is designed by analysis of energy storage system control target;meanwhile,for the unbalanced capacitor voltage energy distribution problem generated with the unbalanced capacitor charging and discharging power,the power and energy model of the energy storage MMC converter is established,and the quasi The power and energy model of the energy storage MMC converter is established,and the fault ride-through strategy based on the inner-loop control strategy of proportional-resonance and the improved power outer-loop control strategy is proposed to achieve the fault ride-through under the power surplus at the receiving end of the energy storage MMC while suppressing the power oscillation at the DC side.Finally,the fault ride-through strategy is improved for the temporary power shortage at the delivery end of the energy storage MMC-HVDC system.When large-scale wind farms are connected to the grid via energy-storage MMC-HVDC,the equivalent DC capacitance is less inertial because there is no large-scale centralized DC capacitance in the MMC-HVDC system,which in turn leads to easy fluctuations in the DC bus voltage under perturbation conditions.Therefore,this paper analyzes the operating characteristics of the AC system at the sending end in case of faults and performs simulation to verify them.The proposed improved control strategy is also described and compared with the conventional control structure to demonstrate the improved fault ride-through capability of the system when the improved control strategy is used. |
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