| With the large-scale integration of high proportion of new energy and the continuous application of power electronic devices in power systems,the system inertia is gradually decreasing,and the problem of improving frequency security has become urgent.Based on the Modular Multilevel Converter High Voltage Direct Current Transmission(MMC-HVDC)with flexible DC transmission system,it has broad application prospects in large-scale new energy grid connection.Using the advantages of fast response and high controllability of MMC-HVDC,active inertia support has been regarded as an important way to improve the system frequency response.Based on the modeling and simulation analysis of the MMC-HVDC system,this paper studies the active inertia support strategy of MMC-HVDC,and the specific research contents are as follows:Firstly,the system structure,mathematical model and basic control method of MMC-HVDC are elaborated.Secondly,the typical structure and working principle of the MMC-HVDC system are analyzed,and based on this,the mathematical model of the MMC-HVDC system is established and the basic control strategy is explained.Finally,an MMC-HVDC simulation model is built on the PSCAD platform,and the established MMC-HVDC model is simulated and analyzed by setting an inverter-side AC fault scenario.A modeling and simulation analysis method for MMC-HVDC applicable to the Qinghai-Tibet DC grid system is proposed.A steady-state operating model of the MMC-HVDC system is established,and based on the actual engineering data of the Qinghai-Tibet DC grid system,the main circuit model and parameters of the MMC-HVDC system applicable to the Qinghai-Tibet DC grid system are explored and designed.Finally,a 21-level true bipolar MMC-HVDC simulation model is built on the PSCAD platform,and simulated studies are conducted on different types of short-circuit faults,verifying the advantages of using MMC-HVDC to solve the problem of DC commutation failure in the Qinghai-Tibet DC grid system.A multi-mode active inertia support strategy for the MMC-HVDC system is proposed.The flexible and highly controllable nature of MMC-HVDC allows it to provide a certain level of active inertia support for the connected AC system.However,using MMC for active support can cause DC voltage fluctuations in the system.Therefore,it is necessary to thoroughly study how to fully utilize the active inertia support capability of MMC-HVDC based on the different frequency changes and frequency modulation requirements of the connected AC system,while adhering to the allowable DC voltage fluctuation constraints.To this end,a multi-mode active inertia support strategy for the MMC-HVDC system is proposed,which considers the DC voltage fluctuation constraints.By establishing the correlation between the DC voltage fluctuation characteristics when MMC provides active inertia support and the changes in AC frequency,the DC voltage is divided into different operation modes according to the allowable fluctuation range.Then,based on the frequency recovery requirements of the AC system,flexible active support is provided to the AC system at the lowest possible cost of DC voltage fluctuation.Simulation results have verified the correctness and effectiveness of the proposed control strategy. |
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