| The survival and development of human beings cannot be separated from the energy on the earth,with the gradual consumption of fossil energy and increasing environmental pollution,vigorously develop clean energy such as wind,light is the key to adjust the energy structure,but the wind and light energy utilization rely too much on the natural environment,AC transmission costs a lot,so the flexible DC transmission become the better choice.Modular Multilevel Converter(MMC)is different from the traditional voltage source Converter.It has a highly Modular structure.MMC’s bridge arm is usually cascade with hundreds of sub-modules.MMC-MTDC(Modular Multilevel Converter based Multi-Terminal High Voltage Direct Current,MMC-MTDC)is widely used in island interconnection and long distance transmission because of its characteristics of flexible operation and high reliability.This thesis mainly studies the decoupling and coordination control strategy of MMC-MTDC system.Modular multilevel converter(MMC)is widely used in the HVDC transmission system.However,some electrical quantities within the MMC are coupled,which makes the design of the control system more difficult and is not conducive to stable operation of the system.In this thesis,the coupling relationship between the AC side current and the internal unbalance current as well as the capacitance voltage of the sub-module is analyzed and a control method to eliminate coupling is proposed to compensate the d-axis as well as q-axis current cross-coupling term in the dq coordinate system.SOGI and Q-PR controller are used to accurately control the double frequency circulation in the internal unbalanced current and compensate the coupling caused by the internal unbalanced current and submodule capacitor voltage in the three-phase coordinate system Finally,the simulation verification is carried out in PSCAD,which is compared with the traditional control method to verify that the proposed method can achieve complete power decoupling.Droop control plays a crucial role in the coordinated control of MMC-MTDC systems.Voltage droop control has become a common control method due to its features of no inter-station communication and high reliability,but it also has problems such as unbalanced power distribution and large DC voltage fluctuation.In order to ensure the coordinated control between the station of MMC-MTDC,maintain a stable DC Voltage,an improved droop control strategy is proposed,the average of each converter DC voltage as a common reference voltage to fix droop coefficient,optimize the power distribution and DC voltage fluctuation under different operating states.PSCAD was used to build a four-terminal MMC-MTDC system,and the improved droop strategy was simulated under two different operating conditions: steady state and transient.The results show that this strategy can reduce the deviation of DC voltage and improve the stability and reliability of the system. |
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