| Modular Multilevel Converter Based Multi-Terminal HVDC(MMC-MTDC)has many advantages such as high waveform quality and easy expansion of sub-modules.It has broad application prospects in the fields of new energy grid connection and long-distance transmission.However,due to the complex topology and control strategy of MMC-MTDC system,it has always been the research focus of scholars at home and abroad.In order to promote the application of MMC-MTDC transmission system and improve the performance of transmission system,this thesis takes MMC-MTDC transmission system as the research object,and conducts in-depth research from the following three aspects: establishing dynamic phasor model of MMC transmission system,improving droop control strategy and optimizing power accurate distribution control strategy.The main contents of this thesis are as follows :(1)A dynamic phasor model of MMC transmission system based on rotating dq coordinate system is established.The model considers the internal dynamic characteristics such as bridge arm circulation and capacitor voltage fluctuation.The dq coordinate system components of system DC voltage,AC voltage and AC current are taken as electrical inputs,and the modulation ratio and circulating current double frequency component are taken as control inputs.The accuracy of the model is verified by comparing the dynamic phasor model with the simulation results of electromagnetic transient simulation software.(2)Due to the fixed droop coefficient in the traditional droop control,the steady-state performance and dynamic performance of the DC system are poor when facing complex working conditions.In view of the above problems,this thesis designs an adaptive droop control strategy considering power margin and voltage buffer.The control strategy adopts the idea of piecewise control droop coefficient,and sets a maximum power deviation value of converter station buffer.When the actual power deviation value of the converter station is less than this deviation value,an improved droop control method in the buffer zone is adopted.The droop coefficient is adaptively adjusted according to the power deviation amplitude of the converter station,so that the change of the system unit power produces smaller voltage deviation and improves the system voltage quality.When the actual power deviation value of the converter station is greater than this deviation value,the droop coefficient is adaptively adjusted according to the real-time power margin of the converter station,so that the DC system has better dynamic performance and the converter station is not easy to reach the power limit.(3)In order to solve the problem of inaccurate power distribution and poor voltage quality in MTDC droop control,this thesis designs an accurate power distribution control strategy considering line resistance and voltage deviation.The control strategy introduces the influence of line resistance into the droop characteristics,and realizes accurate power distribution by changing the droop coefficient of each converter station.The power margin ratio is introduced,and the power is accurately distributed according to the real-time power margin ratio of the converter station,so that each converter station can give full play to its own power output and reduce the overload possibility of the converter station.The voltage deviation adjustment link is set up for the voltage problem.By detecting the voltage of the converter station in real time and changing the voltage reference value,the average DC voltage of the whole system converter station is maintained within the limit. |
