| With the increase of demand for distributed power generation and island supply,the multi-terminal DC technology has received people's attention and has been applied in many projects.However the multi-terminal DC transmission system based on modular multi-level converter(MMC-MTDC)not only has the technical advantage of high voltage and high-capacity that belong to MMC,but also has the advantage of running flexible,stability,and can achieve multi-power supply and multi-load consumption.Therefore,MMC-MTDC system has been known as one of the hot spot.And the content of the research mainly focused on coordination control strategy,fault protection,distributed power generation,power optimization,additional control and so on.In this thesis,the control strategy of MMC-MTDC system is studied deeply.The main contents include:(1)The requirements of the system-level,station level and valve-level control systems of the MMC-MTDC system are analyzed,and the control systems at all levels are designed.At the same time,in order to realize to monitor the running state of the whole system,the monitoring system of the remote and local levels are designed.(2)The control strategy for valve level,station level and the coordination strategy for the MMC-MTDC system is studied.The control strategies such as the valve level,the station level and the coordination control between the converter stations are compared and analyzed,and the appropriate control strategy is selected according to the operation mode of each converter station to ensure the stable operation of the system.The correctness and validity of the control strategies at all levels are verified by simulation and experiment.(3)Considering the situation use the MTDC system supply power for the island,if the load is unbalance,will make the system fluctuations in the power,and the fluctuation power will further cause power quality problems in the power supply and DC voltage.In this thesis,the operating characteristics under unbalance load are analyzed.Then,through the control of the circulation,the sub-module is used to generate the second harmonic power due to the unbalanced of the load,thus realizing the local processing of the fluctuating power,thus avoiding the DC voltage and the transmission side of the converter station AC current power quality deterioration caused by unbalanced load.(4)Considering the load unbalance condition,the traditional AC voltage control strategy can not achieve the balance output of AC voltage.Therefore,in this thesis,the amplitude and frequency characteristics of PI controller and R controller are analyzed,and using PI controller and R controller in parallel structure in the traditional AC voltage control strategy to ensure that the flow station is still able to output balance AC voltage when the load unbalanced,so that the power supply quality is improved. |
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