| In recent years,as massive new energy is integrated into the grid system and power grids are closely interconnected,Modular Multilevel Converter(MMC)is applied to multi-terminal DC transmission technology.As a new type of topology with an easy-to-expand modular structure,MMC can meet high-voltage requirement in transmitting electricity,and will not change the polarity of the DC side voltage when power flow reverse.The multi-terminal DC transmission system based on the modular multilevel converter,which provides electricity for multiple power supplies and places,is highly flexible and reliable.It is now a popular research topic in DC transmission and an important trend in developing an energy network.The key to the safe and stable performance of MMC-MTDC is to ensure coordinated power control between converter stations and the stability of the DC side voltage.But unlike the two-terminal system,the complex multi-terminal system has more converter stations.And disturbances or power fluctuations caused by malfunction will lead to unbalanced power distribution among converter stations and instability of DC side voltage.Aiming at the abovementioned problems,the thesis takes MMC-MTDC as the object of study by focusing on following three aspects.1)By studying three-phase MMC topologies and analyzing principles of each working conditions,the thesis concludes that,the desired output voltage waveform can be obtained by properly inputting or cutting the sub-modules.By simplifying the MMC topology can achieve a simplified equivalent circuit of MMC controlled voltage source.The thesis uses the input and removal status of the sub-modules to introduce the switching function and establishes the mathematical model of MMC in the stationary coordinate system and the dq rotating coordinate system.The superior nearest level approximation modulation,combined with the capacitor voltage sorting algorithm,builds a single-ended MMC model to simulate the effectiveness of the nearest level approximation modulation and capacitor voltage sorting algorithms.Building on the MMC mathematical model,the parallel four-terminal MMCMTDC is mathematically modeled,and the MMC-MTDC hierarchical control system,the converter station-level outer loop and inner loop controller are designed by integrating the control characteristics of MTDC.A two-terminal MMC-HVDC simulation model is built in PSCAD/EMTDC to verify the effectiveness of the above control strategies by taking the example of receiving end supplying power to the passive network and connecting to the AC grid.2)The thesis analyses the principle of the traditional master-slave control,DC voltage margin control,and DC voltage droop control,all of which are MMC-MTDC system-level coordinated control strategies.Simulation experiments are carried out in electromagnetic simulation software to verify their effectiveness.The comparison analysis points out the advantages and disadvantages of traditional control methods.Master-slave control can maintain DC side voltage stability well but requires real-time communication;DC voltage margin control does not rely on communication but the voltage margin value is not easy to select and when the converter station switches into the working mode,the DC voltage and active power will produce a large overshoot;the droop control consists of multiple converter stations for maintaining the system’s active power balance and DC voltage stability,but due to the interaction between the active power and the DC voltage of the droop curve and the everchanging steady operation point,no static difference adjustment of DC voltage and active power can be achieved.3)The thesis offers an improved droop flat control strategy that factors into power margin.Given the shortcomings of traditional droop control and the real-time power margin of droop control stations,the droop coefficient is improved according to the margin of each converter station,so that each droop control station can share unbalanced power in the system and avoid overload in switching converter station.At the same time,in view of the slow dynamic response of the differential link in the traditional direct current control of the current inner loop,the differential flat theory is introduced to reform the current inner loop controller,and the stability of the MMC flat system is analyzed.The controller is composed of two parts:desired feed-forward control and error feedback compensation.Feed-forward control generates dominant control quantity,and error feedback compensation eliminates the influence of system model uncertainty and internal and external disturbances,and corrects the control quantity to accurately track expected value which improves the system response speed.By taking a parallel four-terminal MMC-MTDC DC transmission system as an example,the simulation results show the effectiveness of the proposed control strategy. |
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