| Multi-Terminal Direct Current (MTDC) can achieve the function of multi power sources or multi receiving ends, and placement by electricity. And compared with the2-terminal High Voltage Direct Current (HVDC) system, it is more economical and flexible, and it has obvious advantages in grid interconnection and new energy connecting to the grid. Early MTDC system is based on Line Commutated Converter (LCC), which requires an AC system which has certain strength to realize the commutation exchange, and it also consumes a large amount of reactive power. When MTDC system is used for connecting renewable energy source and supplying power to romote area or islands, it may has problems as reactive power compensation, commutation failure and unable to supply the passive network, which to some extent limits the application and advantages of MTDC.After the1990s, full-controlled switching device based Voltage Source Converter Based HVDC (VSC-HVDC) is taken seriously by many people and rapidly developed for the advantages of adjusting active and reactive power independently and supplying power to the passive network. As the latest generation of the voltage sourced converter topology, Modular Multilevel Converter (MMC) has become the trend of the VSC-HVDC projects for its advantages of less switching losses and independent of thechnology of devices in series.Modular Multilevel Converter Based Multi-Terminal Direct Current (MMC-MTDC) system is able to fully show the technical advantages of the MMC, and it can also make a full use of to the advantages of MTDC system such as the well economy, flexibility and reliability. Compared with the2-terminal HVDC system, the MMC-MTDC system is more complex on simulation modeling and system control and protection strategy. And many of the key issues are yet remained to be solved. Therefore, the paper focuses on the design of MMC-MTDC parameters, simulation modeling, and coordination control and protection strategy to provide some theoretical basis for the implementation of the MMC-MTDC engineering.(1) MMC-MTDC model and control strategyIn order to study a MMC-MTDC system, it is necessary to study on the operating principle and control characteristics of MMC. Based on the operating principle of MMC, a mathematical switching model applied to the transient analysis of MMC and a mathematical model under dq coordinates of the three-terminal MMC-MTDC system which is suitable for control analysis are derived. And based on the dq decoupling control strategy, the paper designed the basic controller respectively for the active sourced side of the MMC and passive sourced side of the MMC, and it also designed the MMC modulation strategy and capacitor voltage balance control strategy, and it tested the controller on the steady-state and dynamic response through the simulation.For one of the key components, the bridge arm, based on the MMC model using the integer function, the paper proposed a designing method for bridge arm reactor parameters to suppress the current volatility on the AC side of MMC, so as to provide a theoretical basis for the bridge arm reactor in engineering. Simulation analysis shows that PSCAD/EMTDC simulation results are very consistent with the theoretical calculation results and the current fluctuations of AC side as well as phase circulation of current are both improved greatly through the optimizing design of the bridge arm reactor.(2) Precision voltage margin control method applied to the MMC-MTDC systemCoordinated control among stations is the key of MTDC system. First, the control principles of master-slave control method, DC voltage droop control and voltage margin control method are researched and the corresponding controllers are designed based on MTDC system. Second, a kind of precise voltage margin control method is proposed to avoid the influence cause by voltage drop and line loss of long DC transmission lines on the margin control, the formula to calculate the margin value of precise margin control is deduced and the corresponding precise margin controller is designed. At last, a3-terminal MMC-MTDC simulation model is established in PSCAD/EMTDC. Based on the simulation model, the operation performance of the general margin control is analyzed, the precise margin controller is validated and the fault characteristic when pole-to-ground fault and pole-to-pole fault occur to MMC-MTDC system is simulated. The corresponding control and protection scheme are given.(3)MMC-MTDC system with large-scale wind farm connectedThe control method of MMC-MTDC system with wind farm connected to is researched. First, the mathematical model of the generator and the model of wind speed and aerodynamic are established and the controller for double-fed wind turbine converter is designed. Considering the control performance of DC voltage control of MMC-MTDC system, a stage-style DC voltage droop control is proposed based on the large power fluctuation of wind farm. The control combines the DC voltage droop control and voltage margin control, different control modes are adapted at different stages of the wind farm operating. At last, the MMC-MTDC system model with wind farm connected to is established in PSCAD/EMTDC and the proposed stage-style DC voltage droop control is validated and analyzed. The simulation results show that when the power of wind farm is large, the MTDC system adapts DC voltage droop control and the fluctuation can be balanced flexibly, when the power of wind farm drops sharply, the stage-style DC voltage droop control can make the master station DC voltage control so that the DC voltage of MMC-MTDC system will not have a significant deviation. (4) System Architecture of a Universal Integrated Control and Protection Platform for Converter StationsMTDC system can be based on LCC, two-level VSC, MMC or other converter types; also it may contain some types of the converters simultaneously, composing hybrid MTDC system. In this dissertation, a novel universal integrated platform of control and protection for converter stations of various HVDC projects was proposed, which can be used for conventional HVDC, VSC-HVDC, and more complex hybrid HVDC systems, etc. The integral function demands and design principles for the universal integrated platform were presented. Then from the function and equipment points of view, two different system architecture schemes were proposed. In each part, the control and protection devices, functions and control areas, and interrelationship of subordination and communication were elaborated. The architecture can make the integration of control and protection functions for different HVDC projects, which lays the foundation for future development of the universal platform. |
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