| China has expansive land and relatively rich energy resources.However,from the perspective of the energy resources' geographical distribution in China,the consumption and production of electricity in China are in reverse distribution.Most of the energy resources are concentrated in the western region of China,while the demand for electricity is concentrated in the eastern region.The optimization of long-distance,large-capacity and high-voltage DC technology has been the hot study issue in China's power grid for a long time.MMC converter station is widely used in HVDC because of its advantages of low loss,small harmonic pollution and easy expansion,however,the transient process of DC line fault goes quite fast with small system inertia and complex nonlinear characteristics.The value of the fault short circuit current is of great importance to the selection of switching devices and DC circuit breakers.The existing MMC DC fault current calculation usually ignores the influence of the AC feed flow and focuses on the calculation of the fault current in a single converter station,which leads to a large error in the calculation of the multi terminal actual current.Therefore,aiming at the short-circuit fault of multi terminal flexible DC system,the fault current analysis of single converter station under different MMC topology and the calculation of fault current under the influence of multi converter station coupling are studied respectively.The main contents of this dissertation includes the following aspects:Firstly,this dissertation describes the modeling method of MMC using PSACD/EMTDC simulation software,and introduces the topology and operation principle of half bridge and full bridge sub-modules used in MMC,based on which,the dissertation describes the operating principle and mathematical model of MMC converter station in detail,and lists the common MMC control strategies applied in each converter station.Besides,the dissertation demonstrates the MMC converter station level control strategy such as inner and outer loop control,and the converter valve level control strategy such as voltage balancing strategy for capacitor and circulation suppression strategy.What's more,the dissertation establishs a four-terminal DC network model based on the parameters of Zhangbei four-terminal flexible DC loop network,which is used for subsequent simulation and verification of the analytical calculation results.Secondly,the DC fault currents of half bridge,full bridge and hybrid MMC including pole-to-ground fault,pole-to-pole fault,and breakage fault are analyzed and verified by simulation,and the fault current variation characteristics of half bridge,full bridge and hybrid MMC under different fault conditions are analyzed.Besides,the fault blocking based on proportions of full bridge sub-modules of hybrid MMC is studied.What's more,based on MMC control strategy,the capacitance of each arm sub module is equivalent to variable capacitance,and the mesh current method is applied to derive differential equation matrix of fault current considering the influence of AC side before blocking.In addition,the MMC fault current value after blocking based on different topologies is obtained,and the calculation method based on above theoretical analysis is verified by the bipolar two-terminal MMC system.Thirdly,the calculation results of multi terminal flexible fault current are derived and verified by simulation.The dissertation analyzes the equivalent circuits of pole-to-pole fault,pole-to-ground fault and pole-to-neutral line fault.The mesh current method is applied to analyze and establish the mesh current differential equation matrix,and the calculated value of fault current is obtained.The dissertation establishes the mesh current differential equation according to different equivalent circuits before and after locking,and proposes the fault current time sequence superposition calculation method,in which the state change of converter station in the fault process is regarded as the time node to divide the fault development stage of the system.The method is verified by simulation. |
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