Research On DC-side Fault Characteristics And Grounding Methods Of MMC-based DC Grid

The DC grid based on Modular Multilevel Converter（MMC）is regarded as a better choice for multi-infeed large-scale transmission grid under the increasing demand for renewable energy.And it is necessary to consider the possibility of forming a DC grid in the future when designing a new high voltage direct current（HVDC）projects.As a large power electronic system,DC grid has the characteristics of low inertia.At the same time,the characteristics of the"grid"also determine that its design requirements are different from the existing HVDC project.At present,countries around the world are still in the exploration and experimental stage of DC grid construction.One of the key aspects in building a DC grid is how to ride through the dc-side fault.The converter stations with fault self-clearance capability topology cannot completely solve the problem that the grid needs to block all converter stations after dc-side fault occurs.Therefore,using the DC circuit breakers（DCCB）to clear grid dc-side faults became the main solution for DC grid fault clearance.However,the dc-side fault characteristics determine that the DCCB needs to break the fault current more than ten times the rated current value in a very short time,thereby causing the research difficulty of the circuit breaker and increasing the total cost.Based on the above background,for the problems in study of the dc-side fault mechanism of HVDC grid,this thesis takes the MMC-based DC transmission grid as the research object,mainly focusing on the dc-side fault characteristics and the grounding electrode design of the DC grid.The main research work of this thesis is as follows:（1）The existing basic structure and protection strategy of DC grid are studied.Firstly,three different converter station topologies based on actual projects are introduced,then the operational principle of these stations under dc-side faults are compared,and it is concluded that the half-bridge submodule+DCCB is the most suitable for the station of the grid;Secondly,the possible network topologys of DC grid and its grounding method are analyzed and found out that bipolar configuration is more suitable for the basic configuration of the grid.Finally,based on the above conclusions,the time sequence of dc-side fault handling strategy is established as outcome for the following fault analysis.（2）The equivalent model and fault current components distribution in dc-side fault analysis are studied.By analyzing the principles of equivalent capacitor model of the converter station after fault and equivalent model of the DC transmission line,the results show that the equivalent capacitor model derived from the conservation of energy during the fault time,RL equivalent model of the overhead line and the PI equivalent model of DC cable can effectively reduce the difficulty of fault current calculation and ensure the accuracy.When studing other fault characteristics,the equivalent capacitor model which adding the station-level control can effectively improve the accuracy of calculating the node voltage and branch current far from the fault location,and the phase domain frequency model of dc line can improve ensure the accuracy of the simulation by considering the traveling wave and line coupling.The component of the fault current is analyzed,and the research shows that the fault current component is divided into two sources:fault stations and non-fault stations.According to the discharge time constant,the farther away from the fault point,the longer the discharge time of the converter station.Finally,the accuracy of theoretical analysis is verified by simulation.（3）A set of evaluation index system for dc-side fault is proposed.Referencing to the concept of protection zone,the index area is divided into three categories:ac-side,converter station,and dc-side.The index types can be divided by type into power,current,voltage,time,energy,and temperature.Despite the differences in AC systems,the most critical factor in ac-side should be if the system is able to dissipate the surplus power after the fault,thus the power transmission intensity ratio R_acis proposed.Base on the limitations of power electronic devices in converter station,the block time T_blockand the capacitance discharge coefficient k_care proposed,which can effectively describe the strength of converter station against dc-side faults.According to the premise of the successful breaking of the DCCB,the fault detection time T_detect,the relevant parameters of the breaking process,the number of blocked converter station N_blockand the distribution factorλwhich describing the ability of power flow redistribution are proposed.Through the Zhangbei DC grid simulation,the results show that these series of evaluation indexes can effectively verify the impact of different fault locations,fault types,fault resistance and other fault parameters on the DC grid,evaluate the fault ride-through capability of the grid,and can be used as design basises of the grid equipment under the transient state.（4）Based on the indexes proposed in the third part,a grounding electrode design method for bipolar DC grid is proposed.The grounding electrode only provides zero potential in normal operation.Different grounding methods will affect the fault circuit after the ground fault on the dc-side and valve-side,thus the design idea mainly considers the fault behavior of different grounding methods under transient state and future expansibility.The equivalent circuits and the doctrinal basis of overvoltage of healthy pole about bipolar HVDC system under dc-side pole-to-ground and valve-side single phase-to-ground fault are established.Based on the rise rate and amplitude of the fault current after the fault,the energy absorbed by the energy dissipation branch of DCCB and the overvoltage level of healthy pole,the pros and cons of the different grounding methods are compared.Finally,by using a three-terminal bipolar DC grid as a simulation example,it is recommended that the small resistor grounding（5-30Ω）can effectively limit the fault current and control the overvoltage level.Besides this,it is recommended to place an inductor at the neutral line in order to limit the fault current.