| The consumption and long-distance transmission of large-scale grid-connected renewable energy put forward higher requirements on the existing power transmission technology.The flexible DC transmission system can realize the asynchronous interconnection of the AC system,and can integrate multiple energy sources to complement each other,and improve the consumption level of renewable energy.It is an important development direction of the future power system.However,the basic unit of the flexible DC transmission system is a modular multi-level converter(MMC).After a DC fault occurs,the MMC is equivalent to capacitor discharge,and the fault current rises rapidly;if no measures are taken to suppress and isolate the DC fault,it may cause the MMC to be blocked,which is not conducive to the safe operation of the system.With the support of the national key research and development plan(2018YFB0904600),this paper conducts a systematic study on the fault analysis,fault current limiting,protection and reclosing of the flexible HVDC system.Firstly,the modulus characteristics of bipolar DC transmission system with ground return and metallic return are established;Secondly,the source-side and grid-side fault current suppression schemes are proposed,respectively.Finally,the fast protection and adaptive reclosing of the DC grid are pproposed.The main work of the thesis is as follows:(1)To decouple the positive and negative lines of the bipolar DC transmission system,two polar mode transformations are used.On this basis,the composite modulus equivalent networks of the bipolar DC transmission systems with ground and metallic return are respectively derived.Based on the frequency domain model of MMC and DC transmission lines,the modulus equivalent circuit of MMC,bipolar DC transmission lines,and fault ports are derived.Subsequently,the composite modulus equivalent network of different fauls is obtained.The frequency domain and time domain expressions of modulus currents of different faults are deduced,and the influence of line coupling parameters on fault current is analyzed.Moreover,the modulus characteristics that can be used to identify different faults are summarized.The derived composite modulus equivalent network can be used for short-circuit current calculation,fault identification criterion selection and protection threshold setting,etc.(2)In order to solve the problems of insufficient voltage regulation capability of the existing dual thyristor fault isolation scheme,a bypass line commutated converter(LCC)and DC circuit breaker(DCCB)bansed source-side fault current suppression and isolation schnem is proposed.The principles and implementation schemes of bypass LCC and hybrid DCCB for suppressing and clearing DC faults are described,and their timing coordination strategies are proposed.The equivalent circuit and discharge time constant of the bypass LCC and hybrid DCCB at different fault stages are studied,and the selection method of key parameters in the proposed topology is analyzed.The proposed method realizes the isolation of the AC and DC sides,accelerates the clearance of faults,and can effectively reduce the breaking current of the hybrid DCCB.(3)In order to solve the problem of MMC block-up during the fault caused by the source-side current limiting scheme,a grid-side fault current suppression scheme based on the Series-parallel power flow controller(SP-PFC)is proposed.The topology of SP-PFC and its connection to the flexible DC transmission system are analyzed,and the emergency control and bypass control strategies of SP-PFC are proposed to enhance the ability of SP-PFC to suppress fault current.The coordination strategy of SP-PFC and hybrid DCCB is analyzed to reduce the breaking current of DCCB.According to the equivalent model of SP-PFC in different fault stages,the influence of different factors on SP-PFC's suppression of fault current is analyzed,and the parameters of SP-PFC are optimized accordingly.(4)In order to solve the problem of fault current suppression affecting the correct operation of the protection,the influence of fault current limiting on the protection is analyzed,and a coordination strategy for fault current limiting and protection is proposed.On this basis,a single-ended protection scheme of flexible DC transmission systems based on the instantaneous differential phase-plane of the modulus current is proposed.This scheme uses the instantaneous differentiation of the modulus current at the initial stage of the fault,which constructs a modulus phase-plane that can be used to distinguish different faults.Besides,it realizes the identification of faults such as internal and external,unipolar and bipolar,and positive and negative grounded fault.Moreover,the influence of boundary elements on modulus current is analyzed,which is used to realize the identification of different fault types by measuring the number of points entering a certain area of the phase-plane.(5)In order to avoid DCCB reclosing on permanent faults causing the DC transmission system to suffer a serious secondary impact,a non-permanent and permanent fault identification scheme based on the residual voltage of the line is proposed;at the same time,to improve the flexibility of reclosing of bipolar DC transmission system with the metallic return,a fault identification scheme based on single-ended "grounding-mode current" and double-ended "metal loop current sum" is proposed.The reclosing and isolation schemes for different types of faults under permanent faults are analyzed,and criteria that can be used to identify pole-to-ground and pole-to-metallic return faults is studied.Based on the difference between the equivalent circuits of non-permanent and permanent faults,the residual voltage characteristics the DC lines are analyzed,which is used to determine the recognition threshold of these two faults.The coordination logic of protection and reclosing is analyzed,which realizes the adaptive reclosing of bipolar DC transmission system that takes into account the identification of metallic faults. |
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