| DC transmission technology plays an important role in cross-regional power transmission and optimal allocation of energy in China.With the development of fullcontrolled semiconductor power devices and automatic control technology,voltage source converters(VSC)based flexible DC technology has become increasingly mature,which will further promote the consumption of renewable generation,improve reliability of power supply and flexible control capability of power system,while solving the technical bottlenecks faced by conventional DC transmission technology and AC grid.However,the flexible DC system still faces many challenges from the perspective of grid interconnection and safe operation.Relay protection is the first line of defense for the safe and stable operation of power systems and is also one of the technical bottlenecks in the development and promotion of flexible DC system.After a transmission line fault occurs in flexible DC system,the fault current will rise rapidly and has a very high amplitude,meaning the fault developed rapidly and has a wide range of impact.So,flexible DC system has very high requirements for the performance of fault detection and fault isolation.However,there are still many deficiencies in the fault detection and isolation methods applied in DC transmission projects: the non-unit traveling wave protection methods cannot reliably detect the DC fault with large transition resistance,while the response time of currently used backup protection method is too long to be effective;there is a direct contradiction between the fault current elimination speed and the economic performance in the existing fault isolation methods.My work started from the above two aspects,aiming to solve the key problems in fault detection and isolation of flexible DC system.Firstly,the protection requirements of flexible DC system and the deficiencies of current protection methods for DC system are analyzed.By comparing and analyzing the fault characteristics of LCC based DC transmission system and MMC based DC system,it is pointed out that the key to the difficulties of fault detection and isolation of the flexible DC system lies in the voltage source characteristics of the converter which leading to the strong impact of DC line fault,and the converter can hardly withstand a large fault current.However,the main and backup protection methods of currently used in DC projects cannot achieve the performances of quickness,selectivity,sensitivity,and reliability.By analyzing the influence of fault location,transition resistance on the fault survivability of flexible DC system,it is pointed out that with the increase of distance between fault point and converter and transition resistance,the requirements for the quickness of protection is reduced and the deficiencies of DC system protection can be made up by improving the performance of unit protection..Aiming at improving the balance of operation speed and sensitivity of current protection for DC system and solving the deficiency of conventional current differential protection in response time and the problem that reliability and sensitivity are cannot to be balanced in non-unit protection,an ultra-fast current differential protection is proposed.The intrinsic properties of HVDC transmission line are analyzed first.Then,essence of ideal current differential protection is revealed and fundamental relations between line models and performance degradation of existing differential protections are discussed.Based on this analysis,a novel current differential protection scheme,including fault detection and faulty pole selection,is developed,and Bergeron model is used to describe the electrical wave propagation process along line,resulting in only simple computations are needed in each calculation cycle.The analysis of response time reveals that the response time of proposed protection method is only consist of the time of fault signals propagating from fault point to measurement devices and the communication delay.Finally,by discussing diversified faults and transient processes,simulation results prove that regardless of fault type,distance and transition impedance,the proposed differential protection can respond correctly.Aiming at relieving the contradiction between the fault current elimination speed and the economic performance of existing DC fault elimination methods,the key that half-bridge sub-module(HBSM)fail in fault elimination and isolation is analysis by comparing the fault current path of HBSM and full-bridge sub-module(FBSM),and the new topology of an reverse-blocking half-bridge sub-module(HBSM)that could withstand bi-directional voltages is proposed to realize the dc faults ride-through and the fast recovery of the system.The working principle of fault current elimination and the fast recovery of system based on the proposed reverse-blocking MMC are analyzed,respectively.Then,simulation model is established in PSCAD,and simulation results show the fault current could be eliminated within 20 ms and the system could be recovered rapidly without vulnerable devices in converter suffering from surge current again.Finally,comparisons between the improved HBSM and existing SMs are conducted,and it is proved that the proposed DC fault elimination method have good performances in fault elimination speed without high investment cost and operation losses.Finally,in view of the lack of research on fault isolation methods of flexible DC system based on DC-DC transformer,a dc fault elimination and isolation method based on isolated modular multilevel DC-DC converter(IMMDT)is proposed.The fault characteristics of HBSM based IMMDT and FBSM-IMMDT are analyzed,and it is revealed that the reason why HBSM based IMMDT can isolate DC fault rapidly but cannot eliminate dc fault is that there is no fault current absorption path.Based on this conclusion,the dc fault elimination method with controlling the feeding current from healthy side of IMMDT to fault side is proposed.According to fault elimination method and reverse voltage requirement of bridge arms,the configuration of reverse-blocking HBSM in IMMDT is analyzed.Finally,a dc distribution model based on the IMMDT with reverse-blocking HBSM is established in PSCAD/ EMTDC,and the simulation results show that the dc fault can be eliminated in several milliseconds while the economic performance of dc system is guaranteed. |
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