| In recent years,flexible DC technology has been widely used in power system,and a large number of flexible DC projects at home and abroad have been put into operation.However,during the operation,when the flexible DC system unintentional loses part or all of the AC power to form an island,the system cannot be shut down due to the failure of the control protection device to switch in time.Under such severe conditions,how to make use of the advanced and controllable nature of flexible DC technology to achieve smooth switching between networking and island conditions,thus providing high reliability and high quality power supply to the grid.It has become a difficult problem in the operation and maintenance of flexible DC engineering.Aiming at the unintentional island problem of flexible DC system,this paper focuses on the island detection and switching control for flexible AC/DC transmission system or flexible DC distribution system.Aiming at the DC island of the flexible AC/DC transmission system,this paper studies the DC island detection and island control of flexible AC/DC transmission system,and the active power distribution method of flexible AC/DC transmission system.Firstly,the DC island detection criterion of flexible AC/DC transmission system is studied,and a main criterion for DC island detection based on switch status combination is proposed.When the position of the circuit breaker is abnormal,an auxiliary criterion based on the voltage-difference limit violation of voltage mutational volume start is proposed.And the DC island detector is designed to realize the rapid detection of DC islands.Secondly,in order to solve the problem that the island direct switching control strategy leads to excessive voltage shock,this paper proposes a DC island smoothing switching method based on the current working condition voltage,so as to realize the DC island smooth switching control of the flexible AC/DC transmission system.Thirdly,in order to avoid the failure of DC island switching caused by the unreasonable distribution of active power before forming an island,this paper proposes an active power optimization allocation strategy aiming at safety and economy,and formulates a specific implementation scheme of reasonable allocation algorithm of active power.Thereby,the optimal distribution of the active power flow of the flexible AC/DC transmission system is realized.Finally,a realtime digital simulator(RTDS)closed-loop simulation system including the control and protection device of an actual flexible AC/DC transmission system is built.The effectiveness of the proposed method is verified by simulation.Aiming at the island of flexible DC distribution system,this paper studies the island detection and island switching control of flexible DC distribution system.Firstly,the access method of equipment for DC distribution system is studied,the medium-voltage converter access scheme is researched.The access method of DC solid state transformer(DCSST)and its grid/island control strategy are studied.The low-voltage converter access scheme is studied.A high-power bidirectional Boost converter topology suitable for energy storage(ES)access is proposed.A control strategy based on the staggered timing trigger mechanism is developed,and the grid/island control strategy of the bidirectional Boost converter is designed.Secondly,the four-terminal “hand-in hand” DC distribution system is constructed,and the start-up,normal shut-down,and the emergency shut-down control scheme are designed.Through the coordinated control of the modular multilevel converter(MMC)and DCSST in the DC distribution system,the start-up,normal shut-down,and the emergency shut-down control of the “hand-in hand” DC distribution system is completed.Thirdly,taking the four-terminal “hand-in hand” DC distribution system as the research object,this paper proposes an unintentional island detection method,which is a combination of the main criterion based on switch status and an auxiliary criterion based on no-current and difference-voltage.And the island detector is designed.Fourthly,an island switching control strategy based on the cooperation of DCSST and ES controller is proposed.Through the cooperation of DCSST and ES controller,the island smoothing switching control of the DC distribution system is realized.Finally,a DC distribution system and island detector model based on RTDS is built.The effectiveness of the proposed method is verified by the simulation of the island detection and island switching control method of flexible DC distribution system,which provides a reference for the application of flexible DC distribution engineering. |
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