Topology Design And Research Based On Current Limiting Multi-port Hybrid High Voltage Dc Circuit Breaker

With the continuous improvement of the quality requirements of renewable energy for flexible DC transmission,new energy sources such as solar energy and wind energy have been rapidly developed in the DC grid.In most cases,the flexible DC grid uses a modular multilevel converter(MMC),which has the advantages of high sensitivity and reliability,and flexible DC transmission has the advantages of fast power decoupling control,The characteristics of low inertia,low impedance,and low harmonic content are the future development directions of DC power transmission.Due to the lack of zero-crossing characteristics of DC current,the difficulty of cutting and clearing fault current continues to increase;therefore,high-voltage direct current circuit breaker(DCCB)is generally considered to be an important means to achieve rapid fault current shutdown and clearing.However,in these conventional two-port DCCBs,the main problem is that one main circuit corresponds to one transfer branch,and the number of power electronic devices is high,which leads to high cost;and in the current DC power grid,due to the high voltage level,the fault current is large.,which makes the capacity of the DCCB too high;in order to reduce the number of power electronic devices input,restrain the rising rate of the fault current,and reduce the current capacity of the DCCB when it is interrupted;therefore,two new multi-port HVDC circuit breaker topologies are proposed.First of all,this paper mainly introduces the general situation of DCCB in DC power grid and the research status at home and abroad.The circuit topology of conventional two-port DCCB and the advantages and disadvantages of its working principle are analyzed and compared in detail;at the same time,the advantages and disadvantages of performance and economy of conventional two-port DCCB are clarified,and it is pointed out that multi-port DCCB can satisfy The demand of the current DC transmission system,and it has a series of advantages such as low on-state loss,small number of devices,strong controllability,and long service life,is the mainstream direction of application in multi-terminal DC systems;The key technologies obtained are described in detail,and the working principles and characteristics of power electronic switching devices,fast mechanical switches,and metal oxide arresters are described in detail,which lays a theoretical foundation and technical support for the subsequent analysis and design of DCCB.Secondly,the circuit topology of Half-Bridge Multi-Port DC Circuit Breaker(HB-MPDCCB)is proposed.The topology of this circuit is that multiple DC lines share a transfer branch.The main circuit of the topology structure uses fast mechanical switches and a small number of power electronic switching devices as the main circuit during normal operation,and has the characteristics of low loss and low inertia during steady-state operation;The current-limiting branch of power electronic devices can significantly reduce the peak value of fault current;adding grounding branch can effectively reduce the energy consumed by the arrester,and indirectly shorten the turn-off current and turn-off time of the arrester.By designing the circuit topology of HB-MPDCCB,the current limiting process,breaking process and energy consumption process of HB-MPDCCB are theoretically analyzed,and the equivalent mathematical model is established.Finally,the corresponding simulation model is built with PSCAD software.The feasibility and correctness of the proposed HB-MPDCCB to remove faulty lines in the DC grid are verified,and the simulation results show that the proposed HB-MPDCCB can effectively limit the rising rate and amplitude of the fault current,and can greatly reduce the DCCB cut off process.energy consumption,with good reliability and economy.Next,in order to further reduce the loss during steady-state operation,reduce the shut-off capacity of the arrester,and make the current-limiting capacitor do not need to be recharged after one input,so as to prepare for the next fault shutdown.On the basis of the proposed HB-MPDCCB,a circuit topology of a full-bridge current-limiting multi-port high-voltage DC circuit breaker(DB-MPDCCB)is proposed.The main circuit is changed from the original half-bridge type to the full-bridge type;a group of thyristors are added to the current-limiting branch.After the DB-MPDCCB is completely turned off,the energy stored in the current-limiting inductor is transferred to the pre-charge capacitor.The DC grid does not need to be recharged after the first input,so as to prepare for the next fault shutdown input.The experimental verification shows that the proposed DB-MPDCCB further satisfies the feasibility and correctness of fault removal in the DC power grid,which proves that the circuit topology has the requirement of further reducing the energy consumption of the arrester.Finally,based on HB-MPDCCB and DB-MPDCCB,this paper proposes a set of DCCB reclosing theory,and analyzes the theory in detail,and uses PSCAD software to build a corresponding simulation model to verify the proposed reclosing the feasibility of the theory.Taking HB-MPDCCB as an example,the closing process of HB-MPDCCB under transient fault and permanent fault is analyzed.When a transient fault occurs in the DC system,the fault disappears,and the HB-MPDCCB recloses and then resumes power transmission;when a permanent fault occurs in the DC system,the HB-MPDCCB will trip again after reclosing.Therefore,this paper only makes a preliminary analysis of the HB-MPDCCB reclosing.