Research On Topology And Control Strategy Of Combined DC Circuit Breaker With Fault Current Limiting Function

High voltage and large capacity DC power grid based on modular multilevel converter(MMC)is an important part of the power system with renewable energy.The fault current of DC power grid rises rapidly and it is difficult to break the DC current.It is an important means to suppress and clear the fault current by using direct current circuit breaker(DCCB)and fault current limiter(FCL).Due to the manufacturing difficulties and high cost of large capacity DC fault suppression and breaking equipment,the key problem to be solved is to develop a combined fault current limiting circuit breaker suitable for multi terminal DC and DC power grid,and improve the comprehensive utilization efficiency of components and the coordinated control ability between equipment.Therefore,this paper has carried out the research on DC fault current calculation,combined current limiting circuit breaker design,reclosing control strategy design and engineering applicability verification scheme design.In the aspect of fault current calculation,a recursive calculation method of DC fault current based on energy transfer between AC and DC sides of MMC is proposed.The internal energy discretization mathematical model of MMC considering the influence of AC side,DC side and controller is established.Based on the dynamic change of DC outlet voltage of MMC,the trapezoidal integral method is used to solve the instantaneous value of fault current at the next moment.This method takes full account of the various factors affecting the fault current and does not need to build a detailed electromagnetic simulation model in the flexible DC system.It has the advantages of high calculation accuracy,simple operation and fast processing speed.It can meet the requirements of parameter optimization design and device selection of current limiting circuit breaker.In the design of DC current limiting circuit breaker,the scheme of using drainage branch to construct active grounding point is proposed to achieve the goal of transferring the fault point current.The main circuit breaker is equipped in the drainage branch to cut off the fault current.The main circuit breaker only needs one-way conduction power electronic switch group,which has good economic advantages.Based on the scheme,two kinds of drain current limiting circuit breakers,resistance type and inductance type,are designed.The design method of resistance drain current limiting circuit breaker is to install superconducting devices at the DC outlet of MMC and drain branch at the DC bus.When a shortcircuit fault occurs in a DC line,the superconducting device can quickly reach the quench state to limit the fault current.At the same time,it can quickly turn on the drain branch to attract the fault current until the fault point is isolated and then turn off the drain branch.Considering the effect of current limiting,the cost of current limiting and the cost of main circuit breaker,the parameters of quench resistance of superconducting devices are optimized,and the optimal matching scheme of quench resistance and the number of switch groups of main circuit breaker is formed,which improves the economy of the whole equipment.The equipment is multi port type,which is suitable for multi terminal DC power grid.The design method of inductive drain current limiting circuit breaker is to provide coupling inductors at DC line and MMC DC outlet for current limiting,and provide drain branch at current limiting switch group.After the DC fault occurs,the coupling inductor is put into operation immediately to limit the current,and the drainage branch is started to attract the fault current.After the fault point is isolated,the main circuit breaker and the current limiter switch group jointly bear the impulse voltage caused by breaking the fault current.Through the reuse of power electronic switch group,the goal of improving the integration and economy of the device is achieved.Considering the current limiting time,current limiting cost and breaking cost,the parameters of coupling inductor are optimized.At the same time,the proposed current limiting circuit breaker is improved to multi port to meet the application requirements of multi terminal DC transmission.In the aspect of reclosing strategy design,a reclosing strategy of coordinated control between DC current limiting circuit breaker and MMC converter is proposed.The reclosing mode of current limiting circuit breakers on both sides of the fault line is designed to ensure the speed of power recovery of DC power grid.At the same time,the current limiting equipment is connected to the circuit in the reclosing process to prevent the electrical stress of breaking permanent fault again.In the phase of line deionization,the scheme that MMC of fault pole and non fault pole is converted to STATCOM mode and VF control mode is designed to make both sides of fault line in equipotential state,maintain voltage stability of AC system,and eliminate reclosing impulse current,so as to improve reclosing success rate.The scheme can effectively support the weak AC power grid such as wind farm in the fault isolation stage,and is suitable for new energy transmission and island power supply.In order to verify the engineering applicability of the proposed topology and control strategy,the engineering applicability verification scheme of the fault current suppression equipment is designed by using the digital physical hybrid simulation platform.A damping impedance matching scheme for DC power interface is proposed.The dynamic equivalent impedance of multi type prototype equipment on the physical side is calculated to realize the real-time impedance matching on both sides of the power interface and ensure the stability of the hybrid simulation platform.Aiming at the problem of interface delay,the slope prediction algorithm of DC signal is proposed,which can effectively improve the fitting degree of electrical quantities on both sides of power interface and reduce the system error caused by delay.Through various types of dynamic experiments,it is verified that the digital physical hybrid simulation verification scheme has high stability and accuracy,and can provide a good engineering applicability experimental verification environment for various types of DC equipment.The prototype of the proposed resistive drain current limiting circuit breaker is connected to the verification platform.Through fault experiments,it is verified that the topology can effectively limit and break the fault current,and has certain engineering applicability.In this paper,the accurate calculation method of fault current for DC power grid system analysis and device design is proposed,the resistance type and inductance type drain current limiting circuit breakers are designed,the cooperative fast reclosing strategy of current limiting circuit breaker and MMC is designed,and the engineering applicability verification scheme of fault current suppression equipment is formulated.The research results can promote the technology improvement of combined current limiting circuit breaker in DC power grid and the safe operation of flexible DC power grid.