Research On Bidirectional Solid State Circuit Breaker For DC Microgrid

DC microgrid is a medium and low voltage power system that uses the DC bus as the power supply carrier.There are two types of DC microgrid operation:grid-connected operation and island operation.In the grid-connected operation state,the DC microgrid can be considered independent large load.In the isolated state,the distributed power supply unit in the DC microgrid can supply power to the load in the system to achieve self-operation.The fault protection technology of DC microgrid has become the basic guarantee for its reliable operation and promotion.Due to the low impedance and small moment of inertia of the DC microgrid system,a large fault current will be generated when a short-circuit fault occurs in the microgrid.If a mechanical switch is used for protection,a high-voltage large arc will occur at the moment when the fault current is cut off,and Burn down electrical equipment.In the context of increasing DC microgrid voltage levels,more efficient and safe solid state circuit breakers need to be studied as protection switches for DC microgrids.According to the requirements of DC microgrid fault protection,a bidirectional short-circuit current solid-state circuit breaker?BSCCB?is studied,and its blocking performance is analyzed and verified.The paper first summarizes the research status of solid state circuit breakers at home and abroad,and then proposes the performance requirements of solid state circuit breakers for DC microgrids based on the characteristics of low impedance and small inertia of DC microgrids,the first point is that the breaking speed should be fast,the second point is the safety of the breaking process.The BSCCB proposed in this paper is based on the above requirements.Secondly,the fault characteristics of the DC microgrid are analyzed.The VSC converter is selected as the research object.When an inter-electrode short-circuit fault occurs on the DC side of the VSC,the fault current rises extremely fast and remains unchanged after reaching a stable value within a short time.If the circuit breaker can block the fault current when the DC-side capacitor is quickly discharged,it can avoid the impact of the fault current on the electrical equipment.When a unipolar ground fault occurs on the DC side of the VSC,the fault current drops to zero after a rapid rise.The load can still work for a while after the DC side capacitor discharge is completed.The unipolar ground fault is characterized by the difficulty of locating the fault point and the long recovery time of the circuit.Thirdly,the working principles of two typical two-way DC circuit breakers are analyzed,and the topology of the BSCCB in this paper is proposed according to the characteristics that the capacitor can be charged and discharged bi-directionally.The fault blocking operation process of BSCCB is analyzed,and the key parameters affecting the blocking performance of BSCCB are studied,and the influence of capacitor on the blocking performance of BSCCB is analyzed.The working characteristics of BSCCB were simulated and verified.The simulation results show that the fault blocking time of BSCCB is 6ms,the blocking speed is very fast.Fourthly,this paper studies the control strategy of BSCCB,proposes the control strategy of current differential protection,and studies the action criteria of current differential protection,and analyzes the working sequence of BSCCB.Fifthly,the BSCCB experimental platform was set up,and the previous-stage signal conditioning circuit,DSP control circuit,and IGBT drive circuit were built.The performance of the prototype was tested under the condition of the experimental voltage of 220V.The experimental result is:the IGBT turn-off signal delay it is 200us,the on-signal delay of the T₁ is 50us,the fault blocking time of BSCCB is 4.2ms,and the bus voltage decreases by 20V during the fault current blocking period,which verifies the correctness of the theoretical analysis and the validity of the simulation results.