Research On Topology Of Multi-port Hvdc Circuit Breaker For Flexible DC Grid

The flexible DC grid developed on the basis of modular multilevel converter（MMC）is well suitable for the delivery and grid connection of distributed clean energy.However,the DC grid has poor tolerance to DC side short circuit.If the fault is not removed in time,the electrical equipment will be burned and the fault range will be expanded.The DC circuit breaker（DCCB）can solve the mentioned problems efficiently and guarantee the operation of the DC grid safely by isolating the fault line quickly and selectively.With the analysis of the research status of HVDC circuit breaker,this paper studies and innovates the hybrid DCCB,and the main research results are as follows.This paper studies the structure and networking mode of DC grid,then the fault current of short-circuit in DC grid is analyzed,which provides a theoretical rationale for the fault current parameters of DCCB,and then the working principle of fault isolation through traditional hybrid DCCB are analyzed in detail.The test circuit is built in the electromagnetic transient simulation software,with the simulation results analyzed,which lay the theoretical and simulation foundations for the research and innovation of hybrid DCCB.A topology of multi-port hybrid DCCB based on capacitor current-limiting is proposed.The energy storage characteristic of transfer capacitor is used to restrict the growth rate of fault-current during the fault isolation,and the capacitor shares the withstand voltage of main breaker branch,which can reduce the number of IGBTs in series of main breaker branch,so as to improve the economy of circuit breaker.Taking the rapid growth of DC grid into account,the expansion of the proposed circuit breaker is simple and practicable.A four-terminal DC grid model is built in the simulation software to verify the breaking capacity and current restriction of the proposed circuit breaker.The results show that the proposed DCCB has strong breaking capacity and remarkable current restriction and economically.Moreover,it can significantly reduce the dissipated energy of the arrester which prolongs the service life of the arrester.A topology of multi-port hybrid DCCB based on double capacitor current-limiting and energy-collection is proposed.Based on the circuit breaker mentioned in the last chapter,a capacitor is added to decouple the attenuation process of fault current from the energy discharge process of energy storage elements in the fault line,so as to shorten the operation time for DCCB to isolate the fault line,and no arrester is used for energy consumption.Instead,the energy of the fault current is stored in the capacitor temporarily.After the fault is removed,the extra energy is released through the discharge circuit.The functions of the proposed circuit breaker are verified by building a DC grid model in the simulation software,the results prove the effectiveness of decoupling the attenuation process of fault current from the energy discharge process of energy storage elements in the fault line to shorten the fault isolation time,which can provide a new thought for the research of DCCB.