Research On Fault Isolation Strategy Of Flexible DC Grid Based On A New Multi Port DC Circuit Breaker

Under the "double carbon" goal,new energy will once again usher in great development,while large-scale new energy grid connection and long-distance power transmission still face challenges.Flexible direct current(VSC-HVDC)transmission technology based on voltage source converter(VSC)has the advantages of independent active and reactive power control,easy networking and so on.It overcomes the shortcomings of conventional DC commutation failure.Therefore,it is widely concerned in new energy grid and transmission,asynchronous system interconnection and other scenarios.However,VSC-HVDC System contains a large number of capacitor energy storage components.Once DC side fault occurs,it will produce a rapidly rising fault current,which poses a threat to power electronic devices.Fast fault isolation is the key to ensure the safety of DC system.However,AC circuit breaker cannot be used because the fault current of flexible DC system does not cross zero.Therefore,the fault isolation of DC system has become a very challenging key problem.At present,the fault isolation of flexible HVDC system mainly depends on AC circuit breaker,DC circuit breaker and self clearing module,but AC circuit breaker can not meet the speed requirements of VSC-HVDC System Fault Isolation,and self clearing module scheme is lack of selectivity.Therefore,double port DC circuit breaker is mainly used in practical engineering.However,with the continuous expansion of DC power grid,the demand for double ended DC circuit breaker is increasing.In order to reduce the engineering cost,the multi port DC circuit breaker sharing the main cut-off branch arises at the historic moment,which reduces the use of high-power semiconductor by sharing the cut-off branch.But it still can not solve the problems of high overvoltage and large transient energy in the process of DC fault isolation.Focusing on the problem of fault isolation of flexible DC transmission lines,this paper proposes a multi port hybrid DC circuit breaker with flexible breaking capability for multi terminal flexible DC system,and develops a prototype of a new type of ring multi port circuit breaker with low cost and high efficiency by building a fault simulation experimental platform.Firstly,the research background and significance of fault isolation for flexible HVDC transmission lines are summarized,and the typical fault isolation schemes under current research are classified,and the improvement space of some fault isolation schemes used in current projects is analyzed;The dynamic response characteristics of single pole and double pole grounding faults in multi terminal DC system are analyzed;To solve the problems of high overvoltage and large transient energy in DC circuit breaker,a multi port DC circuit breaker with flexible breaking capability is proposed.Secondly,the working principle of the proposed topology scheme is described in detail,that is,the capacitor and resistive element on the main cut-off branch are used for commutation and energy consumption respectively,and the charging characteristics of the capacitor are used to buffer the impact of fault energy,so as to realize the flexible cut-off of fault current;When the capacitor charging reaches the turn-on voltage of metal oxide arrester(MOA),MOA is used to absorb the fault energy and parallel resistance is used to accelerate the energy dissipation.The main goal of the proposed scheme is to optimize the current breaking performance and reduce the economic cost.The capacitor is used to replace the multi-stage series IGBT in the mainstream topology research,which greatly reduces the cost of the main current breaking branch;In terms of performance,the "flexible" breaking capacity of DC circuit breaker is realized,which greatly reduces the overvoltage of devices in the process of fault and buffers the forced absorption of fault energy.Finally,in view of the lack of current multi port DC circuit breaker prototype research,a feasible multi port DC circuit breaker scheme is studied,including the design and implementation of fault simulation experiment platform and circuit breaker platform.The voltage level of this experiment is 300 V.The experiment of fault isolation using multi port DC circuit breaker prototype in three terminal flexible DC transmission system is designed and simulated.According to the selection standard,the hardware is selected by numerical simulation,and the physical platform is built;The control circuit and drive circuit of IGBT and other hardware facilities needed in the experiment are designed,and the program to control the breaking sequence of DC circuit breaker is written;Finally,the feasibility of multi port DC circuit breaker is verified.In conclusion,aiming at the problem of fault isolation of flexible HVDC transmission lines,this paper analyzes the shortcomings of the existing fault isolation schemes,and puts forward the optimization solutions;An experimental platform is built to verify the feasibility of a low-cost ring multi port DC circuit breaker topology.