Research On Operation Control Of The Multi-terminal Flexible DC Distribution Network And Engineering Application

The flexible DC distribution network with multi-terminal interconnection is the key technology to build a new power system with new energy as the main body.It has the advantages of strong power supply capacity,large-scale distributed energy consumption capacity and high power supply reliability,which has become the hot spot of academic research and engineering construction.However,the research of multi terminal DC power flow analysis and stability control is a hot spot.Therefore,this paper proposes a complete set of design technology scheme for engineering construction,in-depth research on system operation control technology and power flow analysis,research on microgrid access operation control strategy under multi-terminal interconnected flexible DC distribution network,and summarize and analyze technical problems and engineering solutions in actual operation of the project.The main achievements and innovation points are as follows:(1)A complete set of design systems such as topology,main circuit,key parameters,overvoltage and insulation coordination,control and protection of multi terminal interconnected flexible DC distribution network are proposed.This paper focuses on the analysis and application of an economic and practical star T-connection topology,and defines the key technologies and parameters of engineering construction.(2)In terms of power flow analysis,for the future,when large-scale multi-terminal flexible direct distribution network interconnection and negative impedance constant power load form a complex distribution network system,a new set of theory is proposed,the sufficient conditions for the existence of power flow feasible solution are obtained,the uniqueness of power flow feasible solution is proved based on the uniqueness of limit,and a widely convergent iterative algorithm of power flow solution is proposed.Secondly,aiming at the matching complexity of system network characteristics,load access capability and control parameters,an analytical and numerical existence condition is proposed,which can quickly evaluate the existence of power flow feasible solution of the system after load access and voltage mutation,and reduce the conservatism of the existence condition.An example is given to verify the correctness of the proposed theory and the effectiveness of the proposed iterative algorithm of power flow solution.(3)In the control strategy of microgrid access under flexible DC system,aiming at the problem of reactive circulation in parallel of multiple distributed generators,an adaptive virtual impedance control strategy based on dynamic synchronous compensation is proposed,which takes into account the independent distribution of reactive power and power quality.Aiming at the system transient stability problems caused by fast frequency dynamic response and small inertia,a dynamic frequency stability control strategy based on adaptive virtual inertia is proposed,which can adaptively change the virtual inertia parameters to adapt to various complex operating conditions.The effectiveness and feasibility of the two control strategies are verified by Matlab/Simulink simulation platform and small capacity experimental platform.(4)According to the functional requirements of AC substation voltage loss and emergency support in project operation,a coordinated control strategy of standby automatic switching based on AC and DC is proposed to realize the N-2 function of 110 k V substation voltage loss;Aiming at the problem of power multi-order oscillation when the emergency standby control strategy is adopted for system communication interruption in engineering,an improved coordinated control strategy for flexible DC distribution network is proposed to improve the transient response ability of the system.A three terminal PSCAD electromagnetic simulation model based on practical engineering is established to verify and analyze the effectiveness of the above strategy,as well as the system stability and transient process.(5)The intelligent energy demonstration project,which is the first Internet plus intelligent energy project supported by the National Energy Bureau,which supports the energy consumption revolution.Aiming at the problem of system inrush caused by the redistribution of single station instantaneous blocking power in the project,the inrush suppression technique for temporary blocking of H bridge and DAB module conduction is proposed.The practicability of the proposed method is verified through practical engineering tests.The key engineering tests in the actual engineering test,such as multi terminal normal operation,emergency shutdown,AC/DC fault exit,control strategy mode switching,AC/DC coordinated standby automatic switching and so on,are debugged.The results verify the correctness of the method proposed in this paper and have engineering practical value.