Research On Start-up Strategy Design And Fault Evolution Mechanism In DC Grid

With the depletion of fossil fuels and incremental environmental pressures,China and the world are facing the strategic adjustment of energy structure,large-scale development and utilization of renewable energy is imperative.Wind,solar and other renewable energy large-scale access has became a serious issue which electric power system has to face.Take advantages of flexible high voltage direct current to build DC grid can maximize the integration and utilization of multiple new energy,in order to achieve mass acceptance of new energy.And in this way,renewable energy large-scale access seems to be easier and more feasible.Construction of DC grid faces many challenges in the future,the primary problem is that defines rules for DC grid are various,structure and control objectives are not clearly;secondly,DC grid is of complex structure and great scale,so a safe and effective start-up strategy need to be studied and devolped as soon as possible;finally,DC grid require a ultra fast response time of the protection system,which means exploring the distribution of fault currents in the DC grid is significant for fault location,isolation,removal and system recovery.In this dissertation,a number of key issues mentioned above are amply discussed following a literature review.(1)This paper studied the basic structure and existing control strategy of DC grid.On the one hand,the structure,type and function of each part of the basic structure of DC gird are discussed and divided from four aspects: grid topology,inverter,DC circuit breaker and new energy access.On the other hand,the existing control strategies of DC grid are presented,including master-slave control,multi-point voltage sag control and hybrid control.And their advantages and disadvantages and applicable range are compared and analyzed.(2)A four-terminal DC grid topology with renewable energy access is proposed.From the perspective of system voltage stability,a start-up control strategy for flexible DC grid is proposed,which is suitable for the renewable energy access,taking into account the mutual influence of the network topology.Finally,an electromagnetic transient simulation model is built up in PSCAD / EMTDC,and the feasibility and effectiveness of the proposed control strategy are verified by simulation(3)A new method of calculating the pole to pole fault current in DC grid is proposed.By exploreing the mechanism of pole to pole fault in single-ended MMC,the DC grid can be simplified into the equivalent network of RLC series circuit.Considering the generality of the DC-link topology,a set of unified naming methods for the branches and nodes is defined.On this basis,a set of fault current calculation method is proposed according to the basic principle of KCL and KVL.Finally,the fault test is carried out in the electromagnetic transient model of multi-terminal HVDC transmission network.The simulation results verified the correctness and validity of the proposed fault current calculation method.