Study On Coordinated Control Strategies And DC Fault Characteristics Of DC Grid Based On Voltage Source Converters

In the energy sources structure adjustment all around the world, the advantages of renewable energy lead to its fast development. The research and large-scale application of renewable energy bring new challenges and opptunities. The problem of how to transmit and consume the renewable energy when it connects with the power system has become discussed heatedly. In comparison with AC transmission technology, the DC transmission technology is more adptive to occasions as connection of asynchronous AC systems, large-scale far-distance renewable energy transmission and so on. From the perspective of DC transmission technology development, the voltage source converter(VSC) based DC grid will be the developing direction of DC transmission technology. This thesis chooses the coordinating control strategies and DC fault characteristics of VSC based DC grid as pointcuts and mainly use the software of PSCAD/EMTDC to do the research. The works done and achievements gained mainly include:This thesis introduces the VSC used in the modeling of DC grid and studies the 2-level VSC and modular multilever converter(MMC) fast simulation models which are capable for simulations under steady-state and fault characteristics. For the selection of DC grid topologies in simulation, this thesis studies the flexibility and economical efficiency of the ring, star, meshed and mixed DC grid topologies. The research result shows that these topologies all have their occasions applicable.Based on the mixed-topology five-terminal DC grid, this thesis studies the principles and characteristics of master-slave control strategy, back-up master-slave control strategy, the DC voltage margin control strategy and the DC voltage droop control strategy. Based on these control strategies, the master-droop coordinated control strategy is proposed. The simulation result of the five-terminal DC grid model indicates that, with the master-droop control strategy, the DC voltage and power reference values are precisely tracked under conditions of power input fluctuation and power flow inversion, and the control mode conversion is quick and smooth under the condition of master VSC out of service.The DC grid models of ring, star and meshed topologies are established to study the short-circuit current characteristics of DC pole-to-pole fault at different VSC DC sides. The simulation result show that, the short-circuit current in a DC grid increases rapidly and has a exemely large peak value. Besides, the distribution and peak value of the largest short-circuit current are related to the DC grid topology, the fault location and the line parameters.The detailed and simplified models of the hybrid HVDC circuit breaker are established. With regard to the single DC node connected with AC voltage source, AC load, permanent magnet synchronous generator or doubly fed induction generator, this thesis studies the transient state process of the above circuit breaker and verifies its current cut-off capability.