| With the continuous development of offshore wind farms towards long-distance and large-capacity,all-DC offshore wind farms can avoid the problem of AC voltage reactive power of cables,effectively reduce system construction costs,reduce system losses and improve system efficiency,which is an important means to achieve efficient collection and delivery of large-capacity offshore wind power.However,high-transformation ratio and large-capacity DC converters are still the key equipment of all-DC offshore wind power systems to be further studied.Therefore,proposing a high-voltage DC transformer topology to achieve the stable operation of the all-DC offshore wind power system can improve the energy conversion efficiency and save investment,which is suitable for offshore wind power DC collection-DC transmission.Firstly,this paper proposes a topology of high-voltage DC transformer for all-DC offshore wind power system,including single-phase modular multilevel converter DC transformer(SMMC-DCT)with half-bridge sub-modules and single-phase hybrid modular multilevel converter DC transformer(SHMMC-DCT)with hybrid sub-modules,introduces its stable operation working principle,establish a mathematical model,analyze the operating constraints of DC transformers by power differences between wind turbines according to the energy transfer in the submodule capacitor,and design the ratio of input submodules,submodule capacitance and other parameters in consideration of the system loss,fault current blocking capacity and other factors.A 5kV/150kV all-DC offshore wind power system is established on the PSCAD/EMTDC platform,which verifies the feasibility of the proposed topology and the rationality of the parameter design.Then,according to the correspondence between the DC transformer submodules and the wind turbines,this paper proposes the overall coordinated control strategy of the all-DC offshore wind power system The control modes of dual active bridge DC transformer,machine-side converter and onshore MMC converter are analyzed,and for SMMC-DCT,a capacitor voltage equalization control method of sub-module based on improved nearest level approximation modulation is proposed And two working conditions of power balance and imbalance between wind turbines are set up,and the correctness of the proposed operation control strategy is verified by PSCAD/EMTDC simulation.and the capacitor voltage equalization control method of submodules based on the improvement of the recent level approximation modulation,and the equivalent models of SMMC-DCT using half-bridge sub-modules and single-phase hybrid modular multilevel converter DC transformer(SHMMC-DCT)are established,the fault characteristics are analyzed,the corresponding fault handling strategies are proposed and simulated and verified,and a capacitor voltage equalization control of sub-module based on improved nearest level approximation modulation is proposed.Finally,a fault ride-through strategy based on SHMMC-DCT is proposed.Firstly,the fault characteristics are analyzed,and the fault traversal mechanism of SMMC-DCT and SHMMC-DCT is analyzed respectively.For the occurrence of bipolar short-circuit faults on the medium voltage side and high voltage side lines of DC transformers,fault ride-through strategies are proposed,and the correctness and effectiveness of the strategy are verified by PSCAD/EMTDC simulation. |
