Research On The Topology And Control Strategy Of Hybrid Cascaded Converter Dedicated For Large Capacity Power Infeed

Based on the current situation of energy distribution in China and the requirement of power infeed in load centers,large capacity power transmission becomes a significant problem to be solved in renewable power integration.With the increasing capacity of UHVDC transmission system,the structure of "strong DC weak AC" will appear in the load centers of China,which will affect the stability of the receiving systems.In order to solve the problem of large capacity DC power infeed,this paper proposes a series of hybrid cascaded converter topology by combining UHVDC hierarchical connection with multi-infeed HVDC system,for which the structure of the receiving system can be optimized and stability of the system can be improved.In this paper,according to the application of the hybrid cascaded converter topology in different receiving AC system conditions,the converter structures with different sub-converter combination strategies are proposed,and the operating characteristics are compared.The control strategies of the hybrid cascaded converter based on half-bridge MMC and thyristor group,the hybrid cascaded converter based on LCC and half bridge MMC are studied in detail.The main work is as follows:(1)The basic hybrid cascaded converter topology is proposed,which combines UHVDC hierarchical connection system and multi-infeed HVDC system,so that large DC power can be fed into multiple points of the receiving system and the problem of large DC power evacuation can be solved.This paper analyzes the sub-converter combination strategy of mixed cascade converter valve topology,and compares the operation performance of different sub converter combinations.(2)A hybrid cascaded converter based on half bridge MMC and thyristor group is proposed.The operation characteristics of the topology are analyzed,the multi infeed short circuit ratio and the maximum active power of the receiving power grid are calculated.The control strategy of voltage-balancing control of upper and lower valve is put forward.In view of special working conditions,this paper puts forward many kinds of working modes and their on-line switching strategies.And the DC fault ride through strategy of this topology is analyzed.(3)A hybrid cascaded converter based on LCC and MMC is proposed.This paper analyzes the influence of MMC on the short-circuit current at the AC infeed point of LCC and the maximum multi infeed short-circuit ratio increment of the upper valve.The MMC emergency reactive compensation strategy is proposed to guarantee the operation of upper-valve in case of reactive power loss at the AC side.In view of the fault and maintenance of the sub converter,the on-line switching strategy of the sub converter is proposed,and the DC fault ride through strategy is proposed for the DC line fault.