Research On AC/DC Fault Ride-Through Technology Of Three-Terminal Hybrid DC Transmission Syste

Exhaustion of traditional energy and increasing environmental pressure have become a global problem.The technology of DC transmission has played a considerable role in the advancement and exploit of energy from traditional to renewable sources,and has developed from point-to-point DC transmission to multi-terminal DC transmission,DC grid transmission and multi-point DC transmission,from conventional DC transmission to flexible DC transmission and then to hybrid DC transmission.Focused on three hybrid DC transmission system consisting of the rectifier station using the grid commutation converter with large power transmission capacity,and the two inverter stations using the sub-module hybrid multi-level converter with high power supply quality and reliability,this paper studies the crossover control strategy and fault clearing strategy under the fault conditions of the AC and DC systems,so as to improve the fault traversing ability,power supply reliability and the invulnerable and steady exercise capacity of the transmission system.The main research contents are as follows:1)Aiming at the DC transmission line fault,the development characteristics of DC fault current of converter station with different commutation mechanisms are analyzed,and the expression for the DC current supplied by the converter is also given.By modelling the dynamics of the LCC and MMC hybrid decoupling process,the DC fault current self-eliminating tactics of LCC and Hybrid MMC are unpacked.Further,the FBSM condenser overvoltage hazard and fault eliminating time are comprehensively unpacked for the two fault eliminating measures of Hybrid MMC locking clearing and full bridge sub-module（FBSM）,and the optimal fault clearing scheme is given.2)In view of the AC system fault of the rectifier station,current margin control is proposed for MMC₂ at the load center side.The DC outlet voltage of LCC determines the voltage level of the DC transmission system,and MMC₂ maintains the DC port current as the minimum allowable value;The AC voltage further drops due to the AC near end fault of LCC.It is proposed that the auxiliary converter station MMC₁ adopts voltage margin control,and MMC1 takes over the voltage control of DC system to maintain MMC₂ to draw preset power from DC system,so as to avoid cutting off multiple important loads or frequency collapse at the load center connected to MMC₂.3)In view of the AC system fault of the inverter station,discuss the four modes of converter station operation and propose a PQ-optimal synergism control tactic.By solving the optimal active and reactive power control objectives offline,the control table is made and written into the controller for the online control of the converter valve.In order to meet the requirements of minimum reactive energy input,reduce active energy,minimize and obtain the best overall and positive support for the receiver.In view of the overcharge problem of converter valve submodule caused by the AC fault at the receiving end,this chapter proposes a coordinated control strategy between stations that does not rely on communication.By presetting the DC voltage threshold of each converter station,each converter station switches the operation mode of the converter station respectively at the DC voltage rising stage to achieve the power balance of the DC transmission system and the DC voltage transition to a new stable operation point.