Transient Characteristic Analysis And Control Strategy For MMC-HVDC System With Asymmetrical Fault

Modular multilevel converter(MMC)has been widely applied in flexible DC transmission due to its advantages,such as high modularity,low switching frequency,and high output waveform quality.In actual operation,the fault of the modular multilevel converter high voltage direct current(MMC-HVDC)system,however,cannot be avoided and most of them are asymmetrical faults.Specifically,the short circuit failure probability of converter bridge arm is small,but it seriously jeopardizes the safe and stable operation of the AC/DC system.The single-phase grounding fault on the AC side of MMC-HVDC will lead to the unbalance grid voltage issue which will affect the stable and reliable operation of the system.Under such circumstances,this thesis studies the abovementioned two kinds of asymmetric faults.The main content is as follows:Firstly,this thesis introduces the topology of MMC-HVDC system,and describes the working state of submodule.Then,a mathematical model of AC side of MMCHVDC system in dq coordinate system is established under unbalanced grid voltage.It provides a model basis for the transient characteristic analysis of bridge arm short circuit fault and the research of unbalanced control strategy.Secondly,aiming at the asymmetric fault of converter bridge arm short circuit in MMC-HVDC system,which is lack of detailed study at home and abroad,the transient characteristics of converter bridge arm short circuit are studied under the conditions of unblocked and blocked converters.Aiming at the unblocked converter,the dynamic change process of voltage and current in AC and DC side of the system is analyzed,and the composition of short circuit current in bridge arm is mainly studied.When the converter is blocked,the mathematical model of converter bridge arm short circuit is established and the analytical equation of bridge arm short-circuit current is deduced.The simulation results verify the correctness of the theoretical analysis of the transient characteristics of converter bridge arm short circuit.This part of the work provides a reference for the protection configuration scheme of the converter in MMC-HVDC system and also enriches the research in the field of converter bridge arm fault analysis.Finally,the unbalanced control strategy of single-phase grounding fault on AC side of MMC-HVDC system is studied.This thesis analyzes the transient characteristics of the single-phase grounding fault on the AC side of the MMC-HVDC system.On the basis of inheriting the structural advantages of traditional positive and negative sequence proportional-integral(PI)unbalance controller,an unbalanced control strategy based on the linear adaptive disturbance rejection(LADRC)is proposed.For the problem that the LADRC has too many parameters to set,this thesis provides methods for parameter reduction and adjustment.Compared with the traditional PI control,the designed LADRC unbalanced control system does not depend on the mathematical model of the controlled plant and can realizes the decoupling control of positive and negative sequence current in synchronous reference frame.It also shows good dynamic response characteristics on three control objectives: suppressing negative sequence current,suppressing the double frequency fluctuation of active and reactive power and therefore helps to design the unbalanced control strategy in a new perspective.