Control Strategy Of Flexible System Considering Cycle Coupling Of Electrical Variables Under Asymmetric AC Fault

Modular Multilevel Converter(MMC)has become the mainstream converter topology in the field of flexible DC transmission because of its significant advantages such as high conversion efficiency,high waveform quality and low switching losses.When an asymmetric fault occurs in the AC system,the flexible system is prone to lock up or out of operation,which affects its safe and stable operation.Therefore,it is of great theoretical and engineering value to study the influence mechanism of AC asymmetric faults on flexible DC transmission system and the control strategy under the faults.In this paper,we aim to reduce or eliminate the factors that threaten the stable operation of the flexible system,and focus on the analysis of fault mechanisms and control strategies.The specific research content is mainly divided into the following aspects:(1)The closed-loop analysis theory based on cyclic coupling relationship is used to analyze the influence mechanism of AC asymmetric faults on the flexible DC transmission system.Considering the multi-parameter coupling within MMC,the instantaneous analytical expression of the variables affecting the operating characteristics of the flexible DC transmission system is derived from the bridge arm currents,and the fault equivalent circuit is obtained by extracting the internal potential of arm voltage to quantitatively analyze the mechanism of the impact of AC asymmetric fault on the flexible system.It is found that the negative sequence component under AC asymmetric fault will increase the risk of MMC overcurrent,and the inflow of double frequency zero sequence circulating current will affect the safe operation of the non-fault side.At the same time,the DC current of the bridge arm is no longer balanced among the three phases of MMC.In addition,by building a two-terminal MMC-HVDC simulation system on PSCAD/EMTDC platform,the correctness of the theoretical analysis results is verified.(2)A multi-objective integrated control strategy is proposed to reduce the overcurrent risk of MMC bridge arm under AC asymmetric faults and the impact of fault component on non-fault side: 1)Based on the feedback linearization theory,the positive and negative sequence decoupling of AC current is realized by introducing new variables and performing coordinate transformation,and the negative sequence suppression is carried out on this basis;2)In view of the traditional double frequency circulating current suppression sacrifices the effect of DC voltage fluctuation suppression due to coupling,an improved outer loop voltage control strategy of non-fault side is proposed,which can suppress DC voltage fluctuation more effectively by introducing fault-side power;3)To address the problem that the traditional double-frequency circulating current suppression has poor effect on DC current fluctuation,a new multi-stage DC current suppression strategy based on PI regulator is proposed.This new current control strategy suppresses the fluctuation of DC current during the fault and the stabilization stage by calculating the reference values of PI controller at different stages.Finally,by comparing the simulation with the traditional double-frequency circulating current suppression,it is found that the proposed multi-objective integrated controller can achieve negative sequence current suppression,double-frequency circulating current suppression and DC-side fluctuation suppression,and the DC current and voltage fluctuation suppression is better.(3)Aiming at the problem that the unbalanced distribution of DC current among three phases affects the service life of MMC,a three-phase bridge arm DC equilibrium control strategy considering unbalanced power compensation is proposed.This strategy calculates the unbalanced power caused by fault characteristic component from the perspective of AC and DC power balance,and compensate it in the form of injecting zero-sequence AC modulation voltage.By comparing the simulation results with the original fault wave without control,the proposed three-phase bridge arm DC current equilibrium controller can achieve the control goal of balancing bridge arm DC current under two typical asymmetric faults of single-phase grounding fault and two-phase short-circuit grounding.