| Modular Multilevel Converter Multi-Terminal Direct Current(MMC-MTDC)system transmission lines are mostly overhead lines.When the DC transmission line of the converter station is struck by a non-fault lightning strike,the DC voltage and current of the converter station will experience a high-frequency oscillation with a gradual attenuation of the amplitude in a short period of time.The high proportion of power electronic equipment makes the flexible system show the characteristics of low inertia and weak damping,which may lead to low-frequency oscillation on the DC side,threaten the safe operation of protection devices and power electronic components.In this paper,the receiving end converter station of multi-terminal flexible DC system under non-fault lightning strike is taken as the research object,the oscillation mechanism is studied,and the stabilization strategy is optimized and improved.Firstly,the simulation model of lightning current,the topology of four-terminal MMC and the operation principle of single-terminal MMC are introduced.According to the flow path of lightning current in single-terminal MMC,the corresponding equivalent circuit is constructed,and the changes of DC current and DC voltage are quantitatively analyzed.Based on the simulation platform,the working conditions of fault lightning strike and non-fault lightning strike MMC DC transmission line are simulated,and the variation rules of positive and negative DC voltage and DC current are summarized.According to the different variation rules of positive DC voltage,the non-fault lightning identification criterion is designed,the DC voltage correlation difference between the two conditions is large,the validity of the identification criterion is verified by data sampling.Secondly,the relationship between lightning current amplitude and low frequency oscillation frequency is analyzed.A four-terminal flexible DC equivalent circuit for MMC2 is constructed,and the internal control system is simplified.The low frequency oscillation criterion,low frequency oscillation frequency expression and attenuation coefficient expression of MMC DC side are derived.The influence of MMC system parameters and control parameters on DC side low frequency oscillation is derived by partial derivative.Based on the impedance analysis method,the small signal impedance model of MMC2,the small signal impedance model of HVDC transmission line and the small signal impedance model of three-station equivalent constant power load are derived.The influence of AC side on the stability of MMC system is analyzed by Bode diagram.The simulation model is established by RT-LAB to verify the correctness of the low frequency oscillation mechanism.Inputting no-load lines on the AC side will reduce system stability.Finally,aiming at the low frequency oscillation of MMC DC side caused by non-fault lightning strike,a virtual inertia control method is designed.Firstly,the control block diagram,control mechanism and parameter setting method of the conventional DC additional damping controller are introduced.It is verified by simulation that the non-fault lightning stroke will cause low-frequency oscillation of MMC active power,and the low-frequency oscillation can be regulated by introducing active power.The physical meaning of MMC virtual inertia is analyzed,and the block diagram of optimized additional damping controller based on virtual inertia control is designed.The relationship between virtual capacitance and control parameters is quantitatively analyzed,and the real-time parameter tuning is carried out for the problem of excessive lightning DC voltage change rate.The suppression effect of the controller under several working conditions is simulated by RT-LAB real-time simulation.Virtual inertial control can effectively suppress the amplitude of low-frequency oscillation and shorten the oscillation time of the system,which verifies the effectiveness of virtual inertial control. |
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