| With the development of modern power system technology and the increase of power grid scale,long-distance and large-capacity transmission has become an inevitable trend,among which the advantages of HVDC transmission are increasingly apparent.At present,HVDC technology has been widely used in the actual power grid construction.HVDC system constitutes the key content and core position of smart grid.In HVDC transmission technology,commutation failure is common.The method of avoiding commutation failure by using inverters to trigger early will be constrained by operating conditions.When disturbance occurs on AC side,commutation failure will occur along with some inverters,which will have adverse effects on AC/DC hybrid system.Especially in UHVDC project,the single/bipolar blockade caused by commutation failure will seriously affect the safe and stable operation of power system.In view of this,it is of great practical significance to study the prevention strategy of commutation failure.Based on theoretical analysis and practical application,this paper conducts an empirical study on optimization methods of commutation failure prevention and control for multi-infested HVDC transmission systems.The main research contents are as follows:(1)First,the principle of commutation,the reasons for commutation failure and influencing factors are explained in detail,and the existing commutation failure prevention strategies and methods are analyzed,including the cut-off angle discrimination method,the minimum voltage drop method,the phase angle comparison method,DC voltage zero method and wavelet fault identification method,etc.The defects in the traditional strategy of reducing the risk of commutation failure only by reducing the trigger angle of the inverter device are analyzed.In the case of a significant drop in AC voltage,the commutation angle increases significantly.It is also difficult to reduce the trigger angle value to ensure a certain state of the off angle,resulting in commutation failure.(2)The existing commutation failure prevention methods based on DC current prediction are analyzed.Use the single-phase and three-phase voltage on the AC side or the difference between the AC and DC system currents to predict the commutation failure,use the current to predict the constant increase of the turn-off angle under the fault,and use the voltage to predict the increase under the fault,depending on the fault situation However,the angle increment is directly proportional to the severity of the fault.The DC current and trigger angle in the control phase are selected as the target value,and the AC voltage is used as the solution condition.The optimization method of commutation failure prevention control based on DC current prediction is studied,which is effective for multiple types of connected hybrid systems.Further according to the existing theoretical analysis,optimize the adjustment of the filtering parameters of the DC side voltage during the low-voltage current limiting process to achieve the effect of steady recovery of the DC system.According to the input characteristics of the LCC-HVDC hybrid system,the commutation failure prediction function module(CFPREV)is used to study the commutation failure prevention method based on the improved DC current predictive control.(3)On the basis of theoretical and methodological research,focusing on the multi-infeed HVDC transmission system,in-depth analysis of commutation failure factors during system operation,and the introduction of multi-infeed interaction factor(MIIF)to characterize multi-infeed HVDC The role of converter stations in HVDC.On this basis,the research on the coordination between the commutation terminals for the prediction of commutation failure is carried out,the interaction factor is defined by the nodal impedance matrix,and the concept of the strength and weakness limits of the coupling critical impedance is used to determine the commutation failure.And multi-infeed interaction factors coordinated and cooperated to suppress the multi-infeed DC transmission system commutation failure DC current predictive control method.Simulation test examples are used to verify the applicability of the method in this paper,which shows that the control of AC/DC hybrid system commutation failure(including multiple times)and system recovery control have a good improvement effect.(4)In order to further improve the effect of the method in this paper,for the control link of AC and DC system,the influence of control parameters on the prediction of commutation failure is analyzed,and a control parameter optimization for suppressing commutation failure based on an improved particle swarm optimization algorithm is proposed.The swarm improvement strategy mainly includes the improvement of the inertia weight,the extensive learning strategy in updating the particle velocity,the Gaussian mutation of the global optimal position,and the completion of the improved particle swarm algorithm to optimize the parameters of the DC control system.It is realized by joint call of PSCAD and MATLAB.The simulation shows that the method in this paper improves the dynamic performance index and improves the ability of the DC system to suppress commutation failure.It can better suppress the occurrence of commutation failure in the case of single-phase and three-phase failures.. |
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