Research On Transient Instability Mechanism And Control Strategy Of AC/DC System Under Different Fault Scenarios

In the future,the power grid is bound to become an interconnected power grid coupled with AC power grid and DC power grid.In the AC/DC system,the extensive application of power electronic equipment leads to the qualitative change of the fault characteristics of the power grid,which poses a great threat to the safe and stable operation of the system,how to ensure the safe operation of AC/DC interconnected power grid has become a very challenging topic.The poor tolerance of converter equipment to faults and the incoordination between protection and control make it very easy to have cascading failures and system instability caused by a single simple fault,resulting in serious losses of the system.Therefore,it is urgent to study the transient instability mechanism and control strategy of AC/DC system under different fault scenarios.In this paper,two scenarios of HVDC receiving end AC faults and DC line faults are studied respectively,the main contents and results are as follows:Concerning the instability of transient power angle of HVDC inverter side AC system caused by DC commutation failure due to AC line fault,a method to analyze the transient stability of HVDC inverter side AC system based on energy attenuation intensity is proposed.First,considering the dynamic variation of inverter conduction topology during DC commutation failure,the expressions of the dynamic energy of the receiving end elements under various conduction topologies are analyzed in detail.On this basis,the variation rate of aperiodic component of generator dynamic energy is defined as the energy attenuation intensity,and according to which the stability of system can be assessed.And then,the expressions of energy attenuation intensity corresponding to different conduction topologies of inverter are derived,thus how the dynamic energy absorbed by DC port and fault line in the fault duration affects the energy attenuation intensity is explored,and the cause of transient instability of HVDC inverter side AC system is revealed.HVDC restart failure can result in DC power interruption,which can further cause the transient power angle of rectifier-side AC system to go unstable.Concerning this problem,a transient stability analysis method based on the dissipation rate of dynamic energy is proposed.First,the dynamic energy models of DC terminal and AC filter in the whole process of DC line restart failure are built.On this basis,the variation rate of the aperiodic component of generator dynamic energy is defined as the dissipation rate of dynamic energy,thus the stability of system is assessed according to the dissipation rate of dynamic energy.Then,the effects of DC terminal dynamic energy and AC filter dynamic energy on the dissipation rate of dynamic energy in the whole process of DC restart failure are illustrated,and the cause of transient instability of rectifier-side AC system is revealed.Aiming at the transient stability control strategy of the receiving end system under AC fault scenario,based on the dynamic energy analysis of the receiving end system,the relationship between the dynamic energy of the DC port and the control parameters of the converter is established.It is found that the control parameters affect the trigger angle of the converter,then affect the upper and lower limits of each conduction interval,and finally affect the dynamic energy of the DC port.By analyzing the sensitivity of DC port dynamic energy to control parameters,the influence of different control parameters on DC port dynamic energy can be obtained,which can provide reference for adjusting converter parameters to improve the critical cut-off time of the system;Aiming at the transient stability control strategy of the sending end system under DC fault scenario,the dynamic energy of the whole process of DC restart failure is analyzed.The function of the total energy of the system with respect to the control parameters of the converter is constructed.The objective is to minimize the cumulative value of the total energy of the system in the process of restart and phase-shift triggering.The optimal control parameters are obtained by solving the extreme value of the total energy of the system,and then the objective of improving the transient stability of the system is achieved by adjusting the parameters.