Study On Electromechanical Transient Simulation Of DC Power Considering The Dispersion Of Commutation Failure

As an important means to promote the allocation of resources in the east and west of China,High Voltage Direct Current Transmission(HVDC)is developing rapidly.China has gradually formed a large-scale AC/DC hybrid power grid structure with ‘strong direct current(DC)systems connect to weak alternating current(AC)systems’,and the connection between AC and DC systems is becoming increasingly close.However,in the AC/DC hybrid power grid,AC faults may induce multiple or even multiple DC commutation failures which resulting in serious DC power loss,thus threatening the safe and stable operation of receiving-end AC system.Therefore,this paper aims at the problem of DC power shortage affecting the safe and stable operation of the system under AC faults,from the perspective of realizing fast and accurate simulation of DC power response,studies the dispersion of commutation failure and its impact on DC response.On this basis,a DC power electromechanical transient simulation method that takes into account the discreteness of commutation failure is proposed.Aiming at the problem of insufficient simulation accuracy of DC dynamic response under electromechanical transients,firstly,a DC Quasi-Steady-State mode was established under the electromechanical transient simulation BPA based on an actual DC project.Then,in comparison with the simulation results of PSCAD/EMTDC,it is verified that the root cause of the insufficient accuracy of the DC dynamic response simulation under the electromechanical transient state is the insufficient accuracy of the converter model.Finally,it is concluded that the identification of the commutation failure of the electromechanical transient state at different fault times is single,which leads to the singleness of the simulation of the DC response.Therefore,the system stability calculation error is large,and it is urgent to improve the calculation accuracy.Aiming at the problem of the dispersion of commutation failure and its influence on DC response,firstly,the time dispersion characteristic mechanism of commutation failure is analyzed in detail in theory.Then,the boundary value of the commutation failure fault time under different transition resistances is studied.Finally,the mechanism of the difference in DC response at different fault moments is revealed.The fault time boundary values of commutation failure in the fuzzy zone are obtained,which can be used to judge whether commutation failure occurs at different fault moments.It is found that the difference in DC response under the influence of the discreteness of commutation failure is the result of the mutual game between the fault time and the control system.In order to improve the simulation accuracy of electromechanical transient response to DC power,the calculation method of DC power extreme value based on DC voltage and current response characteristics is firstly proposed.Then,the DC power hierarchical division is carried out according to the type of commutation failure,and the segmented fitting method of hierarchical DC power is obtained.On this basis,an improved DC power hierarchical simulation method considering the discreteness of commutation failure is proposed.Finally,after adding this method to BPA,the comparison with the original BPA,literature method and PSCAD/EMTDC results verify the accuracy and effectiveness of this method.The proposed method can quickly and accurately simulate the hierarchical DC power response under the influence of the discreteness of commutation failure,which improves the accuracy for the stability analysis of electromechanical transients in power system.This paper studies the DC power hierarchical simulation method that takes into account the dispersion of commutation failures.This method can improve the calculation accuracy of the power system stability analysis for electromechanical transients.The correctness and effectiveness of research results have been verified on the actual DC engineering PSCAD/EMTDC simulation model with detailed control and protection modules.This work was supported by the National Grid Smart Grid Joint Fund Project(U1766213)and the National Natural Science Foundation of China(51677073).