| With the promotion of the development of clean energy in China,and considering the situation of the reverse distribution of clean energy and load at a long distance and the obvious advantages of HVDC transmission in clean energy transmission,it is expected that more UHVDC will be put into operation in China,and a multi-UHVDC infeed receiver grid will be formed in some areas.At that time,the coupling strength between AC and DC and between DC and DC will be further increased,and the requirements for coordination among dc controls will be further improved.After the formation of the MIDC transmission system,the ratio of incoming power to the grid at the receiving end increases,while the ratio of power provided by the internal units decreases.The failure will cause a great impact on the grid,which may affect the stable operation of multiple DCs.Meanwhile,the difficulty of system recovery after the failure also increases.Therefore,this paper focuses on the analysis of the fault characteristics and recovery characteristics of the MIDC transmission system after the failure,and further carries out the collaborative optimization of the multi-DCs control system.The main research contents are as follows:This paper studies the cause of the commutation failure after the fault of the multi-UHVDC feed-in power grid.Firstly,the essence of commutation failure is analyzed on the level of theoretical derivation based on the commutation process.Then the factors influencing the failure of commutation are discussed.Then,the simulation model of the Shandong-Hebei UHVAC ring network based on PSD/BPA simulation analysis software was built.The fault characteristics and recovery characteristics of the MIDC transmission system after the fault were simulated by taking the three-phase permanent N-2 fault of the AC line as an example.Based on the theoretical research,the root cause of commutation failure in actual power grid is analyzed.The influence of parameter setting of VDCOL control link on the recovery of MIDC transmission system is studied.First,the control characteristics of VDCOL were analyzed,and the relationship between the parameter setting of VDCOL control link and the dynamic reactive power demand in the DC recovery process was studied,and the influence of VDCOL parameter setting on subsequent commutation failure was further studied.Then,based on the electromagnetic transient simulation software PSCAD/EMTDC,set the doubly-fed into the dc transmission system simulation model.By setting different control strategies and taking the earth short circuit fault as an example,the influence of the multi-VDCOL parameter setting on the system recovery in the MIDC transmission system was simulated and analyzed,and its rules were summarized.A VDCOL control parameter setting method to suppress subsequent commutation failure of MIDC transmission system was studied.Firstly,based on the analysis of the influence of VDCOL parameter setting on system recovery,combined with the MESCR,the optimization strategies of integral modulation and differential modulation were proposed.Then,from the perspective of easy regulation,a method is proposed to change the characteristics of VDCOL control by adjusting the recovery voltage threshold U_L.Aiming at the problem that the control measures can inhibit the subsequent phase conversion failure,an optimization model is proposed by increasing the fault severity when the subsequent phase conversion failure occurs for the first time.In view of the specific problem of the optimization model,combining with the optimization strategy of the overall modulation and the differential modulation,the specific solution method is given.Finally,simulation was carried out in the small system built by PSCAD/EMTDC and the large system built by PSD/BPA to verify that the proposed method can effectively suppress the subsequent multi-dc commutation failure caused by faults at different locations,so as to restore multi-dc coordination. |
