Influence Of Sub-synchronous Oscillation On Distance Protection On The Remote Side Of Straightening And Suppression Strategy

With the continuous development of flexible DC transmission technology,China’s AC and DC power grids have gradually become complete.High-power transmission between large power grids requires flexible DC transmission as a technical support.It also provides more complete control methods and safer and stable operation compared to traditional power grids.However,due to changes in the structure of the power grid,a large number of new electrical characteristics will appear during the transient operation,which poses a threat to the reliability of the distance protection of the traditional AC power grid.Among them,sub-synchronous oscillation(SSO)is one of the common problems that affect the safety of AC and DC power systems.According to actual operating data,sub-synchronous oscillation may invalidate the distance protection in traditional AC systems,and the system operation risk is increased and serious Because the fault cannot be removed immediately,it triggers a cascading reaction of the DC grid,which seriously affects the safe operation of the system.Therefore,it is of great significance and practical value to study the impact of sub-synchronous oscillation on distance protection and suppression strategies in AC and DC systems with MMC＿HVDC.This paper first analyzes the topology,mathematical model and operation control principle of the MMC＿HVDC of the modular multilevel converter,and analyzes the fault current characteristics when different fault types occur on the far side of the AC side of the flexible system.The external characteristics presented will directly affect the operation of the distance protection on the AC side.Then it analyzes the adaptability of the AC power grid in the AC and DC system as the distance protection of the sending end and the receiving end respectively,and combines the AC and DC interconnection electromagnetic transient simulation model on the PSCAD of the Northeast Chongqing AC power grid and the Chongqing-Errous straightening project.The simulation test of the impact of the next synchronous oscillation of the straight system on the distance protection on the AC side has been carried out.Studies have shown that before the sub-synchronous oscillation,the flexible system will still have a certain degree of impact on the distance protection device,and even more after the sub-synchronous oscillation occurs.Serious consequences for misoperation and refusal to distance protection.Secondly,it analyzes the safety risks brought by the sub-synchronous oscillation to the system operation after the failure occurs and the protection refuses to operate.Finally,combined with the current control strategy of MMC＿HVDC,an additional damping control strategy based on active and reactive cooperative control is added to suppress the continuous fluctuation caused by sub-synchronous oscillation,and the effectiveness of its method against the system is verified,and the safety caused by this phenomenon Solutions to hidden problems.