The Sub-synchronous Oscillation Mechanism And Characteristics Of Multiple DC Transmissions System

Torsional interaction exists between HVDC system and thermal generation units,and the constant current controller of HVDC may provide negative electrical damping in the range of the torsional vibration frequency of the unit,which will cause subsynchronous oscillation(SSO). More and more DC projects with multiple thermal power units are put into use in China, the synchronous oscillation caused by multiple DC transmissions should be more concerned.Firstly,the paper discusses the system time subsynchronous electrical component characteristics between the machine-net and AC-DC transfer by derivation.In the single HVDC system,the unit interaction factor(UIF), the complex torque coefficient method using CPCM and the time domain simulation are used to analyze the impacts of the system short circuit capacity,DC power level and the parameter settings of the HVDC controller on the electrical damping. It is verified that UIF preliminary judging, the complex torque coefficient method and the time domain simulation to further analysis is a suitable method system for dc transmission sending system of SSO analysis.Secondly, the SSO characteristics of a typical two HVDC system are analyzed by the improved complex torque coefficient method using CPCM.In detail, the paper analyzes the impacts of the AC system strength, parameters of HVDC rectifier controllers, DC power level,and the electrical distance of the DC transmissions.The results indicate that the impacts of the AC system strength, DC power level are similar to the single HVDC system,but the controller parameters have no significant effect.Finally,for Ningxia power grid with multiple thermal power units and multiple DC transmissions(Ningxia–Zhejiang UHVDC, Ningxia–Shandong DC) tightly coupled, a simplified equivalent sending system is built to study the damping characteristics of the units nearby the inverters and how they are coupled by the improved complex torque coefficient method using CPCM. It is verified that reducing the dc power level or improving the strength of the AC sending end and the electrical coupling between dc systems are helpful to reduce the risk of subsynchronous oscillation.And the paper introduces how to design a controller(SSDC) to reduce the SSO risk of the turbine-generators in Ningxia power grid.