Research On DQ Impedance Characteristic And Small Signal Stability Of MMC-HVDC System

Modular multilevel converter based high voltage direct current(MMC-HVDC)has advantages of low harmonic content in output voltage,flexible and scalable structure,no commutation failure problem and so on,thus has been widely applied to scenarios such as power grid asynchronous interconnection,large-scale delivery of renewable energy and weak system power supply.However,due to the complex internal harmonic characteristics of MMC,oscillation can be easily caused by the variation of system operation mode,inappropriate parameters setting and interference of other external factors.which brings severe challenge to the safety and stability of the power grid.Based on the background,research on DQ impedance characteristic and small signal stability of MMC-HVDC system helps to improve the safety and stability of the power system.thus has good theoretical research value and engineering application prospect.In order to provide basis for the system stability research,the DQ impedance model of the MMC system is established.MMC converter station adopts outer loop control,inner loop current control,phase-locked loop and circulating current suppressing control.The model is validated by comparing the theoretical results with the frequency scan results of electromagnetic transient model in PSCAD/EMTDC,Based on the DQ impedance model of the MMC system,the impacts of each controller bandwidth on the DQ impedance characteristic of MMC converter station are analyzed,and the impacts of each controller bandwidth on MMC system phase margin under different short circuit ratio(SCR)are investigated.It is found that the variation of the system phase margin is characterized by 'non-monotonic' trend with the change of-active power control and inner loop control bandwidth.Finally.further study is conducted on the above-mentioned 'non-monotonic'variation charactertistic and its mechanism.The correlation between the 'non-monotonic' charactertistic and the interaction among controllers(phase-locked loop,inner loop control and outer loop control)is explained.The charactertistic brings challenge to the design of control parameters,hence the parameter design method of active power control and inner loop current control for improving stability under different SCR conditions are proposed.The feasible region and design method of controller bandwidth are verified by detailed electromagnetic transient simulation.