Analysis Of Subsynchronous Oscillation Characteristics And Suppression Strategies Of Direct-drive Wind Farms With MMC-HVDC System

With the rapid development of new energies represented by photovoltaic and wind power generation,the dense grid connection of electronic and electronic equipment has caused the problem of subsynchronous oscillation in the power system.Due to the increase in wind power penetration rate and limited environmental resources,offshore wind power technology will become one of the future development trends.Due to its technical advantages,flexible DC transmission technology based on modular multilevel converters will become an ideal method for long-distance transmission of large-scale offshore wind farms.The interaction between the direct drive wind turbine and the MMC-HVDC system makes the phenomenon of subsynchronous oscillation more complicated and prominent.Therefore,in order to ensure the safe and stable operation of the power system,the small disturbance stability analysis of the grid-connected system has important practical engineering value.This paper focuses on the research on the dynamic characteristics and suppression measures of the subsynchronous oscillation of the direct-drive wind farm via the MMCHVDC grid-connected system.The research content and main results are as follows:(1)A small disturbance dynamic model of direct-drive wind farms connected to the grid via MMC-HVDC is established.This model covers the wind turbine grid-side converter station,the soft-straight wind farm side converter station and the corresponding control system.Among them,the wind turbine grid-side converter station adopts phase-locked loop and VSC double closed-loop control,and the MMC wind farm side converter station adopts AC voltage control and circulating current suppression control.The correctness of the built model is verified by the PSCAD/EMTDC electromagnetic transient simulation software.(2)Analyze the subsynchronous oscillation characteristics of the direct-drive wind farm connected to the grid via the MMC-HVDC system.For the established small disturbance dynamic model,eigenvalue analysis is used to study the relevant dominant oscillation modes.Through the participation factor method,the main electrical parameters that affect the oscillation mode of the system are identified.Using eigenvalue sensitivity analysis,the dominant control parameters that affect each oscillation mode are obtained.Then further analyze the influence of the main operating parameters of the system and the dominant controller parameters on the stability of the system with the help of characteristic root locus.The results show that the relevant operating parameters and control parameters of the direct drive fan side and the flexible side will affect the damping or stability of the system,and there is a damping coupling between the controller parameters.(3)The measures to suppress the subsynchronous oscillation of the direct-drive wind farm via the MMC-HVDC grid-connected system are studied.Aiming at the small disturbance instability of the system under different operating conditions,an optimization scheme for the control parameters of the circulation suppressor in the converter station at the side of the flexible and straight wind farm is proposed.PSCAD/EMTDC time domain simulation is used to verify the effectiveness of the suppression scheme.