Subsynchronou Oscillation Analysis Of DFIG Based Wind Farms Connected With The Series Compensated Transmission Network

Vigorously developing renewable energy is an important strategy to realize the low-carbon transformation of energy.Among them,wind energy is the renewable energy with the most commercial development potential,which has been widely developed and utilized worldwide.In China,the wind energy rich area is far away from the load center,which must be transmitted and absorbed in a long distance.The series capacitor compensation can help to improve the long-distance transmission capacity,but the introduction of series capacitor will induce the risk of subsynchronous oscillation(SSO)in power system,threatening the safety of the system operation.Therefore,grasping the mechanism and inducing conditions of SSO of wind farms connected with series compensation is important for effective prevention of operational risks.This paper focuses on SSO of doubly-fed induction generators(DFIG)based wind farms connected with series compensated transmission network.The main work is as follows:The equivalent model of DFIG wind farms for SSO analysis is studied.At first,the mathematical model of DFIG is established.Then,based on the similarity transformation theory,the commonly used single-machine and multi-machine equivalent models of DFIG wind farms are improved,it is deduced that the DFIG turbines with the same(near)working condition can be composed of two parts: single-machine infinite subsystem and single-machine multiplier networking subsystem.This method can maintain the dominant dynamic characteristics of DFIG wind farm,reflect the oscillating mode among DFIG turbines,and reduce the amount of modeling and simulation calculation.Therefore,the equivalent method of DFIG wind farm can be obtained as "single machine equivalent under similar conditions,group equivalent under different conditions".At last,the effectiveness of equivalent method is verified on PSCAD/EMTDC simulation platform.The SSO characteristics of DFIG wind farms are studied.At first,based on the system linearization model,the influence of wind speed,series compensation,the number of DFIG turbines and controller parameters on the system dynamic characteristics is qualitatively analyzed.Then,based on the characteristic root analysis method,the validity of the equivalence model is further verified,the results show that the single-machine infinite subsystem can reflect the oscillation mode of DFIG turbines(mode of DFIGs),while the single-machine multiplier networking subsystem can reflect the dynamic behavior of the interaction between DFIG wind farm and power grid(mode between DFIGs and grid).The SSO mechanism of DFIG wind farms is studied.At first,based on the induction generator effect,the reason of SSO is explained as "negative resistance effect".Then,based on the space vector and the system state space equation,the frequency domain impedance model of the system is established and verified on the PSCAD/EMTDC.The results show that the impedance model based on the system state space equation is more accurate.At last,from the perspective of system equivalent impedance,the influence mechanism of various factors on the SSO characteristics is revealed.The phenomenon of SSO in Guyuan area is analyzed,and the theoretical analysis of SSO is supplemented from the perspective of actual engineering analysis.At first,based on the actual recording data when the SSO occurred in Guyuan area,the characteristics of the SSO in Guyuan area are analyzed.Then the Guyuan area simulation model is built in the PSCAD/EMTDC simulation software to reproduce the actual SSO phenomenon.