Analysis of Sub-synchronous Oscillation of DFIG-based Wind Farm Integrated to Grid Through VSC-HVDC System Based on Torque Method

In this paper, the sub-synchronous oscillation of DFIG-based wind farm connected to VSC-HVDC is studied from the perspective of electromagnetic torque. Firstly, the amplitude-frequency relationship between electromagnetic torque and speed is deduced. It is found out that the stator current is related to the VSC controller, that is, the coupling between DFIG and VSC-HVDC is realized via electromagnetic torque. Then the frequency response of the electromagnetic torque of the model is analyzed. Results show that the negative damping characteristics of the electromagnetic torque in a specific frequency range cause the sub-synchronous oscillation of the system. The influence of controller parameters and steady state conditions on system stability is further studied. It is found out that increases of the proportional coefficient of the VSC and decrease of integral gain are adverse to system stability. Finally, correctness of the theoretical analysis is verified with PSCAD/EMTDC based time-domain simulation. © 2020, Power System Technology Press. All right reserved.