Study On Interaction And Stability Criterion Of DFIG-based Wind Farm Integrated To Grid Through VSC-HVDC System

There is no time to delay the development of clean energy,and the vast territory of our country is bound to bring about the problem of long-distance and large capacity transmission of wind power.AC transmission is a weak current network connection,series compensation transmission is prone to sub-synchronous control interaction,while the traditional DC thyristor commutation needs strong grid to provide commutation current,so the wind farm through the VSC.HVDC transmission grid has a development prospect.With the development of power system to high proportion of renewable energy and high proportion of power electronic equipment,the problem of stability becomes increasingly prominent.In this paper,the interaction and stability criteria of wind power grid connected system are studied from three aspects:Component Level.This paper takes the rotor side converter of doubly fed induction generator as an example,derives the inertia expression of the converter and explanation of physical meaning of inertia,meanwhile compares it with the traditional inertia of power system: rotor inertia and electromagnetic inertia of inductance and capacitance,and a unified differential equation describing the inertial elements of power system is obtained.At the same time,the criterion of converter instability is deduced according to the differential equation.Combined with the stability criterion,the instability mechanism of DFIG connected to grid through series compensation and the reason of time varying of DFIG oscillation frequency are explained.Finally,the inverter model of DFIG and the VSC.HVDC converter model are built to verify the correctness of the stability criterion.The interaction between rotor converter(RSC)and voltage source converter(VSC)is studied.Firstly,the small signal model of the system is built,through the analysis of the vibration mode,the vibration mode that the DFIG and the VSC.HVDC participate in is identified.Then,according to the electromagnetic equation and the control strategy,the RSC output is converted into the electric potential in the DFIG,so that it has a direct electrical connection with the VSC,forming a dynamic equivalent circuit of the two fans.This paper reveals the mutual feedback behavior between DFIG and VSC-HVDC through voltage power coupling,and gives the transfer function model block diagram.In addition,according to the input and output characteristics of the equivalent circuit and converter,the reasons of RSC and VSC output reverse and VSC output lag RSC during the dynamic interaction are explained,and finally verified by simulation waveform.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.