Research On Harmonic Modeling And Harmonic Unstable Of The Converter In DFIG

When the Doubly Fed Induction Generator(DFIG)is connected to the power grid,there will be a lot of harmonics in the back-to-back type double Pulse Width Modulation(PWM)converter.These will not only cause harm to the power system in itself,but also produce other more serious negative effects.The harmonic instability will happen when the harmonic distortion gets to more serious,which will seriously affect the power quality and stability of DFIG,and influence the normal operation of the system.So it is of great significance for the research of harmonic generation mechanism and harmonic instability of the double PWM converter in DFIG,and they are also the basis of the harmonic elimination and prevention of harmonic instability.First,the harmonic mathematical models of the converter in DFIG are deduced,which includes low-order harmonics and high-order harmonics.The low-order harmonics of grid side converter(GSC)are calculated according to the feedback control and power conservation equations.Then,the low-order harmonics of rotor side converter(RSC)can be obtained via the double Fourier transform algorithm.The high-order harmonic models of GSC and RSC are established by the switching function of space vector pulse width modulation and DC currents based on the same generation mechanism.Meanwhile,this harmonic mathematical model is verified by the simulation via the real time digital simulator(RTDS).Second,in order to analyze the stability of the converter in DFIG,the impedance modeling is used.Since the control and circuit model of RSC do not involve the electrical quantity of DC side,the small signal transfer functions in the circuit,control and phase-locked loop modules of RSC are first derived.Then the small signal transfer functions in each module of GSC can be obtained by using the interaction relationship of DC side.The closed-loop impedance transfer function of the whole converter is derived based on the proportional relationship between the stator voltage and AC voltage of GSC.Finally,the harmonic stability of the converter in DFIG is analyzed by using the generalized Nyquist stability criterion.The influence trend and the stability boundary of each parameter are obtained by changing the parameters of circuit,control and mechanical modules in the system.Then the impedance mathematic modeling is verified by the simulation of RTDS.In addition,the conditions for the occurrence of harmonic instability are revealed based on analysis results including the control strategy and operation mode of DFIG.