| With the rapid growth of renewable energy generation,a large number of power electronic devices are applied to the power system.The interaction between power electronics equipment and the power system may cause oscillation problems.The oscillation problem has seriously affected the safe and stable operation of power systems including grid-connected converters,such as renewable energy power generation and high-voltage direct current transmission.The oscillation frequency of oscillation problem in the renewable energy grid-connected system is independent of the fundamental frequency of the system or the switching frequency of the converter.Most of the oscillation problems occur suddenly,and at the beginning,the oscillation amplitude diverges,eventually forming a sustained oscillation phenomenon whose oscillation amplitude is almost constant,or the divergence oscillation triggers the system protection,causing the system to stop.The impedance analysis method shows that the small damping system will excite the oscillation,but positive damping system is stable according to classical control theory.And damping analysis and the mechanism of the sustained oscillation have not been studied in depth.In this thesis,an impedance is built with nonlinear frequency couplings,and an improved impedance-based analysis is used to judge the system damping,and a mechanism of sustained oscillation is proposed to fill the research gap of sustained oscillation analysis.That is to say,negative damping system excites oscillation and reveal the mechanism of sustained oscillation process after excitation oscillation.The accurate impedance-based analysis can direct the design of grid-connected system,then the system stability can be improved.Compared with the traditional first-stage theoretical simulation and experimental analysis methods,the system studies the sustained oscillation process,firstly analyzes the key influencing factors through experimental measurement,and then theoretically study the mechanism of sustained oscillation.For the complex conditions of the existing sustained oscillation problem,the experimental results show that the existing impedance analysis method can not correctly analyze the oscillation conditions.Using the experimental method to reproduce the sustained oscillation phenomenon,it is found that the system gradually stabilizes from the divergent oscillation in the process ofsustained oscillation;according to the analysis,it is found that there is a phenomenon of frequency coupling in the oscillation,and the coupling component can not be ignored.Different from the existing impedance matrix and sequential impedance modeling methods,this thesis proposes an impedance modeling method to establish a self-admittance/impedance and accompanying admittance/impedance model with uniform positive and negative sequence frequencies,using complex variable reprentation.And the impedance model can accurately describe the nonlinear frequency coupling which exists in phase-locked loop.In this thesis,the inverter system is equivalent to a single-input and dual-output system,and two impedance models are described to the frequency coupling characteristics of the system,and have a clear physical meaning.The built impedance model provides an analytical basis for analyzing the damping and oscillation problems of the system.For the modeling of asymmetric controller and outer loop control,the modeling method of decomposing the asymmetric controller into two symmetric controllers is proposed,and the frequency domain impedance model of asymmetric controller and outer loop control is established.This thesis proposed an improved impedance analysis method based on self-admittance and accompanying admittance to judge system damping and accurately determine the conditions of oscillation generation.The impedance model with frequency coupling is used to build the equivalent impedance circuit of the system.The accompanying admittance introduced by frequency coupling is combined with the self-admittance to obtain the equivalent output impedance with frequency coupling,and then the Nyquist criterion is adopted.The larger the phase angle margin,the larger the positive damping of the system,the relative damping of the analysis system,and the more accurate system oscillation.Combined with experimental research,it is found that the small positive damping system is stable and the negative damping system excites the oscillation.This thesis revealed the mechanism of sustained oscillation of new energy power generation systems.The oscillation is excited by the negative damping of the system,which produces divergent oscillations.In the process of oscillation divergence,the negative damping system gradually becomes a critical damping system,eventually forming a sustained oscillation phenomenon.By building a small-signal model of steady-state sustained oscillation,the characteristics of the sustained oscillation can be described,and the amplitude of the oscillation can be predicted.Finally,considering the effects of frequency coupling,for phase-locked loop,asymmetric current controllers and voltage feedforward control,equivalent impedance analysis is used to analyze their effects on system damping.In different control,more accurate system damping is obtained by considering the frequency coupling.And the influence of different controls on the oscillation is studied.Then optimize the design of PLL parameter,proportional feedforward and asymmetric current control to appropriately improve system damping and reduce the risk of oscillation. |
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