| In the demand of long-distance and high-capacity renewable energy power supply,the flexible high-voltage direct current(HVDC)transmission technology with modular multilevel converter(MMC)has been developed rapidly in China,and gradually integrates with traditional high-voltage AC power grid to form a power electronics dominated AC/DC hybrid power system.However,compared with the traditional AC power grid,power electronic equipment such as MMC significantly broadens the response bandwidth of the hybrid power system,intensifies the interaction between power grid equipments,and repeatedly causes broadband oscillations from several Hertz to thousands of Hertz in the HVDC projects that have been put into operation,which seriously affects the power safety.Stability analysis and oscillation suppression are the key problems to be solved in AC/DC hybrid power systems.In order to solve the above problems,this dissertation studies the mathematical modeling and model simplification methods of MMC,and explores the broadband oscillation suppression technology of MMC-HVDC system based on impedance analysis,which provides theoretical support for reducing the instability risk of AC/DC hybrid power system.Accurate mathematical model is the basic tool for system stability analysis,but the complex harmonic coupling phenomenon inside MMC brings great challenges to its modeling process.Firstly,based on the topology and operation principle of MMC,this dissertation analyzes the small-signal response of MMC under AC and DC side purturbations,and reveals the complete coupling mechanism of internal harmonics in MMC.On this basis,an impedance modeling method based on sequence selection matrix,frequency shift matrix and harmonic state space(HSS)is proposed,which unifies the modeling process of MMC AC and DC loops under closed-loop control,and finally obtains the infinite-dimensional impedance matrix model of MMC.The proposed modeling method can cover the control modes such as power control,AC and DC voltage control that commonly used in MMC-HVDC projects,and can be extended to other control structures,with strong universality.In addition,the model can not only directly reflect the frequency and sequence coupling within MMC,but also reflect the coupling relationship between the AC and DC power grids connected by MMC,laying foundation for the stability study of AC/DC hybrid power system.After obtaining the complete MMC infinite-dimensional impedance matrix model,it still needs to be reduced and simplified before it is used in practice.In this dissertation,MMC is decomposed and transformed into a signal flow graph to obtain the analytic transfer function of its internal signal path.The systematic simplification method of MMC impedance model is studied from three aspects of model establishment,model truncation and model application.In the aspect: model establishment,signal flow graph is used to reveal the conjugate symmetry and phase dependency between elements of MMC impedance matrix,and an intuitive judgment method of element symmetry and phase offset is proposed to simplify the process of obtaining matrix elements.In model truncation,a method of MMC harmonic quantization estimation based on forward gain of signal flow graph is proposed,which provides a theoretical basis for truncation and order reduction of infinite dimensional impedance matrix.In the aspect of model application,the multidimensional impedance matrix of MMC is equivalently transformed into a one-dimensional impedance,avoiding the use of the generalized Nyquist criterion to simplify the stability analysis process of MMC.Based on the simplified model,this dissertation,starting with the problem of low and medium frequency oscillation of MMC-HVDC system,analyzes the system impedance and control loop characteristics under various control modes,and reveals the oscillation risk of MMC under the influence of harmonic coupling.By introducing capacitor voltage fluctuation compensation into the MMC control,the coupling harmonics in the AC and DC output voltages are eliminated,and the frequency characteristics of the low-frequency impedance and control loop transfer function in MMC are reshaped.Meanwhile,the fluctuation compensation enables MMC have harmonic blocking and impedance isolation capabilities,effectively preventing the spread of oscillation,eliminating the coupling between AC and DC systems,and improving the system stability.Based on the decoupling effect of fluctuation compensation,this dissertation constructs the analytical expression of MMC model,and gives the control parameter design constraints of MMC from the perspectives of control performance and stability,providing theoretical guidance for the stability control and parameter adjustment process of MMC-HVDC system.In addition to low-and medium-frequency oscillation,MMC also has a danger of high-frequency oscillation(HFO)due to its long communication link and large control delay.In this dissertation,a unified model of high-frequency impedance is proposed through approximation,which can reflect the influence of various control strategies on the high-frequency impedance of MMC in a unified mathematical form,so as to facilitate the general analysis of HFO.Based on the unified model,this dissertation compares the HFO risk of MMC under different control modes,and reveals the influence mechanism of feedback low-pass filter order on high-frequency negative damping convergence,which provides a basis for the selection of filter order.On the other hand,through the quantitative analysis of the negative damping,filter bandwidth and control bandwidth of MMC,a high frequency oscillation suppression method based on the joint design of low-pass filter and passive damper is proposed.This method can avoid the sacrificing control bandwidth as much as possible on the premise of ensuring the high frequency stability of MMC,and can maintain stability when the control strategy and power grid environment change,with strong adaptability.For occasions where dampers cannot be installed,this dissertation also proposes an oscillation suppression method based on time delay adjustment,which can improve the system stability by moving the frequency range of MMC negative damping distribution out of the risk zone. |
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