| With China proposing the goal of energy conservation and emission reduction and then advancing its process rapidly,the main way to achieve this goal is constructing the modern power system featuring high proportion of renewable energy and high proportion of power electronics.Differing from the traditional power system dominated by synchronous generators,the renewable energy and its supporting power electronic devices in the modern power system have small capacity and exchange electromagnetic energy with the grid rapidly,which leads to poor overload capacity and low rotational inertia.At the same time,the topology structure,control strategy,installation quantity and location of renewable energy and power electronic equipment also show complex and diversified characteristics in different demands of the power generation,transmission,transformation and distribution scenarios.Therefore,with the batch access of diversified power electronic equipment to the grid,the time scale of the internal energy transformation,transmission form of energy and topology of the network will change significantly.The above characteristics bring great challenges to the control and stable operation of modern power system.As renewable energy and power electronic equipment are heavily connected,it is urgent to establish a grid-connected system model suitable for the access of diversified power electronic equipment to capture the dynamic behavior of the modern power system,to predict and suppress the interactive oscillation phenomenon between the power electronic equipment and the grid.Supported by the National Key Research &Development Program of China and other projects,this thesis focused on the study in the following four aspects: a unified complex circuit modeling methodology for the modern power system considering multi-frequency coupling effect(m-FCE),online measurement technology of impedance/admittance model,system oscillation mechanism analysis from the perspective of complex circuit and oscillation suppression methods are studied.The problems of modeling,oscillation mechanism analysis,and oscillation suppression for the renewable energy power generation gridconnected system considering m-FCE are solved,which provides the theoretical basis and practical guidance for accelerating the construction of modern power systems with renewable energy and power electronic equipment as the main body.The innovations and contributions of this thesis are concluded as follows:(1)To depict the m-FCE caused by asymmetric structure and unbalanced operating conditions,the plausibility of using complex-valued vector and complex-valued phase domain admittance in the rotating polar coordinate frame to characterize m-FCE existing in the single-phase grid-connected system is illustrated.The phase-shifting transformer is introduced to quantitatively characterize the strong and weak coupling and decoupling of the system,and then a unified complex circuit modeling methodology for depicting m-FCE in polar coordinate frame is proposed.The unified complex circuit models of single-phase converter-grid system and three-phase converter-grid system in unbalanced working condition considering m-FCE are established step by step.It is found that the phase dynamics will form additional positive feedback to participate in generating m-FCE,hence it is necessary to consider both the amplitude and phase dynamics to accurately characterize m-FCE.The model shows the complex circuit branch and coupling component interaction branch of each order coupling frequency in the system,and the closed-loop characteristic equation of the system is in the form of single input and single output(SISO).(2)Based on the established complex circuit model of single-machine system,considering the different initial phase of each unit and m-FCE,the global multi-phase domain coordinate frame and complex-valued vector mapping relationship in this coordinate frame are proposed and constructed.Then the plausibility of extending the unified complex circuit modeling methodology to the multi-machine grid-connected system considering m-FCE is illustrated.The unified complex circuit models of an isomorphic multi-machine grid-connected system concluded by three-phase gridfollowing converters and a heterogeneous multi-machine grid-tied system contained with grid-following/grid-forming converters are established.The proposed methodology in the multi-machine system scenario has the advantages of modular modeling,clear physical meaning and compact form of mathematical model.(3)In view of the difficulty in determining the accuracy of complex-valued phasor domain admittance which constitutes the unified complex circuit model,the basic principle of online measurement of phasor domain dmittance in the rotating polar coordinate frame is given.The technical difficulties of online measurement of the complex-valued phasor domain admittance considering m-FCE are analyzed,and the online measurement scheme of the unified complex circuit model considering m-FCE is proposed.The experimental online measurement of the unified complex circuit models of single-phase and three-phase grid-following converters are carried out,and the simulation online measurement of the unified complex circuit model of isomorphic and heterogeneous multi-machine systems are also carried out,which verified the feasibility and accuracy of the proposed unified complex circuit modeling method and its measurement scheme.(4)Based on the established unified complex circuit model,the generation mechanism of m-FCE and oscillation mechanism of single-phase,three-phase grid-tied converter systems,isometric and heterogeneous multi-machines systems connected to weak grid are analyzed from the perspective of complex circuit analysis.The asymmetric control structure of phase-locked loop and outer voltage loop will lead to m-FCE,and the larger the bandwidth of the two controllers,the more easily the system becomes unstable and oscillates.In unbalanced operation situations,the greater the voltage unbalance factor(VUF),the stability of the three-phase grid-following converter system is enhanced.This phenomenon mechanism is illustrated that the unbalanced voltage in the grid provides stronger grid support for the converter.When the isomorphic multi-machines are connected to the weak grid,the magnitudes of the diagonal and off-diagonal impedance elements in complex-valued phasor domain impedance matrix of the unit will decrease as more units accessing the network,and thus the isomorphic multiple machines are more likely to interactively oscillate with the grid.However,if the grid-forming converter is accessed in and form the heterogeneous multi-machine system,the diagonal and off-diagonal impedance characteristics in the complex-valued phasor domain impedance matrix of the unit are improved,and the unit is not easy to have interactive oscillation with the grid.There are two characteristics of m-FCE can be concluded as follows: firstly,the high-order frequency coupling component is generated from the low-order frequency coupling component;secondly,the dynamic behavior of the coupling component weakens with the increase of order of coupling frequency.The generation mechanism of m-FCE is also concluded as follows: the mismatch of phase angles and the admittances between the same order coupling components,and the “chain reaction” among different orders coupling components lead to m-FCE.The simulation and experimental results all validate the analysis of oscillation mechanism.(5)An oscillation suppression approach developed by the Active Disturbance Rejection Control and complex-valued amplitude-phase correction technology is proposed.Since the mechanism of phase-locked loop impacts on m-FCE and oscillation revealed in the chapters of modeling and stability analysis,the first-order linear ADRC is introduced into the phase-locked loops of single-phase and three-phase gridconnected converters to suppress the m-FCE caused by the prefilter-based phase-locked loop.Moreover,consider the influence of background harmonics in unbalanced operating conditions,the complex-valued amplitude-phase admittance characteristics of the converters and the grid are both modified to improve the operation stability of the system.Simulation and experiment verify that the proposed method can effectively reshape the complex-valued amplitude-phase admittance characteristics of the systems and thus improve the operating stability in both single-machine and multi-machine systems. |
PDF Full Text Request