Research On Interactive Resonance Suppression Between Three-phase Grid-connected Converter And Power Grid

Voltage source converters(VSCs)are widely used in new energy generation,AC/DC power transmission systems.Due to the wide geographical distribution of renewable energy power generation units and the improvement of power levels,the power grid gradually exhibits weak grid characteristics,and its grid impedance cannot be ignored.In a single VSC system,the coupling between VSC and grid impedance may cause interactive resonance and the coupling between VSC control unit and grid impedance and the mutual coupling between VSCs control units may cause resonance in multi-VSCs paralleled system.From the point of impedance analysis,the resonance suppression with high gain controller and with high PLL bandwidth is studied in this paper.1.The circuit topology of three-phase LCL grid-connected converter is introduced and its working principle is analyzed.The mathematical model in the stationary coordinate system is established.On this basis,the mathematical model in the synchronous rotating coordinate system is established by means of small signal perturbation.The parallel system is divided into source subsystem and load subsystem.The possible resonance frequency points are found by impedance analysis,and then the output impedance characteristics of VSC are reconstructed to make it passive at the resonance frequency points to avoid the interaction resonance with the power grid.2.Harmonic distortion rate is an important index of power quality.In order to actively suppress grid-connected current harmonics or compensate for non-linear load harmonics,high-gain controllers,such as repetitive controllers,are often used in converters.The traditional repetitive controller design does not consider the impedance interaction between the converter and the power grid,which may result in the stable operation of a single VSC and the resonance of multiple parallel VSC.In this paper,an output admittance remodeling method of grid-connected converter based on embedded repetitive controller is proposed.The converter designed by this method has better adaptability to the impedance variation of power grid and can operate stably in parallel system.The effectiveness of this method is verified by experiments and simulation.3.The current control of grid-connected converter needs phase-locked loop to provide synchronous signal.The influence of PLL bandwidth on current control in dq coordinate system is greater than that in alpha beta coordinate system.PLL will affect the reference current in current control in alpha-beta coordinate system and the reference current and modulation voltage in dq coordinate system.When the PLL bandwidth is high,it will affect the q-axis output impedance of VSC and may interact with the grid impedance.In this paper,a compensation method based on q-axis modulation voltage is proposed under the condition of high PLL bandwidth,which can keep VSC stable even when the PLL bandwidth is high.The validity of this method is also verified by experiment and simulation.