Impedance Modeling Of Grid-Connected Converter System And Research Of Its Voltage Tracking Ability Based On Phase-locked Synchronization

The renewable energy generation is helpful to solve the problem of energy crisis and environmental pollution.The grid-connected converter has good controllability and a variety of flexible operation mode.It can output sinusoidal and bidirectional current.It can aslo realize the decoupling power control and maintain operation with unity power factor.It is an important interface of renewable energy access with multi functions and many advantages.Therefore,the safe and reliable operation of the high-power grid-connected converter has become the important guarantee of safe and reliable operation of the modern power system with gradually increased permeability of electronics technology and power electronic device.Good voltage tracking ability of grid-connected converter plays a very important role in the safe operation of the grid-connected converter system during grid faults or disturbance.The voltage tracking of the grid-connected converter system is realized by the phase-locked control and the inner current control.Some research shows that the phase-locked control technology will have a certain influence on the stability and power quality of the grid-connected converter system.Therefore,it is meaningful for the safe and reliable operation of the grid-connected converter system to study its stability analysis based on the phase-locked synchronization method.The study of the influence of phase-locked control technology on the voltage tracing ability of the grid-connected converter is also necessary.This dissertation is supported by National Natural Science Foundation of China.The research is focused on the impedance modeling and voltage tracking ability of the grid-connected converter system based on phase-locked synchronization.This dissertation made some groping and innovative research on these three parts:the influence of parameters of synchronous reference frame phase-locked loop on the analysis of grid-connected converter safety,the modeling of sequence impedance/admittance of grid-connected converter considering the effects of phase-locked loop,the output angle compensation method of phase-locked loop to improve the voltage tracking ability of grid-connected conveter.The main work and the results accomplished by this dissertation are as follows:(1)The basic principle and mathematical model of the synchronous reference frame phase-locked loop is studied.The time domain analysis of phase-locked loop of the three-phase voltage source PWM rectifier system during single-phase grid voltage dip is given.A second-order nonlinear differential equation to calculate of the output of phase-locked loop is deduced.The validity of the equation is virified by the calculation and simulation of the software.The influence of the different PI parameters of the phase-locked loop on its output is analyzed.The change rule is also summarized.The hardware-in-the-loop simulation verifies the validity of the simulation.A qualitative analysis of the influence of the parameters of phase-locked loop on the security of the grid-connected converter during unbalance grid voltage fault is presented.(2)A series of the impedance/admittance modeling method of grid-connected converter based on harmonic linearization method considering the effects of phase-locked loop is given.The small-signal impedance model of grid-connected converter under balance grid voltage is obtained according to harmonic linearization method.Then,the small-signal stability of grid-connected converter system can be analyzed.The sustainable resonance will occur when the small-signal stability criterion is not satisfied.The existing small-signal impedance stability analysis method can only analyze the stability of the grid-connected converter system.However,the frequency and the amplitude of the sustainable resonance is unable to make quantitative analysis.Therefore,the large-disturbance impedance of the grid-connected converter is defined.The large-disturbance impedance can be easily obtained through the experiment and simulation methods.The method for estimating the frequency and the amplitude of the sustainable resonance based on the large-disturbance impedance is given.The mathematical modeling of large-disturbance impedance can be obtained the quasi-harmonic linearization method based on the describing function.The production mechanism of the sustainable resonance is deeply discussed.The quantitative analysis of the sustainable resonance is also done.Finally,the admittance matrix of grid-connected converter under unbalanced grid voltage considering the coupling of the positive and negative sequence component is given.The stability analysis method using the copupled-admittance is aslo illustrated.Simulation and experiment verify the effectiveness of the proposed analysis method.(3)The influence of phase-locked loop on control system of grid-connected converter can be neglected when the grid is strong.The voltage at PCC will couple with grid-connected converter via phase-locked loop through modeling the dynamic characteristics of the voltage at PCC when the grid is weak.The coupling will become greater when the grid impedance become bigger.Therefore,it is necessary to decrease the coupling between the voltage at PCC and the grid-connected converter by improving the output performance of the phase-locked loop to enhance the phase-locked control ability of grid-connected converter.Then,the voltage tracking ability of grid-connected converter is improved.The grid-connected converter will have a large distortion of harmonic current and deteriorating performance under unbalanced grid condition.It is harmful to its security operation.The main components lead to the distortion of current is the third-order current caused by the negative voltage at fundamental frequency.The output angle disturbance of the phase-locked loop due to negative voltage at fundamental frequency is one part of the reason of the production of the distortion of current.By analyzing the mechanism of harmonic current,the compensation method of synchronous reference frame phase-locked loop under unbanlaced grid condition based on the harmonic linearization method is proposed.It can work properly under unbanlaced grid condition and has good dynamic performance.The phase-locked control ability of grid-connected converter is enhanced.The amount of third-order harmonic current is decreased obviously.The current control ability of grid-connected converter is enhanced without the attenuation of its dynamic performance.The proposed compensation method obviously enhanced the voltage track ability of grid-connected converter and also its security operation.Cycle-by-cycle simulation verifies the validity of the proposed method.