Stability Analysis Of Grid-Connected Converters By Apparent Impedance

The rapid consumption of primary energy and serious environmental pollution have accelerated the research and application of new energy,such as solar energy and wind energy.The new energy generation which is unstable has a negative influence on the reliability and power quality of the power grid.With the rapid development of new energy generation and the demand for flexible and controllable power in power system,electrical and electronic inversion technology has been deployed more widely in power systems.Three-phase PWM converter is widely used in new energy access and motor drive because of the advantages of flexible control,continuous power factor and low harmonic pollution.There is no interaction between the converter and the power grid since the impedance of the power grid is negligible in the ideal grid.However,as for grid-connected inverters access to the new energy distributed grid and rectifier loads connected to the distribution network,the impedance of the grid cannot be ignored.The power grid shows the characteristics of the weak grid.Since the influence of the grid impedance is not considered during the parameter design process,the grid and the converter will interact with each other through the impedance when connected to the grid,which causes the stability problem when the impedance is mismatched.In recent years,due to the mismatch of the impedance,the oscillating accident happened sometimes.Aiming at the problem above,the more mature stability analysis method is an impedance method.With the deepening of research,its shortcomings gradually appear.Since the apparent impedance based analysis method has no need to measure impedance of the grid and converter separetly,researchers are paying attention to it.However,in the existing research,the analysis method based on apparent impedance is only applied to simple DC systems and three-phase systems.No literatures analyze the stability of complex systems with the method.Based on this,it is necessary to investigate the effectiveness of apparent impedance based method in the stability analysis of complex systems.For this reason,the paper uses the impedance analysis method and the apparent impedance analysis method to analyze the stability of the converter in complex system.This paper first introduces the impedance method,which mainly includes three aspects:the principle of impedance method,the acquisition method of grid impedance,and the modeling of the impedance of the converter.Harmonic current injection method is accurate in measuring the grid impedance,hence it is suitable for software simulation and system design applications.The basic principle of measuring grid impedance is analyzed,and the measurement process is intorduced.The effectiveness of the harmonic current injection method for measuring grid impedance is verified through two simulation models.Measurement of converter impedance is introduced.With the method of harmonic linearization,the input(output)impedance model of the converter is deduced based on the relationship between the small signal perturbation voltage excitation and the output current response.Emphasis is placed on the apparent impedance method,including the definition of apparent impedance,the principle of stability analysis,and the flow of analysis.The apparent impedance method is a stability analysis method that does not depend on the position of the measurement point.It is assumed that a voltage or current is injected into a certain node of the system.Taking a three-phase system as an example,the injection point divides the system into two subsystems.The two subsystems are represented by the frequency-dependent Thevenin equivalent circuit or the Norton equivalent circuit.The apparent eigenvalue of the system is estimated by vector fiiting with the impedance value measured from the injection point,namely,the apparent impedance.And the stability of the grid-connected converters is analyzed by evaluating the apparent eigenvalues in the complex plane.Based on the Nyquist criterion of impedance method and the eigenvalue criterion based on apparent impedance,the grid impedance is connected with the converter impedance,the apparent impedance and the simulation time-domain waveform.At the same time,the theoretical analysis is used to explain the simulation results,and the simulation results are used to verify the theoretical analysis and form a complete system of stability analysis of the grid-connected converters.Simulation analysis shows that the stability analysis method based on apparent impedance has good adaptability in complex systems.