Black-box Impedance Modeling And Small-signal Stability Region Analysis Of Grid-connected System For Voltage Source Converter

With the increasing demand for energy in our country,vigorously developing renewable energy is a necessary way to promote the transformation of our country’s energy consumption structure.The large-scale grid connection of renewable energy has led to the gradual transformation of the modern power system from a traditional synchronous generator-besed power system to a power electronic based power system,followed by a series of unknown mechanism of oscillation accidents.As a typical power electronic device,the stability of the three-phase voltage source converter greatly affects the stability of the power system.Impedance method is widely used in engineering practice because it can ignore the structure and parameters of the system and can judge the small disturbance stability of the system through impedance measurement.However,since the impedance model obtained by impedance measurement is strongly related to the operating points of the device,that is,when the operating points of the device changes,the impedance of the device needs to be re-measured,which brings inconvenience to the use of the impedance method.Therefore,it is urgent to study the dependence of impedance and operating points and then analyze the impact of operating points on system stability,which has great value for studying the small disturbance stability of modern power systems.This paper takes the "black-box" voltage source converter as the research object and focuses on the relationship of its dq impedance and the operating points.Then it establishes the operating points decoupling impedance model and its corresponding parameter identification algorithm.Finally it combines the model to carry out the small disturbance stability analysis under variable operating points.The specific research content has the following aspects:(1)Starting from the transfer function block diagram of the voltage source converter,the dq impedance model is introduced.Then,the impedance model is verified by the frequency sweep method,which illustrates the highly dependent relationship between the impedance of the voltage source converter and the operating points.(2)A unified modeling framework for the "black-box" voltage source converter is established,and from this,the operating points decoupling impedance model of the "black-box" voltage source converter is established,and the operating points and system parameter information in the impedance model are completely decoupled and separated,and then a parameter identification algorithm is proposed,which can identify the system parameter information based on the results of a finite set of impedance measurements,so as to realize the impedance prediction of the "black-box" voltage source converter at any operating points.Then,extensive simulation tests and hardware-in-the-loop experiment verified the accuracy and wide applicability of the operating points decoupling impedance model.(3)According to the established operating points decoupling impedance model,the smallsignal stability of a typical voltage source converter under all operating conditions is further analyzed,and the small-signal stability region under all operating conditions is obtained.The results show that the small-signal stability region under all operating conditions is non-convex,that is,when the voltage of the point of common connection remains unchanged and the active power output by the system is increased,it is possible for the system to change from stable to unstable and then stable again.The small-signal stability region under all operating conditions is negatively correlated with the equivalent reactance of the gird,that is,the increase in the equivalent reactance of the gird will reduce the proportion of the small-disturbance stability region under all operating conditions.Finally,time-domain simulations are performed to verify the above conclusions.