Frequency-coupled Impedance Modeling Of Virtual Synchronous Generators And System Stability Analysis

The integration of distributed generating sets and power systems based on renewable energy has been developing rapidly,but the low inertia and damping of power systems have brought serious stability problems.Virtual Synchronous Generator(VSG)technology plays an important role in this process.However,VSG is also composed of fragile power electronic devices.At the same time,new energy power generation is remote,has a wide distribution range,a long transmission distance,and complex operating conditions.It often presents a greater sensibility at the Point of Common Coupling(PCC)between VSG and the power grid.Therefore,the interaction stability between VSG and the power grid is usually studied by impedance modeling method.Voltage-controlled VSG contains power controllers,Root Mean Square(RMS)of AC voltage,voltage and current double closed-loop,such as structure,in which the output sequence-impedance of VSG will be coupled.Although the couplings are relatively small,they still have a significant impact on the stability analysis.However,there is no research on this aspect in the existing publications.The authors all regard it as an independent decoupling system without considering the possible Mirror Frequency Effect(MFE).To fill this gap,this paper describes the causes of frequency coupling effect in VSG,and then two typical VSG systems,namely Proportional Resonant control(PR)and dual-loop control are modeled and analyzed based on harmonic linearization,which include the main circuit and control structure of the VSG.Then,the correctness of the proposed method is verified by the sweep method.The couplings in the system are considered in the model,which can be used to analyze the MFE in the VSG.The relationship between power controller and the MFE will be the focus of this paper.Through the deduced VSG impedance expressions,the relationship between different controlling parameters and system stability is analyzed by using impedance stability criterion.Finally,the corresponding simulation model is built in Matlab/Simulink,and the relevant experiments are completed on the experimental platform of VSG.Both the simulation and the experiment verify the feasibility of the sequence impedance modeling and the correctness of the theoretical analysis in this paper.