Research On Standard Modeling And Interaction Issues Of Grid-connected Power Electronic Converters

With the development of power electronic technology,high penetration of different types of power electronic equipment in the power system introduces much uncertainty to power system,especially for issues in stability and power quality.Modeling and simulation is one of the key methods to analyze the impact of numerous power electronic devices integrated into the power system.The traditional switching model is too complicated to adapt to large syste m simulation.Therefore,it is necessary to simplify the model reasonably.In addition,in the weak power system,the power electronic transformation devices are coupled with the power grid,which may cause the grid oscillation.This paper focuses on the standard modeling and interaction issues of grid-connected power electronic converter.First,a unified model for DC boost converter,AC two-level,three-level and modular multilevel converters are established.The proposed average modeling method realizes the rapid simulation in the large system,which greatly saves the simulation resources.Averaged modeling could be achieved by relacing the switching device with the controlled voltage and current source,and the other parts of the circuits remain unchanged.The theoretical basis,modeling process,simulation verification or experimental verification of the boost converter,three-phase six-switches circuit,T-type three-level circuit and modular multilevel circuit are given in the paper,respectively.Second,stability is of great concern for flexible grid integration with power electronic transformer.In the case of power electronic transformer integration with flexible grid emulator,this paper first does detail derivations of small signal impedance models of power electronic transformer and in the synchronous rotating reference dq frame,which is further verified by comparing the measured impedances in software.Then the impedance-based Nyquist stability criteria is used to determine the stability of the interconnected system.Two simulation cases both in time domain and frequency domain were given,showing stable and unstable conditions by adjusting the control parameters---modifying the ratio of the source and load impedances.Final experi mental results verify the practicability and validity of the proposed models.Third,aimed at the potential subsynchronous and supersynchronous oscillations in the interconnected system of medium-voltage AC power grid and MMC grid inverter,the output admittance model of the grid-connected MMC inverter in the positive sequence and negative sequence coordinates are developed,which is the basis of the stability analysis of the grid-connected MMC inverter.Combined with the impedance analysis theory,two si mulation cases in time and frequency domains predict subsynchronous/supersynchronous oscillation with parameters variations,which validates the subsynchronous/supersynchronous oscillation analysis.