Research On Impedance Measurement Of Three-phase Converter Considering Frequency Coupling

In recent years,the occurrence of a large number of grid-connected oscillation accidents has not only seriously affected the safe and stable operation of grid,but also affected the development of the renewable industry.Three-phase converters are important interface devices of these system.The common analysis method that analyzes the stability problem of grid-tied system bases on the impedance of three-phase converter and grid.The impedance of three-phase converters is the basis of stability analysis.In general,there are two ways to obtain the impedance of three-phase converters.The first way is that obtaining the impedance mathematical model in the stationary frame or the rotating domain by establishing the transient equation of three-phase converters and using some corresponding linearization methods,but this way is limited by complexity of controller of three-phase converters.If the control loop of converter is complex,it will be difficult to get the mathematical model of the three-phase converter;in addition,in most cases,the converter manufacturer will not disclose control parameters of their three-phase converter,it's impossible to apply the mathematical model without the control parameters of three-phase converters.The second way is to directly obtain the impedance of three-phase converters through the corresponding impedance measurement instrument.At present,most of the related research focuses on impedance measurement of converters in the synchronous rotating coordinate system.However,measurements in the synchronous rotating coordinate system require a phase-locked loop to provide grid phase information,which inevitably introduces new errors.And impedance measurements in the stationary coordinate system will be affected by frequency coupling,the measured impedances are equivalent impedances.A small part of the study considers the effects of frequency coupling,but they could not directly obtain the self-impedance and accompanying-impedance of three-phase converters.This thesis uses the three-phase two-level grid-connected converter as the research object,studies the impedance measurement of the three-phase converter in the stationary coordinate system,and the influence of frequency coupling on the impedance measurement of the converter is considered.Firstly,basing on the harmonic linearization method and the main circuit equation of three-phase converters,the impedance mathematical model of three-phase converters with the influence of the PLL is developed,and the reason of frequency coupling that is caused by PLL is analyzed.The fundamental reason for the frequency coupling is that the D axis gain and Q axis gain are different.The thesis gives the reason way the measured impedance is equivalent impedance in the stationary coordinate system under ideal grid condition,and the difference and correlation between the equivalent impedance and the self-impedance is analyzed when grid is not ideal.The impedance measurement considering the frequency coupling effect is studied,and a simple and effective impedance measurement method of three-phase converters is propose.The method considers the frequency coupling effect,and injects a single-phase disturbance in PCC,only needs one frequency sweep cycle,and the self-impedance and accompanying-impedance of three-phase converters can be obtained by calculating the measured voltage and current data.The proposed method does not need three-phase injection disturbance,which reduces the complexity of the measurement system,avoids the interference caused by the asymmetry of the three-phase disturbance,and the measurement result is accurate and is not affected by grid impedance.The simulation model of three-phase converters are established in MATLAB\SIMULINK.The proposed impedance measurement method is verified under different working conditions,which proves the validity and accuracy of the proposed method.And the experimental results also prove that the proposed method can accurately measure the impedance of three-phase converters.Basing on the self-impedance and accompanying-impedance of three-phase converters measured by the proposed method,some simulation and experimental applications are given.It is proved that the converters impedance measured by the proposed method can be used as basis of stability analysis of grid-tied system.