Research On Key Technologise Of Broadband Three-phase Grid Impedance Online Measurement

With the ongoing development of the distributed generation systems,the application of grid-connected inverters is extremely widespread.The existence of the grid impedance will deteriorate the control of inverter and affect the system stability,which makes the harmonic resonance between the grid-connected inverter and the grid more and more frequent.Under complex and changing grid conditions,the system stability can be improved by adaptive control of the inverter,the significant guarantee of which is to realize the online measurement of the grid impedance rapidly and accurately by the inverter itself.Based on a three-phase inverter,technologise of broadband three-phase grid impedance online measurement were studied in this paper.The grid impedance online measurement scheme based on a three-phase inverter was firstly given.Key technologies about grid impedance online measurement,as well as the online implementation of FFT algorithm,were introduced in detail in this paper.In order to achieve broadband grid impedance,two typical pseudo-random binary sequences(MLBS?DIBS)were studied in this paper,including the theory and synthesis of the sequences,the power spectrum and parameters design.Noise existing in signals will influence the measurement accuracy of high band grid impedance.Three methods to improve SNR were introduced in this paper.The average method in frequency domain,which remains the time domain amplitude and frequency resolution,can help achieve better accuracy of gird impedance.When measuring only positive or negative sequence impedance,the traditional signal injection based on d/q axis will generate both positive and negative harmonic component,which will lower power quality.By appropriate design of signal injection,a new orthogonal signal injection method based on dq axes was proposed in this paper to improve the power quality.Considering spectral leakage of fundamental signals and noise in pratical,a hybrid grid impedance online measurement method based on different designs of different bands was given in this paper to achieve broadband grid impedance with high accuracy.Furthermore,the simulation model of a three-phase grid-connected inverter and a digital control based experimental prototype were built in this paper.The effectiveness of broadband grid impedance online measurement technologies given in this paper were verified by simulation and experiments accordingly.