| With the development of power electronics and new energy generation technology,more and more distributed power sources and DC loads are connected to the grid,and the traditional AC distribution grid is facing more severe challenges in terms of new energy consumption capacity,flexibility and high efficiency.DC distribution grid will become the new distribution trend in the future due to its low line loss,high efficiency,high reliability and smooth access to distributed power sources.The uniqueness of DC power distribution network in terms of topology and operation control also causes the unique characteristics of DC power quality problems in terms of formation mechanism and manifestation,etc.The analysis and study of DC power quality problems are beneficial to the promotion and development of DC power distribution technology.Thus,this paper analyzes and studies the formation mechanism,ripple detection and disturbance identification of some DC power quality problems.First,the current status of domestic and international research on DC distribution grid power quality and disturbance detection and identification is described.The characteristics of various topologies of DC distribution networks are introduced.Based on Matlab/Simulink simulation software,a simulation model of DC distribution network is built to analyze the relevant characteristics and working principles of wind power generation system,photovoltaic power generation system and energy storage module in DC distribution network.Since there is no unified definition and national standard for DC power quality problems,this paper draws on the experience of AC power quality problems and existing literature to provide a preliminary definition and description of DC power quality problems such as voltage ripple,voltage fluctuation,voltage transient drop and transient rise in DC power distribution network,and analyze the related formation mechanism.The simulation model of DC distribution network is also used to simulate and study the effects of different working conditions on DC power quality problems.Next,the characteristics,formation mechanism and detection method of ripple in DC distribution network are analyzed.The detection method based on the Modal Variable Modal Decomposition(MVMD)algorithm is proposed to achieve the accurate detection of ripple signals.Firstly,the optimal decomposition scale is calculated by the orthogonal exponential method or the spectral feature correlation method to realize the adaptive decomposition of the signal,and then the DC disturbance signal is decomposed into a series of eigenmode components(IMF)by the variational modal decomposition algorithm,and then processed by the Hilbert transform(HT)to detect many parameters of the ripple components in the DC disturbance signal,such as the instantaneous frequency,amplitude and onset time of each ripple component.amplitude and start/stop time of each ripple component.Finally,the method is compared with the empirical modal decomposition(EMD)algorithm,and the comparison experiments show that the method can effectively decompose the ripple signal,and has high detection accuracy and noise robustness.Finally,to identify the power quality disturbances in DC power distribution network,six types of power quality disturbance waveforms of DC power distribution network simulation model under different working conditions are analyzed,and five characteristic quantities of DC power quality disturbances are extracted as the basis of identification and classification according to their waveform characteristics,time domain and frequency domain characteristics,and the multi-level support vector machine(M-SVM)hierarchical identification method is adopted to realize the accurate identification of DC power quality disturbances.The accuracy and effectiveness of the adopted method are demonstrated by the simulation example. |
