Fault Analysis And Protection Strategy Research Of DC Distribution Network Integrating Photovoltaics

With the rapid development of power electronic technology and photovoltaic power generation technology and the increase of DC load,compared with the traditional AC distribution network,the DC distribution network has the advantages of energy saving and more convenient access to emerging DC energy sources.Among the emerging DC energy sources,photovoltaic power supplies solve the power supply problem in areas not covered by the public grid.While alleviating the dependence on non-renewable energy sources,it is also conducive to environmental protection.When a photovoltaic-integrated DC distribution network fails,different DC regions will present different fault states,among which pole-to-pole faults are the most serious of DC faults,which seriously threaten the safety of the DC distribution network.Therefore,this article focuses on the inter-electrode fault response characteristics of different DC fault areas of the DC grid integrated with photovoltaics and the formulation of corresponding protection strategies.The main research work is as follows:1.Establish a DC distribution network model that integrates photovoltaics.The current research status of the voltage level and topology structure of the existing DC distribution network is described,and the topological structure of the ±10k V medium voltage double-ended DC distribution network is selected.The voltage source converter(VSC)is used as the AC-DC bus conversion device,and the DC solid-state transformer(DC solid-state transformer)composed of multiple dual active full bridge(DAB)converters is used.State transformer,DCSST)is used as the conversion device between medium voltage DC bus and low voltage DC feeder,and photovoltaic is selected as the main DC load of this paper.Construct mathematical models of voltage source converters,DC transformers,and photovoltaics.Based on this,establish a simulation model in PSCAD to simulate the stable operating state of the DC distribution network.Failure analysis laid the foundation of the model.2.Inter-pole fault analysis in the medium and low voltage DC fault area of the DC distribution network integrated with photovoltaics.Based on the established PV-integrated DC distribution network model,according to the different fault characteristics of each area,the fault area of the PV-integrated DC distribution network is divided.The fault response characteristics of each stage of the VSC converter when the inter-pole and ground faults occur in the medium voltage DC region are analyzed with a theoretically equivalent method.Based on the comparative study of the fault current and voltage of the two faults,the inter-pole fault is selected as the main research of this paper.Fault;analyze the fault response characteristics of each stage of the dual active full bridge and photovoltaic power when the photovoltaic power supply has an inter-electrode fault in the low-voltage DC area controlled by the constant voltage method and the disturbance observation method;and perform the fault characteristics in PSCAD/EMTDC Simulation verification,and finally the theoretical verification of the fault characteristics in MATLAB.3.Research on the protection strategy of the medium and low voltage DC fault area of the DC distribution network integrated with photovoltaics.Through the analysis of the fault between the poles in the medium and low voltage DC region,it can be seen that the fault has a large fault current differential value.With the change of the fault distance,the fault current differential value only changes in amplitude,which is beneficial to the extraction and extraction of fault characteristic quantities in the protection strategy.Tuning.To meet the adaptability of the protection strategy and combine the fault characteristics of the DC distribution network,on the basis of the traditional inverse time overcurrent protection,this paper proposes an improved very inverse time protection based on the differential value of the fault current as the characteristic quantity.However,due to the double-ended power supply of the faults between the poles of the distribution network,the improved very inverse time protection cannot meet the reliability of the relay protection.Therefore,a protection strategy that combines the directional pilot protection and the improved very inverse time protection is proposed to prevent failures.The current reversal at the end of the line is the criterion for the directional pilot protection,which can overcome the shortcoming of the improved very inverse time overcurrent protection that is susceptible to adjacent lines.Finally,verify the above protection strategy in PSCAD/EMTDC.