| Distributed generator(DG)has significant advantages in energy conservation,environmental protection,transmission and distribution loss,flexibility,reliability,etc.,so it is increasingly connected to the distribution network,which breaks the original single-power radiant network pattern of the distribution network and becomes an active distribution network with multi-terminal power supply structure.The control strategy and access mode of distributed power supply also make its grid-connected fault characteristics more complex,which affects the reliability of line protection and the safe and stable operation of the distribution network.Therefore,through the analysis of the distributed power control strategy,different access mode of fault characteristics and the influence of traditional differential protection,this topic put forward is applicable to consider distributed power access to power full waveform line differential protection algorithm,and meter and unchecked branch and the influence of harmonics in power system to further improve the algorithm.Specific research contents are as follows:(1)the structure and characteristics of the distributed generation system are analyzed.Considering the latest grid-connection requirements of distributed power supply,the double closed-loop control strategy of its grid-connected inverter with low voltage ride through capability(LVRT)is studied in depth.In the Matlab/Simulink platform to build the corresponding photovoltaic(pv)grid model,the simulation results show that this subject adopts meter and LVRT grid inverter control strategy can avoid the distributed power supply network,improve the level of voltage,at the same time and node failure characteristics of the distributed power in different degrees by the point of failure types,location and the influence of grid inverter control strategy.(2)the influence of the distributed power supply on the traditional differential protection under the connection modes of T connection to the 10 k V line of the public grid and the 10 k V bus of the public grid substation is analyzed.The former is more likely to cause misoperation or rejection of the protection,while both of them may cause protection rejection with the increasing capacity of photovoltaic grid-connection.Therefore,this paper chooses the latter connection mode and builds the distribution network simulation model,so that the traditional differential protection has a good adaptability in the application scenario of the distributed power supply access substation 10 k V bus,which provides a solid foundation and a simulation verification platform for the subsequent research.(3)proposed the line differential protection algorithm based on full current,and applied it to the distribution network application scenario where the distributed power supply is directly connected to the substation 10 k V bus.Based on the full waveform characteristics of fault current,a protection criterion is designed to meet the requirements of "in-area fault protection is not rejected and out-of-area fault protection is not misused".Considering the reliability,selectivity,sensitivity,quickness and moderate margin of protection,the relevant parameters in the criterion are adjusted reasonably.The proposed algorithm and the traditional line differential protection algorithm based on power frequency are respectively verified and analyzed for their quick performance and sensitivity.The simulation results show the superiority and effectiveness of the proposed algorithm.(4)through the simulation analysis did not detect branch access and the impact of the power system harmonic of differential protection algorithm,improved the current line differential protection algorithm is proposed,so as to reduce the area of failure unchecked power supply branch makes the possibility of protection maloperation,avoid when not testing load branch for a large number of motor protection misoperation of this particular case,has positive significance to the promotion of protection of reliability and sensitivity. |
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