Selective Protection Considering Prediction Of Short-Circuit Current And Its Breaking In Low-Voltage Multilevel Distribution Systems

As one of the most common power faults in low-voltage distribution systems,short-circuit fault poses a serious threat to the quality,reliability and continuity of power supply.Selective protection is the most effective way to clear the short-circuit fault rapidly and reliably,and ensure the continuous and reliable power supply in the non-fault areas as far as possible.Limited by the time and current selective principle and the breaking technology,the conventional selective protection technologies can only achieve local selectivity and can not balance the rapidity and the selectivity.This dissertation puts forward a novel low-voltage multilevel selective protection technology,which takes the full-range dynamically coordinated protection of short-circuit fault into comprehensive consideration,including fault detection and short-circuit current prediction,selective and coordinated communication,and short-circuit current breaking.The following works have been carried out:(1)The system principle of the novel low-voltage multilevel selective protection technology and its’ key techniques,including fault detection and short-circuit current prediction,selective and coordinated communication,short-circuit current breaking,are all expounded.According to its working principle,the overall hardware scheme of the novel selective protection is designed.(2)The short-circuit fault early detection technology with morphological wavelet algorithm is achieved on DSP,and then the detection time dispersivity of fault detection modules in different levels and branches is analyzed.According to the mathematical model of short-circuit current in low-voltage distribution systems,a method of trend extend prediction for short-circuit current is presented.The five-element filter matrix based on short data window Fourier algorithm is mainly designed to separate the AC and DC components of short-circuit current and to extract the characteristic parameters.The sequence combining fault detection with short-circuit current prediction is designed,and the waveforms of short-circuit current are imported to an arbitrary waveform generator to provide short-circuit current signals for the sampling and processing of DSP,then the online prediction effect of short-circuit current based on fault early detection is tested.(3)The selective protection mechanism based on CAN(Controller Area Network)bus communication is put forward,and the program of selective coordinated communication between the fault detection and short-circuit current prediction modules and the central control modules is designed in detail.Taking both the fault positions and the orders of detecting out fault into consideration,the selective coordinated process between the modules is tested and the selectivity and rapidity of fault protection is analyzed.(4)The fast breaking technology of short-circuit early current based on eddy current repulsion principle is expounded,and its driving circuit is designed.Co-operating with the fast breaking mechanism based on the eddy current repulsion principle,the selective protection mechanism based on CAN bus communication is applied in the low-voltage distribution experimental system to carry out the confirmatory experiments of multilevel selective and fast breaking of short-circuit current.In conclusion,the dissertation puts forward a novel low-voltage multilevel selective protection technology with prediction of short-circuit current,which takes the full-range dynamically coordinated protection of short-circuit fault into comprehensive consideration.And the method of trend extend prediction for short-circuit current achieves the organic combination between fault early detection and short-circuit current prediction,which provides the online prediction of short-circuit current with a new idea.The selective protection mechanism based on CAN bus communication balances the rapidity and the selectivity of fault protection,and the co-operation with the fast breaking mechanism lays a technological foundation for the multilevel selective and fast breaking of short-circuit current.