Research On Operation Modes And Fault Control In Cable Network

With the economy developing rapidly, distribution network construction increases and land for line-corridors in the city is getting less and less, land costs are increasingly rising, so laying of cables become the main ways for line construction. Meanwhile, in order to meet the needs of the urban landscaping and power distribution reliability, urban distribution lines have been converted from the original overhead lines into underground power cable lines completely or partly. With the proportion of power cable in urban areas distribution network increasing quickly, capacitive current of distribution network is increasing gradually, so that fault current increases when single-phase ground fault occurs, and some difficulties have been brought to the actual operation units and the design departments to some extent. Because there are various kinds of single-phase protection ways, and a big controversy arises on the choices about single-phase protection way, in the paper, the methods of synthesis evaluation on the single-phase protection ways and the approaches to protecting it from fault were formulated, which was expected to provide a theoretical basis and reference to the actual operation units and the design departments.The causes resulting in different faults were obtained through analysis of different types fault models in cable networks, and the processes of cable insulation damage to burn was studied. Based on the study above, a permanent ground fault model and the arc grounding model were formulated; using a controlled switch turn-on and turn-off single-phase ground arc, and using variable resistor simulated arc resistance, the arc faults were established during the cable breakdown to burning based on the criterion of the fault disappearance or transformation. The model establishment provided a theoretic basis for calculation and analysis on single-phase ground fault, and the first cable arc burnout model based on thermal factors was derived, which was a basis for the calculation of the single-phase ground default control effect against protection methods response time.Based on the establishment of the cable network fault models, the factors affecting single-phase ground fault analysis were calculated in random combinatorial way. After analyzing the results of the cable network capacitive current, grounding resistance, ground point location and grounding protection methods on the degree of influence system operation have been obtained. After comparing the system voltage and current under different conditions, it's found that the neutral grounding via low resistance mode protected systems best during transient process of single-phase ground fault, while during steady process based on shunt devices protected systems best.A large number of single-phase ground fault calculation results were used to establish a single-phase ground comprehensive evaluation based on IAHP. Through the analysis on different cable networks single-phase ground fault ways, the criterion of distinguish single-phase ground fault protection ways were obtained; based on establishing hierarchy structure of comprehensive evaluation, hierarchical decision-making table was set up, and judge matrix was obtained by using pairwise comparison, and consistency of matrix should be checked, and the comprehensive weight of overall goal was obtained, at last, the comprehensive evaluation weight of each protection way was obtained by collating. IAHP was applied to single-phase ground fault protection methods as an evaluation way for the first time in this paper, which can effectively avoid the errors caused by human factors and achieve fault protection approach purpose accurately and provide reference for the selection of protection means to specific cable network.In order to verify different protection methods on the control effects of single-phase ground fault and the characteristics of arc extinguish, simulation experiment platform was structured based on 380V power system. System component parameters were obtained through simplifying to be equivalents based on 10kV equipment, and the experiment proved that the experimental platform could effectively simulate 10kV distribution system. PIC control system was used to control the earth-fault beginning and protection mode response time accurately. The simulation experiments on the platform proved that the simulation results and the comprehensive evaluation results were correct, and verified the protection methods for single-phase ground fault controlling effects and extinguishing arc role.Grounding tests based on streaming device fault protection methods carried out in a substation, the results of which proved that the grounding protection methods played the role of arc-suppression and suppression over-voltage when single-phase ground fault occurred and verified the results of comprehensive evaluation based on IAHP correctness.