Study On The New Grounding Modes Of The Medium Voltage Level Distribution Networks

Neutral grounding of medium voltage level distribution networks is a comprehensive technical problem, which is important to the system's overvoltage, insulation coordination, communication jamming, fault line selection, reliability and safety .Recently, with the rapid development of the economy, increasing of the expansion of power system, the vast amount of using of electric cable, the traditional grounding encountered a lot of new challenges because of the capacitance current distribution in phase short circuit to ground increasing sharply. For example, there are difficulties in fault line selection and overvoltage suppression, and the old Petersen Coil no longer meet the current. In view of the questions mentioned above, flexible grounding and distributed compensation come up.Capacitive current is compensated by the flexible grounding, and overvoltage is reducing by a parallel low resistance. At the same time, it can be helpful for fault line selection, which includes the advantages of neutral earthing via Petersen Coil and neutral grounded with Low-resistance. Installing the Petersen Coil at the end of line, distributed compensation could fix up the problems in expanding the capacity of Petersen Coil, lower cost.After introducing the brief theories of flexible grounding, based on the PSCAD software, a simulation model on the neutral flexible grounded in distribution network is presented, which includes intermittent arc earthing model, multiple runs model, and so on. The overvoltage of system is calculated by the model, thinking about single phase-to-ground type, the time of paralleling resistor, and the moment of fault occurring. The results indicate that, fault type can be determined by the curve of faulty feeder, the overvoltage of system would be smallest if the small resistor is paralleled when neutral voltage is zero.After introducing the brief characteristics of distributed compensation, flexibility of distributed arc suppression coil is studied by the location and capacity, and cooperation with arc extinction coil of principal station, and a simulation model is calculated. The result shows that fault current is smallest when distributed compensated at the end of line with suitable capacity. Broken line overvoltage can be restrained by over compensation and adding damping resistor.Combined with the theoretical research, this paper introduces the Structure and Principle of the grounding transformer and the arc suppression coil integrated devices, and does an experiment. This device has features such as simple structure, low cost, convenient installment. Through test and actual application shows that i t can fulfill the requirement.Flexible grounding and distributed compensation could compensate the fault current, improve the security and reliability of the system, servicing for the distribution network.