Research Of Neutral Point Grounded Mode For Medium Voltage Network

The grounded mode of power system is related to technical, economic, secure and other various aspects and it is one of the foundations of security and stability operation of the power system. It is great significance to the secure and economic operations of the power system. With the growing capacity of grid and increasing voltage levels, the original neutral point grounded mode can not meet the requirements for the development of the power system.In the power system with all sorts of voltage levels, phase-to-ground fault is the main form of the fault, occupied 70% of the total to malfunction. Since no short-circuit loop is formed so that the fault current is small and the three phase line to line voltages are still symmetric when single-phase to earth fault occurs in a non-solidly earthed network, the load can be in normal operation for a period of time. But after the fault, the non-grounded fault phase voltage would increase, discontinuity grounded arc still can produce arc voltage, long-term operation may damage the insulation and serious phase-to-phase fault.Aiming at this problem this thesis thoroughly analyses grounding principles and basic characteristics of three kinds of grounded mode (ungrounded, grounded through a resistance, compensated grounded) used in medium voltage network. An automatic tuning suppression-coil device with on-line tap-changer is developed for the capacitive current compensation and faulted line selection in case of an earth fault. The results from field test show that the measurement of the capacitive current is accurate and the effect of compensation is very good when single-phase grounded fault occurs.