Research On Ferroresonance Overvoltage In Small Current Grounding System

There are many breakdowns always occurring in small current grounding system such as single phase grounding, single phase break and so on, when the breakdown happened, it will produce a large overvoltage on bus owing to the nonlinear of the magnetic inductance in potential transformer, which will cause certain influence to the safe operation of power grid. So it is necessary to analyze the ferroresonance overvoltage in small current grounding system.This paper based on the excitation of single phase grounding, using the parameters of35kV Shanshi substation in Wenling power supply bureau. Firstly establish the mathematic model of magnetic inductance in potential transformer by Piecewise least squares curve fitting. Secondly, write the state differential equations when the system under normal operation and fault condition, then choosing the improved Euler method to calculate the state differential equations. Finally, use Visual Basic6.0as the programming tool to calculate the ferroresonance overvoltage.This paper analyzes two influence factors of ferroresonance overvoltage: the size of earth capacitance and the size of resistance when single phase earth fault occurs. We can find that with the increase of earth capacitance, the system will occurs fundamental resonance and frequency resonance. The results show that when system occurs fundamental frequency resonance, two phase of voltage will increase, one phase voltage will decrease, at the same time will accompany the virtual grounding phenomenon; when system occurs frequency resonance, three phase of voltage will increase and present cyclically. Finally, this paper discuss two kinds of most common method to restrain ferroresonance at present: connecting a grounding resistance at neutral point in potential transformer and four potential transformer method. We can concluded that when system occurs single phase grounding, choosing connecting a grounding resistance at neutral point in potential transformer as elimination resonant method is suitable.