Research on main transformer neutral point grounding with small reactance and overvoltage protection

Transformer neutral point by small reactance grounding in the line to reduce the single-phase short-circuit current at the same time, improve the neutral point of the overvoltage level, small reactance resistance value is selected whether it is reasonable, will be directly related to the safe and stable operation of the power grid. In this paper, a 220 kV neutral point by small reactance system using ATP-EMTP (Alternative Transient Program - Electromagnetic Transients Program) software modeling, through the setting of single-phase faults in the station, quantitative analysis of the neutral point in series with different values of small reactance before and after the overvoltage and short-circuit voltage. The changes of industrial frequency overvoltage and short-circuit current are quantitatively analyzed before and after series connection of small reactors with different resistance values at the neutral point. Calculation and simulation results show that, with the increasing value of series reactance, the short-circuit current in the neutral point of the transformer decreases and gradually tends to be saturated, and the risk of insulation rises. If the purpose of connecting small reactance in series is to reduce the overvoltage and short-circuit current of the neutral point, to avoid the loss of ground overvoltage and to eliminate the insecurity caused by the discharge gap, it is recommended to choose transformer zero sequence reactance), which can ensure that the neutral point of both main transformers is connected to a small reactance. It is recommended to choose transformer zero sequence reactance), so as to ensure that the zero sequence reactance and short circuit current of the system remain unchanged after both main transformers are grounded by small reactance.