Overvoltages Induced by Single-Phase Ground Faults in 110 kV Networks

Abstract

Overvoltages induced by single-phase ground faults in 110 kV networks operating in different neutral grounding modes are investigated. Owing to low values of resistance and high values of inductive and capacitive reactances, the transients in power lines and transformers that occur in 110 kV networks differ essentially from similar processes in 6 to 35 kV networks. This difference results in that the transient overvoltage free components have higher amplitudes, and that they have lower attenuation coefficients.

It has been established that 110 kV networks cannot operate with the isolated neutral, because the overvoltage in this case may exceed seven times the nominal phase voltage, which would greatly complicate the transformer winding phase insulation. The use of a neutral grounded through resistance or inductive reactance with a value equal to the zero-sequence impedance of the considered circuit significantly reduces the level of ground fault currents and especially overvoltage. Neutral grounding through resistance leads to more efficient results, because the overvoltages and ground fault current have in this case fairly low values. Neutral grounding through inductive reactance results in somewhat higher overvoltage and ground fault current values. If the neutral is grounded through a nonlinear inductance, the overvoltages have still lower values, but the ground fault current approaches the value of the short-circuit current as in the case of the solidly grounded neutral. Comparison of the relevant results relating to various grounding methods of 110 kV networks has shown that the best results are obtained in case of using the neutral grounded through resistance with the value equal to the zero-sequence impedance of the considered network circuit. In this case, the overvoltage levels in the phases do not exceed 1.78 of the phase voltage value, and the ground fault current is much less than the short-circuit current in case of using the solidly grounded neutral. However, in choosing the neutral grounding method, it should be constantly borne in mind what particular task has to be solved: to limit the short-circuit current or to limit overvoltages. In the second case, the use of the neutral grounded through a fast-saturating reactor is a more acceptable choice.