Abstract
The paper analyses how the characteristics of the medium-voltage network with insulated neutral and the conditions under which the single line-to-ground fault occurs (insulation condition and value of the fault resistance) influence the zero-sequence components of the voltage of the transformer station medium-voltage bus bar, the fault current, the currents of the faulted line and the currents of the other non-faulted power lines connected to the transformer station bus bars. Assuming the waveform of the medium voltage bus voltages in the transformer station is known, the influence of the fault resistance on the total distortion coefficient of the fault current is analysed. This establishes the conditions under which a single line-to-ground fault can be detected by controlling the total distortion coefficient of the fault current and the currents of the fault-free lines. It also examines the conditions under which a single line-to-ground fault can be selectively detected by checking the effective value and direction of flow of the zero-sequence currents of the medium-voltage lines during the fault. The results obtained by the calculation were verified experimentally in the medium voltage network considered in the study. The acceptable differences, considering the degree of accuracy with which the parameters of a medium voltage network are known, between the results obtained by calculation and those obtained experimentally show that the simplifying assumptions accepted for the mathematical models used in the calculation are correct.
Highlights
Medium-voltage power grids extend over long distances, which is why the number of faults in these grids is very high
The main findings of the study are presented below: (a) A single line-to-ground fault occurring in a medium voltage network with isolated neutral can be selectively detected by checking the harmonic content of the fault current only if the resistance at the fault site does not exceed 100 Ω
As a result, controlling the harmonic content of the phase current does not provide a reliable procedure for selective single line-to-ground fault detection. (c) If the fault site resistance exceeds 100 Ω, it is advisable to use the actual value of the zero-sequence current and its direction to sense a fault in a medium voltage network with isolated neutral using a directional current relay
Summary
Medium-voltage power grids extend over long distances, which is why the number of faults in these grids is very high. During an ideal single line-to-ground fault (the valu√e of the resistance at the fault location is zero) the voltage of the fault-free phases increases 3 times, which is why there is a risk that the fault will turn into a multiple fault with important consequences on the network and on continuity of the supply to consumers. For this reason it is important that the detection of such a defect be done selectively and in a short time. In [13] the transient regime caused by the fault is analysed for single line-to-ground fault detection
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