The maximum unbalanced zero sequence current in the overhead lines and the cable lines of a 10 kV low-resistance grounding system is approximately 0.37 A and 0.26 A, respectively . Furthermore, the starting current is set as 0.45 A in this study. It is higher than the maximum unbalanced zero- sequence current.
The conclusions are summarized below: (1) The zero-sequence current ratio coefficient, which is independent of transition resistance, is used to distinguish the faulty feeder from the healthy ones. The significant difference between these ensures sensitivity in the event of high-resistance ground faults.
It is defined as the ratio of the zero-sequence current of the feeder to that of the neutral branch. Nonetheless, both zero-sequence voltage and zero-sequence current are affected by the transition resistance, The influence of transition resistance can be eliminated by calculating this coefficient.
Therefore, it is necessary to prevent the generation of this current. The maximum unbalanced zero sequence current in the overhead lines and the cable lines of a 10 kV low-resistance grounding system is approximately 0.37 A and 0.26 A, respectively . Furthermore, the starting current is set as 0.45 A in this study.
The expression for inverse-time zero-sequence over- current protection is t I I ( ) 1.2 15 3 75= <      1.8 3 3 15 0.6 3 75 < I I 0 0 0 ≤ ≤ > (20) The zero-sequence current of the faulty feeder is higher than that of the healthy one. Hence, the protection of the faulty feeder would function first owing to the marginal delay .
That is, U f =5.77kV. The grounding resistance of the neutral point Rg is 10 Ω. Therefore, according to equation (16), the zero-sequence voltage is related only to the transition resistance and system admittance. Assuming that the transition resistance is constant, the only variable in equation (16) is ωC0∑.
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