Zero-sequence current and new method of fault line location in distribution network

In distribution networks with an isolated neutral, when a single-phase grounding occurs on one of the outgoing lines, the zero-sequence capacitance current(ZSCC)of the faulty line appears from the fault point toward both the source sending end and the remote end. It has an oblique distribution along the line, and decays to null on both ends. The ZSCC of every unfaulty line starts from the sending end to the remote end with similar current distribution to that of the faulty line. All ZSCCs of the unfaulty lines are superposed in the segment of the grounding point to the sending end; hence the ZSCC of this end is greater than that of each of the unfaulty lines and thus the faulty line can be detected accordingly. Although this method has been applied in some substations, it becomes very difficult to use when one or several lines are cable lines or very long or when there is a large grounding resistance because the difference of the above mentioned currents may be very small in these situations. Nevertheless, this method is the simplest and most reliable method for fault detection if the current difference is sufficient for measurement. This paper proposes a new approach to further promote this method for fault detection. The basic idea is to utilize a portion of the capacitance compensator at the substation or add a group of three-phase capacitances with appropriate capacity to the busbar in star topology. Earthing the neutral of the capacitances is similar to connecting a long unfaulty line, and thus will result in a significant and sufficient amplification of the zero-sequence current of the sending end of the faulty line. This amplified signal can then be detected easily. In resonance earthed networks, when a single-phase grounding occurs, the inductive current of the arc-extinguishing coil compensates for the ZSCC of the lines completely, and thus reduces the ZSCC of the sending end of the faulty line significantly. Recently, it has been proposed to connect a resistance in parallel with the coil in order to achieve sufficiently large zero-sequence current of the sending end of the faulty line. But this resistance consumes enormous hot energy and hence requires a special construction. On the contrary, the approach proposed in this paper does not have this problem and the additional capacitances required can be easily achieved. The arc-extinguishing coil should be switched off when earthing the neutral of capacitances to avoid any unfavorable influence to the fault detection.