配电线路的零序电流和故障选线新方法

为进一步研究配电线路单相接地后的故障选线问题,分析了配电线路的零序电流,即配网中某线路发生单相接地时,故障线路本身的零序电容电流分别从故障点向线段的首端电源侧和末端流出,端部电流为零,沿线电流按照斜线的规律分布;非故障线路的零序电流从各自的首端流向末端,末端电流为零,沿线亦按斜线的规律分布。全部非故障线路的零序电流叠加到故障点到首端的线段中,故首端的零序电流比每条非故障线路的为大,据此可检出故障线路。但当线路很长或接地电阻较大时,上述零序电流间差别不大,难以进行故障选线。为此,提出了一种新方法,即利用母线上的一部分无功补偿电容,或在母线上投入一定容量的三相星形电容器组,在判定永久性单相接地时将其星形中性点临时接地,相当于投入非故障的长线路,以使故障线路首端的零序电流显著增大而被检出。对于谐振接地电网,可将电容器临时接在电网中性点上,并在其投入或在上述星形中性点接地的同时切除消弧线圈,以免削弱故障线路首端的零序电容电流。

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.