Doaa Khalil Ibrahim

In medium voltage distribution network, the phenomena of having a simultaneous false trip on healthy feeder/feeders during a real fault on another feeder is common. It affects badly the continuity of service. In this paper, three different causes of this unnecessary simultaneous tripping in distribution networks are discussed. The earth fault element problems caused by these phenomena are also evaluated. A practical solution for this problem is proposed by using the custom logic scheme exists in the digital protection relay. The proposed method is applied to an actual system utilizing the existing protective digital relays. Actual records for faulty phase current and calculated zero sequence currents were captured and analyzed by means of digital fault recorders exist in the protection relay. The obtained results are encouraging and indicate the suitability and advantages of the proposed method.

This article presents a central relay based on wavelet transform for high- impedance earth fault detection, zone identification, location, and classification in part of the Egyptian 500-kV transmission network. The scheme recognizes the distortion of the voltage and current waveforms caused by the arcs usually associated with high-impedance earth faults for unsymmetrical faults, whether single line to ground fault or double line to ground fault. The proposed discrete wavelet transform based analysis yields three phase voltages in the high-frequency range and zero-sequence root mean square current in the low-frequency range that are fed to fault detection and location algorithms, respectively, while phase currents in the high frequency range are fed to the classification algorithm. The fault detection algorithm is based on the recursive method to sum the absolute values of the high-frequency signal generated over one voltage cycle, while the zone identification and fault location algorithms use unsynchronized zero-sequence root mean square currents.On the other hand, the fault classification algorithm is based on the currents in the high frequency range for one- side data of the faulted line at the local relay after the detection and location process. Characteristics of the proposed central relay are analyzed by extensive simulation studies that clearly reveal that the proposed relay can accurately determine the network faulted line and can calculate fault distance with an acceptable error that does not exceed 5%. All simulation studies are carried out using a high-impedance earth fault model of a distribution system that is modified for transmission systems. An available real high-impedance earth fault case study is used to check the performance of the fault classification algorithm to classify phase and earth faults.

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