Protection of major transmission lines using travelling-waves
An ultra high speed relay for the protection of long EBY transmission lines is described in this thesis. The need for ultra high speed relays is first discussed. From a brief review of protection based on "impedance" distance algorithms or methods using post fault transients, it is shown that at present, there is no truly ultra high speed relay available for the protection of long EBY transmission lines. The proposed relay operates on the incident fault transient travelling waves received at the relaying point only. After the arrival of the first incident transient at the relaying point all subsequent incident travelling waves are detected using the cross-correlation of the first reflected wave with the incident transient waves. From the amplitudes of the subsequent incident transients, two fault resistance estimates are obtained. These two fault resistance estimates are in agreement only for the subsequent incident wave which is caused by direct reflection from the fault. The fault location can then be determined from the time of arrival of this wave. Additional checks based on the ground mode delay or the line round trip wave amplitude, are incorporated to enhance further the security of the scheme. Good fault discrimination is shown to be possible over a large range of fault resistances for symmetric three phase faults to ground, phase to ground faults and phase to phase faults. Double circuit transmission lines and compensated transmission lines can also be protected. The relay has good noise tolerance and a reasonable bandwidth requirement. A real time implementation of the basic algorithm for an internal phase-a to ground fault shows that an ultra high speed relay response with good accuracy can be achieved using currently available digital hardware.