ADVANCEMENTS IN DIGITAL DISTANCE PROTECTION

Abstract

The day is not far off when digital distance protection would be widely used in the protection of power transmission lines for its low costs, higher reliability and better system coordination. Digital distance relays utilize fault distance estimation in making tripping decisions. Therefore, fast and accurate fault distance estimation is of critical importance to digital protection. Although many algorithms have been proposed previously, distance protection still remains a challenge. Unsyncronized distribution systems protection, untransposed lines and parallel lines protection, digital filtering and phase selection have been long-standing puzzles. This thesis is aimed to propose better solutions to these barriers. Two-terminal system instead of single-terminal one is often used to improve the fault location estimation accuracy. However, a two-terminal system needs strict synchronization and failure to do so will result in. very large errors. Algorithms have been proposed to eliminate the errors brought about by unsynchronization. Compared to the existing methods, these methods have great computational advantages and high accuracy. Accuracy and computational efficiency are two essential requirements on distance relaying algorithms. The symmetrical components based method has been well accepted in the past. An improvement to this method has been made to enhance its computational efficiency without compromising any accuracy. A new modal components based method has been designed for the protection of untransposed lines which frustrate the symmetrical components based method. Parallel transmission lines have been extensively utilized in modem power systems to enhance reliability and security for bulk transmission of electrical energy. But the protection for the parallel lines has not been well figured out. A new distance relaying algorithm for parallel lines has been developed in the thesis. It is independent of fault resistance, remote infeed and source impedances and found to be very suitable for parallel lines protection. The rest of the thesis has been dedicated to the introduction and application of a new mathematical tool, i.e., wavelets transform, to the digital distance protection. Novel B-Spline wavelets based digital filters of high accuracy and little delay have been developed and may be used instead of the traditional low-pass digital filters to enhance the relaying speed. Finally, a combined approach to phase selection using fault generated high frequency noises and wavelets transform based feature extraction has been developed. The good time and frequency localization characteristics of wavelets transform make the proposed method more simple and reliable than the previous FFT based methods for phase selection. In conclusion, the thesis has been focused on unsyncronized distribution systems protection, untransposed lines and parallel lines protection, digital filtering and phase selection. Extensive simulation studies have verified that the proposed techniques are promising for digital distance protection.