Fault Analysis And Its Influence On Relay Protection Of Transmission Lines On Same Tower

As an effective transmission mode, the transmission lines on the same tower have beenwidely adopted in the power grid of China because of its efficiency in land use, highertransmission capacity and lower construction cost. Compared with other transmission modes,this transmission mode features strong zero-sequence mutual inductance and great possibilityof cross-line fault, which in turn demands more effective and accurate relay protection. Bystudying the fault characteristics of transmission lines on the same tower under differentoperating conditions of power system and transmission lines, this dissertation sets out toexamine the influence of the zero-sequence mutual inductance and the cross-line fault on theperformance of faulty phase selection element, distance protection, zero sequence currentprotection, pilot zero sequence directional element et al, assesses the operation characteristicsof relay protection from different viewpoints. Based on the above analysis, this dissertationpresents feasible suggestions to improve the performance of relay protection of transmissionlines on the same tower. The main research works and results are listed as following:1. By analyzing the fault characteristics of transmission lines on the same tower, a typicalfault analysis model has been built, which is used in this paper for systematic research of thefeatures of zero-sequence mutual inductance and cross-line fault and their influences on theperformance of relay protection. On the basis of the analyses, the countermeasures on theconfiguration and setting of relay protection have been put forward as well.2. As one of the inherently features of transmission line on same tower, the zero-sequencemutual inductance influence on fault characteristics and relay protection has been studied.According to the analysis, the conception of magnetically-strong and electrically-weakconnected power network has been proposed in this dissertation. By introducing the concept of“electromagnetic competition” to the fault characteristic analysis, the criterion ofmagnetically-strong and electrically-weak detection is put forward to study the influence ofzero-sequence mutual inductance on the zero sequence directional protection. Moreover, anovel method has also been presented, which utilizes the efficiency factor to analyze thezero-sequence mutual inductance influence on the performance of relay protection. 3. In order to improve the performance of relay protection used on transmission lines onthe same tower, the characteristic of cross-line fault and its influence on relay protection,especially the different-phase cross-line grounding fault, has been studied. By analyzing theinfluence of cross-line fault on the performance of various types of relay protection elementssuch as the pilot protection, the distance protection, the phase selection element etc, thisdissertation draws practical conclusions on the methods to improve the sensitivity, selectivityand reliability of the relay protection, especially the operation border demarcation under thecircumstance of magnetically-strong and electrically-weak system.4. Based on the collected data in power system operation, the inherent defects of existingrelay protection methods used in the transmission line on same tower have been analyzed.Meanwhile, this paper puts forward several precautionary measures and solutions to the relayprotection suitable for the transmission line on same tower and discusses novel configurationand setting schemes of the relay protection.5. By utilizing the practical relay protection devices and the dynamic simulation of RTDS(Real Time Digital Simulation), the effectiveness and reliability of the proposed methods andmeasures of relay protection used on the transmission lines on same tower are examined andvalidated.