Research On Fault Detection And Location Of Transmission Line Based On Wavelet Theory

As transmission lines are responsible for the power delivery, they are lifeblood of the entire power grid. However, in the poor operating environment and the vast distribution area, transmission lines are subject to faults. Once a fault of the transmission lines occurs, not only the agricultural and industrial production and people's daily life are affected, but also power system safety and stable operation are also imperiled. Therefore, if fault detection and location are performed timely and reliablely, the duration of finding fault can be shorten, the fault can be handled early, power restoration can be assured quickly, and the weak link of transmission lines are founded timely with low economic loss due to transmission lines failure.Firstly, this dissertation introduces the background, present situation, the purpose and significance of fault detection and location of the transmission lines briefly, and describes fruits achieved and problems present at home and abroad in detail, and proposes the main research contents. The relevant researches show that the transient traveling wave is a signal with mutation and singularity when the transmission line fault cccuring, wavelet transform has feature of analyzing partial signals in time domain and frequency domain, especially being suitable for mutation and non-stationary signals. The principle of multi-resolution analysis and Mallat algorithm theory are studied, and with the combination of wavelet transform singularity detection theory, the integration of theories above is applied in feature analysis of transient traveling wave of transmission line fault. The analysis of the equivalent circuit of transmission line fault shows that transmission line fault network can be divided into the non-fault network and additional fault network, so the analysis of transmission line fault network is regarded as the additional fault network analysis, and the transient process of short-circuit in three-phase power supply system is discussed. Meanwhile, the dissertation describes the basic traveling wave theory of lossless single-phase transmission lines and three-phase transmission lines, and presents the mathematical expression of the wave equation. According to refraction and reflection principle of traveling wave, the faulty traveling wave is analyzed by the grid method. As the electromagnetic coupling between three-phase transmission lines exist so closely that it becomes very difficult to solve the wave equation of three-phase transmission lines. Therefore, by using Karenbauer transform, three-phase power system with existence of the electromagnetic coupling between them in phase domain is transformed into that of no electromagnetic coupling, obtaining the zero-modal current and line-modal current. With the earth as current loop, the zero-modal current is large in impedance, and the traveling wave runs slowly, so the line-modal current signal caused by the transmission line fault is processed with wavelet transform. After that, the location methods of transmission line fault are summarized. Aiming at the inaccurate fault location caused by the velocity uncertainty, a fault location algorithm with no effect of travelling wave velocity is deduced. In the actual transmission lines, since a lot of noise interferences exist, the fault moment can not be extracted so accurately that the accuracy of fault location is affected. Considering shortcomings above, the dissertation proposes the threshold algorithm based on wavelet transform coefficient estimation. The algorithm can decompose fault signals with noises into multi-level wavelets, and make coefficients squared of the wavelet transform in fine scale, therefore the features of fault signals in the singular point are highlighted.Finally, the model of actual transmission lines are built in the Matlab, and in the case of noise and no noise, the fault detection and location of transmission lines are simulated. Theoretical analysis and simulation data indicate that the location algorithm and denoising algorithm selected can reduce the location error effectively and make the fault location accurately.