| With the accelerating pace of UHV power grid in our country, the proportion of UHV transmission lines will be increasing in the future. It has the advantages of large transmission capacity, long transmission distance and it can effectively reduce the transmission losses. UHV transmission line distributed capacitance is relatively large because of the use of split conductors. At the same time line is arranged in parallel reactor. These measures make UHV transmission line transient process very serious These characteristics brought a lot of new problems to the original protection based on the amount of frequency, but the transient fault protection based on transient signals will not be affected, and the speed of protection can be evidently improved. This method has become one of the research focuses of power system relay protection in recent years. When UHV transmission line fault, a lot of important information was contained with the fault current and voltage. It is very important to select the suitable signal processing method for the protection of UHV transmission lines and determine whether protection can be quickly and reliably remove the fault line.In this paper, the transient process of UHV AC transmission line fault is completely analyzed through calculation and simulation, and the influence of system components on the transient current signal is investigated. Commonly used fault signal processing algorithms of UHV transmission lines are analyzed and compared, and aiming at the existing problems, put forward to adopt Hilbert-Huang transform(HHT) applied to nonlinear signal analysis to extract the UHV transmission line fault transient signal analysis. EMD in HHT transform, which has the problem of mode mixing and the large amount of data computation, combined with the multi resolution in the wavelet transform, use a multi-resolution based MEMD decomposition method to solve, and apply to the protection.After combining the theoretical analysis of the previous chapters of the paper, by applying the multi-resolution EMD of HHT transformation to obtain the spectral density ratio of malfunction signal in the high frequency components and low frequency components, we constitute a new UHV transient current protection algorithm based on marginal spectrum energy density. Finally, protection criterion is verified through a large number of simulation results that shows the influence of shunt reactor, fault type, fault distance and fault time on the protection criteria. The results show that protection can be reliable operation. |
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