A Novel Adaptive Distance Protection Scheme Based on Variable Data Window

摘要

In recent years, with increasing voltage level and transmission capacity of power grid, the requirement for fault clearing time has become higher and higher in order to safeguard the system stability. Though pilot protection can fast trip the whole line fault, it is hard to reduce the trip time less than one frequency period under the consideration for reliability and the restriction of the rate of data exchange between two sides. Therefore, to fast clear the close-in fault, we still need to resort to distance protection which is operated based on single-end electric quantity. But traditional distance protection which is based on stable state quantity and integral period algorithm can not trip rapidly in some cases [1]. In this context, how to improve the trip speed of distance protection without impairing system safety has become a hot issue in research. Researchers have made extensive study on ultra-speed distance protection. In order to break through the constraint placed by stable state quantity, researchers proposed to utilize the fault location information in fault transient component to realize distance protection. A typical example is traveling wave distance protection, which is capable of tripping in 5ms [2]. The key problem of traveling wave distance protection is to accurately identify the first backward traveling wave reflected from fault point and to determine the wave's precise arrival time at protective relay. To tackle this key problem, a series of algorithms are introduced into study, such as correlation analysis, maximum likelihood estimation, wavelet transformation, morphology, etc. [3, 4]. However, all of these algorithms demand high sampling rate, which is hard to achieve under current hardware level. What's more, the traveling wave distance protection is non-directional, and due to the effect of the bus structure on both ends of the line, the traveling wave distance protection has some difficulty in identifying the reflected wave from fault point, contralateral bus, and transmitted wave from dorsal bus. By now most of the study on traveling wave distance protection and other protection based on transient component is theoretical study and numerical simulation, which has a long way from application. Another effective way to speed up distance protection is to retain the use of stable state quantity and to introduce superior variable data window algorithm to form the inverse time operation characteristics of zone 1 of distance protection. As for variable data window algorithms, the estimation precision of these algorithms depends on the length of data window and the magnitude of transient components [5]. Generally speaking, the longer the data window and the smaller the transient component is, the higher the accuracy will be; on the contrary, the shorter the data window and the larger the transient component is, the lower the accuracy will be. In order to prevent overriding under extreme conditions, the inverse time operation characteristics of zone 1 of distance protection needs to take step mode, this means when the data window is short, the scope of protection will be comparatively small; when the data window is long, the scope of protection will extend. However, with this mode, part of the line, particularly near the end of protected line will see no obvious improvement in trip speed.