Applied Studies Of Self-adaptive Principles And Modern Digital Signal Processing Techniques On Transmission Line Protections

The self-adaptive protection aims at achieving the best performance in the event of changes of power system operation modes. This paper introduces the basic concept of adaptive protection, as well as the issues of classification, the meanings of research and the applications in power systems. Besides, the applications of adaptive technique in the field of phase selector, superimposed distance protection, as well as auto-reclosure are dissucssed in details.Mordern signal processing technique, as a powerful tool to analyze the fault information of the power systems, has been widely applied in various fields. And it also provides a powerful means for self-adptive technology to be used in protective relaying. Mathematical morphology (MM) is one of the latest techniques in advanced signal processing fields. MM is developed from set theory and integral geometry, and is widely used in geometrical analysis and description. It is concerned with the shape of a waveform in time domain rather than frequency. Morphological filters are nonlinear signal transformation that locally modifies geometrical features of signals and image objects. Morphological calculation needs only the comparison and addition operations without multiplication, the computational complexity, therefore, is much smaller than conventional digital filter algorithms. Thus, it has the excellent foreground in protection and even the other areas of power system.A novel superimposed phase selector aiming at improving the performance of phase selector in the event of a wide range change of power system operation mode is presented in this dissertation. In this new scheme, the fault component of phase-phase voltage difference is polarized with the fault component of the third phase current. With this arrangement, this element not only adapts to various operating modes of power systems, but is superior to the traditional superimposed phase selectors. For instance, the superimposed phase-phase current difference based phase selector or the superimposed phase-phase voltage difference based phase selector, in terms of selectivity and sensitivity. The advantages of this phase selector can be highlighted especially in the case of the positive-sequence impedance not being equal to the negative-sequence impedance. In addition, this phase selector can reliably identify the single-phase-earth fault and the phase-to-phase fault. The results of EMTDC simulation tests show that the outstanding phase selection performance can be achieved even when the negative-sequence system impedance is extremely unequal to the positive-sequence system impedance. Besides, this phase selector has higher reliability compared to the traditional superimposed based schemes.Aiming at the defects of the conventional superimposed distance relay, for instance, the mal-operation resulting from an external earth fault occurring while the transmission system operates at large power angle, two novel self-adaptive superimposed distance protection scheme is proposed. Utilizing information of the wide area phasor measurement units (PMU), the two schemes can adjust the operation threshold dynamically, depending on the real-time parameter measurement of two equivalent power systems. In the first scheme, the boundary conditions of mal-operation of previous criteria are deduced in theory. Then, a new criterion to swithch the threshold value depending onα=90°is put forward. In the second scheme, the accurate threshold value for operation is caculated in theory, is used to adjust the setting in real time. By virtue of this arrangement, the reliability and sensitivity of the superimposed distance protection of Zone I are improved. The effectiveness of the new criterion is verified with the results of ATP simulations. Besides, the work of this paper also provides a new path to introduce the PMU technique into the protective relaying.A new integrated directional relaying principle for transmission line and busbar is presented. This novel directional relay, which integrates the protective functionality of line directional protection and busbar protection, is implemented with the polarity comparison of the current travelling waves. The function of line directional protection of the integrated directional relay (IDR), which is actually the protective decision of IDR, is defined as the virtual directional protection (VDP). Using morphological gradient to characterize the initial current travelling wave into spike signal the direction of a fault can be determined by the polarity comparison of the formulated spike signals. A busbar fault can be detected if all the compared spike signals have the same polarities. Otherwise, a forward fault will be identified by the VDP of one of the transmission lines connected to this busbar. In addition, the integrated directional relay is used to consist of the duplicated main protection scheme combined with the classical directional protection, classical distance protection and differential protection of busbar. By virtue of the individual protection principles and the measurements, the proposed duplicated protection scheme is promising to obtain higher reliability compared with the conventional schemes.At present, most self-adaptive auto-reclosure schemes are dependent on the high-frequence power energy information sampled from the transmission lines to distinguish between the instant faults and the permanent fault. Aiming at the defects of the conventional schemes, for instance, the single-window transient energy ratio based scheme is difficult to identify the fault using the single threshold value and the high-frequency noise can not effectively decrease by the conventional filter, two individual criteria of dual-window transient energy ratio (ER) are proposed. This ER approaches to 1 during the steady state whereas it increases greatly around the time instants at which the signal appears the singularity. Therefore, the setting of the criterion is easy. To effectively extract the high-frequency energy of the modal information, two different morphological algorithms (GMMG and COOCG) are put forward. EMTP based simulation results show that the GMMG and COOCG based auto-reclosure scheme improves the reliability of discrimination and is promising to be used in the real power systems.All the theoretical innovations and application studies are summarized in Chapter 8, where a few prospective issues about self-adaptive technology and MM worthy studying are also proposed.