Studies On New Principles And New Criteria Of High Performance Main Protection Applied To UHVAC Long Transmission Lines

With the rapid development of society and economy,the demand of electric power has been growing continuously.During this period of time,both the total capacity of the power grid and the area covered by power grid keep expanding,and both the capacity and distance of power transmission increase as well.In order to achieve optimal allocation of energy resources across the country,it is urgently needed to resolve the problem of power transmission in terms of long-distance,large-scale,and huge-capacity between energy source centers and load centers.Ultra-High-Voltage(UHV)power transmission is the best solution in respond to the above challenge.UHVAC and UHVDC are two main ways to implement the UHV power transmission,playing different important roles in different application scenarios.UHVAC long transmission lines served as the backbone of UHVAC synchronous power grid,therefore,the safe and reliable operation of UHVAC long transmission lines is of great significance to the safety and stability of the power grid.By this virtue,a series of novel criteria and principles of main protection of UHVAC long transmission line are studied in this thesis.For conventional UHVAC transmission lines,under the premise of selectivity,the new criteria that can further improve the operating speed and sensitivity,together with reliability in view of complex electromagnetic environment,are studied.For UHVAC half-wave-length transmission lines,existing local main protection is no longer applicable.If only relying on the pilot main protection,the line will lose its main protection once the communication channel becomes invalid.Considering the existing study of the half-wave-length backup protection is nearly blank,lose of main protection is a fatal threat to the power grid.Even if communication channel works well,the inherent extra long time delay due to information exchange will also result in an unacceptable time delay of main protection.Therefore,it is urgent to study the new principle of ultra-high-speed main protection only based on local measurement of UHVAC half-wave-length transmission lines.The main work is summarized as follows:For conventional UHVAC transmission lines,aiming at the problems that the action speed of protection based on phasor measurement is restricted by the time window length of the filtering algorithm,together with and the algorithm vulnerability to false data,a new pilot main protection criterion for transmission lines based on waveform similarity is proposed.By comparing the discrete point sets of current sampling values,a waveform similarity-based algorithm named as Hausdorff Distance capable of identifying the difference of currents between both sides of the protected transmission line is used to realize fast identification and removal of the fault.Then,on the foundation of the proposed waveform similarity criterion,Bergeron model is utilized to completely compensate the capacitive current,and an improved waveform similarity based criterion independent of fixed threshold,together with a supplementary CT saturation recognition and unblocking criterion,is designed.By this means,the sensitivity and reliability of protection can be enhanced further on the premise of guaranteeing the action speed.For UHVAC half-wave-length transmission lines,the problem that the action speed of halfwave-length transmission lines pilot main protection is slowered inherent time delay is discussed firstly,and the shortcomings of existing local measurement based main protection suitable for UHVAC half-wave-length transmission lines are analyzed.According to the theory of travelling wave based fault location and the fault propagating feature of half-wavelength transmission line,a novel fault-location-type local main protection principle is proposed.By recognizing the time difference between line modal and zero modal of travelling wave,the fault position can be determined.Thus,most internal fault can be distinguished from external fault of forward direction.This new protection criterion is capable of covering nearly full range of the line.Furthermore,aiming at the problem that the above protection principle cannot detect ungrounded faults and symmetrical faults,an enhanced protection principle is put forward.Indeed,there still exists the difference between arrival times of different frequency components of line modal after propagating through an ultra-long distance,which can be used to implement the protection criterion capable of detecting ungrounded faults and symmetrical faults.Firstly,a wavelet multi-resolution analysis based algorithm is designed to detect the time difference of the arriving times between various frequency-band components contained within the line modal travelling wave,and an improved fault-location-type local main protection principle capable of identifying all kinds of faults is proposed.Finally,a comprehensive main protection scheme combining above two fault-location-type main protection principles suitable for UHVAC half-wave-length transmission lines is proposed,which can realize instantaneous detection and removal of the fault occurring at any point the entire transmission line,by using this scheme the operating speed fast than existing pilot main protections can be achieved.