Fast Protection Based On Full Fault Information For AC Transmission Lines In UHV Grid

In order to meet the sustained and rapid growth of power demand,the world's largest and most complex UHV AC/DC hybrid power grid has been formed in China,which puts forward higher requirements for the protection equipments as the first line of defense to ensure the operation security of the power grid.Fast fault clearance can enhance the transient stability of interconnected large power grids and make full use of the super large power transmission capacity of UHV lines.Fast removal of AC line faults on the inverter side of HVDC transmission project can also reduce or avoid the occurrence of continuous commutation failure.Power-frequency based protection(PBP)has high reliability but slow operation speed,while transient based protection(TBP)possesses fast operation speed but low reliability.In view of the current situation that the operation speed of PBP is difficult to improve and the reliability of TBP is poor,this dissertation makes a thorough exploration from two aspects.(a)The possibility utilizing the primary equipments unique to UHV power grid to form the new line boundary,and the new line boundary characteristics can be utilized to improve the fault discriminating ability of non-unit ultra-high-speed boundary protection.(b)The possibility utilizing full fault information from time and space dimensions and the protection scheme with high-speed and high-reliability can be realized.The major contributions of this dissertation are as follows.(1)Full fault information of transmission line and its effective extraction method are studied deeply.Mastering the formation,types and characteristics of full fault information comprehensively,the fault information is divided into transient and power frequency quantities,and the basic requirements for extracting full fault information are proposed.Through the study on wavelet transform and mathematical morphology in depth,a series of processing tools which are suitable for fault transient extraction are presented.a)Discrete wavelet transform has the characteristics of frequency division,which is suitable to obtain the wide specific frequency band of fault traveling waves.b)Synchronized wavelet transform achieves high time-frequency resolution by compressing in frequency domain,which can be used to extract narrow frequency band information of fault traveling waves.c)Morphological gradient can be used to detect the fault occurrence,accurately reflect the polarity of the initial traveling wave,and label the time when the initial traveling wave reaches the protection installation position.d)Morphological filter can remove high frequency noise while keeping the initial traveling wave characteristics unchanged.(2)Boundary characteristics of AC lines in UHV grid are analyzed in detail.The boundary of UHV AC lines consists of busbar stray capacitance,shunt reactor and/or series capacitor.The boundary of AC line on the inverting side of UHVDC project consists of busbar stray capacitance,AC filter,reactive compensation capacitor,DC filter and smoothing reactor.Based on Peterson principle,the natural frequency characteristics of the AC line boundary in UHV grid are obtained.UHV AC line boundary imposes a significant attenuation on the low-frequency components and the high-frequency components,and thus is similar to a band-pass filter.AC line boundary on the inverting side attenuates the high-frequency components and the characteristic frequency components remarkably.The above boundary characteristics provide a theoretical basis for novel non-unit ultra-high-speed boundary protection.(3)A novel non-unit ultra-high-speed boundary protection is proposed.According to the inherent frequency characteristics of the line boundaries in UHV grid,novel boundary protection principles utilizing directional traveling waves are proposed.The post-fault time range is quantitatively given to ensure the validity of protection principle.For UHV AC lines,the forward internal and external faults are discriminated according to the product of high frequency energy and low frequency energy of modal backward traveling waves.Further the discrete wavelet transform based protection algorithm is established.For AC lines on the inverting side,the forward internal and external faults are distinguished by the product of high frequency energy and characteristic frequency energy of modal backward traveling waves,and synchronized wavelet transform with better time-frequency resolution is applied in boundary element algorithm.In order to improve the right discriminating rate for internal faults effectively,the adaptive setting method in terms of fault type is presented.Simulation results show that the boundary protection possesses stable performance under various fault situations,and the operation time is less than 2 ms,which is appropriate as the non-unit ultra-high-speed protection element.(4)The full fault-information fast protection of AC lines in UHV grid with both high speed and high reliability is proposed.Through the time-sharing utilization of the fault information existing in the whole fault process,the fault information fusion is conducted at three levels,and the fourfold protection defense line is constructed within post-fault 25 ms.Full fault-information fast protection schemes for UHV AC lines and inverting-side AC lines are proposed respectively.This protection scheme includes transient starting element,non-unit ultra-high-speed boundary element,unit high-speed pilot protection elements(polarity element,distance element and directional element),faulted phase selection element,and the coordination of PBP principles.Directional element,faulted phase selection element and boundary element achieve the non-unit TBP function,which distinguishes most forward internal and external faults less than 2 ms after fault occurrence.Unit high-speed pilot protection elements realize unit TBP function,which makes full use of fault polarity,distance and direction information,and can protect the entire line in less than 5 ms,while maintaining a high reliability level because of the use of "selecting 2 from 3" logic.Mathematical morphology and wavelet transform are well applied in protection algorithms.The coordination of TBP and PBP complements each other and achieves a balance between operating speed and reliability.Simulation results show that the proposed protection scheme is feasible and can obviously enhance the line protection performance.