| With the increasing power demand in east China economic developed area, it isnecessary to construct ultra high voltage direct current (UHVDC) transmissionengineering and implement large capacity, long distance and―transmit power from thewest to the east‖strategy. UHVDC transmission line has higher voltage, farther distance,and usually crosses large mountains as well as complex environment, so it has muchprobability to suffer lightning strike and grounding fault. In order to improveprotection action reliability of UHVDC transmission line, it’s important to quicklyfind fault location and to ensure the safty and stable operation of power system. Takingthis issues of UHVDC transmission line fault mechanism, transient protection newprinciple, lightning strike identification and fault location as a cule, this dissertationmade a deep research for providing technology support to enchance the performance ofUHVDC transmission line protection and fault location.In order to find the reason that the UHVDC transmission line protection may notact correctly at present. This dissertation discusses the fault mechanism of the travelingwave process, transient process of control system regulation and recovery process ofUHVDC transmission line. The results show that: High resistance grounding fault at theend of the transmission line is the main reason for the traveling wave protection failure.With the increase of transition resistance, the electric parameters, the variation and thevelocity of variation of faulted voltage wave become decrease, and response of thecontrol system also becomes slow. The increase of the fault distance affects the electricparameters and response time of the control system. To avoid AC system failure, delayof the differential protection and the regulation of control system，valve protection mayact earlier than differential protection to block fault polar. Thus the differentialprotection will lost the role of the backup protection.In order to solve the problem that UHVDC transmission line can’t act correctly, thisdissertation presents a new method based on one end electrical transient signal andprovides three criterions to correctly identify external or internal fault, relay starting andpolarity selsction. The new mehtod is based on: the physical boundary bombined withsmoothing reactor and DC filter has stop-band characteristic to the high-frequency ofUHVDC traveling wave. When external fault occurred, the energy of high-frequencytransient voltage will be weakened when it travels through boundary and is small at thepoint of relay installation. While internal fault occurred, the energy of high-frequency transient voltage at the point of relay installation is much higher. The identificationcriterion of internal and external fault is based on the differences. The criterion ofstarting is used that wavelet transformation modulus maximum increases with the levelincreasing. Ratio between the positive and negnetive polarity wave area is used to selectfault line. Simulation results show: the new method has good ability to stand highresistance.In order to make traveling wave protection and transient protection to practical, thisdissertation proposes a new transient criterion to identify the lightning strikes on theUHVDC transmission line. According to the difference between lightning strikes andgrounding fault, lightning disturbance identification criterion is formed. The newcriterion is based on: when UHVDC transmission line suffer lightning strike whithoutfault, the correlation degree of the voltage traveling-wave and the direct voltage is large.When it is lightning strike with fault or grounding fault, the voltage’s value of faultpolar is cut suddenly in fault point and the correlation degree is small. There are morehigh-frequency components of the lightning strike with fault than the grounding fault,and their low frequency components is approximate equal. According to this, it caneffectively identify the lightning strike with fault and grounding fault via comparing theratio of the high frequency energy and low frequency energy in5ms-time window. Thesimulation results show that the criterion can identify lightning type correctly, quicklyand realiably.To be able to locate fault point quickly and clear the fault in time after the fault ofUHVDC power transmission line. This dissertation puts forward a new fault locationmethod of UHVDC transmission line using energy spectrum identification. This methodis based on the following points: The physical boundary of UHVDC system isopen-circuit to the high frequency signal making the the traveling wave reflects betweenfault point and boundary at the end of a transmission line. These results in the wavemeasured at relay being a periodic signal, and thus the nature frequency is formed. Thenature frequency of fault traveling wave is related to the fault distance. Energy spectrumof fault transient signal has high density around the main nature frequency. The faulttransient energy includes much fault distance information. Energy spectrum of one endline mode voltage change with the fault location conditions. So ANN can be used to fitthe relationship between the energy spectrums of line mode voltage to locate fault point.The reliability of fault location is improved by changing the point characteristic ofnature frequency into band characteristic of wavelet energy. |
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