Study On Fault Mode Recognition Of Transmission Lines Based On Improved Fractal Theory

Statistic data show that 80% of faults in large capacity power transmission system are transient. Although the generic fixed time-limited reclosure can guarantee transient stability in power system and reliability in power transmission, it might be companied with a second breakdown, then, the research of adaptive reclosure are of great significance and urgency. Shunt reactors installed on EHV transmission lines are basically to absorb reactive power, stable operating voltage and suppress secondary arc current. The introduction of shunt reactor could weak the coupling voltage, accelerate the extinguish of the fault arc, then limit application of voltage and electric arc criterion, make determining fault nature in single-phase adaptive reclosing harder. To solve this problem, this paper will start with beat-frequency oscillation.The swift identification of beat frequency oscillation is the core in beat-frequency criterion for adaptive reclosure. After studying the multifractal property of beat frequency phenomenon when transient fault takes place on transmission lines with shunt reactors, it is discovered that beat-frequency is not consistent with the self-similar properties, then Multifractal detrended fluctuation analysis(MFDFA) method can be used to determine fractal parameter and analyze the fractal property of H(q) in different situations. The H(q) curve of terminal voltage in single-phase transient faults satisfies the arctangent distribution while H(q) curve in permanent faults trends to constant. According to the fractal contrast, this paper proposes a new beat-frequency criterion which is based on MFDFA to distinguish transient faults from permanent faults, and it is achieved by defining critical values ratio δ to describe that variance. The criterion has strong anti-noise ability, its validity and reliability are unaffected by fault condition or DC component, and are proved by simulation and measured data.At the same time, that transient fault which has a threat on EHV transmission lines security frequently accompanied by strong beat-frequency is an universal viewpoint, but weak beat-frequency phenomenon has been discovered already by analysis that oscillatory condition may cause misjudgment of criterion which could cause serious consequences in power network. This paper proposes a completely beat-frequency detection method which could be used to extract nonlinear characteristics from fault current recorded in shunt reactor to solve this indicated problem. Specifically, fixing best fractional Fourier transform domain(FrFT) firstly, in which nonlinear characteristics can be enlarged far better, then calculating the extended Hurst index of current waveform in that domain as a nonlinear quantitative criterion for distinguishing transient fault from permanent fault. This method has excellent immunity to noises, and can obtain reliable detection result fleetly no matter whether strong or weak beat-frequency occurs in fault waveform after single-phase transient fault. The method has been proved to be effective and reliable by numerous EMPT simulations.