Distributed strain sensing, employing apodized π-phase shifted FBG: Application in power transformer oil breakdown detection

Abstract

Early detection of power transformer oil breakdown can save the transformer and power grid from any potential damages. In this article, a novel distributed strain sensing system based on apodized π-phase-shifted Fiber Bragg Grating (FBG) is presented in order to detect power transformer oil breakdown. The main objective is to detect high-frequency acoustic waves and ultra-small strains resulting from partial discharge (PD) occurrence. To this end, optical fiber under various time and position dependent strain conditions is simulated by employing transfer matrix method. Strain is incorporated in the equations by employing photo-elastic constants. For a single FBG sensor case employing a laser at a fixed wavelength of 1550 nm, it is shown that incorporation of a π phase shift drastically increases the sensitivity, reaching values as high as 60 %/0.1μstrain in reflected light intensity. However, maximum measurable strain is limited to only 0.25 μstrain. Afterwards, a suitable Gaussian apodization function is applied to the FBG which drastically increased the strain measurement range to almost 0.45 μstrain by a complete suppression of sidelobes in the reflected light spectrum. It is shown that the proposed system is capable of detecting high-frequency PD-induced acoustic waves of up to 500 kHz.