Ultra-low concentration detection of dissolved C2H2 in oil using fixed-frequency RF-assisted calibration-free wavelength modulation spectroscopy

Abstract

The monitoring sensitivity of dissolved C2H2 in transformer oil is crucial to ensuring the safe operation of electric power systems. A highly sensitive sensor system based on fixed-frequency radio frequency assisted calibration-free wavelength modulation spectroscopy (FF-RF-WMS-2f/1f) for dissolved C2H2 in transformer oil was developed and evaluated. Lowering the headspace sampling pressure to 0.1 atm effectively increased the outgassing ratio and detection signal of dissolved C2H2, while also reducing background interference. By employing a novel fixed-frequency RF-assisted modulation technique, we effectively suppressed the amplitude and influence of optical fringes and improved the system’s linear fitting R-squared value for gas-phase concentration from 0.9997 to 0.9999. Furthermore, the minimum detection limit (MDL), as calculated by Allan deviation, was reduced from 8.58 ppb to 4.36 ppb, while the optimal integration time increased slightly from 62 s to 79 s. The equivalent MDL for dissolved C2H2 reached 0.9 ppb. The comparative experiments showed that the error was less than 0.2 ppm or 8 %, the relative standard deviations (RSDs) of continuous measurement were less than 3 %, and the RSD at ultra-low concentration was less than 8 %. The experiments verified the high sensitivity and accuracy of the sensor system, making it suitable for ultra-low concentration dissolved C2H2 detection in transformer insulating oil.