In-situ detection of dissolved C2H2/CH4 with frequency-division-multiplexed fiber-optic photoacoustic sensor

Abstract

An in-situ detection system of dissolved C₂H₂/CH₄ with a frequency division-multiplexed fiber-optic photoacoustic (PA) sensor (FOPAS) is designed for diagnosing failures of large power transformers. The system relies on a fluorinated ethylene-propylene (FEP) membrane to extract gases and an all-optical gas sensing element, which has the advantages of non-consumption of oil, anti-electromagnetic interference and passive operation. The oil-gas separation unit and the PA excitation-detection unit are closely integrated into an independent system, communicating with a dual-frequency demodulator through two optical fibers. Two lasers emitting at 1532.83 nm and 1650.91 nm operate simultaneously, exciting PA signals of C₂H₂ and CH₄, respectively. The modulation frequencies of the two lasers are 1486 Hz and 1490 Hz, and twice the frequencies fall within the resonant frequency band of the fiber-optic microphone. A custom-designed dual-channel digital lock-in amplifier is embedded in the demodulator to avoid crosstalk between frequencies, which achieves dual-component synchronous detection. The experimental results show that the temperature increase can promote oil-gas separation. The system can reach equilibrium within 2 h at 60 ℃. The minimum detectable concentrations of dissolved C₂H₂ and CH₄ are both about 0.1 μL·L⁻¹, which meet the detection requirements of dissolved gases in transformer oil. The system has the potential for real-time monitoring of dissolved C₂H₂/CH₄. The excellent detection performance provides technical support for the more accurate real-time condition monitoring and early fault warning of large power transformers.