Development of a Cold Trap-Photoelectron Ionization Time-of-flight Mass Spectrometer for Detection of Trace SF₆ Gas Decomposition Products

SF₆ gas insulated electrical equipment has been widely used in power gird for its fast installation, low space consuming, high reliability and low maintenance work. However, the partial charging inside of the equipment has significant impact on its insulating performance and can even lead to insulation breakdown. The internal fault of SF₆ gas insulated equipment can be found effectively through detection for decomposition products of SF₆ gas. In the early stage of internal fault, the decomposition products of SF₆ gas are usually in very low concentration due to the adsorption process by the adsorbents in the equipment. And these decomposition products with low concentration can be barely detected by the existing in situ detecting technique, which lead to the influence on early diagnosis. In this work, a portable device based on cold trap (CT) combined with photo-electron ionization time-of-flight mass spectrometry (PEI-TOF MS) was designed to provide a promising technique for detection of trace SF₆ gas decomposition such as SO₂ and SO₂F₂, which could be applied for early diagnosis of SF₆ gas insulated equipment. The workflow of enriching detection for trace SF₆ gas decomposition could be generally divided into four steps: preflushing, sampling, injection and aging. The trace amount of SO₂ and SO₂F₂ in SF₆ were enriched by adsorption and desorption process. Carbopack B and Carboxen-1000 were employed as adsorbent in this work. The enrichment ratios for typical decomposition products such as SO₂ and SO₂F₂ were 136 and 49 times, respectively. The detection limits for SO₂ and SO₂F₂ were 0.01 and 0.1 μL/L, respectively. The suitcase CT-PEI-TOF MS has wide application prospect on establishing an earlywarning for early diagnosis of SF₆ gas insulated equipment.