# Development of an Online Detection Setup for Dissolved Gas in Transformer Insulating Oil
## 1 Introduction  
Transformers are the central electrical component of power grids, providing a reliable power supply, and their stable operation is of great importance to grid safety [1, 2]. Transformer insulating oil is composed of paraffin with 20 or more carbon molecules, which play the roles of cooling, insulating, and arc extinguishing during transformer operation. When alkanes are subjected to the energy levels generated by the transformer, their chemical bonds are broken, resulting in the formation of corresponding gases (methane, ethane, ethylene, acetylene, carbon monoxide, and carbon dioxide) [3]. The gases are fully dissolved in the insulating oil in a confined space. Therefore, the status of the transformer can be assessed by the type and content of the dissolved gases [4]. The development of transformer maintenance plans based on dissolved gas analysis (DGA) in transformer insulating oil is currently one of the main methods in the power industry for improving the reliability of the grid power supply [5]. As a result, maintenance decisions in the grid can be optimized by obtaining more timely information on transformer operation through the online detection of dissolved gases in transformer oil. The gases that reflect information regarding the transformer's condition are those dissolved in the insulating oil. Consequently, online detection of dissolved gases differs from conventional online gas detection. Both oil-gas separation and gas detection need to be considered. Oil-gas separation methods are divided into two main types: Vacuum degassing and dissolved equilibrium. Vacuum degassing requires the use of equipmentwith a high vacuum level. Therefore, the dissolved equilibrium method is the most commonly used separation method [6]. Membrane separation is the main method used in online oil-gas separation equipment. It has a simple structure but requires a long degassing time [7]. This degassing method does not provide information regarding the transformer in real time. The headspace degassing process, a type of dissolved equilibrium method, has the advantages of fast degassing and high repeatability. This method is meant to be used by an online testing device. Volatile organic compounds (VOCs) detection conventional methods include catalytic combustion [8], semiconductor gas sensor [9], thermal conductivity detection [10], electrochemical detection [11], and gas chromatography [12]. These methods have certain limitations, especially in multicomponent gas detection. With the development of VOCs detection technology, some scholars use various types of sensors for multi-component gas detection [13] in many fields, such as purified essential oil detection in commercial plant products [14], alcoholic beverages and perfume quality control [15], bakery food product classification [16], and coffee processing [17]. However, in the field of DGA real-time detection, it is necessary to use the high anti-interference, fast response time and high-performance detection method that does not consume the test samples [18]. Therefore, optical detection technology can effectively satisfy the above requirements. [PERSON], [PERSON], Mt, [PERSON] et al. [19; 20] used UV-visible and FTIR spectroscopy to measure dissolved gases in oil. [PERSON], [PERSON] et al. [21] used Raman spectroscopy to evaluate the aging state of transformer insulation oil. [PERSON], [PERSON]. et al. [22] used Fourier spectroscopy to analyze oil samples and a fuzzy algorithm to estimate the concentration of dissolved gas. However, these spectroscopic devices are complicated and unsuitable for field applications. As a result, the photoacoustic spectroscopy (PAS) technique has been used for the online detection of dissolved gases in transformer oil [3; 23]. The detection sensitivity of the photoacoustic phenomenon is heavily influenced by background noise, which cannot be eliminated in field applications. To reduce the effect of field noise on optical detection sensitivity, absorption spectroscopy techniques are used.