## 1 Introduction  
Condition monitoring and fault diagnosis is crucial in ensuring the long-term safe and stable operation of power transformers. Latent faults in the power transformer in early stages can be found by monitoring the operation state of the insulation system, which mainly consists of oil and paper [1]. The occurrence of incipient thermal or discharge faults cause the C-H and C-C bond cleavage of oil-paper insulation system, leading to the formation of various dissolved gases, such as H\({}_{2}\), CH\({}_{4}\), C\({}_{2}\)H\({}_{6}\), C\({}_{2}\)H\({}_{4}\) and C\({}_{2}\)H\({}_{2}\)[2]. These gases turn into insulated defects, which reduce the insulation strength of the oil-paper insulation system and develop into permanent faults. The composition and content of these dissolved gases must be detected in order to identify types and severity, as well as to accurately assess the insulation status of power transformer [3; 4; 5]. In addition, Acetylene (C\({}_{2}\)H\({}_{2}\)) is a colorless combustible hydrocarbon. It is the simplest alkyne, with a distinctive odor. It is widely used as a fuel and in many other industrial applications. For example, it is an important raw material for organic synthesis such as in 2-chloro-1,3-butadiene; in lighting; in welding and metal cutting; and in dry-cell batteries, etc. However, it is extremely unstable in its pure form and it is easy to incur fire or explosive accidents when it is liquefied, compressed, heated or mixed with air. Thus, the safety monitoring of C\({}_{2}\)H\({}_{2}\) is vital during its production and handling. Use of gas sensors is one of the most widely and effectively used methods to detect the composition and content of C\({}_{2}\)H\({}_{2}\) gas [6]. Metal oxides (MOs) that change their electrical conductance while exposed to ambient gas can be utilized as gas sensing materials [7]. Many MOs are applicable to detect the reducing and oxidizing gases, for instance, the conductance of CuO, NiO, In\({}_{2}\)O\({}_{3}\), WO\({}_{3}\), TiO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), etc. show different response to gases. However, ZnO and SnO\({}_{2}\) are the main gas sensing materials for commercial applications and experimental research [8]. Pure SnO\({}_{2}\) is sensitive to all the dissolved gases, but it still has some shortcomings, such as high heating temperature, unsatisfied response and poor selectivity [9].Research on how to efficiently improve gas sensing properties is focused on surface modification [10], doping [11] and morphology or size control [12; 13]. Recently, graphene has attracted much attention because of its two-dimensional nanostructure with extremely high specific area and electron mobility. The graphene sensor is expected to exhibit excellent gas sensing performance [14]. However, pure graphene is composed of periodic, closely-packed carbon atoms with a benzene ring structure, which is unfavorable for the adsorption of gas molecules. In this regard, the graphene derivative, which is decorated with oxygen functional groups to increase the adsorption site for gas molecules, was named reduced graphene oxide (rGO) [15]. In particular, many groups have reported the gas sensing properties of rGO-based materials, loaded with MOs nanoparticles, with respect to various gases. The NiO-rGO nanocomposite exhibited high response, good repeatability and acceptable selectivity toward CH\({}_{4}\) gas detection [16]. ZnO-rGO exhibited significantly higher responses than pure ZnO with respect to various gases, including O\({}_{2}\), SO\({}_{2}\), NO\({}_{2}\), CO, C\({}_{6}\)H\({}_{6}\), and C\({}_{2}\)H\({}_{5}\)OH [17]. The MO-rGO (SnO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\) and NiO) composite porous films exhibited much better sensing performances than those of the corresponding pure oxide films in detecting C\({}_{2}\)H\({}_{5}\)OH gas [18]. TiO\({}_{2}\)-rGO shows good sensitivity and excellent selectivity to different concentrations of NH\({}_{3}\) from 5 ppm to 50 ppm at room temperature [19].