## 1 Introduction  
The transition to an industrial structure requiring high power is accelerating due to the Fourth Industrial Revolution and the COVID-19 pandemic. Therefore, the scale of electric facilities is gradually expanding, and accidents and blackouts at power facilities can cause enormous economic losses in a highly power-dependent society. As the number of long-term power facilities has increased, so has interest in evaluating the deterioration state of insulation in major power plants. Underground power transmission cables are essential for supplying electricity generated from thermal power, hydropower, nuclear power, and renewable energy to industrial sites and households. Depending on the insulating material used, these cables are typically classified into oil-filled (OF) cables made from paper and oil and cross-linked polyethylene (XLPE) cables made from XLPE. Compared to OF cables, XLPE cables pose a lower risk of environmental pollution and fire due to leakage. Therefore, XLPE cables have become increasingly popular as underground power transmission cables in recent years. Moreover, XLPE cables are replacing OF cables due to improved efficacy and longevity. XLPE is polyethylene (PE) with a cross-linked structure that is produced by reacting PE with organic peroxide under high pressure to generate free radicals. These radicalscreate cross-links and a network structure in the polymer [1]. XLPE is used as an electrical insulating material due to its hydrolysis resistance, high insulation properties, abrasion resistance, and excellent mechanical properties [2; 3; 4; 5; 6]. In addition, it is used in piping systems for chemical storage, in heating and cooling systems, etc., due to its high extrusion speed and low cost. When XLPE is exposed to the power cable operating environment, deterioration progresses and initial characteristics degrade. The main causes of XLPE cable deterioration are electrical [7], thermal [8], mechanical [9], and environmental factors [10], while the cable itself deteriorates due to aging and abnormal phenomena. Age deterioration is caused by heat generated by conductor resistance, and the typical abnormality is partial discharge (PD), which also generates heat. Therefore, determining the deterioration characteristics of XLPE when exposed to heat is necessary for cable operation and management. Since XLPE is a polymer material, various analytical methods, such as physical, chemical, thermal, and electrical analyses, are used for characterization. Physico-mechanical methods, such as tensile strength and elongation, are generally used as standards for determining the longevity of XLPE insulating materials [11]. Chemical analysis methods include Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), etc., thermal analysis techniques are thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), etc., and electrical analysis methods include breakdown voltage [12; 13]. Many studies have been conducted on the characterization of XLPE cable insulation using these analysis methods. Typically, the characteristics of XLPE insulation were confirmed according to the operating period of a 110 kV power cable [14]. As a result, it was confirmed that the XLPE insulation of cables operating for more than 10 to 15 years was noticeably deteriorated, and mechanical, chemical, and electrical properties were reduced. In addition, research was conducted to accurately determine the degree of cable deterioration through machine learning, such as through principal component analysis (PCA) and artificial neural network (ANN), using characteristic analysis data on deterioration [15]. In order to use artificial intelligence to diagnose the deterioration of power cables in the future, accurate and multifaceted deterioration characterization of XLPE, the main insulating material of XLPE power cables, is required. Accelerated degradation is generally performed to confirm the properties of polymers, including XLPE.