## 1 Introduction  
An electrical power system is generally considered as a large network that connects power plants to customers; it consists of electricity generators and electrical power transmission and distribution networks. One of the most important components in electric power systems is insulators for high-voltage power cables. Cross-linked polyethylene (XLPE), which is a form of polyethylene with crosslinks, is widely used as an insulation material because of its excellent advantages, including high resistance against weather, moisture, and temperature, high mechanical strength, and remarkable electrical properties [1, 2].  
All dielectrics, including XLPE, undergo several alterations in their physical or chemical properties as a result of internal or external environmental factors, e.g., mechanical constraints, humidity, temperature, and partial discharges. Aging can lead to the dielectric breakdown of power cables with XLPE insulation and increase the instability of the electric power system [3, 4, 5, 6]. Therefore, the study of the aging of XLPE is becoming increasingly important. In the macroscopic regime, measurements of space charge, insulation resistance, and temperature have been applied for detecting the characteristics of the aging of XLPE, whereas in the microscopic regime, X-ray diffraction has been typically used to characterize XLPE insulation [7]. Above all, it is important to study the influence of defects on the aging phenomenon and the degradation of the insulating performance. The partial discharges induced by defects in the XLPE insulation usually reduce insulation performance in the long term and can act as a factor that further impedes the stability and safety of the overall electric power system. Measurements of partial discharges have been used in one important way to detect defects in XLPE insulation [8].  
Recently, terahertz (THz) spectroscopy and imaging techniques have attracted attention as new methods for studying insulation materials, as well as for realizing nondestructive testing and inspection [9; 10; 11; 12; 13; 14]. In particular, the photon energy of THz waves, e.g., 4.1 meV at 1 THz, corresponds to the energy range of intermolecular interactions, lattice vibration, and molecular motions such as vibration and rotation. This property enables the investigation of aging effects in XLPE using THz time-domain spectroscopy [15; 16; 17; 18]. More importantly, THz techniques can be applied to detect defects inside XLPE insulation. Defects such as voids and conducting particles in XLPE insulation can lead to strong and inhomogeneous electric fields around the defects, internal discharges such as treeing discharge, and acceleration of the aging process in polymeric insulated power cables, in order. Detecting the defects using an internal nondestructive examination method is thus particularly important. In addition, the advantage of continuous-wave (CW)-THz imaging systems is that they are able to work contactless and target insulators with extremely low electrical conductivity. Therefore, THz technologies can provide testing methods safer than existing methods that need to get the electrodes in contact such as electrical impedance tomography [19], or cannot be used for detecting defects in power cables such as Eddy-current testing [20]. Furthermore, the THz imaging system is expected to be an easy and simple testing method, so it does not require any careful attention by experienced technicians, unlike ultrasonic testing [21]. In this paper, we used a CW THz imaging system as a nondestructive examination tool for detecting void defects inside XLPE insulation.