## 1 Introduction  
With the ever-increasing power demand as a result of modern industrial development, high-voltage transmission lines are required to transmit large amounts of electricity; consequently, this raises the demand for high levels of insulation for power lines. Moreover, although composite insulators have been developed in recent years, porcelain insulators remain in use in extremely high-voltage transmission applications in Korea. Porcelain insulators are devices for mechanically securing and electrically isolating power lines in live transmission towers. They perform an important function in determining the reliability of transmission lines, as well as ensuring that insulation intervals are provided between transmission towers and transmission lines [1]. Of the total 1,223,538 porcelain insulators (manufactured by foreign companies) used in the 154 kV transmission lines throughout Korea, approximately 65% or 797,659 have exceeded their 30-year service life [2]. Although there is no immediate degradation in their insulation performance or mechanical failure as a result of exceeding their service life, utilities are concerned about aging insulators because these assets are key factors in the reliability of the power system. Hence, utilities prefer fast and accurate methods for detecting insulator defects. In the past, insulator damage has been primarily investigated from an electrical standpoint; however, with this approach, mechanical conditions are difficult to measure. For example, one of the most commonly used methods is the insulation resistance measurement, in which each insulator in the string is tested by the continuous application of high voltage. With this technique, the resistance of the insulator is obtained by simply dividing the DC voltage applied to the insulator by the sum of the charging current and leakage current generated [3]. Another technique that is used by a number of utilities is based on the principle of electric field measurement. When the insulator has an electric fault or low level of insulation capacity, the axial and radial distributions along the insulator of the surrounding electric field produce distortion; the foregoing field condition is then detected and compared with that of the non-fault standard electric field. If no distinct difference is observed, then the insulator is judged normal; otherwise, it is deemed defective [4; 5; 6]. In order to detect the mechanical defects of insulators, investigation is underway to classify fault-state by various techniques, such as ultrasonic wave method with contact-type ultrasonic probes [7], noise measurement method using contactless microphones [8], and temperature measurement method using infrared cameras [9]. In addition, small damage inside the insulator is detected with high accuracy using radiation such as CT (Computed Tomography) and MRI (Magnetic Resonance Imaging) [10]. In recent years, a method of detecting defects in appearance through imaging of an insulator using an unmanned aerial vehicle (UAV) has been studied [11]. However, most of these methods are significantly influenced by environmental conditions, such as seasons, weather, illumination, temperature, humidity, and solar flux [3; 12; 13; 14]. The use of ultrasonic testing also has its limitations: It is problematic and time consuming to scan the entire surface [7]; using radiation is a problem in test time and cost, and there is a high risk in the test method [10]; and using UAV is easy, but it can only detect insulators breakdown [11]. In this study, the frequency response function (FRF) method is employed to facilitate mechanical damage measurements and minimize the influence of the surrounding environment in such measurements. In the field of machinery, frequency response analysis (FRA) using FRF is mainly used for setting the resonant frequency of automotive frames [15].