## 1 Introduction  
Composite insulators, with favorable electrical properties, have been widely used in AC and DC transmission systems [1; 2; 3; 4]. By the mid-1980s, the number of composite insulators applied in the power grid in America had reached 2 million [5].  
Although composite insulators were initially considered to be maintenance-free, the performance of silicone rubber material on the surface of composite insulators deteriorated during the aging process in poor operating environments. With the increase of aging time, composite insulators lost their hydrophobicity and allowed cracks to appear in the surficial material, resulting in an increase of the probability of pollution and icing flashover accidents [6; 7; 8; 9].  
A significant amount of research of aging composite insulators has been carried out by scholars, proposing a series of methods to evaluate the aging effects of composite insulators. In Reference [10], scholars measured the hydrophobicity of silicone rubber to evaluate the aging effects of composite insulators aged for different numbers of years. In Reference [11; 12], researchers studied the surface roughness of silicone rubber to investigate the aging degree of composite insulators.  
On this basis, some scholars proposed to use one or several parameters summarized from test results to characterize the aging effects of composite insulators qualitatively or quantitatively. In Reference [13; 14], the static contact angle in the surface material was proposed to characterize aging effects of composite insulators. In Reference [15; 16], a new method was proposed to characterize the aging degree of composite insulators by analyzing the test results of Fourier transform infrared spectroscopy (FTIR), along with hardness and images captured by scanning electron microscope (SEM). In addition, some scholars [17] established lifespan prediction methods based on 371 composite insulators from the lines in high temperature and humidity regions. In these studies, most composite insulators sampled were running in the same transmission line where operating environments have some differences over the large length of the line. Unfortunately, due to the great influence of operating environments on aging performance of composite insulators, there are some deviations among the results calculated by the prediction methods and actual results.  
In this study, composite insulators of the same batch of Factory A exposed for 1-12 years in a high altitude area experimental station of Hunan province (as shown in Figure 1) were sampled. The changing law of lifespan prediction parameters was investigated by means of widely accepted methods, including the hydrophobicity classes test method (HC test), the static contact angle (CA) method, the hardness method, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and the salt-fog flashover test. Test results are presented in Section 2. In Section 3, by means of correlation calculation, parameters significantly correlated with aging time are obtained. On this basis, a prediction method based on a back propagation (BP) neural network is proposed for determining the aging time of composite insulators.  
Corresponding Parameters of Composite Insulators under Different Aging Years for Lifespan Prediction  
In this research, composite insulators (recorded as A\({}_{1}\), A\({}_{2}\), A\({}_{3}\), A\({}_{4}\), A\({}_{5}\), A\({}_{6}\)) exposed for 1-12 years in a high altitude area experimental station of Hunan province were sampled. In order to investigate the changing law of lifespan prediction parameters under different years, several widely accepted methods, including the HC method, the CA method, the hardness test, SEM, FTIR, XPS and the salt-fog flashover test, are employed for composite insulators aged for different years in this section [18].