Analysis of the growth conditions of icicles during insulator icing

Highlights

• The parametric method of wet-growth icing with icicles are proposed.

• The mathematical model of the water film flowing during insulator icing is established.

• The simulation shows the water film on insulator surface is mainly generated at the droplets collision area, which supplies the water of icicle growth.

• The critical condition of insulator icicle growth is obtained through the calculation of water film overflowing.

Abstract

In this paper, the accretion processes of different icing types of insulators are analyzed, and the insulator icing types are creatively redefined into three types according to the characteristics of water film and icicle growth. Based on which, a new parametric method of wet-growth icing and icicles is proposed and a mathematical model of water film flowing on insulators is established. The method and model make it possible to simulate the uneven distribution of water film on insulator and precisely predict the insulator icing. The simulation results show that the water film on the insulator surface is mainly generated at the collision area of water droplets, the edge of insulator shed and the steel cap on the windward side. The overflowing water expands the icing area on the insulator surface and at the same time supplies water for the growth of icicles. The greater the wind velocity, the liquid water content of air and the median volume diameter of water droplets, the faster water film will overflows to insulator shed edge and the greater the probability of icicles.

Introduction

Icing seriously threatens the safe operation of transmission lines in various ways. Icing can increase the mechanical load on wires and towers, resulting in accidents of wire break, wire galloping, pole and tower collapsing [1], [2], [3], [4]. Insulator icing mainly reduces the insulation properties of insulators, causing partial discharge or flashover accidents [5], [6]. The flashover of ice-covered insulators has always been the focus of researchers at home and abroad, including the ice flashover characteristics [7, 8], the factors affecting ice flashover voltage [9], and the ice flashover models [10], [11], [12].

In 1970, M. Kawai [7] conducted AC flashover tests on ice-covered insulator strings. It is found that the icing on insulator strings always appears unevenly. When the ice thickness increases to a certain value, the flashover can occur under the working voltage. In the study of references [8, 13], the AC ice flashover tests were conducted on composite insulators, and the results show that the AC ice flashover voltage of composite insulators decreases with the increase of ice thickness. When the icing on an insulator string reaches a certain value, the downtrend of ice flashover voltage slows down and gets saturation. Similarly, the study in reference [14] found that the DC flashover voltage of insulators decreases with the icing thickness. Moreover, the ice flashover voltage is also affected by factors such as the surface contamination of insulators, the structure of the insulator shed, and the voltage polarity. The research results of Chongqing University [15] show that the icing weight W of an insulator string and its minimum flashover voltage U_f (W) satisfy a power function relationship U_f (W) = U_f (0) × e-^mW, where U_f (0) is the minimum flashover voltage when there is no ice on the insulator string. In addition to the effects of ice weight and thickness, the icing type will also affect the discharge characteristics of ice-covered insulators. The research of N. Sugawara [16] in 1993 shows that different icing types can lead to different flashover voltages, and the withstand voltage of insulator strings covered with icicles is significantly lower than that without icicles. M. Farzaneh [17] and Shu Lichun [18] pointed out that during the process of insulator icing, the presence of icicles will seriously distort the distribution of external electric field of insulators. As a result, the electric field strength at the tip of icicles will increase, making it easier to form partial discharges and ice flashover arcs. As shown in Fig. 1, the researchers equivalently calculate the residual resistance of the ice layer with a semi-cylindrical structure, thus establishing an ice flashover model of insulators. However, the equivalent method is only applicable to the case where the icicles severely bridge the insulator sheds, and is no longer available to other icing types. Otherwise, as present in reference [19], [20], some methods like nanotechnology science can be used for increasing self-capacitance of silicon rubber (SiR) insulators thereby voltage and electric field distribution become more uniform along the string. And how the ice and icicle would affect the electric distribution of insulator strings with improvements need further research.

Therefore, different icing types have significant differences in the damage degree to the insulator's insulation performance. Particularly, the presence of icicles plays a key role. Previous studies have mainly focused on the flashover characteristics of ice-covered insulators and the establishment of ice flashover models. There is a lack of targeted research on the growth mechanism of different icing types on insulators [21], [22].

Makkonen [23] summarized the icing process on conductors into three aspects, namely the collision process of supercooled water droplets on the conductor surface, the capturing process of collision water droplets, and the freezing process of the captured water droplets. To calculate the icing rate, Makkonen [24], Fu Ping [25, 26], and Jiang Xingliang [27], etc. used three efficiencies, namely collision efficiency β₁, capturing efficiency β₂ and freezing efficiency β₃, to characterize the icing efficiency. The insulator icing also satisfies these three processes. But with a much complicated structure, the icing on insulators is significantly different. Based on the difference in density, the insulator icing can be divided into five categories: glaze, hard rime, soft rime, hoarfrost, and snow. The division is conducive to identifying the essential characteristics of icing, but is not good for numerical simulation.

Another division is based on the formation mechanism of ice, and the icing is divided into dry growth and wet growth. For dry-growth icing, the time for liquid water droplets to freeze into solid ice is extremely short, and there is no water film formed on the insulator surface. For wet-growth icing, the water droplets captured on the insulator surface will not freeze completely, and the freezing rate is relatively slow with an unfrozen water film forming on the ice layer. The division of dry and wet growth icing is closely related to the thermal equilibrium process of freezing, which provides an entry point for the numeral calculation of icing and is helpful for the analysis of icing mechanism and the establishment of icing models.

Considering the serious threat of icicles to the operation of insulators, this article creatively redefines the insulator icing into three types according to the characteristics of water film and icicle growth. By analyzing the relationship between the droplet collisions, ice freezing, icicle growth and the flowing of water film, a numerical model is established to simulate the critical condition for icicles growth. The work in this paper can be used in the precise prediction of insulator wet-growth icing with icicles. The research also reveals the formation mechanism of wet-growth icing with icicles, which lays the foundation for the establishment of a three-dimensional numerical model of different insulator icing types.