Analysis and Suggestions for the Self-blast Problem of Glass Insulators on Transmission Lines

By virtue of many advantages such as good self-cleaning ability, outstanding mechanical performance, and excellent anti-vibration-fatigue ability, the glass insulators have been widely used in various alternating-current and direct-current transmission lines. However, the self-blast incidents of glass insulators have frequently occurred in transmission lines with various voltage levels, seriously affecting the safe operation of transmission lines. Herein, the main causes of self-blast problem of glass insulators are summarized and analysed from different perspectives, and some corresponding suggestions have been proposed to reduce the self-blast of glass insulators. In addition, the relevant prospects are provided for the future research work about the glass insulators. The viewpoints in this article have important theoretical and practical significance for improving the safety and reliability of transmission lines and ensuring the stable operation of electrical power systems.


Introduction
Transmission lines are a critical component of electrical power systems, undertaking an important role in transmitting electricity and attracting widespread research enthusiasm [1,2].With the improvement of technology and the increase of demand, the scale of transmission lines has also rapidly developed.As a special insulation device, the insulators play an irreplaceable role in transmission lines.Benefiting from many advantages such as good self-cleaning ability [3], low aging, outstanding mechanical performance [4], and excellent anti-vibration-fatigue ability [5], the glass insulators have presented their unique safety and superiority in operation [6,7], and have been widely used in numerous alternating-current and direct-current transmission lines with various voltage levels.
However, the self-blast incidents of glass insulators often occur during service.The self-blast problem of glass insulators refers to the phenomenon that the balance between compressive stress and tensile stress of glass pieces is disrupted to produce uneven stress distribution [8,9], resulting in rapid cracking and subsequent fragmenting of glass pieces under stress.On the one hand, the self-blast problem of glass insulators is closely related to their own quality [10].On the other hand, transmission lines may face harsh natural environmental conditions such as large temperature difference and lightning [7], making glass insulators susceptible to various complex factors and increasing the risk of self-blast.At present, the self-blast problem of glass insulators is relatively prominent, such as the concentrated self-blast and collective self-blast, posing a serious threat to the stable operation of transmission lines and electrical power systems [11].
In recent years, the research about glass insulators has attracted extensive attention from scholars.Zhang et al. provided a detailed summary and introduction of glass insulators from aspects such as materials, structural design, and manufacturing technology [12].Bai et al. analysed the main defects in the production process of glass pieces, and proposed some solutions for these defects [13].Wang et al. introduced the design and elimination tests of glass insulators [14].Tong et al. analysed the internal stress and life cycle of glass insulators [8].Sun et al. conducted research about the operating performance of glass insulators through a series of well-designed experiments [15].Some scholars have studied the recognition and location methods for the self-blast of glass insulators [16,17].In addition, some researchers have also conducted the research and analysis on the self-blast problem of glass insulators [18,19].These studies provide helpful references for understanding the intrinsic characteristics of glass insulators and dealing with the self-blast problem of glass insulators.
In order to minimize the self-blasted rate of glass insulators, the operation staffs of transmission lines and the manufacturers of glass insulators have taken some improvement measures.But, the selfblast of glass insulators still occurs from time to time, making it urgent to seek methods to reduce the self-blast of glass insulators.Based on those, this article provides an in-depth summary and analysis for the causes of self-blast of glass insulators, and proposes a series of corresponding measures and suggestions for reducing self-blast of glass insulators.Furthermore, the relevant prospects are provided for the future research work about glass insulators, in order to provide valuable guidance for reducing self-blast of glass insulators and improving the reliability of transmission lines.

The insufficient product quality
The high-quality glass pieces are essential for glass insulators.If the glass pieces contain impurities, the probability of self-blast of glass insulators will greatly increase [10,11].Due to the unique nature of raw materials and manufacturing technology, there is inevitably a type of nickel sulfide impurity that cannot be completely removed in glass pieces.During the service process of glass insulators, nickel sulfide frozen in glass due to quenching treatment will gradually undergo phase changes.This phase transition is also accompanied by volume change of nickel sulfide, which causes the glass to withstand enormous stress of phase change, leading to the self-blast of glass insulators [20].In addition, the defects such as bubbles and creases in glass pieces, as well as the pores and cracks in adhesive cement may also lead to the self-blast of glass insulators.

The accumulation of surface dirt
On transmission lines operating in heavily polluted areas, the surface of glass insulators is prone to accumulating a large amount of dirt.When encountering high humidity weather such as heavy fog, the accumulated dirt will be soaked in moisture, leading to a decrease in the insulation performance of glass insulators.Under the action of electric field, the leakage current will be generated on the surface of glass insulators.The leakage current will generate heat, causing surface moisture to evaporate and forming local dry band [7,21].Due to the significant increase of surface resistance in the dry bands, the intensity of electric field will increase significantly, resulting in arc discharge and corona phenomena.The chronic partial discharge can lead to micro-cracks and long-term heating of glass pieces, ultimately leading to the self-blast of glass insulators [22,23].

The impact of environmental conditions
The thermal conductivity of glass is relatively poor.When the ambient temperature undergoes drastic changes, due to the large temperature difference between the inner and outer layers of glass pieces, the different stress will be faced by the inner and outer layers of glass pieces.When the stress derived from temperature difference exceeds the strength that the glass pieces can withstand, the glass insulators will self-blast.Especially for the glass pieces with internal impurities, the difference in the coefficient of thermal expansion between impurities and glass pieces can easily disrupt the uniform distribution of internal stress of glass pieces when there is a drastic change in temperature, thus leading to the self-blast of glass insulators [7,24].In addition, the glass insulators may also be subjected to lightning strike [11,22].If the energy of lightning strike is too high, it will cause glass insulators to crack, breakdown, and even self-blast under the action of strong arc current.

The effect of external damage
The glass insulators are a special device mainly composed of tempered glass.During the processes of production, storage, transportation, and installation of glass insulators, the improper placement and collision may cause damage to the stress layer and mechanical strength of glass pieces.If the glass pieces have been damaged and continues to withstand the mechanical and electrical loads during the service of glass insulators, this damage may be further expanded, eventually leading to the self-blast of glass insulators [3].In addition, on transmission lines that have already been put into operation, the glass insulators may also be damaged by other external forces, such as attacks from shotguns or slingshots [25], causing serious damage to glass pieces and leading to the self-blast of glass insulators.

Improving the quality of glass insulators
The manufacturers of glass insulators should improve the manufacturing technology and the structures of products in a targeted manner to further improve the quality of glass insulators [26].The suggestions for improving the quality of glass insulators are as follows: 1) replacing fossil fuels with electricity in melting furnaces [20], and avoiding using the manufacturing equipment containing nickel element to prevent the introduction of nickel sulfide in the manufacturing processes of glass pieces; 2) strengthening the selection and management of raw materials for glass pieces to prevent the inclusion of impurities in raw materials [9]; 3) strictly implementing the homogenization processes to fully eliminate glass pieces containing nickel sulfide [24]; 4) strengthening the inspection and testing of glass pieces to strictly eliminate glass pieces with defects such as bubbles and creases; 5) optimizing the cementing technology of glass insulators to reduce air holes in the adhesive cement and improve the coaxiality of various components; 6) strictly controlling the quality of adhesive cement to ensure that it has excellent dimensional stability and thermal stability.

Strengthening the response to pollution
For newly-built or renovated transmission lines, the selection and configuration of glass insulators should be carried out scientifically based on the distribution map of pollution areas in the power grid.In the areas with high pollution levels, it is not recommended to use the glass insulators [27].If necessary, the suitable anti-pollution glass insulators should be selected, and tracked and evaluated during service.In addition, since the surface dirt is one of the important factors leading to the selfblast of glass insulators, it is necessary to clean the surface dirt of glass insulators in time.The operation staffs of transmission lines should arrange reasonable cleaning and treatment for glass insulators with severe pollution based on their service conditions to eliminate the surface dirt of the glass insulators, reducing the external incentives that cause the self-blast of glass insulators [26,27].

Enhancing the protection against lightning
The glass insulators in the areas with strong lightning should be properly protected against lightning to avoid the self-blast caused by lightning [11].The following measures can be adopted to enhance the protection against lightning: 1) installing lightning arresters reasonably near the glass insulators to enhance their ability to resist lightning and reduce the risk of damage caused by lightning strike; 2) strengthening the construction of the grounding devices of lightning arresters to ensure that the lightning arresters are well grounded and can quickly introduce lightning to the earth; 3) regularly inspecting and maintaining the glass insulators on transmission lines to ensure that their lightning arresters and grounding devices work properly, and to identify and repair potential problems in time.

Avoiding the external damage
During the processes of production, storage, transportation, and installation of glass insulators, it is necessary to strengthen the safety awareness of workers and standardize their operating behaviors, which will reduce the damage to glass insulators caused by external forces originated from human factors.During the transportation and installation of glass insulators, in view of possible external damage factors, the appropriate protective measures should be taken, such as setting protective covers and installing protective sleeves, etc., to enhance the ability of glass insulators to resist external force damage.In addition, in order to further avoid external damage to glass insulators, the operation staffs should also increase protection efforts to strengthen inspection for transmission lines and publicity about power regulations, enhancing the awareness of public in protecting power facilities [28].

Conclusion and outlook
In summary, the reasons for the self-blast of glass insulators come from many aspects, such as the insufficient product quality, the accumulation of surface dirt, the impact of environmental conditions, and the effect of external damage.Regarding the above causes, the corresponding suggestions and measures have been provided to address the self-blast problem of glass insulators, including improving the quality of glass insulators, strengthening the response to pollution, enhancing the protection against lightning, and avoiding the external damage.The viewpoints of this article will be of great significance for improving the safety and stability of electrical power systems.
In the future, in order to further reduce the self-blast of glass insulators and improve the reliability of glass insulators, the following work is worth carrying out: (1) Conducting the researches about the self-blast mechanisms of glass insulators.It is significant to explore the influencing factors that lead to the self-blast of glass insulators, and to deeply analysis the self-blast mechanisms of glass insulators.The studies about the self-blast mechanisms of glass insulators can provide scientific basis and theoretical guidance for optimizing the manufacturing technology and formulating the preventive measures of glass insulators.
(2) Strengthening the innovation in inspection methods for the quality of glass insulators.It is recommended to develop or optimize non-destructive testing techniques for more effectively detect the quality defects of glass insulators.By developing new inspection techniques, the quality of glass insulators before and after entering the power grid can be effectively detected, thereby improving the safety and reliability of transmission lines.
(3) Implementing the overall supervision measures.It is necessary to conduct the technical supervision during the stages of manufacturing, arrival, installation, operation, and retirement of glass insulators to ensure that the product quality meets standards, the installation and construction meet specifications, the mechanical and electrical performances meet requirements, supervising the quality of glass insulators throughout their entire lifecycle.