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Article

The Icing Characteristics of Post Insulators in a Natural Icing Environment

by
Zhijin Zhang
*,
Jiahui Tu
,
Yuanpeng Zhang
,
Xingliang Jiang
and
Zhenbing Zhu
Xuefeng Mountain Energy Equipment Safety National Observation and Research Station, Chongqing University, Chongqing 400044, China
*
Author to whom correspondence should be addressed.
Atmosphere 2025, 16(1), 64; https://doi.org/10.3390/atmos16010064
Submission received: 5 December 2024 / Revised: 2 January 2025 / Accepted: 6 January 2025 / Published: 9 January 2025
(This article belongs to the Special Issue Advances in Atmospheric Icing: Predictive Models, Thermodynamics, and Mitigation Strategie)
Browse Figures
Figure 1
<p>Physical diagram of post insulators.</p> "> Figure 2
<p>Xuefeng Mountain Energy Equipment Safety National Observation and Research Station.</p> "> Figure 3
<p>The rotating conductor and vernier caliper.</p> "> Figure 4
<p>Glaze icing at different times for Type I to VII insulators: (<b>a</b>) t = 5 h; (<b>b</b>) t = 10 h; (<b>c</b>) t = 15 h.</p> "> Figure 5
<p>Glaze icing at different times. Glaze icing on the windward side and lateral side of type VI post insulator: (<b>a</b>) windward side; (<b>b</b>) leeward side.</p> "> Figure 6
<p>Changes in environmental parameters during the glaze icing process: (<b>a</b>) temperature; (<b>b</b>) wind-speed; (<b>c</b>) relative humidity; (<b>d</b>) rainfall.</p> "> Figure 7
<p>Mixed-phase icing at different times for type II, III and V insulators: (<b>a</b>) t = 7 h; (<b>b</b>) t = 20 h; (<b>c</b>) t = 30 h.</p> "> Figure 8
<p>Mixed-phase icing on different sides of type II post insulator: (<b>a</b>) Windward side; (<b>b</b>) Leeward side.</p> "> Figure 9
<p>Changes in environmental parameters during the mixed-phase icing process: (<b>a</b>) temperature; (<b>b</b>) windspeed; (<b>c</b>) relative humidity; (<b>d</b>) rainfall.</p> "> Figure 10
<p>Curve of glaze icing parameters changing with time: (<b>a</b>) Variation curve of icicle length over time; (<b>b</b>) Variation curve of icing mass over time; (<b>c</b>) Variation curve of rotating conductor icing thickness over time; (<b>d</b>) Relationship curve between icing mass and rotating conductor icing thickness.</p> "> Figure 11
<p>Curve of mixed-phase icing parameters changing with time: (<b>a</b>) Variation curve of icing mass over time; (<b>b</b>) Variation curve of rotating conductor icing thickness over time. (<b>c</b>) Relationship curve between icing mass and rotating conductor icing thickness.</p> "> Figure 12
<p>Comparison of the icing morphology between post insulator (I) and suspension insulator (II): (<b>a</b>) glaze icing; (<b>b</b>) rime icing.</p> ">
Versions Notes

Abstract

:
Icing significantly reduces the electrical performance of insulators, and grid failures caused by insulator icing are common. Currently, most research on the flashover characteristics of insulators under icing conditions focuses on artificially iced suspension insulators, with limited studies on post insulators under natural icing conditions. The shed spacing of post insulators is smaller, making them more prone to bridging by icicles in the same icing environment, which exacerbates insulation problems. Therefore, investigating the icing characteristics of post insulators is crucial. In this study, natural icing growth was observed on seven different types of post insulators at the Xuefeng Mountain Energy Equipment Safety National Observation and Research Station. The icing morphology and characteristics of these insulators were examined. The main conclusions are as follows: (1) the icing type and morphology of post insulators are influenced by meteorological conditions, with more severe icing observed on the windward side. (2) The icing mass and icicle length of the insulator increase nonlinearly with icing time. Specifically, during the glaze icing period from 0 to 8 h, the ice mass on the Type V composite post insulator was 3.89 times greater than that during the 13-to-18 h period. (3) Within the same icing cycle, the icing growth rate on composite post insulators is faster than on porcelain post insulators. (4) Compared to suspension insulators, the sheds of post insulators are more easily bridged by icicles. Notably, when the sheds of post insulators are bridged by icicles, the length of icicles on suspension insulators is only half of the gap size.

1. Introduction

With the rapid growth of China’s economy, the demand for electricity is increasing. A high-voltage direct-current (HVDC) transmission, due to its advantages of low loss, high capacity, and suitability for long-distance transmission, is better able to meet the huge electricity demand driven by the fast-paced development of China’s socio-economic landscape [1].
With the rapid development of ultra-high-voltage direct-current (UHVDC) transmission projects, an increasing number of transmission lines, substations, and converter stations will pass through areas prone to ice and snow. Every winter, extreme icy weather can cause outdoor electrical equipment such as insulators to grow ice, and in severe cases, lead to flashover incidents, which can result in power outages. Both domestically and internationally, there have been several instances of insulator flashover accidents caused by harsh snow and ice conditions, leading to significant economic losses. As early as the 1990s, countries with cold climates such as the United States, Canada, and Norway reported incidents of insulator flashovers caused by icing [2,3,4]. In early 2008, a large-scale ice damage caused by extreme snow and ice weather led to significant economic losses in the power grids of several provinces, with Hunan province being the most affected [5,6]. In recent years, there have also been records of ice damages. In 2018, the ±800 kV Qishao line P4491 tower in the phase V composite insulator string was covered by ice, resulting in 2/3 length of the insulator string arcing phenomenon [7]. In November 2020, Liaoning and Jilin provinces experienced a large-scale ice disaster, which posed a serious threat to normal heating and power supply for residents [8]. In February 2021, an extreme snow and ice storm swept through the power system in Texas, USA, severely affecting electricity usage for local residents [9]. A large amount of research has been conducted both domestically and internationally on insulator flashovers caused by icing [10,11]. Studies have found that ice build-up causes a decrease in the flashover voltage of insulators. Additionally, the flashover characteristics of insulators are closely related to the extent of icing. Therefore, researching the icing characteristics of insulators is of great importance.
The types and severity of insulator icing, as well as the key factors influencing the icing process, include temperature, humidity, wind speed, and precipitation. Both domestic and international researchers have conducted extensive studies on the icing growth process of insulators, analyzing the factors that affect their growth characteristics in icing environments. These factors can be categorized into three main types: environmental temperature, wind speed, and liquid water content in the environment. (1) Impact of Environmental Temperature: Farzaneh et al. [12] found that within the temperature range of −10 °C to 0 °C, lower temperatures accelerate the heat exchange rate between water droplets on the insulator surface and the surrounding environment, leading to a faster growth of icing thickness and icicle length. However, when the temperature drops below −10 °C, water droplets in the air condense directly into ice crystals, which are difficult to adhere to the insulator, thereby slowing ice accumulation. Huang et al. [13] reached similar conclusions. Zhang et al. [14] observed that when the temperature dropped from −2 °C to −5 °C, the increase in icing thickness and growth rate was minimal. However, when the temperature continued to fall from −8 °C to −11 °C, the icing growth rate on the insulator string increased significantly. (2) Impact of Environmental Wind Speed: Huang et al. [15] found that an increase in wind speed enhances the roughness of the ice layer on the insulator surface, blowing more water droplets toward the insulator. This increases the heat exchange between the droplets and the surrounding environment, accelerating condensation. They also observed that the angle between the wind direction and the insulator affects the icing rate, with angles between 20° and 40° accelerating the icing process. Liu et al. [16] found that the wind direction influences the distribution of icing, with the most severe icing occurring on the windward side, particularly along the shed edges and at the base of the pole. (3) Impact of Environmental Liquid Water Content (LWC): Huang et al. [13] suggested that an increase in LWC significantly raises the amount of ice on the insulator. A higher LWC means more water droplets in the air, and as droplets collide with the insulator surface due to airflow, the ice mass increases. They also found that as the median volume diameter (MVD) of the droplets increases, the ice mass increases. Larger droplets, with higher potential energy, are more likely to collide with the insulator surface. Han et al. [17] reached similar conclusions.
Icing on insulators poses a significant threat to the safe operation of power systems, making insulator de-icing a critical area of research. He et al. [18] reviewed various de-icing technologies, highlighting robotic de-icing as a recent focus of study. Refs. [19,20] designed deicing robots for power transmission lines using neural network learning and deep learning, while Liu et al. [21] explored image characteristic extraction and monitoring techniques for iced insulators. Zhu et al. [22] conducted research on image recognition techniques for measuring the icicle length of iced insulators. Research on the growth characteristics of icing on insulators is essential for assessing the degree and stage of icing, which is crucial for advancing de-icing technologies.
Most icing research to date has focused on suspension insulators, with relatively fewer studies addressing the icing characteristics of post insulators. In contrast to suspension insulators, post insulators feature more densely spaced sheds, which increases the likelihood of shedding bridging during icing. Additionally, much of the existing research relies on artificial icing conditions, with limited exploration of the icing behavior of post insulators under natural conditions. Natural environments, with their fluctuating weather patterns, present significant differences from the controlled conditions of artificial environments. To address this gap, this study focuses on experimental research examining the icing characteristics of post insulators in natural environmental conditions. The research includes capturing the icing morphology of post insulators in natural settings, measuring icing parameters and their variations, and analyzing the relationship between icing morphology and environmental changes over the icing period. The study also compares the icing differences between post and suspension insulators, with the goal of determining the natural icing characteristics of post insulators. These findings will provide valuable insights for the selection and design of insulators in regions prone to icing.

2. Test Method for the Natural Icing of Post Insulators

2.1. Sample and Test Platform

In this paper, seven kinds of post insulators with different structures are selected as the experimental objects, of which four are porcelain post insulators, numbered the I, II, III, or IV type, and three are composite post insulators, numbered the V, VI, or VII type, of which the physical drawings and structural parameters are shown in Figure 1 and Table 1, where D is the diameter of the pole, P1, P2 is the size of the umbrella outstretched, H is the height of the structure, L is the creepage distance, and S is the surface area.
Relying on the Xuefeng Mountain Energy Equipment Safety National Observation and Research Station, the observation of the natural icing growth characteristics of the above seven types of pillar insulators was carried out, and the full view of the observatory is shown in Figure 2.
The Xuefeng Mountain Energy Equipment Safety National Observation and Research Station is located in Huaihua City, Hunan Province, at an altitude of 1450 m. The icing period of the station can be more than 10 times per year, from the end of November to mid-February of the following year. The location of the research station belongs to the subtropical monsoon climate, with cold and sufficient precipitation in the winter, the variable climatic conditions can form a variety of glaze, rime, mixed-phase ice, and other types of ice, the station assembled with the six elements of the meteorological instrument observed at any time to cover the ice of the changes in environmental conditions. In this paper, during the ice covering observation process, several post insulators of the same type are placed vertically in the open space of the observatory, and in order to prevent the ice covering process of the post insulators from interfering with each other, the intervals between the placements are more than 2 m.

2.2. Natural Icing of Post Insulators and the Measurement of Its Parameters

Before the test, the insulators are cleaned with water. After cleaning, any remaining surface water is absorbed using a clean sampling fabric, and the insulators are then left to air dry in a stationary position. Initially, each insulator is covered with a cover, which is removed once icing conditions are met to allow for natural icing. When the surrounding temperature drops to around 0 °C, the protective cover can be removed from the insulator.
During the icing period, since the post insulators used in the test are relatively short and the insulators are arranged in essentially the same environment, the icing on the same insulator of the same type in the same direction can be considered uniform. In order to observe the icing pattern and icing growth of the post insulators, the icing condition of the insulators was photographed every 2–3 h. The length of the icicles of one set of sheds of the insulators was measured using vernier calipers (SMCT, measuring range: 0–150 mm, accuracy: 0.02 mm), and the average value was taken as the length of the icicles of the insulators. At the same time, to enhance the sample’s representativeness, the ice on the middle set of sheds is meticulously dislodged and precisely weighed using an electronic balance (Sartorius, measuring range: 0–10.2 kg, accuracy: 100 mg). This weight was then multiplied by a proportional factor to estimate the total ice weight on the insulator. The proportional factor is the ratio of the total structural height of the insulator to the structural height of the measured sheds. This method enables the gathering of relatively abundant and reasonably accurate data within the available timeframe and conditions.
In this paper, the icing mass of the post insulator and the icing thickness of the rotating conductor were used to quantify the degree of insulator icing. This method is a recognized approach for characterizing the degree of ice coating on insulators, and relevant research has verified its rationality [23,24]. In order to accurately characterize the degree of insulator ice coverage, the rotating conductor is placed adjacent to the post insulators during the test process in this paper, and the rotating conductor used is made of stainless steel, with a length of 500 mm and a diameter of 28 mm, and rotates at a uniform speed of 1 r/min, as shown in Figure 3, and the thickness of the rotating conductor is recorded simultaneously with the measurement of the insulator icing parameter.
During the insulator icing process, changes in environmental temperature, environmental humidity, wind speed, wind direction, atmospheric pressure, and rainfall are recorded using a six-parameter weather instrument, with a recording frequency set to once per minute for all experiments. This instrument can measure temperatures from −40 °C to 60 °C with an accuracy of ±0.3 °C, relative humidity (RH) from 0% to 100% RH with an accuracy of ±3% RH, wind speed from 0 to 60 m/s with an accuracy of ±0.1 m/s, and wind direction from 0° to 360° with an accuracy of ±2°. It also measures atmospheric pressure in the range of 30 to 110 kPa with an accuracy of ±0.25% and rainfall from 0 to 4 mm/min with an accuracy of ±4%. These precise environmental measurements provide essential data to support the analysis of insulator icing characteristics.

3. Morphological Analysis of Natural Icing on Post Insulators

3.1. Morphology Analysis of Glaze

The glaze on insulators forms when supercooled rain or drizzle falls onto insulators at subfreezing temperatures. This type of ice has a density between 0.8 and 0.917 g/cm3 and is hard, transparent, and pure.
Figure 4a–c shows the glaze on a post insulator at different times during an icing period, while Figure 5 shows the front and side views of the icing on a Type VI post insulator. Through observation, the following can be seen:
1.
When glaze formed on the insulator, icicles appeared along the edges of the sheds. Icicles on the windward side were generally longer and thicker than those on the leeward side. This is because the windward side of the insulator blocks most of the supercooled rain, causing the majority of the droplets to run down along the windward edges of the sheds, which allows for icicles to grow continuously. On the leeward side, however, only supercooled water carried by airflow can be captured. As the icing duration increased and the thickness of the ice deepened, the effective diameter of the insulator grew, enabling the windward side to block more raindrops, further widening the icing disparity between the windward and leeward sides.
2.
In the early stages of icing (t = 0 to 5 h, when there was no rainfall), the icicles on the upper part of the insulator were longer and thicker than those on the lower part, with the thickness of the ice on the sheds followed a similar pattern, as shown in Figure 4a. However, this phenomenon became less pronounced in the middle and later stages of icing. This is because in the early icing period, rainfall was relatively low, and the upper sheds of the insulator partially blocked the incoming flow, preventing some rain from reaching the lower sheds. This reduced the likelihood of raindrops colliding with the lower sheds, resulting in a slower icing growth rate on the lower sheds. In the middle icing period (t = 5 to 8 h, as rainfall increased), there was ample supercooled water, excess droplets flowed directly downward along the icicles, providing more moisture for the lower sheds and allowing the icicles there to grow more rapidly. Consequently, in the middle and later stages of icing, the icing levels on the upper and lower parts of the insulator became essentially uniform. Therefore, when rainfall is low, the icing growth rate on post insulators is slower, and the icicle distribution between the upper and lower parts is uneven. When rainfall is higher, the icicle distribution becomes more uniform.
During the icing period on the insulator, observations reveal that environmental parameters play a decisive role in determining the type and extent of natural icing on the insulator. Figure 6 shows the environmental parameters recorded by the weather instrument over an 18 h period during the glaze icing on the aforementioned post insulator. Altitude and atmospheric pressure are factors that can potentially affect various environmental conditions, including temperature, the distribution of water vapor in the air and enthalpy [25], which, in turn, could influence the icing process on insulators. The atmospheric pressure during experiment ranged from 85.56 kPa to 86.01 kPa, with a variation of less than 1%, so the impact of altitude and atmospheric pressure can be ignored. Chemicals can also influence icing. Salts, such as sodium chloride (NaCl), lower the melting point of ice, which is why they are commonly used in road de-icing applications [26,27]. The station is situated far from urban areas and pollution sources. The conductivity of the melted ice was tested after the experiment, with values ranging from 10 to 20 μS/m, indicating that the chemical impact on our results is minimal.
As seen in Figure 6,
1.
Before and during the rainfall, the relative humidity around the insulator remained close to 100%. However, after the rainfall ended, the relative humidity in the environment gradually decreased to about 80%. Based on actual observations, there was no fog on Xuefeng Mountain during this period, indicating that the icing type was primarily glaze icing.
2.
During the period of glaze icing, when icicle growth predominated, the temperature in the icing environment ranged between −4 and 0 °C, and wind speed remained relatively low, typically between 1 and 4 m/s. The wind accelerated the heat exchange rate between the droplets captured by the insulator and the surrounding environment, which facilitated the freezing of the droplets into ice. With continuous rainfall, the environment maintained a high liquid water content, allowing for icicles on the insulator to grow rapidly and continuously. The post insulator stands upright on the ground, and the wind direction can be considered perpendicular to the axis of the insulator. Under constant wind speed and humidity, changes in wind direction led to variations in the icing morphology of the insulator. However, the rate of water droplets impacting the insulator and the rate of heat exchange between the insulator and the environment remain the same, resulting in no significant change in the mass of ice accumulation. As can also be seen from Figure 5, the icing on the insulator has a distinct windward side and a leeward side, indicating that there were minimal changes in wind direction during the experiment.
3.
During the initial icing period (t = 0 to 2 h), the ambient temperature on Xuefeng Mountain was between −3 and −1.5 °C, with a relative humidity at 100%. Under the influence of wind, moisture in the air directly condensed on the insulator surface, forming a thin layer of ice that enveloped the insulator. Excess moisture flowed along the upper surface of the sheds toward the edges, where it eventually froze into small icicles, forming the initial shape of the icicles along the shed edges. At this stage, the larger sheds had more small icicles, while the smaller sheds had fewer. This is likely because the larger sheds blocked some of the windborne moisture, preventing the smaller sheds from capturing additional supercooled water. As icing continued, the temperature steadily dropped and wind speed increased, causing the ice layer on the insulator to thicken. The small icicles gradually grew into full icicles and the ice layer on the insulator became increasingly solid.
4.
During the icing period from t = 2 to 8 h, rainfall provided a continuous supply of liquid water to the insulator. Rain predominantly fell on the windward side, causing the sheds on this side to ice over more quickly, with icicles growing at an accelerated rate. The continuous flow of liquid water allowed for smaller sheds to capture enough moisture to freeze. Due to the shorter distance between the icicles on the edges of the smaller sheds and the larger sheds, some icicles began bridging between the two, as shown in Figure 5a. As icing progressed, most of the smaller sheds became fully bridged with the larger sheds and began growing laterally. However, the greater distance between the larger sheds meant that fewer icicles bridged between them. From t = 8 to 15 h, wind speed initially increased and then decreased, while rainfall significantly diminished, slowing the icing rate. The ice layer on the sheds thickened and approached saturation. The length of the icicles continued to increase until full bridging between sheds occurred, and the icicles expanded in thickness, ultimately forming the icing pattern shown in Figure 4c.

3.2. Morphology Analysis of Mixed-Phase Ice

The type of ice accretion on insulators varies depending on the icing environment. In conditions of heavy rainfall, glaze icing primarily occurs through wet growth. In contrast, under foggy or overcast conditions without rainfall, rime icing forms via dry growth. In complex climates with alternating rain and fog, mixed icing develops. Mixed-phase icing is a composite process shaped by fluctuating weather conditions, with alternating rime and glaze phases ultimately resulting in a mixed-phase ice coating. This type of ice has a density ranging from 0.6 to 0.8 g/cm3 and typically appears as transparent or opaque, layered, or tabular structures. Figure 7a–c show mixed-phase icing on Type II, III, and V post insulators at different times during a particular icing period, and Figure 8 displays the windward and leeward side views of icing on the Type II post insulator.
From Figure 7a–c and Figure 8, it can be seen:
1.
During the period from t = 0 to 7 h, rime formed on the windward side of the insulator, taking on a shrimp-tail shape of white ice. The rime primarily accumulated on the upper surface of the sheds, with relatively less ice on the lower surface. Similar to glaze icing, the growth rate of icing on the windward side during mixed-phase icing was higher than on the leeward side. This was due to the supercooled water droplets being blown toward the windward side, allowing it to capture more moisture and freeze more readily, while the leeward side, being sheltered, captured less moisture, resulting in a slower icing growth rate.
2.
During t = 7 to 20 h, the rime on the windward side of the insulator sheds continued to grow in a dry manner. On the windward side, the trend of ice growth at two sides was faster than that in the middle. Meanwhile, there was no significant difference in icing between the upper and lower surfaces of porcelain insulators, and bridging occurred at the sides. In contrast, for composite insulators, icing mainly occurred on the upper surface of the sheds without bridging between them. This is because the spacing between sheds and the smaller rod diameter in porcelain insulators caused water droplets in the middle to flow more easily toward the sides under the influence of wind, resulting in faster ice growth at the sides and a tendency for bridging. In contrast, composite insulators had larger shed spacing and rod diameters, making it difficult for water droplets in the middle to flow to the sides, leading to a uniform distribution of ice on the upper surface of the sheds on the windward side.
3.
During t = 20~30 h, glaze and rime alternately appeared, transforming the insulator from rime icing to a mixed-phase icing form, as shown in Figure 7c. This type of icing was dense, opaque, and formed block-like ice with strong adhesion. During the formation of mixed-phase icing, glaze fell on the rime-covered insulator, causing the initially brittle rime to condense tightly as supercooled water flowed in. As more supercooled water infiltrated, the gaps between the insulator sheds gradually filled with ice, posing a serious threat to the insulator’s safe operation.
4.
Unlike glaze icing, there was minimal difference in icing between the upper and lower parts of a single post insulator during rime icing. This can be attributed to the relatively short length of the post insulators used in the test, resulting in nearly identical environmental conditions at both the upper and lower parts, and consequently, a similar icing growth rate across the entire insulator. In contrast, during the early stages of glaze icing, the icing growth rate on the upper sheds was faster. This was due to the rainfall, with the upper sheds partially blocking rain from reaching the lower sheds. Additionally, some supercooled water likely froze before it could flow down to the lower sheds, further accelerating the icing growth on the upper sheds.
During the icing process of the post insulator, diverse changes in environmental parameters led to the formation of mixed-phase icing. Figure 9 shows the 30-h environmental parameters recorded by the meteorological instrument during the formation of mixed-phase icing on the post insulator. The atmospheric pressure ranged from 85.56 kPa to 86.01 kPa, with minimal variation, as previously mentioned.
From Figure 9, we can observe the following:
  • During the formation of mixed-phase icing on the insulator, temperatures ranged from −6.5 to −3 °C, and wind speeds were relatively high, mostly between 4 and 10 m/s. The relative humidity of the air remained high, generally above 95%, though the rainfall was significantly lower than during glaze icing, and the icing formation time was also longer.
  • The observations indicate that during the formation of mixed-phase icing, rime ice initially formed in foggy conditions, which then transitioned to mixed-phase icing as rain alternated with fog. Actual observations show that at the beginning of icing, the insulator, which was in low-temperature condition captured water droplets carried by the wind, which froze upon contact with the surface of insulator. The high humidity of the air provided a steady supply of supercooled droplets, allowing for continuous icing. Under the influence of wind, the icing rate on the insulator surface accelerated. The effect of wind direction has been discussed in Section 3.1 and will not be repeated here.
  • Figure 9c,d illustrates that during the first 20 h of icing, no rain occurred, but the relative humidity remained high. During this period, rime ice primarily formed through dry growth on the windward side, exhibiting brittle, low-density structures such as ‘shrimp-tail’ and ‘pine-needle’ shapes. Under strong winds, the icing on the windward side of the insulator grew more rapidly at the edges than in the center. This could be due to supercooled water droplets carried by the wind toward the windward side, with droplets in the center likely flowing toward the sides, thus slowing the icing rate in the center. Between 20 and 30 h, the rime ice became fully saturated with rainwater as rainfall began. Under the influence of strong wind and low temperatures, the brittle rime ice solidified into denser, harder ice due to the supercooled water. As rime and glaze alternated, the insulator developed a mixed-phase ice coating, which was opaque, dense, and strongly adhesive, as shown in Figure 7c.

4. Analysis of the Natural Icing Characteristics of Post Insulator

4.1. Theoretical Analysis of Insulator Icing Mass

The related work in reference [28] studied the icing characteristics of transmission lines and proposed relevant formulas. In this paper, we extend these formulas to insulators based on reference [23]. Using these formulas, the trend of how various parameters affect the icing mass can be understood, and the analysis of icing on the test insulators can provide useful insights. The icing mass of the conductor per unit time can be expressed by Equation (1) [23]:
m = ∫ 0 t   α 1 ⋅ α 2 ⋅ α 3 ⋅ w ⋅ U ⋅ D ⋅ L ⋅ d t i
In the equation, α1 is the collision coefficient, α2 is the collection coefficient, typically set to 1, α3 is the freezing coefficient, w is the liquid water content, U is the wind speed, D is the diameter of the icing cylinder, and L is the length of the cylinder.
α1 can be expressed by Equation (2) [28]:
α 1 = A − 0.028 − C ( B − 0.0454 ) ,
where
A = 1.066 K − 0.00616 e x p ⁡ ( − 1.103 K − 0.688 ) B = 3.641 K − 0.498 e x p ⁡ ( − 1.497 K − 0.694 ) C = 0.00637 ( φ − 100 ) 0.381 .
A, B, and C are determined by two dimensionless parameters, K and φ, which can be express by Equation (4).
K = 2 D d 2 ρ d U 9 μ D φ = R e 1 2 K R e 1 = ρ a d v 1 μ
In the equation, Dd is the droplet diameter, ρd is the droplet density, U is the wind speed, μ is the air viscosity, and D is the conductor diameter, Re1 is the droplet Reynolds number, v1 is free stream velocity.
According to reference [23], the expression for the freezing coefficient α3 is
α 3 = π h ( T s − T ) + 0.62 π h L e c a p [ e ( T s ) − e ( T ) ] α 1 w U [ L f + c i ( 273.15 − T s ) + c w ( T s − T ) ]                         + 4 π ε σ R T 3 ( T s − T ) α 1 w U [ L f + c i ( 273.15 − T s ) + c w ( T s − T ) ]                             + c w ( T s − T ) + c w ( 273.15 − T ) − h r c U 2 c a α 1 w − U 2 2 L f + c i ( 273.15 − T s ) + c w ( T s − T )
In the equation,
e T = 0.61121 e x p ⁡ 18.678 − T 234.5 257.14 + T × T N u = C R e 2 n P r 1 / 3 h = k a N u D i R e 2 = ( D + 2 d i ) U / v 2 P r = v 2 / a
In Equations (5) and (6), h is the convective heat transfer coefficient, J/(m2·K); T is the ambient temperature, K; Ts is the surface temperature of the ice during dynamic equilibrium, K; Le is the latent heat of evaporation or sublimation at Ts, Le = 2.51 × 106 J/kg; ci is the specific heat capacity of ice, ci = 2090 J/(kg·°C); ca is the specific heat capacity of air, ca = 1014 J/(kg·K); w is the liquid water content, g/m3; U is the flow velocity, m/s; Lf is the latent heat of fusion of ice, typically taken as 3.35 × 105 J/kg; cw is the specific heat capacity of water, cw = 4.2 × 103 J/(kg·K); rc is the local viscous heating recovery coefficient at the surface of the cylindrical conductor, taken as 0.79; ka is the heat transfer coefficient of air, ka = 0.0244 W/(m·°C); Di is the diameter of the iced cylindrical body, m; σR is the Stefan-Boltzmann constant, σR = 5.670 × 10−8 W/(m2·K4); D is the diameter of the icing cylindrical body, m; di is the ice thickness, m; v2 is the air viscosity, v2 = 1.328×10−5 m2/s; a is the thermal diffusivity of air, a = 1.88 × 10−5 m2/s; Nu is the Nusselt number; C and n are constants, with C = 0.683 and n = 0.466 in icing environments; Pr is the Prandtl number.
For insulators, during icing, they are equivalently modeled as a cylinder with the same height and surface area [23]. The equivalent diameter is
D 1 = S 2 π + h 2 4 − h 2
In the equation, S is the surface area of the insulator and h is the height of the insulator.
From Equations (1)–(7), it is evident that for cylindrical conductors, the icing mass per unit time is strongly influenced by environmental factors such as liquid water content (w), wind speed (U), conductor diameter (D), and length (L), all of which exhibit a positive correlation. In other words, as these factors increase, the icing mass on the conductor also increases. Under surface icing dynamic equilibrium in natural environments, lower temperatures promote faster icing growth on the conductor. However, due to the more complex structure of insulators, including variations in the sheds design, differences in icing are expected. The use of Equation (7) for equivalence may therefore lead to discrepancies with real conditions of the Station. Furthermore, the significant variation of these influencing factors in natural environments complicates the application of these formulas for quantitative calculations. As a result, these equations are employed in this study primarily for qualitative analysis. The main focus is to examine the actual icing growth curves of different insulators based on their specific icing characteristics, with Section 4.2 and Section 4.3 providing a detailed discussion on the icing growth patterns under various conditions.

4.2. Analysis of Glaze Icing Characteristics

To analyze the growth characteristics of glaze ice on post insulators, this study uses Type II, III, and V post insulators as examples, observing changes in icicle length and icing mass over the icing period. Additionally, the thickness of ice on a rotating conductor was recorded. The experimental results are shown in Figure 10a–d. Figure 10a–c depict the variation curves of icicle length, icing mass, and rotating conductor ice thickness over time, while Figure 10d illustrates the relationship between insulator icing mass and rotating conductor ice thickness.
From Figure 10a–d, we observe the following:
  • The icicle length and icing mass of the insulators exhibit a nonlinear growth pattern over time, initially increasing rapidly before slowing as they approach saturation. For example, during the icing periods of 0–8 h, 8–13 h, and 13–18 h, the icicle length of Type V insulator increased by 48 mm, 18 mm, and 16 mm, respectively, while the icing mass increased by 6.92 kg, 3.66 kg, and 1.78 kg, respectively. This pattern is likely due to the substantial rainfall during the 0–8-h period (as shown in Figure 6), which provided an abundant supply of supercooled water. Combined with relatively high wind speeds, this increased the collision rate between water droplets and the insulator, accelerating heat exchange and enhancing the freezing coefficient, thereby facilitating ice formation. During the 8~13 h period, the rainfall diminished, leading to a slower icing growth rate. After 13 h, the rainfall further decreased, and most of the icicles had bridged, with their length remaining constant. At this stage, the icing on the insulator reached saturation, filling the gaps between the sheds and reducing the surface area, which in turn caused a decrease in the effective diameter. This trend was most pronounced in the Type V insulator.
  • When comparing the three types of insulators during the 0–8-h icing period, the icicle lengths for Type II, III, and V insulators were 34 mm, 38 mm, and 48 mm, respectively, with icing masses of 2.64 kg, 3.89 kg, and 6.92 kg. These results indicate that the Type V composite insulator had the fastest icing growth rate, while the Type II porcelain insulator had the slowest. The primary reason for this difference is that the composite insulator exhibits excellent hydrophobicity, which prevents water droplets from spreading across its surface and promotes the direct formation of icicles along the shed edges. In contrast, the hydrophilic surface of the porcelain insulator causes water droplets to spread into a film, hindering icicle formation. Additionally, the Type V composite insulator has the largest rod diameter, surface area, shed extension, and shed spacing, which enables it to capture more water droplets. The thinner shed edges of the composite insulator further facilitate the formation of droplets that freeze into icicles, resulting in a faster icing growth rate compared to porcelain insulators.
  • The icing thickness trend on the rotating conductor over time is consistent with the icing parameters of the insulators. Figure 10d presents the relationship curve between insulator icing mass and the rotating conductor icing thickness, indicating an approximately linear relationship between the two. Thus, the icing thickness on the rotating conductor can serve as an indicator of the insulator’s icing level.

4.3. Analysis of Mixed-Phase Icing Characteristics

To analyze the growth characteristics of mixed-phase icing on post insulators, this study takes Type II, III, and V insulators as examples, observing the variation in icing mass during the icing period and recording the icing thickness on the rotating conductor. The experimental results are shown in Figure 11. Panels (a) and (b) display the variation curves of insulator icing mass and rotating conductor icing thickness over time, respectively, while (c) illustrates the relationship curve between insulator icing mass and rotating conductor icing thickness.
From Figure 11a–c, the following can be observed:
  • The icing mass of the three types of insulators exhibits an initial rapid growth, followed by a slower phase, and then a resurgence of faster growth as icing time progresses. Taking the Type V insulator as an example, the increase in icing mass during the periods of 0–10 h, 10–20 h, and 20–30 h were 15.48 kg, 4.88 kg, and 14.92 kg, respectively. This pattern can be explained as follows: During the 0–10 h period (as shown in Figure 9), the low environmental temperature and high wind speed speed accelerated the heat exchange rate between the water droplets on the insulator surface and the surrounding environment. As the insulator’s effective area increased with icing, the likelihood of collision with water droplets also increased, leading to faster icing growth. During the 10–20 h period, the environmental temperature and relative humidity remained relatively constant, while wind speed gradually increased to 10 m/s. At this point, the increase in the insulator’s effective diameter reduced the probability of supercooled water droplets colliding with the insulator. The decrease in collision probability outweighed the increase in effective area, resulting in a slower icing growth rate. In the 20–30 h period, rainfall began, causing a significant amount of rainwater to infiltrate the rime on the insulator surface. This trapped water then froze under the influence of low temperatures and cold winds, transforming the brittle rime into a dense, block-like mixed-phase ice, which increased both the density and mass. As a result, the icing growth rate accelerated again during this stage.
  • The icing growth rate of insulators is influenced by their structural design. For example, during the icing period from 0 to 10 h, the icing mass increments for Type II, III, and V insulators were 7.24 kg, 10.38 kg, and 15.48 kg, respectively. This indicates that, within the same icing period, the Type V insulator experienced the largest increase in icing mass, while the Type II insulator showed the smallest. This difference can be attributed to the Type V insulator’s larger rod diameter, surface area, and shed spacing, compared to the smaller dimensions of the Type II insulator. As a result, under identical conditions, the surface of the Type V insulator captures more water droplets, leading to a faster icing growth rate, while the Type II insulator has a relatively slower growth rate.
  • Similar to glaze icing, the rotating conductor icing thickness over time shows a trend consistent with the insulator icing mass, with an approximately linear relationship between the two.

5. Icing Difference Between Post Insulators and Suspension Insulators

Figure 12a,b compares the rime icing and mixed-phase icing formations on a post insulator (I) and porcelain suspension insulator (II) over the same time period, where the icing duration in (a) is 14 h, and in (b), it is 22 h.
As shown in Figure 12,
  • For both rime icing and mixed-phase icing, icing was more severe on the windward side than on the leeward side. The reasons for this are consistent with the analysis in Section 3 and will not be repeated here.
  • By comparing the rime icing morphology of post insulators and suspension insulator strings (Figure 12a), it can be observed that under the same icing conditions and time, the gaps between the sheds on the windward side of the post insulator are densely filled with rime ice. In contrast, the suspension insulator string exhibits more ice accumulation on the upper surfaces, with a more dispersed distribution. This difference can be attributed to the greater number of large and small sheds on the post insulators, as well as the shorter spacing between them, which facilitates the densification of rime ice in the air gaps between the sheds under the same icing conditions. Reference [29] suggests that the wedge angle can influence icing. Additionally, the angles between the sheds and the core rod of post insulators, as well as those between the sheds and the steel feet or caps of suspension insulators, vary. These differences may contribute to the variations in ice accumulation.
  • Figure 12b shows the icing morphology of post insulators and insulator strings under mixed-phase icing conditions. A comparison of the two reveals that on one side of the post insulator, the gaps between the sheds were completely filled with mixed-phase ice, while on the other side, icicles had formed bridges between the adjacent large sheds. In contrast, the mixed-phase ice on one side of the suspension insulator string did not fully fill the gaps between the sheds, and the icicles on the other side only spanned half the distance between the sheds. This indicates that under the same icing conditions, the air gaps between the sheds of the post insulator are more easily filled with ice and bridged by icicles, posing a more severe insulation risk.
  • Reference [13], which investigates FXBW4-220 composite insulators, found that factors such as the shed angle, rod diameter, and the shed ratio also influence ice accumulation. Notably, the maximum ice thickness was observed on the windward side of the core rod, which contrasts with the icing characteristics seen in both post and suspension insulators.

6. Conclusions

  • This study, conducted at the Xuefeng Mountain Energy Equipment Safety National Observation and Research Station, investigated natural ice accretion on seven types of post insulators to analyze their icing morphology and characteristics. The main conclusions are as follows.
  • Icing distribution: In the early stage of glaze icing, icicles are longer and thicker on the upper portion of the insulator, with the difference diminishing over time. For rime and mixed-phase icing, growth initially occurs on the upper surfaces and becomes more uniform in the later stages. Icing is consistently more severe on the windward side.
  • Icing growth rates: Under glaze icing, ice mass initially increases rapidly, then slows down. For type V composite post insulators, the ice mass increased by 6.92 kg during the first 0–8 h, 3.66 kg during the 8–13 h period, and 1.78 kg in the 13–18 h period. Notably, the icing rate during the first 5 h (without rain) was similar to that from 5 to 8 h (with rain). In mixed-phase icing, the growth rate follows a nonlinear pattern: fast, slow, and fast again. During the 0–8 h period, the increase in ice mass was 15.48 kg, significantly higher than the 4.88 kg observed during the 10–20 h period. A sharp increase in ice mass (14.92 kg) was noted in the 20–30 h period, driven by rainfall that triggered the transition from rime to mixed-phase icing. The ice mass on rotating conductors exhibited a nearly linear increase, similar to that observed on the insulator.
  • Environmental effects: Icing growth is influenced by temperature (below 0 °C required), wind speed, humidity, and precipitation, with higher values accelerating ice accumulation.
  • Insulator comparison: Post insulators, with shorter shed spacing, accumulate ice more quickly than suspension insulators under the same conditions. After 14 h of rime icing, post insulators show ice filling the gaps between sheds, while suspension insulators still have significant gaps. After 22 h of mixed-phase icing, post insulators have fully bridged, while suspension insulators have only bridged half of the gap between sheds.
  • This study has some limitations. Primarily, it relies on experimental observations and lacks a theoretical analysis of energy transfer and related processes. Future research should focus on developing simulation models grounded in theoretical frameworks. Additionally, the impact of altitude and atmospheric pressure on icing characteristics has not been addressed in this study. It is recommended that future experiments be conducted at observation stations situated at varying altitudes to investigate these factors.

Author Contributions

Conceptualization, Z.Z. (Zhijin Zhang) and X.J.; methodology, Z.Z. (Zhijin Zhang); validation, Z.Z. (Zhijin Zhang), J.T. and Y.Z.; investigation, J.T.; Visualization, J.T.; writing—original draft preparation, J.T. and Y.Z.; writing—review and editing, Z.Z. (Zhijin Zhang) and J.T.; supervision, Z.Z. (Zhijin Zhang) and X.J.; project administration, Z.Z. (Zhijin Zhang) and X.J.; funding acquisition, Z.Z. (Zhenbing Zhu). All authors have read and agreed to the published version of the manuscript.

Funding

This research is sponsored by the Natural Science Foundation of Chongqing, China (No. CSTC2024YCJH-BGZXM0060, CSTB2023NSCQ-LZX0021) and the Central University Basic Research Funds Project (No. 2023CDJYXTD-005).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are unavailable due to privacy.

Acknowledgments

The authors would like to extend their heartfelt gratitude to the professors and teachers in their research group (Jianlin Hu, Lichun Shu, Qin Hu, Hualong Zheng, Guolin Yang, and Yutai Li) for their guidance and support during the experiments for this thesis. Their insightful suggestions greatly contributed to the progress of their research, and they are deeply thankful. The completion of this thesis would not have been possible without the assistance and collaboration of the fellow group members. The authors sincerely thank their colleagues, and senior and junior fellow students, for their help with the experiments. It was through everyone’s efforts that the experiments were successfully conducted and completed.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Physical diagram of post insulators.
Figure 1. Physical diagram of post insulators.
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Figure 2. Xuefeng Mountain Energy Equipment Safety National Observation and Research Station.
Figure 2. Xuefeng Mountain Energy Equipment Safety National Observation and Research Station.
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Figure 3. The rotating conductor and vernier caliper.
Figure 3. The rotating conductor and vernier caliper.
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Figure 4. Glaze icing at different times for Type I to VII insulators: (a) t = 5 h; (b) t = 10 h; (c) t = 15 h.
Figure 4. Glaze icing at different times for Type I to VII insulators: (a) t = 5 h; (b) t = 10 h; (c) t = 15 h.
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Figure 5. Glaze icing at different times. Glaze icing on the windward side and lateral side of type VI post insulator: (a) windward side; (b) leeward side.
Figure 5. Glaze icing at different times. Glaze icing on the windward side and lateral side of type VI post insulator: (a) windward side; (b) leeward side.
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Figure 6. Changes in environmental parameters during the glaze icing process: (a) temperature; (b) wind-speed; (c) relative humidity; (d) rainfall.
Figure 6. Changes in environmental parameters during the glaze icing process: (a) temperature; (b) wind-speed; (c) relative humidity; (d) rainfall.
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Figure 7. Mixed-phase icing at different times for type II, III and V insulators: (a) t = 7 h; (b) t = 20 h; (c) t = 30 h.
Figure 7. Mixed-phase icing at different times for type II, III and V insulators: (a) t = 7 h; (b) t = 20 h; (c) t = 30 h.
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Figure 8. Mixed-phase icing on different sides of type II post insulator: (a) Windward side; (b) Leeward side.
Figure 8. Mixed-phase icing on different sides of type II post insulator: (a) Windward side; (b) Leeward side.
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Figure 9. Changes in environmental parameters during the mixed-phase icing process: (a) temperature; (b) windspeed; (c) relative humidity; (d) rainfall.
Figure 9. Changes in environmental parameters during the mixed-phase icing process: (a) temperature; (b) windspeed; (c) relative humidity; (d) rainfall.
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Figure 10. Curve of glaze icing parameters changing with time: (a) Variation curve of icicle length over time; (b) Variation curve of icing mass over time; (c) Variation curve of rotating conductor icing thickness over time; (d) Relationship curve between icing mass and rotating conductor icing thickness.
Figure 10. Curve of glaze icing parameters changing with time: (a) Variation curve of icicle length over time; (b) Variation curve of icing mass over time; (c) Variation curve of rotating conductor icing thickness over time; (d) Relationship curve between icing mass and rotating conductor icing thickness.
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Figure 11. Curve of mixed-phase icing parameters changing with time: (a) Variation curve of icing mass over time; (b) Variation curve of rotating conductor icing thickness over time. (c) Relationship curve between icing mass and rotating conductor icing thickness.
Figure 11. Curve of mixed-phase icing parameters changing with time: (a) Variation curve of icing mass over time; (b) Variation curve of rotating conductor icing thickness over time. (c) Relationship curve between icing mass and rotating conductor icing thickness.
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Figure 12. Comparison of the icing morphology between post insulator (I) and suspension insulator (II): (a) glaze icing; (b) rime icing.
Figure 12. Comparison of the icing morphology between post insulator (I) and suspension insulator (II): (a) glaze icing; (b) rime icing.
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Table 1. Structural parameters of the test post insulators.
Table 1. Structural parameters of the test post insulators.
Serial Number of InsulatorsD (mm)P1/P2 (mm)H (mm)L (mm)S (cm2)
I11072/561150380522,717
II13850/351200321518,048
III10067/521500502724,966
IV10649/341500390918,074
V36868/521800595580,920
VI18575/591200419131,483
VII88.450/261230352413,586
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Zhang, Z.; Tu, J.; Zhang, Y.; Jiang, X.; Zhu, Z. The Icing Characteristics of Post Insulators in a Natural Icing Environment. Atmosphere 2025, 16, 64. https://doi.org/10.3390/atmos16010064

AMA Style

Zhang Z, Tu J, Zhang Y, Jiang X, Zhu Z. The Icing Characteristics of Post Insulators in a Natural Icing Environment. Atmosphere. 2025; 16(1):64. https://doi.org/10.3390/atmos16010064

Chicago/Turabian Style

Zhang, Zhijin, Jiahui Tu, Yuanpeng Zhang, Xingliang Jiang, and Zhenbing Zhu. 2025. "The Icing Characteristics of Post Insulators in a Natural Icing Environment" Atmosphere 16, no. 1: 64. https://doi.org/10.3390/atmos16010064

APA Style

Zhang, Z., Tu, J., Zhang, Y., Jiang, X., & Zhu, Z. (2025). The Icing Characteristics of Post Insulators in a Natural Icing Environment. Atmosphere, 16(1), 64. https://doi.org/10.3390/atmos16010064

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