Icing Flashover Voltage Research Articles

AC flashover characteristics and arc development process of glaze ice-covered insulators in natural environment

Ice accretion on the insulators can reduces the creepage distance to a certain extent as low as the dry arc distance, which may cause flashover trip and subsequent blackout. Extensive research on insulator icing has been carried out in a controlled-climate room. Due to great differences in environmental parameters between laboratory simulation and field tests, icing characteristics and discharge development of ice-covered insulator obtained in the laboratory cannot veritably reflect natural icing law and arc propagation process. Consequently, in the present study the glaze-icing on energized insulators was investigated on site, and the mechanism of icicle growth on the insulator energized with different voltage levels was revealed. Meanwhile, AC flashover performances and arc discharge development of ice-coated insulators for the two test conditions were compared and discussed. Results show that for a weak electric field, icicle firstly grows vertically, and then bends to the axis of the insulator when its length reaches to a certain level. Nevertheless, icicle develops towards the axis at the beginning under a high electric field. AC flashover voltage of ice-accreted insulators increases exponentially with an increasing applied voltage, while the relation of flashover voltage and ice weight fits well with a negative power function. Icing flashover voltage of the insulators obtained in natural environment is 14%～21% higher than that in the chamber. Compared with the flashover in the laboratory, during the discharge of the insulators naturally covered with ice, arc extinction-reignition and floating phenomena are more significant, and the ice-melting is more severe.

Novel dc flashover model for predicting flashover voltage of the iced insulators

This work first proposes a novel flashover model based on the deduced residual resistance of the ice layer. The most important finding of the field icing tests is that the double-layer ice-melting water film exists on the inner and outer surfaces of the ice layer. The conductivity of melting water on the inner and outer surface of the ice layer has been measured, and the corresponding surface conductivity has been calculated. According to the proposed flashover model, the ratio η of the resistance of the ice layer to that of the pollution layer gradually increases with the increase of the ratio k of the resistance of the outer surfaces to that of inner surfaces, and eventually approaches saturation. Furthermore, the icing flashover criterion has been modified, and the voltage gradient of the inner surface is less than that of the outer surface. Test results show that the ice flashover voltage is lower than the pollution flashover voltage between 2.18–9.69% and 14.75–20.32% under light ice and semi-cylindrical ice, respectively. The simulation results based on the proposed model and the classic model are compared with the field test data. Results show that the proposed model could attain a high accuracy of dc flashover voltage and demonstrate the effectiveness of the proposed model.

Study of inverted T-shape insulator strings in icing conditions

For power transmission lines operation in icing conditions flashover problem of insulator strings is a serious and intractable problem. Once it happens, can lead to power line face tripping and longtime power outage. This paper focus on lab based experimental study of analyzing inverted T-shape insulator strings configuration in icing conditions. Experimental results show that inverted T-shape insulator strings have better insulation performance under icing conditions compared to ordinary suspension insulator strings (I-shape insulator strings). Analysis shows that inverted T-shape configuration leads to improvement in 50% icing flashover voltage range from 4.79% to 11.92%. Furthermore, 50% icing flashover voltage of inverted T-shape insulator strings satisfy negative power function with both freezing water conductivity and equivalence ice thickness. Results indicate that different ice morphology and flashover development process lead to the insulation improvement of inverted T-shape insulator strings. Based on the analysis on Ice Stress Parameter (ISP) and insulation improvement, some suggestions on configuration of inverted T-shape insulator strings are presented in this paper.

Preliminary study on icing and flashover characteristics of inverted T-type insulator strings

Insulator icing flashover has always been one of the important factors influencing the safety of power grids in winter. Due to the limitation of tower structure and distance between the conductor and the ground, the existing anti-icing flashover method has certain effects but still cannot fully meet the safety requirements of power grids. Based on the idea of changing the ice coverage and discharge path of insulator string, a brand-new insulator string that arranged as inverted T-type is proposed in this paper to achieve the purpose of reducing ice coating and increasing the ice flashover voltage. The icing and flashover experiments are simulated in an artificial climate room. The experiment results show that the ice amount of inverted T-type insulator is smaller than that of the same type and same creepage distance of suspension I-string, and the inclined portion of the inverted T-type arrangement is basically free from bridges under glaze and mixed-phase ice conditions. In the case of the same creepage distance, dry-arc distance, the same degree of contamination, and the ice-covering condition of the same type of insulator, the melting flashover voltage gradient E50 of the inverted T-type string is 14.8–16.3% higher than that of the suspended I-string. The results show that after the insulator adopts inverted T-type arrangement, the ice coating can be significantly reduced and the ice flashover voltage can be increased. The conclusions of the study provide a new solution for the anti-icing flashover treatment and design of the overhead lines.

Voltage Distribution and Flashover Performance of 220 kV Composite Insulators under Different Icing Conditions

Composite insulators are widely used in modern power systems to provide electrical insulation and mechanical support for transmission lines and substations. However, the insulation strength will decrease greatly under the combined conditions of ice-covering and contamination, and icing flashovers may take place under these serious conditions. In this paper, AC flashover tests of different artificially ice-covered 220 kV composite insulators were carried out in a multi-function artificial climate chamber under energized ice accumulation conditions. The test results indicate that, with the increasing of ice thickness, the flashover voltages decrease and tend to saturation. The icing flashover voltages can be increased by adding booster sheds, but excessive booster sheds can lead to lower flashover voltages under heavy icing conditions. The voltage distributions of the iced insulators were measured using experimental methods. The results show that, the air gaps withstand most of the applied voltage. The zinc oxide (ZnO) resistors that are contained in the insulators can influence the voltage distributions of the iced insulators, but have little affect on the icing flashover voltages. The work done in this paper can provide reference for the design and type selection of outdoor composite insulators in cold climate regions.

Experimental study on ice-covered samples of composite material tower

Ice accretion of overhead transmission composite tower is the most occurred and serious problem in power system. In order to decrease the icing effect, hydrophobic coatings are normally implemented on the surface of composite towers. This paper conducted a series of experiments on composite tower samples that coated with hydrophobic coatings of PRTV, IC701 and DS-02 respectively. Firstly, the adhesive force at the ice-coating interface was studied and the results indicated that PRTV leads the rank and has the lowest adhesion which is 20% less than pure aluminum sample and then it is closely followed by IC701. The experiment of coating effects on icing process illustrated that coated samples have insignificant thinner ice covering and less ice amount where DS-02 samples own the thinnest and IC701 samples own the minimum ice shedding time. Additionally, three icing flashover tests were carried out and it is found that IC701 and PRTV own much higher flashover voltage for both horizontally and vertically arrangement tests and vertical arranged samples have a higher discharge voltage than the horizontal arranged ones. Besides, it was observed that the discharging channel most occurs along the ice melting formed water flow path. The last experiment displayed that PRTV and IC701 coated samples have much better voltage resistance performances and the ice shedding time is shorter than the blank and DS-02 sample. In consequence, coatings for composite towers especially PRTV and IC701 can improve the icing flashover voltage to a certain extent and have an effective anti-icing characteristic.

DC flashover performance of ice‐covered insulators under complex ambient conditions

The electrical characteristics of ice-covered insulators in coexisting conditions of high altitude, icing and pollution are a basis for external insulation selection of transmission lines in icing regions. Ice accretion on insulators has been studied for decades, while few studies focus on the flashover performance of ice-covered insulators polluted by two types of contamination, that is the solid layer method and icing water conductivity method, simultaneously. On the basis of the investigations on DC flashover performance of ice-covered insulators in the artificial climate chamber, the influence of different parameters, such as insulator string length, icing water conductivity, salt deposit density as well as atmospheric pressure on DC icing flashover voltage of insulator strings is studied. According to the experimental results, the relationships between the flashover voltage and each of such factors are determined and analysed. Moreover, the correction formula for DC flashover voltage of ice-covered insulators in complex ambient conditions is proposed.

Crystallisation effect of conductive ions in freezing water during phase transition and its effect on ice flashover voltage

Icing experiments on three units of XP4-160 insulator string are conducted to reveal the influence of the crystallisation effect of conductive ions during phase transition on melting water conductivity and ice flashover voltage, and to analyse the motion of conductive ions and its spatial distribution. The influence of different parameters, such as freezing water conductivity, electric field and pollution, on crystallisation effect is examined and analysed. Results of the tests indicate that the melting water conductivity of the ice layer and icicles is considerably greater than that of freezing water, particularly for a polluted insulator, because of the crystallisation effect. The electric field has a significant influence on the crystallisation effect, that is, the melting water conductivity under a non-energised condition is higher than that under an energised condition. Finally, through 35 kV insulator flashover tests, the reason for the propagation of partial arcs over icicles is determined, and the flashover process is illustrated. The crystallisation effect during phase transition changes the ion distribution in a solid ice layer, thereby increasing water film conductivity, and is also a critical reason for the decline of ice flashover voltage.

Effects of icing degree on ice growth characteristics and flashover performance of 220 kV composite insulators

Icing degree is one of the important factors influencing insulator's icing flashover performance. But there may be differences between the effects of icing degree under non-energized and energized icing conditions. For this purpose, the comparative tests of non-energized and energized glaze ice accumulation are conducted on 2 types of typical 220kV composite insulators under different icing degrees in this paper. Combining with the numerical calculation of electric field distribution, the monitoring of leakage current, the measurement of the ice-melting water conductivity and the observation of flashover arc paths, the influences of icing degree on ice growth characteristics and flashover voltages of composite insulators are deeply analyzed. Results indicate that with the increase of icing degree, all relevant ice parameters gradually increase but their regularities are related to the shed configuration and the position of sheds as well as whether the ice deposits on non-energized or energized insulators. These variations lead to differences of the quantity and length of icicle air gaps and the conductivity of the water film on the ice surface, and then affect the flashover arc path and icing flashover voltage. The icing flashover voltages of energized ice-covered insulators are higher than that under non-energized condition and their difference becomes larger with the increase of icing degree while the effect of shed configuration on the icing flashover performance gradually becomes less apparent when the icing becomes heavier.

Comparison of AC icing flashover performances of 220 kV composite insulators with different shed configurations

The shed configuration has obvious effects on the flow field characteristics and electric field distributions around composite insulators, and then impacts their icing flashover performance. In this study, the AC flashover comparative tests for 12 types of 220 kV composite insulators with different shed configurations under non-energized and energized ice accumulation conditions are conducted in the artificial climate chamber. The influences of shed configuration on icing flashover voltages and flashover arc paths are analyzed. Results indicate that with the increase of the icing stress, both the flashover arc path and the ratio of the air gap arc gradually decrease. These variations lead the icing flashover voltage to gradually decrease and then approach to saturation. For ice-covered composite insulators with different shed configurations, the ratios of the air gap arc to the total flashover arc are different due to the differences of their shed spacings, icicle lengths, differences of shed diameters, (extra) large shed diameters. These result in the differences of icing flashover voltages and their characteristic exponent c describing the influence of ice thickness. Besides, for insulators iced under energized condition, their icing flashover voltages and flashover arc paths are different from that under non-energized condition. Hence, the energized icing test, which is also the only normative reference of icing test proposed by the IEEE standard, should be carried out to optimize the shed configuration of composite insulators used in ice areas.

Ice flashover performance and its characterization parameter of composite insulator with booster sheds

In order to investigate the ice flashover performance and characterization parameter of composite insulators, numerical calculation was applied to simulate the icicle growth and make comparison with different structures. It was found that the number of booster sheds existed an optimized range and saturated icicle bridged degree of big sheds or booster sheds could be considered as the characterization parameter to reflect the ice flashover voltage. Booster sheds applied to 500 kV composite insulators show its advantage on anti-icing performance. However, when design for lower structural height insulators, such as 110 kV composite insulators, excessive booster sheds could form a shorter flashover path. Experiments show there is a good corresponding relation between flashover voltage and saturated icicle bridged degree. Moreover, when applied to 500 kV composite insulators, saturated icicle bridged degree of big sheds dominated the flashover process, while the value of booster sheds played more important role for 110 kV samples.

Influence of shed configuration on icing characteristics and flashover performance of 220 kV composite insulators

Shed configuration is one of the important factors affecting the icing characteristics and icing flashover performance of composite insulators. In this paper, the non-energized and energized glaze icing tests are conducted on 12 types of typical 220 kV composite insulators with different shed configurations in the artificial climate chamber. Combining with numerical calculations of electrical field distributions, the monitoring of leakage current, the measurement of the ice-melting water conductivity and the shooting of flashover arc paths, the influences of shed configuration on the ice growth and icing flashover voltages of composite insulators are comprehensively analyzed. Results indicate that a larger shed diameter brings about the faster ice growth and the lower average electric field strength across the icicle air gap, which leads to the reduction of the icing flashover voltage. When the shed spacing is larger, both the icicle length and ice thickness become larger, and the ice-melting water conductivity during the flashover is higher. But the ratio of icicle air gap arc to the flashover arc is higher and its effects on the icing flashover performance are more obvious, which leads to the improvement of the icing flashover voltage. When the ice accretes on the energized insulator, the ice-melting water conductivity is smaller while the ratio of icicle air gap arc to the flashover arc is larger than that under non-energized icing condition. These variations lead to the higher icing flashover voltages for energized insulators. It is thus suggested that the icing and flashover tests under different icing degrees should be conducted on energized composite insulators to deeply study the influences of shed configuration.

Effect of grading ring on ice characteristics and flashover performance of 220 kVcomposite insulators with different shed configurations

The energized icing tests for five kinds of 220 kV composite insulators with and without the grading ring (GR) at the high voltage (HV) end are performed in the artificial climate chamber. Combining with the numerical simulations of electric field (E-field), the differences between the influence of GR and shed configuration on the ice characteristics and flashover performance of composite insulators are compared and analyzed. Results indicate that GR can improve both the ice growths and E-field distributions near the HV end. The improvements of GR on the icing flashover voltages under light icing degree is relatively obvious, while with the increase of icing degree, these effects gradually become less apparent. For different insulators, the effects of icing degree on icing flashover voltages under the presence of GRs are all higher than that under the absence of GRs. Moreover, the influence law of icing degree among different insulators remains unchanged under the presence and absence of GRs. In general, the improvement of optimized shed configuration for the icing flashover performance is more obvious than that of GR, which can be considered as one main measure to improve insulation performance of composite insulators in ice regions.

Influences of grading ring arrangement on AC flashover performance of 220 KV ice-covered composite insulators

The grading ring (GR) can adjust the potential distribution along the composite insulator and reduce the electric field (E-field) strengths near its end fittings. However, little research has been conducted about the effects of GR on the insulation performance of composite insulators under icing condition. Therefore, the ac icing flashover performance of two kinds of 220 kV composite insulators under different GR arrangement patterns are tested in the artificial climate chamber. Combining with the numerical simulation, the effects are analyzed. The results indicate that when GR is installed at the high voltage (HV) end, the E-field strength close to the end fitting and the icing uniformity on the whole insulators are both improved. These cause the effect of icing degree on the icing flashover voltage to be larger than that without GR at the HV end. When GR is installed at the grounded end, the perpendicular icicles on the GR will impact the flashover arc path, which causes the icing flashover voltages to be lower than that without GR at the grounded end. When different GRs are only installed at the HV end, the characteristic exponents b (characterizing the influence of freezing water conductivity on icing flashover voltage) are all larger than that without GR. Moreover, the value of b increases with the decrease of GR’s diameter and with the raise of GR’s height. But b changes slightly when there are variations in GR’s pipe diameter and structure. The research can provide reference for the application of GR for 220 kV composite insulators in icing regions.

Effects of electric field distribution on icing and flashover performance of 220 kV composite insulators

The energized icing of composite insulators has been studied by the researchers around the world, but not much about the effects of electric field distribution on their icing and flashover performance. For this purpose, the ice accretion and flashover performance of 220 kV composite insulators with two shed configurations on occasions of different electric field distributions considering the effect of grading rings are analyzed in this paper. The test results indicate that the intensity and homogeneous distribution of electric field have positive effects on the appearance, density and weight of ice; the improvement of electric field distribution after installing grading ring has more effects than the icing characteristics, such as quality, density as well as appearance of ice, on flashover voltages of ice-covered composite insulators; the icing flashover voltages are increased by the improvement of electric field distribution and the increasing ranges of icing flashover voltage were relied on shed configuration. Icing accumulation patterns have less effect on flashover performance of composite insulators with alternatediameter or widely spaced shed configurations.

Comparison on ac icing flashover performance of porcelain, glass, and composite insulators

The electrical performance of insulators under the comprehensive conditions of low air pressure, pollution and icing, is an important basis for the selection of external insulation of transmission lines and substations in icing regions. However, little research has been dedicated to the comparison on ac icing flashover performance of composite, porcelain and glass insulators in this environment. Based on the investigations carried out in the artificial climate chamber on three types of iced insulators, the ac flashover performances of insulators were researched in this paper. In addition, the paper analyzed and compared the effects of various factors, including ice thickness, pollution and air pressure on the flashover performance of three types of iced insulators. The experimental results show that the flashover voltage of three types of insulators decreased with the increase of ice thickness, pollution, and the altitude. The characteristic exponent characterizing the influence of the ice thickness, pollution and atmospheric pressure on the flashover voltage were related with the insulator types. The effect of ice thickness and atmospheric pressure on icing flashover voltage was more apparent for composite insulator than porcelain and glass insulator. The characteristic exponent characterizing the influence of pollution on the flashover voltage was small for composite insulators. Under the same condition, the flashover voltage gradients of ice-covered composite insulators are slightly greater than porcelain and glass insulators.

Effect of shed configuration on DC flashover performance of ice-covered 110 kV composite insulators

Composite insulators have been used in power system for more than 20 years in China. Compared to porcelain and glass insulators, the pollution flashover performance of composite insulators is better, whereas the icing flashover performance is worse. Shed configuration is a key factor that influences flashover voltage of composite insulators covered ice or snow in icing regions. The DC flashover tests of artificially ice-covered 110 kV composite insulators with different shed configuration were carried out in the multi-function artificial climate chamber. The test results revealed that there are huge differences between the icing flashover voltages of the composite insulators with different shed configuration. The test results indicate that icing flashover performance of the improved composite insulators for icing regions is not always superior to that of the standard composite insulators.

Study of DC flashover performance of ice-covered insulators at high altitude

This paper examines the direct current (DC) icing flashover performance of typical DC porcelain, glass, and composite insulators in a complex environment of high altitude, icing, and pollution. The effects of the insulator string length, ice thickness, pollution level, freezing water conductivity, air pressure, and arrangement of insulator strings on the DC icing flashover performance are analyzed, and the correction formula for the DC icing flashover voltage is obtained. The DC flashover process of ice-covered insulators at high altitude is classified into dynamic changes of surface arcs, air-gap arcs, and residual resistance of the ice layer, as observed through a high-speed camera, and a DC flashover circuit model explaining the flashover performance for ice-covered insulators at high altitude is established.

Mechanism of salt migration in icicles during phase transition and its impact on ice flashover

This paper addresses the salt migration during phase transition of cooling water in the icicle growth period. This paper contains three parts, first, application of Fick's law in salt migration process; second, salt concentration measurement of icicles in artificial climate chamber; third, ice flashover experiments during melting period. Based on freezing model analysis, cooling water started to freeze from the water-insulator interface. Fick's diffusion law was applied to deduce the change of salt content in the liquid water. It was investigated that the melting water conductivity of icicle tips was proportional to the applied water conductivity. Freezing velocity (in mm/h) played important roles on the salt migration process during the growth of icicles and the melting water conductivity of an icicle tip under 2.6 mm/h was 1.66 times higher than under 3.6 mm/h. Special attention was paid to the ice flashover under non-bridged and bridged icicles. Icing experiments under energized condition showed that the conductivity of icicle tips increased by 70 percent when icicles grew to bridge the gap between adjacent sheds, which was a critical reason for the decline of the corresponding ice flashover voltage.

Field and laboratory tests of insulator flashovers under conditions of light ice accumulation and contamination

Investigations have been made of the performance of composite insulators because of the repeated flashover accidents on the lightly iced insulators in winter in the north of China. Based on the field research, a new test method was developed by taking into account the combined influences of the light ice accumulation and the secondary contamination deposition on the ice surface, and a different view of reproducing the atmospheric and environmental conditions in laboratory was provided accordingly. The test results confirmed that the icing flashover voltages of composite insulators with secondary contamination deposition were considerably lower than the contamination flashover voltage when the insulators were both precontaminated with the same equivalent salt deposit density (ESDD). In these areas where similar accidents happened, the increase in the insulation strength should be considered.