Composite Insulator Hydrophobic Performance

• Pretreatment is often done prior to measuring hydrophobicity of composite insulators, though the necessity of this is questionable.
• The investigation focuses on surface roughness related to light/heavy pollution, chalking, and its impact on hydrophobicity characteristic (HC).
• Surface roughness affects the pretreatment effect on HC level and static contact angle.
• The pretreatment effect on HC level and static contact angle is obvious.
• Static contact angle percentage changes by up to 15.3% when surface roughness varies.
• Ra and RSM of varying pollution levels changes at rates of 130% and 269%.
• High surface roughness degrees reaching 3.242µm and 400µm causes a permanent loss of hydrophobicity.
• Surface roughness causes water drops to transition between two states, changing the sample static contact Angle up to 31% within 10 minutes, leading to varying measurement results.
• It is suggested that surface roughness of composite insulators be added as a parameter in hydrophobicity tests.

Hydrophobicity

• Excellent composite insulator hydrophobicity enhances contamination resistance.
• Hydrophobicity leads to small water drops instead of films, greatly improving surface resistance.
• Critical voltage increases and industrial frequency decreases, ensuring long-term safety and reliability of transmission lines.
• Hydrophobicity decreases after pollution flashover accidents.
• Hydrophobicity is measured per DL/T 864-2004 to replace insulators before it decreases to level HC6.
• On-site measurement is difficult, so lab experiments are used.
• Hydrophobicity characteristics include classification, recovery, and migration.
• Silicon rubber jackets are often covered with pollution and powder, pretreatment removes it to measure hydrophobicity of the silicon rubber itself.
• Pretreatment affects measured results, so in-lab experiments should mimic practical operating states to grasp hydrophobicity.
• This paper uses surface roughness as a quantifiable characterization parameter for varying degrees of pollution and pretreatment effect.
• Surface roughness measurement can provide the basis for hydrophobic classification.
• Surface roughness changes also causes measurement errors within the static contact angle method and hydrophobic sizing method
• Hydrophobic research has mainly focused on classification, recovery, and migration, with less research on surface roughness.
• This paper examines whether pretreatment of hydrophobic measurement affects the hydrophobic characteristic of composite insulators.
• The relationship between hydrophobic changes and various surface roughness caused by pretreatments is analyzed, along with the impact of different types.

Surface Roughness Measuring Method

• The surface profile method is used for measuring surface roughness measuring.

• Surface profile refers to the intersection between a plane and the actual surface.

• Measuring involves detecting the surface of insulators using a stylus to get the surface profile and parameter calculations.

• Surface profile parameters include amplitude, distance, hybrid, curve, and related parameters.

• The research focuses on amplitude and distance parameters

• Amplitude parameters can be denoted as Ra using the arithmetic mean between sampling lengths of profile peak and profile valley.

• Distance parameters are denoted as RSM using the mean value of profile element width within sampling length Xs.

• During testing, the surface roughness of rubber jackets are averaged across three measurements, the stylus force should be less than 0.5N.

Testing Equipment and Samples

• Relevant standards require using a watering can in the hydrophobic classification method (HC).
• Hydrophobic Angle measurement uses optical contact Angle measuring system of type CAM200.
• Type A Shore durometer and Type TR-200 surface roughness instrument are used, with a sampling length is set as 0.8mm.
• 15 operating composite insulators are selected from 12 different voltage grade lines ranging from 100kV to 500kV in 12 different provinces, where operating environment includes eight typical climate regions and five major domestic manufacturers.
• Rubber jacket sampling points are all on the high voltage terminal.

Hydrophobic Characteristic and Static Contact Angle

• Some believe pretreatment reduces interference of pollution for hydrophobic measurement; others believe it is not needed to maintain surface state.
• Half of every rubber jacket surface is pretreated, and the hydrophobicity test results are presented in tab.1.
• A1, A6, A9, A14, A15 operate in the light pollution level
• A2, A3, A4, A5, A8, A10, A11, A12 operate in the heavy pollution level
• A7, A13 are operating in chalking composite insulators
• Hydrophobic classification results with and without pretreatment differ, and the static contact angles also differ by 15.3%.
• There exists a certain relationship between hardness and its change tendency.
• It is suggested that pretreatment is not required in hydrophobic measurement of composite insulators, results must match practical operation state.

Roughness Comparison Before and After Pretreatment

• Surface roughness tests before and after pretreatment of the rubber jacket used TR-200 surface roughness instrument.

• Before pretreatment, Ra was between 0.288µm to 0.876µm, RsM was between 66.7µm to 193.5µm, the surface profile is more uniform.

• After pretreatment, Ra is between 0.293µm to 0.893µm, RSM is between 66.7µm to 193.5µm.

• Before pretreatment, Ra was between 0.458µm to 0.911µm, RSM was between 125.5µm to 304.2µm, the surface profile is more uniform.

• After pretreatment, Ra is between 0.494µm to 0.985µm, Rsm is between 135.4µm to 329.0µm

• Roughness measurement of seriously pulverized surface profile shows a deeper amplitude parameter (Ra is between 1.284µm to 1.724µm), and a smaller distance parameter(RSM is between 95.4µm to 114.7µm).

• After the pretreatment, Ra (1.656µm ~ 3.878µm) and RSM (281.0µm ~ 352.2µm) all have obvious increase.

• After pretreatment, roughness Ra in high hardness value decreased, while Ra in low hardness value increased.

• No matter how hard the chalking surface gets, there will be obvious changes before and after the pretreatment.

Relationship between Surface Roughness and HC

• There is a clear relationship between hydrophobic (HC) classification measurement result and distance parameter RSM.
• The relationship between HC level and amplitude parameter Ra is unclear and widely dispersed.
• Amplitude parameter of chalking surface Ra is higher, surface crack is filled with superfine silica powder and dust, making RSM small enough to present homogeneous surface profile.
• Surface crack prevents water drops from crossing silica powder, ejecting water drops caused by HC level performs as Cassie model with a great hydrophobicity.
• Powder filling surface crack gets removed after pretreatment, amplitude parameter Ra remains high and distance parameter RSM increases.
• Ejecting water drops move into surface crack with a poor hydrophobicity.

Surface Roughness and Hydrophobic Classification

• Sand paper was used to create a surface with Ra=3.242µm and RSM =400µm, the static contact angle descent 89.0°~93.5°.
• Surface hydrophobicity cannot restore, as other parts can return to HC1~HC2, even when put in the oven at temperature 30°C for 1 month
• When roughness reaches certain values (R≥3.242µm, RSM≥400µm), hydrophobicity of material itself cannot play leading role.

Roughness Impact on Hydrophobic Recovery

• Hydrophobic recovery characteristic of rubber jacket differs with or without pretreatment.
• Hydrophobicity of some pretreated rubber jackets may recover in 48h, while those without pretreatment cannot.
• Static contact angles are greater than 90°, the surface roughness measurement shows increases in Ra after immersing in deionized water for 96h, but RSM shows no obvious change.

Roughness Impact on Hydrophobic Migration

• Inert material diatomite used in hydrophobic migration is between 106µm~150µm in diameter, with which surface structure gets more complicated.
• Size, shape, adhesion and coating way of diatomite particle directly affect Ra and RSM, further influences measurement results.
• Most tested sample rubber jackets' hydrophobicity cannot migrate onto the surface, while new samples show better migration

Surface Roughness and Statics Contact Angle

Statics contact angle method one of hydrophobicity measurements, can cause:

• Statics contact angle lag, the angle changed from 126.0° in 10s to 87.0° in 10min
• Analysis shows process is classical Cassie model slowly transforming to Wenzel model, caused by various surface roughness
• Impact of roughness change on statics contact angle is smaller than HC level, results conflict on high roughness surface
• Quantity and strength of ejecting water in HC level may change the Cassie model to Wenzel model, while the statics contact angle remains in Cassie model. The conflict is not obvious on smaller roughness surface.
• Statics contact angle shows no classification function, existing standards only require 0av≥90°,0min≥85° to meet hydrophobicity.

Conclusion

• Surface roughness on hydrophobic characteristic of composite insulators is obvious
• HC level results and distance parameter RSM have obvious corresponding relation, corresponding relation with Ra isn't ideal because of surface powder filling
• Rubber jacket roughness varies before/after pretreatment, trends are all related to hardness o Whether chalking surface: obvious changes take place before/after
• Obvious changes in both hydrophobicity + statics contact angle before/after jacket pretreatment o pretreatment should not be a requirement to avoid result deviation
• Methods of statics contact angle and HC level have inequitable results on high roughness surface o HC level suggested due to better similarity with operating conditions
• Surface roughness measurement suggested in HC level to correspond to hydrophobic hierarchical relationship + degree of aging on rubber jacket