Hao Shen

Bio: Hao Shen is an academic researcher from Tsinghua University. The author has contributed to research in topics: Physics & Insulator (electricity). The author has an hindex of 2, co-authored 4 publications receiving 22 citations.

Preparation and basic properties of superhydrophobic silicone rubber with micro-nano hierarchical structures formed by picosecond laser-ablated template

Abstract: Hydrophobicity is the most important property of silicone rubber for outdoor isolation. Controlling wettability, along with interface and adhesion studies, has been widely investigated for technological applications, such as self-cleaning surfaces, by modifying the geometric structure and chemical composition of surfaces. Superhydrophobic surface with self-cleaning and anti-icing properties is ideal for outdoor insulation material. This paper reports on an effective strategy to modify the surface structure of silicone rubber. Microcraters with nanotexture can be fabricated on a mold steel using high-power picosecond laser incorporated with a high-speed scanning mirror. This template is used in direct replication process in which the surface morphology of silicone rubber can be changed during vulcanization. Two accessible methods are used to implement the modification process. Single-point pulse method is used to prepare regular papilla and abundant nanotexture on open architecture-type superhydrophobic surface, with contact angle reaching 151.5°±1.7°. Line scanning method is used to prepare connected papillae on enclosed-type surface, forming the secondary structure. In this case, nanotexture is richer and has a smaller scale (100 nm) compared with open architecture-type surface. Moreover, the contact angle of the enclosed-type superhydrophobic surface is 150.3°±0.9°. The contact model of water on superhydrophobic surfaces follows the Wenzel and Cassie coexisting model as shown by the air pockets, which are found experimentally and by calculation. Sliding angle of superhydrophobic surfaces reaches 5°, which indicates that this insulating surface possesses good self-cleaning properties. Repeated and large-area fabrication of superhydrophobic silicone rubber can be achieved by the proposed methods. These methods can be performed for large-scale application.

Investigation on tracking wheel test of large-diameter composite insulators under AC voltage

Abstract: The tracking wheel test specified in IEC/TR 62730:2012 provides a method that can be used to evaluate the resistance of composite insulators to tracking and erosion for rejecting inadequate materials or designs. Currently, most of the tests have been conducted on composite line insulators and some other insulator types with relatively small diameters. As the usage of large-diameter composite insulators, such as composite station insulators and composite cross-arms, increases rapidly, it is urgent to investigate if the tracking wheel test method is also suitable for their evaluation. In this study, a tracking wheel device for composite insulators with different diameters, designed and constructed in accordance with the IEC standard, is presented. The relationships between the leakage current and NaCl content as well as the average diameter of the specimen are investigated. It is shown that the test parameters specified in the IEC standard are not suitable for specimens of relatively large diameters. Accordingly, a relevant modification of test parameters has been introduced and verified, showing similar phenomena during the verified test as those observed during previous studies on composite line insulators. It is thus claimed that the modified test procedure is feasible for being applied on large-diameter insulators.

Study on influencing factors and laws of infrared temperature measurement of silicone rubber composite insulator

Abstract: Silicon rubber composite insulator is favoured by power system because of its excellent anti-pollution flashover performance. Among them, the core rod is an important structural component to bear the mechanical load of composite insulator. In recent years, there have been many brittle fracture faults of composite insulators in China, which directly affect the reliability of transmission lines. The composite insulator is usually accompanied by abnormal heating during the aging process. As an effective live detection method, infrared temperature measurement can find the defects of composite insulator in time and reduce the occurrence of string breaking accidents. In this paper, by artificially simulating the abnormal temperature rise of composite insulator, infrared temperature measurement was carried out under the conditions of different observation distance, different observation angle, different sheath thickness and different heat source temperature, so as to explore the influencing factors and laws of infrared temperature measurement of composite insulator. The results show that due to the difference of heat loss, sheath barrier and radiation ratio in thermal radiation propagation, the measured temperature decreases and the absolute temperature difference increases with the increase of observation distance, sheath thickness and observation angle. The built-in heat source temperature increases the total internal heat, which can partially weaken the influence of sheath thickness on the measured temperature and significantly expand the difference caused by observation angle.

Introducing Chlorine Into Epoxy Resin to Modulate Charge Trap Depth in the Material

Abstract: The epoxy insulators in dc gas-insulated transmission lines (GIL) tend to accumulate surface charges, which causes insulation flashover. Increasing the surface conductivity of epoxy resin, which can restrain the accumulation of surface charges on the epoxy insulator, is a potential method to improve the insulation performance of dc GIL insulators. The conductivity of polymer dielectric is strongly influenced by the charge trap characteristics of the polymer. In this work, we introduce chlorine with strong electronegativity and a larger atomic radius into the epoxy group segment of epoxy resin to improve the distributed energy level structure, which in turn reduces the electron trap depth, to increase the surface conductivity of epoxy insulating material without affecting its intrinsic dielectric strength. Based on the results of quantum chemistry calculation, the modulation laws of introduced chlorine (including the chlorine in the form of a hydrolyzable chlorine atom and a nonhydrolyzable chlorine atom) on distributed energy levels of epoxy resin molecule are anticipated. These laws are explained from the microscopic perspectives of electron energy structure and electron cloud offset. Both the inductive effect of the chlorine atom and the conjugation effect of the 2p electron orbital of the oxygen atom in the epoxy group impact the distributed energy levels by changing the spatial distribution of electron cloud density between and on valence bonds.

Influence of sheath thickness and core rod diameter on fault temperature rise of composite insulator

Abstract: Infrared temperature measurement is an effective means for insulator health monitoring, and the theoretical basis and the subsequent diagnosis methods have been widely studied. However, in practical applications, the influence of insulator itself on fault temperature rise is rarely considered. In this paper, the temperature rise experiment of composite insulator was conducted to study the influence of sheath thickness and core rod diameter on fault temperature rise. According to the experimental results, the evolution law of fault temperature rise with the change of sheath thickness and core rod diameter was obtained. On this basis, the design strategy for composite insulators was proposed.

Improving the outdoor insulation performance of Chinese EHV and UHV AC and DC overhead transmission lines

Abstract: Power girds in different countries face different types of threats and in China the threats severely affecting grid reliability were in the past large power outage accidents caused by pollution flashovers. First records of pollution flashover accidents in China can be traced back to 1950s in industrial areas in Northeast and East China. Thereafter, such accidents occurred occasionally, but after 1970s they started to expand and were recorded in most of the country's provinces. The very first large area outage accident caused by pollution flashover happened in early 1974 in Northeast China around the local largest cities Shenyang and Fushun. Between 1971 and 1994, 3542 pollution-caused line trip-outs (several trip-outs on one overhead line (OHL) happening the same day is counted in China as a single event) were recorded on 35–500 kV OHLs and 1768 substation trip-outs. A total of 44 large area outage accidents caused by pollution flashovers took place during the 30 year period before 2007, as shown in Figure 1, while the outages recorded between 1981 and 2001 accounted for 43% of the total number. Among them, outages of super large character occurred every 5–6 years, e.g., in 1990 in North China and Northeast China, in 1996–1997 in East China and Central China, 2001 in North China and Northeast China [1–3]. Here we refer to the large area outage accidents as flashover events on several main transmission overhead lines and substations, appearing during one or several consequent severe meteorological events and continuing for several days. They affect several big cities within one province and stretch over an area of less than 100 thousand km2. For super large area outage accident, we refer to the accidents stretching across several provinces over the area of 200–300 thousand km2 and resulting in hundreds of pollution flashover events.

A superhydrophobic surface with aging resistance, excellent mechanical restorablity and droplet bounce properties.

Abstract: The use of a silicone rubber composite insulator has become an important aspect to ensure the safe operation of an electrical power grid. This study introduces a preparation method of a superhydrophobic silicone rubber surface using a simple preparation process at low cost and with excellent performance, which can be used in the mass production of silicone rubber composite insulators. In this study, the combination of a compression molding process and a template method was used to prepare the product. A microstructure composed of numerous boat-shaped grooves was constructed on the surface of silicone rubber. The modification of a low surface energy material is not required. The static contact angle with water after the high-temperature treatment exceeds 150°, and the rolling angle is under 10°. Excellent performance has been observed in terms of self-cleaning effect, aging resistance, and mechanical and droplet bounce properties. It has been shown that the loss of superhydrophobic properties, due to the prolonged immersion in water, can be restored by a high temperature heating process.

Suppression of surface charge on micro- and nano-structured superhydrophobic silicone rubber

Abstract: Laser-ablated template and fluoroalkyl silane modified composite coatings are used to prepare a micro- and nano-structure on silicone rubber surfaces. By testing the surface potential accumulation and dissipation process, we assess the suppression of surface charge on superhydrophobic silicone rubber. The surface physicochemical properties and resistivity are examined. The electron and hole trap distribution are analysed using isothermal current decay theory. Peak density of traps associated with the superhydrophobic surfaces are found to be promoted to lower energy levels relative to unmodified silicone. Surface micro-structure increases trap density and reduce trap depth; nano coatings are shown to further reduce surface trap depth. The improvements shown to suppress surface charge accumulation is beneficial to a range of electrical industries and helps mitigate surface flashover on insulation.