Jia Zhidong

Study on the hydrophobic transfer based on the theory of absorption and cohesion

Room temperature vulcanized (RTV) silicone rubber (SIR) coatings are used increasingly on outdoor HV porcelain and glass insulators and bushings when subjected to heavy pollution. An important property of the RTV-SIR coating is its ability to restore the hydrophobicity of the surface after a layer of contamination has built up on it, and thus continues to suppress the development of leakage current, dry band arcing and flashover. As hydrophobicity is the key to the superior contamination performance outdoor insulation, the present study was undertaken to better understand this phenomena. In this paper, the hydrophobic migrations of RTV SIR coating with the pollution of kieselgur and kaolin were measured. The virgin kieselgur and kaolin and them with hydrophobicity were analyzed using Scanning Electron Microscope. The theory of absorption and cohesion is introduced and applied to explain the mechanisms of hydrophobic transfer.

Flashover mechanism of RTV coated insulators

The use of room temperature vulcanized (RTV) silicone rubber (SIR) coatings for improving the contamination performance of existing ceramic insulation has become popular. RTV SIR coatings work on the principle of providing a high surface resistance even in the presence of moisture and contamination, hence, limiting the leakage current to harmless levels. This paper deals with the flashover mechanism and performance of contaminated RTV SIR coated insulators under wet conditions. Four types of profiles of insulators with RTV SIR coating were evaluated. The withstanding voltage and flashover tests were performed on these insulators. The equivalent salt deposit density and the non-soluble deposit density were measured. Based on the analysis of leakage current monitored, the flashover mechanism of RTV SIR coated insulators was proposed in this paper.

Influence of fillers on silicone rubber for outdoor insulation

HV outdoor insulating systems such as line and station insulators, surge arresters and bushings, using silicone rubber for outdoor insulation have become very popular. This paper presents the mechanical behavior and electrical behavior of silicone rubber blend with different fillers. Various loadings of alumina trihydrate (ATH) and silica are added to silicone rubber and their performance are evaluated. Different types of silica such as fumed silica and precipitated (ppt) silica are studied. It was found the type of different silica filler makes difference in mechanical properties and dielectrical properties. And it was also found silane treatment of precipitated silica have great influence on the performance. The test results show that the more addition of ATH, the better the erosion resistance is achieved, but will result in declined mechanical properties.

Loss and recovery of hydrophobicity of RTV silicone rubber coating

The loss of surface hydrophobicity of room temperature vulcanized (RTV) silicone rubber (SIR) due to immersion in acid, alkali and saline solutions has been studied as a function of immersion time and concentration. The loss and recovery of the hydrophobicity is correlated with the solute and the absorption of water during immersion and its subsequent desorption in air. The hydrophobicity is determined by measuring the static contact angle between the tangent to a droplet of distilled water and the horizontal RTV-SIR coating surface. The surface tensions of solutions were measured before and after the RTV-SIR coatings were immersed in solutions. The values show the effect of solution on RTV-SIR coating.

Controlling the electrospinning process by jet current and Taylor cone

To prepare uniform nanofibers from poly (ethylene oxide) (PEO), a voltage of 0 to 15 kV was applied to a distance of 15 cm to 30 cm between tip and ground collector. In this study, jet current and Taylor cone were used to express the electrospinning stability. It is showed that an electric current during electrospinning affects the fiber morphology. SEM and jet current monitoring were used to study the jet stability with positive power. It is suggested that it may be concluded that there is a voltage range for spinning with these conditions. The most important one is the Taylor cone shape. When jet current is at the smaller ratio of slope and the Taylor cone still has a part out of the nozzle, the fibers are the most uniform fibers.

Study on the influencing factors of natural degradation of liquid silicone rubber used for external insulation

In recent years, liquid silicone rubber (LSR) has been widely used in the external insulation field. However, further study on the degradation mechanism and the influencing factors of liquid silicone rubber is still required. Especially when it is used in southern parts of China, of which the high temperature and humidity environment could accelerate the degradation process. It is reported that cracking and chalking phenomena were observed on the liquid silicone rubber sheath of PT and CT. In this paper, a series of ageing experiments were designed and carried out to test the performance of liquid silicone rubber when exposed to factors such as high temperature, high humidity, sun exposure and corona discharge. After the experiments, characteristics such as hardness, hydrophobicity, glossiness and mechanical properties were tested and SEM was used to study the change of micro structure. It is concluded that high temperature, high humidity and sun exposure play significant roles in the degradation of liquid silicone rubber, which could lead to the rise of hardness, loss of hydrophobicity, decrease of glossiness and surface cracking. The results also show that the change of mechanical properties of liquid silicone rubber were tiny.

Research on Testing of Over-exciting to Evaluate the Tolerated DC Currents of Transformers

During monopolar operation of HVDC systems with ground return, large currents returned from earth may enter into neutrals of transformers, which causes DC bias of transformers and influence transformers running. The reason of DC bias of transformers and the factor of tolerated DC bias are analyzed in this paper. Great capacity transformers constructed in the form of three-phase groups or three-phase five-legged are more prone to DC bias saturation because the air gaps between laminations are small. According to simulation of a single phase two-winding transformers model the exciting currents of transformers under AC over exciting and DC bias analyzed. More odd components appear in the symmetrical exciting current under over-exciting. Some even components appear in the asymmetrical exciting current under DC bias. Saturation effect caused by over-exciting and DC bias is similar in the influence of transformers. Exciting current can be as the indicating depicting the change of magnetic field within transformer. The testing of over-exciting in transformers can be used to evaluate the tolerated DC currents of transformers. This method of evaluation based over-exciting is simple and available

Controlled deposition of electrospun poly(ethylene oxide) fibers via insulators

Electrospinning is a subjecting of a fluid jet to a high electric field process, by which polymer fibers with nano-scale diameters lower than 100 nm are formed. Significant progress has been made in this area throughout the past few years and this technology has been exploited to a wide range of applications. However, the efficiency is still very low because the repelling force could not be controlled between two needles. For the investigation, two needles were used. And 3 kinds of insulators, including polytetrafluoroethylene (PTFE) tube, polyvinyl chloride resin (PVC) tube and silicon rubber tube, were invited to overcome the repelling force. It is concluded that it is possible to provide a more controllable and easily achieved electrospinning process by using insulators.

Effect of Relative Humidity on DC Corona Pulse Characteristics at Different Air Pressure

It has been investigated that the effect of relative humidity on statistical parameters of negative corona pulses at different air pressure conditions. A rod-plane electrode configuration has been used. The range of air pressure and relative humidity is 0.06 MPa~0.10 MPa and 30%~90% respectively. The variation of average amplitude and standard deviation of negative corona pulses is illustrated. Average amplitude and standard deviation of negative corona pulses are lowered, as the air pressure or relative humidity rises. It is demonstrated by the ratios of average amplitude under different conditions that the degree of influence of air pressure (or relative humidity) on average amplitude becomes more pronounced at higher relative humidity (or air pressure). The linear relationship exists between average amplitude and the square root of the product of air pressure and relative humidity. While between standard deviation and the product of air pressure and the square root of relative humidity are the second order relationship. It means that the degree of influence of air pressure and relative humidity is the same on average amplitude, but different on standard deviation

A new method to remove the aging RTV coatings on glass insulators

RTV coatings had been widely used in electric insulators to prevent pollution flashover performance at external insulation. For the bad weatherability of polymers, RTV coatings will be aging during the long serve time. To remove the aging coatings, many methods had been used, such as solvent dissolve, high pressure water flushing, and dry ice flushing. But it is not easy for these methods to remove the coatings entirely and easily. In this paper we described a new method-laser cleaning, which is a very easy and useful method. We used the continuous laser with Ytterbium Laser System (YLS-2000) and the pulsed laser with IPG fiber laser system (Nd:YAG) in the experiment. Different scanning speed of the laser irradiating and different energy density had been tested to find Optimization parameters which could remove the coatings and not damage the surface of the glass insulators. The both laser irradiating can remove the coatings clearly in the experiment. But the continuous laser will leave the carbon clusters on the surface, which is easy to remove but bad for the insulators. The scanning speed can be higher than 10 mm/s that means the glass insulators with 20mm diameter can be dealt in 3 minutes with pulsed laser. The laser cleaning is a good method to remove the RTV coatings on the glass insulator surfaces up and down. With the cost of laser technology decreases, it will be a better method as a coating removing method.