The introduction

With the increase of China’s power grid transmission and distribution voltage level, more and more anti flash-over composite insulators are applied in the state grid, for better application of the composite insulators, the reference as below:

1  Composite insulator application selection

1.1  Application status

Composite insulators are light in weight and well suited to a variety of environment, they not only reduce the worker’s maintenance workload, but also raise production rate; Moreover, the insulation performance is significantly improved and the pollution flash-over accidents are greatly reduced. As a result, the composite insulators are applied more and more by virtue of their advantages. But because the operation experience of composite insulators is relatively less, the selection of the composite insulators is based on the experience of traditional porcelain insulators, it is mainly used in the area at second and third pollution classification levels and the leakage distance is generally lower 20% than that of the porcelain insulator.

1.2  Selection of varieties

Throughout the composite insulator market, although its varieties no less than a dozen and the mainstream products are high-temperature silicon sulfide rubber composite insulators, after the completion of the upgrade, and the products withstand large scale pollution flash-over. The main two varieties as below:

1.2.1  High quality products

Some quality products with good impact resistance , earthquake resistance and explosion-proof are most favored, its internal epoxy draw rod has extremely high tensile strength and is as strong as 2 times as that of ordinary steel and 8 ~ 10 times of high strength porcelain, which greatly improve the product’s mechanical properties. Its end and mandrel connection adopts advanced crimping type, the process ensures the reliable mechanical strength of the product. The silicone rubber is used, the material has excellent high and low temperature resistance, weather resistance and heat aging properties and corrosion resistance, solid structure with epoxy lead rod, ensure the inner insulation is protected from moisture. The operation is maintenance-free, no need to clean. In addition, this kind of products use standard ball and socket structure, they are very easy to be transported and installed.

1.2.2  Products with rapid technological progress

Some manufacturers step up the product replacement cycle almost every year and there are new models on the market.

For example, one kind of product based on the theory of electric field distribution, a conductor is added to an insulator, which to draw a portion of the power line around the insulator into the insulator, so the distribution of the electric field inside and outside to achieve a certain balance, it can also withstand the extreme high flash-over voltage. Another example is that a sensor is installed in an insulator: once an insulator flash-over occurs, the signal is transmitted in an instant by the sensor to the headquarters to prevent the accident from spreading further. Besides, some adopt the whole extruding package sheath to wear umbrella, stress corrosion resistant mandrel, excellent grading ring type, recombination material formula, high-tech inner seal, etc.

1.3  Usage type

The shape and installation of composite insulators directly affect the deposition sludge quantity after they put into use, so the user need to pay attention to the type selection to ensure the insulators withstand the test of actual operating conditions.

Two main types that are commonly used as below:

1.3.1 Choose from the surface shape of the outer insulated umbrella skirt

The user prefers to choose the composite insulator whose external insulation surface structure anti-fouling performance is strong. According to the material properties and characteristics used in the insulation of products, combined with the degree of influence of various factors in the operating environment of the product, the user determines which one will be selected. The favored one is shown as (a) , the upper surface of the umbrella skirt has a small inclination angle and the lower surface is flat without dripping edge, which can significantly reduce the accumulation of contaminants that make water droplets are easy to form a spherical state away from the surface, but also can prevent the umbrella skirt structure from being deformed; However the insulator in which the lower surface of the concave and convex with water drops along the absolute edge (b) , the grooves between the edges are prone to fouling, so it is not regarded as a good prospect. Large umbrella skirts are also popular to enhance the gap insulation performance of the product exterior insulation surface structure characteristics; to increase the surface insulation performance, small diameter high pollution flash-over voltage umbrella skirt is added between large umbrella skirts.

FIG A  Umbrella skirt structure of composite insulator

 1.3.2  Choose from the insulator string installation mode

The dangling string Insulators, horizontal string insulators and V-shape string insulators are three types of installation, see FIG B, the preferred is the horizontal string insulator, whose upper and lower surfaces can be washed by rain, the best self-cleaning; Secondly, V-shape string insulators, its degree of contamination is intermediate , suspension string is seldom used because of the most serious cFIG B  Installation method of composite insulator.

FIG B  Installation method of composite insulator

 2  Maintenance of operation and damage fault

For a long time, many power enterprises believe that for prevent the transmission and distribution line pollution flash-over, as long as the composite insulator is cleaned regularly, the good result can be achieved. Actually a new concept on maintenance and repair is already formed, according to the degree of air pollution, fouling feature and easy-fouling season , different scientific and technical modification will be chosen to improve the flash-over voltage of composite insulators and avoid malignant power failure accidents.

With the increased number of composite insulators used year by year, the related number of flash-over and damage is increasing, except for construction and external forces, two reasons for the composite insulator default: firstly, the mechanical and electrical properties can not meet the safe operational requirements, such as interface breakdown, mandrel brittle fracture, mandrel fracture, mandrel breakdown and installation damage , which accounts for about 6% ~ 37% ; secondly, as same as the flash-over of traditional insulators. The former type damage occurs in the early products, the reason is that the selection of materials and process are not mature enough, the annual damage rate is about 0.5/10000. Now, the repairing of damaged composite insulator can adopt some advanced technology to increase the work efficiency.

2.1  Technical modification to solve interface breakdown

Insulator interface breakdown damage accidents account for more than 60% of all electrical damage accidents (mostly occurs in the case of lightning strikes), such accidents occur to the suspension products with early use of glue and extruding process, the 110 kV cross arm products, and recent extruded through umbrella and integral injection of composite insulation. it is necessary to transform insulator technology to solve the problem of coupling agent default, due to the wrong selection of the coupling agent and manufacturing default causes the interface breakdown.

2.2  Strengthen monitoring and maintenance, eliminate the pollution flash-over

In the continuous fog, continuous rain, dew and melting ice and snow conditions, the flash-over occurs, such flash-over accounts for 6% of the total number of flash-over identified. Although there are some of the reasons are controversial, but the use of composite insulators can eliminate pollution flash-over is biased. In fact, the silicone rubber surface fouling layer is too thick and the hydrophobicity is difficult to migrate to the surface of the pollution layer, under a long period of wet conditions, it will appear gradually weaken or even temporarily disappear; moreover, various contaminants also contribute significantly to its hydrophobic migration. Therefore, the salt density of insulator string should be measured regularly, the leakage current recorder can be installed if conditions allow, according to the amplitude of the leakage current and Pulse number to monitor the operation of polluted insulators, the instrument can help the user to get the alert signal in advance. There are three main maintenance methods: the first is that increase disk number of the series disk suspension insulator (e.g., 4 ~ 5 pieces for 35 kV line, 8 ~ 10 pieces for 110kV line); The second is the use of anti-fouling expanded umbrella suspension insulator; the third is that increase the number of umbrella for rod insulator or adopts special umbrella shape.

2.3  Use stress and corrosion resistant mandrel

The fracture of the mandrel glass fiber caused by acid erosion occurs in the the mandrel fracture accidents accounts for 74.5 % and the 500 kV line is the most prominent. Some brittle fractures of mandrel occur within 1 ~ 3 years of operation, Some occur after 8 ~ 9 years. The most typical accidents reason with some 500 kV alternating current lines is that the end part Improper sealing and moisture intrusion. Therefore, on one hand, it is necessary to improve the core stress corrosion (acid) resistance of the rod, on the other hand, the user must ensure the seal is firm and long – acting to prevent the sheath from being damaged. At present, the user always selects stress and corrosion resistant mandrel for DC line, the end mandrel sheath is connected to the hardware by applying high temperature silicone rubber on the basis of crimping process to significantly reduce the occurrence of brittle fracture.

2.4  Quality control

Further standardize the transportation, storage and installation measures of the composite insulator, strengthen the quality check before the insulator put into use, the faulty composite insulator is forbidden to be used on line, the online composite insulator with damaged mandrel must be replaced in time.

2.5  Add facilities to prevent bird pecking

Guano flash-over takes the second place in the total number of identified flash-over, accounts for about 29%. Generally available from the umbrella skirt surface and pole tower whether there are remnants of bird droppings, and it can be further determined by the movement of surrounding birds and the migration regularity of birds. The main danger of Guano flash-over is the instantaneous feature of bird excrement, so the common practice is that install prevent bird thorn on the tower top cross arm , and add a large umbrella plate porcelain insulator to the top of the insulator .

2.6  Properly increase the string length to resist lightning strikes

Lightning flash-over trip occupies the first place in the external insulation flash-over, accounting for about 45% of the total number of flash-over and the voltage range of the most suffered lines is 35 ~ 110kV. Compared with porcelain insulator string of the same length, the 35 ~ 110 KV line composite insulators have less discharge distance due to the less umbrella diameter, for 220KV and above products, it always related to the insulator with shortened grading ring position. Therefore, within the allowable range of the tower window size, under the premise of increasing investment and ensuring safety, the composite insulator string length must be increased appropriately.

3  Improvement of design and manufacturing defects

The operation history of Composite insulators in China is more than 20 years and the technology is mature now, if the composite insulator manufacturer can make efforts in the following three areas,the manufacturer will win more users:

3.1  Prolong the service life of insulators

Polymer materials have weaker bonds than porcelain, which means that they are easy to age and changes with various stresses during operation. Its service life expected value is difficult to assess, long-term reliability is not clear, and the insulator failure detection is difficult. Insulator performance is determined by the material collocation, product design and the manufacturing process. Manufacturers should fully consider the mutual impact between the material formula and product design, try not to use easily degradable materials in the manufacturing process, and minimize such aging and enhance the long – term reliability of composite insulators.

3.2  Reduce the production cost of insulator

Polymer raw material costs are much higher than porcelain, the materials mix together with fillers and additives not only reduce the cost, but also enhance performance and facilitate production. Because the glass fiber rod or barrel mechanical strength is high, polymer material breakdown voltage is also very high, the mandrel size can be smaller than the porcelain body, and the skirt can be made very thin, the sub-material consumption is much less than that of porcelain insulator, even though the cost of composite insulator raw materials is high, the cost of composite insulation products is as good as porcelain insulators, or even better than porcelain insulator with the same voltage class.

3.3  Improve the after-sales service

The manufacturer is expected to develop suitable products for a variety of complex environment

and leave enough margin to the insulator, even design specific mandrel, fittings, sheath of umbrella skirt and adhesive layer to ensure the quality of products . Apart from the products manual, the manufacturer had better provide a maintenance process guide book to the user, in order to increase the user’s skills and better trace the after- sales service.

Conclusion

Only the proper selection of insulator and scientific use and maintenance of insulator is done, the safety and stable operation of power line can be secured.