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Analysis on the low level of 6 kV box sealed insulation in power plant

Modern large and medium-sized power plant from the high-voltage factory transformer and start / standby transformer to 6 kV switchgear basically choose the 6 kV cable bus or 6 kV box sealing connection. Now has been built or under construction in power plant, the total contracted by foreign and domestic individual self built power plants, power plants, such as Zhuhai Shajiao C power plant and Wu eight diameter with 6 kV cable bus, while the vast majority of power plants with 6 kV total box seal. Factor is the higher cost of cable bus, is the total box seal mother 3~ 5 times, on the other hand is the domestic cable winding radius, a cable bus shape, especially for the coexistence of large scale and placement is more difficult, and the imported cable further add cost, so choose more common box bus. However, due to the poor sealing structure of 6 kV box, the insulation performance is affected. This article tries to 6 kV common box sealing mother shortcomings, contact engineering examples, put forward personal opinion to deal with doubt.
 
6 kV common box sealing mother shortcomings
NPB in three-phase busbar are made of rectangular bus body is exposed, only can close the sealing function is good, can effectively prevent moisture and dust into the. 6 kV the total box seal basically the traditional porcelain insulator, porcelain insulator is a silicate sintered body, many factors affect the quality of skills, it is difficult to ensure, porcelain structure is not fine, multi crystal are difficult to prevent the existence of its internal space, irregular oscillation in a long time under the conductor open space, gradually expanding, easily under strong electric field breakdown insulation down. Even the porcelain insulator intact, because the outer surface adsorbed water film and easy adsorption in the air, salt and dust, a conductive path, then the leakage current through the insulator surface, partial discharge, resulting in bus insulation is greatly reduced, and the running voltage discharge for the development of porcelain insulator surface flashover arc. A trouble. Large power plants built on the sea due to high humidity, high salt content in the air, the most serious. On the basis of the Taishan power plant, Zhanjiang power plant and other power plant records, insulation value are very low humidity in the air, the larger outage unit 6 kV total box seal, was less than 1 M (the lowest acceptance value should be more than 6 M), it is necessary to take the necessary measures to start the unit, and then extended. The unit launched the moment, add the launch cost. In the network load is not full, the unit starts frequently, 6 kV box seal insulation has affected the safe operation of the unit.
 
Comment on the handling of 6 kV box closure defects
Taishan power plant planned capacity of 5600 MW + 51000 MW units, is now Asia's largest planned power plant, unit 1, No. 2
Group has been put into operation, 3~ unit 5 is under construction. 1, unit 2 total box seal only according to the conventional order, package heat shrinkable sleeve only in the bus, but the trial operation and after the operation has repeatedly due to the 6 kV total box seal insulation level is too low in the unit, the unit starting effect. 3~ No. 5 machine orders, in addition the bus heating belt, still not really worried because treatment of low insulation level of power plant in this question of doubt, made an investigation, to eventually withdraw and tropical, instead use full polyester glass fiber reinforced plastic (DMC) insulator.
 
To find the best way, below may deal with 6 kV common box sealed mother insulation several ways to comment.
 
3.1 bus appearance package heat shrinkable sleeve
The plan is to package the heat shrinkable sleeve in three-phase bare bus, the bus and the porcelain insulator is added between the insulating layer, and then improved bus insulation. The approach is relatively simple, very little increase in cost, but can not handle the insulator surface constitute a conductive loop of doubt, to improve the insulation of all bus boxes to help little. Taishan power plant 1, unit 2 is a heat shrinkable sleeve, but the moisture in the air when the insulation is still very low is confirmed.
 
3.2 in the closed bus box device plus tropical
The first is an electric heating box in bus passive unilaterally change from first to last, total box seal mother operation conditions, the start-up of the unit when put into use, the progress of the temperature in the box, the box in the boring, porcelain looks difficult to form a conductive loop, then the progress of busbar insulation. But this plan is difficult to control heating with input time, and increase the power loss and the additional amount of maintenance, and a doubt is that practice is not as good as expected, with the heating power can not be too large, the temperature is not very high, the moisture is not easy to discharge, so the insulation of the bus also helps a little progress, Tianjin Panshan Power Plant on the former selected this approach, but the effect is not ideal, the final switch to DM C insulator to deal with doubt.
 
3.3 porcelain insulator exterior coating RTV coating
RTV is a room temperature vulcanized silicone rubber, because of its unique molecular structure and has excellent hydrophobicity and hydrophobicity migration. 
 
The characteristics of RTV are as follows:
Good hydrophobicity. The non polar solid medium, cohesion force and the appearance of molecular water molecules less than water molecules themselves, so this kind of medium moisture appearance often become isolated droplets, water film is difficult to form a series, the contact angle is greater than 90!
 
Hydrophobicity transfer good. Hydrophobicity transfer RTV is hydrophilic dust accumulated RTV coating appearance also showed hydrophobicity in a certain period of time, namely RTV coating hydrophobicity migration to the pollution layer. The RTV is composed of hydroxy terminated two methyl siloxane condensation, its basic structure is a three-dimensional network of silicon oxygen bond, including a lot of small molecular groups (e.g., methyl - CH3) at the same time, including many of the free state and free silicone small molecular group. Because the silicon oxygen chain belongs to the flexible chain, the molecular motion is large and the density is low, which causes the molecular chain segment to bulge the sewage layer and form a free film above. Therefore, the surface of the hydrophilic pollution layer becomes hydrophobic, showing excellent resistance to pollution flashover and humidity flashover function.
 
# voltage spread evenly. Because of its strong hydrophobicity and hydrophobicity migration, it is difficult to form a series of conductive path, so it will not show uneven voltage distribution, constitute the umbrella skirt arc scene. The porcelain insulator coated with RT V coating, basically to prevent the formation of porcelain appearance of water film, prevent condensation occurs, then the The bus through the porcelain insulator to the box (to) the current channel can not leak or very weak, greatly improve the insulation resistance value of busbar.
 
Because of the RTV coating also has excellent hydrophobicity transfer, even after the appearance of porcelain in paint spraying dust dirty, it will not cause the appearance of porcelain wet, damp, insulation function is not affected by the bus. Zhanjiang power plant, Whampoa  power plant in the 6 kV box seal mother porcelain insulator painted RTV coating, just coated after the inspection of insulation is very high, are more than 100 G, the role is very ambitious. But the approach is only a temporary measure, on the one hand it is difficult to ensure uniform coating, on the other hand can not afford the time detection, RTV coating life as long as 3~ for 5 years, and even in the short term beginning of powder drop, can not deal with questions from the foundation, Zhanjiang power plant construction. After 3 years, the insulating function drops a lot.
 
4 unsaturated polyester glass fiber reinforced plastic insulator (DM C)
From the power plant operation experience, which absorb moisture, condensation and creepage flashover of insulators is the primary factor of doubt, as long as the insulator is connected with the outer casing and other space are insulated, may not constitute a discharge space. In order to deal with the common box sealing mother insulation low, only the use of new insulators to prevent the shortcomings of porcelain insulators.
 
DM C is the choice of insulator with high temperature and high pressure molding skills will not be full of polyester glass fiber reinforced plastic bar and a molding die holder, aluminum alloy fasteners, forming the appearance of smooth lubrication, no bubble crack. DM C insulators and RTV coatings are the same as non-polar solid media, with excellent hydrophobicity and hydrophobicity migration, compared with RTV coating porcelain insulators have the following advantages:
 
High mechanical strength. Porcelain is brittle material, compressive strength is good, bending, tensile strength is low, porcelain insulators will grow with the loss of mechanical support capacity, the so-called insulator aging or deterioration. While the impact strength DM C 30 kJ/ insulator m2, bending strength
MPa, much better than porcelain insulators.
 
High insulation resistance. In normal and soaking for 24 h, 101012 R, will not be affected by humid.
 
# flame retardant, arc resistance, high resistance to tracking and heat, can be in 155 by long time C.
 
Per cent crawl distance. For 6 kV common box seal mother, in the case of constant device size, creepage distance increased by 40% ~ ~ 60%, the conventional porcelain insulator creepage distance is about m/ MV, and DM C insulator reached 38 m/ MV.
 
long term. RT V his paint itself is not long, and the construction of life may be shorter, aging after finishing very difficult, basically to replace the insulator. DM and C insulator is itself has excellent self purification, hydrophobic and hydrophobic transfer information, life is very long, from the beginning of 2000 in the domestic use, so far is not insulated effect of ambition, loss of sight found.
 
Lightning instead, a plurality of small section head needle, it uses its base pressure head separated by the needle upward leader branch lightning upward leader, in the lightning downward leader and lightning upward leader between close air stage, greatly accelerate the thundercloud charge of lightning downward leader leak loss rate, high drop in potential lightning downward leader, to terminate the lightning downward leader and lightning upward leader to eliminate lightning scene.
 
It should be noted that the lightning suppression needle can only be limited within the scope of maintenance of lightning. The surface maintenance radius of lightning suppression needle can be estimated by 0.707 HR [1~ 4]. In the meantime, HR lightning strike distance, about one or two, three types of lightning protection construction, HR can be taken separately for 30 m, 45 m, 60 M. On the lightning protection of transmission lines, in the whole line, part of the line segment, easy to hit the tower, across the tower (across the highway, railway, river), each pole top transformer terminal tower set 1~ 2 lightning eliminator (single lightning line tower 1, double lightning line 2) 1 sets of multi-storey short diffuser type grounding device can be set.
 
The utility model has the advantages of complete independent intellectual property right, reasonable design, simple manufacture and construction, low cost and good lightning protection effect, and is suitable for large area popularization and application.
 
From the recent engineering experience, select DM C insulator initially received the desired role. Inner Mongolia, Tianjin Datang Panshan Power Plant Power Plants and other power plants 6 kV total box seal in the insulator are all replaced by DMC insulator, role of ambition. The goods have been further popularized in Guangdong area: No. 5 unit of Taishan 3~ when ordering on the selection of DMC insulator, then orders for Zhuhai power plant, Huizhou power plant LNG were also selected DMC insulator.
 
Conclusion
To sum up, the selection of DM C insulator has the incomparable advantage compared with other methods of increasing the sealing mother insulation level. Although the pre planned investment is large, but from the foundation treatment of insulation low doubt, after put into use to go from the insulator, insulation resistance basically will not decline, are maintained at above 1000 M, on the other hand do not have to clean the blackout,
 
Added benefit. So choose DMC insulator is the most reliable, no security hidden trouble 6 kV common box sealing mother insulation low fundamental method.