Introduction
Insulators used in 1000kV UHV substations require higher mechanical strength, good electrical performance and anti-pollution performance. At the same time, 1000kV insulators are required to have higher operational reliability due to their longer string and extremely inconvenient detection and replacement.
1000kV porcelain insulator string has the disadvantages of many pieces, long structure and large weight. At the same time, due to the poor hydrophobicity of electric porcelain materials, porcelain insulator strings are prone to flashover in wet environment (such as rainy and fogy days). Therefore, measures such as sufficient zero-value insulators or additional coating of RTV or PRTV antifouling flashover materials are often taken to increase the creepage distance of insulator strings. However, this will further increase the length of insulator string and maintenance workload. The application of synthetic insulators in power grid has also become a research hotspot because of the advantages of high pollution lightning voltage, high utilization rate of creepage distance and good pollution accumulation characteristics. Synthetic insulators (including pillar insulators and suspension insulators) have been used to replace porcelain insulators in some substations of 750kV and below. The 1000kVGIS bushings of 1000kV substations, such as Jingmen and Wannan stations, also partly use synthetic materials, and they work well.
Development status of porcelain insulator string
In the UHV substations that have been built or are under construction, the specific layout type of 1000kV distribution device is combined. Double-umbrella type or triple-umbrella type porcelain insulator with large climbing distance is selected as the cross-line tensioning insulator string in the station. For the dangling insulator string, in order to control the string length and ensure the safe clear distance between the lead line and the equipment below it, the three-umbrella type porcelain insulator is selected. The single-piece creepage distance of the three umbrella type porcelain insulator for suspension insulator is 545mm.
The number of insulator strings depends on the pollution level of the substation. Creepage distance method or pollution flashover withstand voltage method is used to determine: but when it is difficult to determine the pollution resistance characteristics of insulators through field tests, the number of insulator strings is also determined or checked by cooperating with the insulation level of line insulator strings.
For UHV substations located in national standard d dirty area, according to GB/T 26218.2-2010 "Selection and size determination of high voltage insulators for use under dirty conditions Part 2: The unified creepage specific distance specified in "Porcelain and Glass insulators for AC system" is 43.3mm/kV. After considering zero-value insulators, the number of 1000kV suspended insulator strings is 59 pieces. There are also some UHV substations located in the Level d2 contaminated area. Combined with GB/T 26218.2-2010 and the relevant regulations of the electric power operation company on insulation configuration, the unified creepage specific distance is set at 48.5mm/kV. At this time, the number of dangling insulators should be increased accordingly. When the level of contaminated area is further increased, other measures should be taken as the existing 1000kV porcelain post equipment is unable to meet the requirements of external insulation configuration. Therefore, it is no longer meaningful to simply increase the creepage distance of insulator string.
According to the results of the study on the swaying of 1000kV wire at different periods, the influence of the wind deflection of the wire on the distance between phases is limited. Single I, double I or V-type tensioning insulator string can be used for the cross-line of the incoming and outgoing lines of 1000kV distribution device. V-type insulator string is generally used for the main bus line, and single I or V-type dangling insulator string can be used for the lead down line and jumper. In the UHV substation that has been built, the 1000kV suspended insulator string mainly adopts V-shaped structure to limit the swaying of the wire and reduce the width of the frame. For the UHV substation in the D-level contaminated area, the 1000kV V-type dangling insulator string group string generally selects 160 kN three-umbrella porcelain insulator sheet, with the single structure height of 160mm, creepage distance of 545mm, and the number of insulator sheets of 59 pieces.
1000kV suspended insulator string in UHV substation is mainly used to hang the lead wire from the incoming and outgoing lines to the GIS bushing, and the insulator string is equipped with tensioning clip. Through calculation, it is found that only when the included Angle of the V-type insulator string is greater than the maximum wind deflection Angle of the wire, can the insulator string be relaxed under the maximum wind deflection of the wire, and prevent the insulator from falling off or being damaged under pressure.
Suspension insulator string scheme
The 1000kV suspended insulator string in UHV AC substation adopts V-type suspension structure. The insulator mass is considered as 160kN. Determination of creepage distance for synthetic insulators. According to GB/T26218.3 -- 2010 "The selection and size of high voltage insulators for use in dirty conditions Part 3: According to "Composite Insulator for AC System", due to the characteristics of shape and material, the pollution resistance of silicone rubber umbrella insulator is usually better than that of porcelain insulator or glass insulator when the creepage distance is the same. If the pollution tolerance or flashover is only considered, the creepage distance can be reduced when synthetic insulator is used. However, compared with traditional porcelain materials, Silicone rubber materials are more prone to deterioration under the action of environment, electric field and arc, which will reduce the pollution performance and service life of insulators under certain conditions. Therefore, synthetic insulators with the same creepage distance as porcelain or glass insulators are recommended to avoid deterioration, flashover problems and improve pollution performance. For the national standard d filthy condition, the creepage distance of 1000 kV synthetic hanging insulator is recommended to be no less than 545x59=321.55(mm).
According to the technical conditions, refer to the porcelain insulator string scheme, through the manufacturer investigation.
The assembly of synthetic insulator string is basically the same as that of porcelain insulator string. Due to quality restrictions, the U-shaped hanging ring, extension ring, connecting plate, adjusting ring and tensioning clamp used in the assembly of synthetic insulator string are the same as those used in porcelain insulator string. Considering the difference in electrical properties between the synthetic material and the porcelain material. The voltage equalizing ring and shielding ring used for insulator string may be different, which needs to be determined after specific electric field analysis of the assembly and connection scheme. Because the synthetic insulator string with hard connection mandrel, for the whole connection structure. It is convenient to install and construct the string on site. After the installation, there will be no "U" shaped effect caused by the long length of porcelain insulator string. Therefore, composite insulator string is adopted. It is possible to replace saddle rings with simpler racetrack rings for the V-hanging string of pressure equalizing rings.
Type V hanging insulator string
The use of "V" insulator string for compact and compact transmission lines can effectively limit the swing amplitude of the wire, thus reducing the size of the tower head and the dynamic and static width of the corridor. Due to the close distance between the three-phase wires, the natural transmission power and electromagnetic environment of the lines are better than that of ordinary lines. The Angle of "V" insulator string directly affects the size of tower head and the stress of insulator string. According to a large number of research and design data collected at home and abroad, the included Angle of "V" type string is basically between 70° ~ 120°. The load of "V" insulator string is complicated, which is not only related to the vertical load, but also related to the included Angle, horizontal load and longitudinal load of "V" insulator string. The larger the included Angle of the "V" insulator string, the larger the vertical, horizontal and longitudinal load.
Technical and economic analysis
After the use of synthetic materials, 1000kV suspension insulator string length can be shortened by nearly 1m. Based on the arrangement and connection scheme of 1000kV distribution unit, the height of inlet and outlet line frame can be reduced from 41m to 40m. At the same time, considering that the mass of a single synthetic insulator string is about 1.2t lower than that of porcelain insulator string, by re-optimizing the design of the incoming and outgoing wire frames, each pin of incoming wire frame (width 49m) could save about 2t of steel and each pin of outgoing wire frame (width 51m) could save about 2.5t of steel. Calculated at the steel price of 10,000 yuan /t, each pin of incoming frame could save about 20,000 yuan and each pin of outgoing frame could save about 25,000 yuan. Combined with insulator material investment estimate and manufacturer's quotation. The price of a single piece of porcelain insulator is 139 yuan, and the price of a single synthetic insulator is 4600 yuan. If the assembly of the string of insulator is the same, the investment of each string of synthetic insulator is about 7200 yuan less than that of porcelain insulator string.
Although 1000 kV synthetic insulator string has certain technical and economic advantages compared with porcelain insulator string, the running time of synthetic insulator is shorter. How to delay the aging effect of synthetic materials and improve the manufacturing and technological level of insulators is still an important factor restricting the popularization and application of synthetic insulator in UHV stations. Continuous follow-up studies are needed. In addition, the application of 1000kV synthetic insulator string should be combined with electric field analysis to further optimize the design of voltage balancing ring and shielding ring of insulator string. On the international research results in recent years. Synthetic insulators are not a maintenance-free product. Operation experience shows that the hydrophobicity of synthetic insulators will decrease to varying degrees after operation for a certain number of years under the action of ozone, ultraviolet light, humidity, high and low temperature electrical stress and other external factors, which is the main reason for the "unknown flashover" in the operation of composite insulators. Therefore, after the large-scale application of synthetic insulators. How to do well on - line monitoring of its running performance is also worth paying attention to.
Summarize
Considering the weight of the insulator as 160kN, V-type suspension structure is still adopted. Under the condition of national standard d pollution, the creepage distance of the insulator should be no less than 32155mm. Considering the weight and connection mode of 1000kV suspended insulator string, 1000kV synthetic insulator string and porcelain insulator string can be assembled in the same way. The included Angle of V-shaped string is still considered as 65°, so the insulator string will not be damaged by compression and meet the design requirements.