Lowering the natural frequency of higher voltage class bushings using rubber dampers on flanges has been done in the past. For example, this design was used mainly for Italian transmission utility, Enel, to satisfy very high transformer short circuit mechanical stresses. However, this solution has since been abandoned due to high production costs. Using extra high strength porcelains, supplied by the most competent insulator manufacturers, some bushings can be made to satisfy almost all requests in this field. By contrast, for extremely severe seismic conditions or on special request, OIP bushings can be equipped with composite insulators that, due to lower weight, higher flexibility and excellent mechanical characteristics, offer advantages compared to porcelain-housed bushings. Indeed, this design approach is prominent for GIS bushings. Seismic tests performed on 550 kV rated gas bushings at the highest seismic level in the standards demonstrated excellent performance, without any internal or external damage that could compromise service reliability.
Design Aspects for HVDC Bushings
Fire in the valve hall is the most critical event that could cause a long forced outage in a UHV DC facility. To minimize risk of such a serious event, plant designers increase project safety margins by specifying bushings based on solutions that minimize risk of transformer oil spilling into the valve hall in case of failure. This also serves to minimize risk of oil leakage from wall bushings and other equipment. In some cases, technical specifications require a bushing arrangement that allows installation of the transformer totally outside the valve hall. To comply with this requirement, transformer bushings are equipped with a long metallic extension on flanges in order to pass through the wall of the valve hall. This leads to very long bushings for UHV DC applications.
The main solutions provided for DC applications can be based on one of the following: purely solid insulation (RIP); hybrid types that are either a solid insulation combined with gas (RIP+SF6) or an oil-impregnated paper insulation combined with gas (OIP+SF6); or a purely a gas (SF6) solution. One technical solution for HVDC transformer bushings is an innovative hybrid OIP+SF6 system based on a fully-sealed OIP condenser bushing that interfaces with the transformer and a gas-filled hollow composite housing for the air side, i.e. the portion that protrudes into the valve hall (see Fig. 13)
This solution is particularly optimized from the safety point of view and its advantages include:
• the oil-impregnated paper grading capacitor is fully sealed and it ensures a complete separation between the gas compartment and the transformer turret;
• gas leakage inside the transformer is not possible since this is blocked at the level of the grading capacitor and detected by suitable oil pressure gauges;
• there is no risk of transformer oil spills from the turret to the valve hall in case of bushing failure due to the triple barrier system ensured by the composite insulator and the two sealing barriers of the grading capacitor;
• in case of damage to the barrier on the SF6 side, the risk of oil spill in the valve hall is avoided by the presence of the composite insulator and the pressure gauges;
• total free oil quantity inside the bushing is extremely low, minimizing risk of fire;
• the oil inside the grading capacitor permits better cooling of the bushing – especially useful for high current ratings up to 6300 A and for the presence of harmonics;
• the grading capacitor winding is 100% protected from risk of particle contamination during installation of the bushing onto the transformer since it is totally sealed.
A particular HVDC application that requires special attention from the safety point of view is the wall bushing. The schematic in Fig. 15 shows the different solutions applicable to wall bushings. The traditional OIP solution is no longer requested due to possible oil spills and fire risk in the valve hall. Other possibilities are RIP-SF6 technology and pure SF6 gas solutions.
The solution selected by GE Grid is the pure SF6 gas technology, which is felt to offer the following advantages:
• simple design based on a high technology solution;
• lower weight compared with equivalent solution RIP-SF6;
• less different materials, therefore lower electrical risks under DC and polarity reversal stresses;
• no ageing of the internal insulation system due to limited quantity of organic materials;
• fast production and delivery time thanks to simplicity of the solution;
• limited new investment for production;
• limited maintenance and on-line monitoring of gas status;
• excellent characteristics from the safety and pollution points of view, thanks to adoption of composite insulator housings.