Hydroelectric generating stations often require transmission of the energy over large vertical distances....
Since it is usually more economical to transmit the power at transmission voltages rather than the generation voltage, the step-up transformers are often located at the lowest elevation while the transmission system is most accessible at higher elevations.
At transmission voltages, there are four possible methods to transmit this power:
• Air insulated bus |
• Oil cable |
• Solid dielectric cable |
• SF6 Gas Insulated Bus |
The air-insulated bus would require large clearances. Shaft sizes would need to be very large to accommodate this solution. In addition, there is a significant safety hazard due to the exposed high voltage conductors. While the oil cable would solve the size problem, it has two distinct disadvantages: (1) Large vertical drops result in very large pressure heads, and (2) Since oil burns easily, a real fire hazard exists should a fault occur in the cable system. If a fault were to occur in one cable circuit, the potential fire would spread to the adjoining circuits eliminating all power transmission capabilities. Solid dielectric cable, while not as flammable as oil cable, still offers a fire hazard. Close attention to support over the vertical distances would be required to prevent the cable from stretching and failing, especially at field splices.
The utilization of SF6 CGIT bus for vertical shaft applications offers the following advantages relative to the other technologies:
• Since the SF6 bus is enclosed and grounded, clearances can be small thus reducing the required shaft size. Also, the dead-front construction eliminates any high voltage safety hazards. |
• SF6 is an inert gas and will not burn, so there is no fire hazard. Since SF6 is a gas, the pressure head is very low; typically about 10psi per 1000 feet of vertical head. |
• The design of SF6 bus enclosures offers little risk of stretching or mechanical damage. Since the joints offer the same mechanical strength as the balance of the bus system, no elaborate support methods are required. |
• Since the joints offer the same mechanical strength as the balance of the bus system, no elaborate support methods are required. |
The CGIT system concept has been successfully applied in vertical installations. An existing installation at the Southern California Edison Balsam Meadows Station, operates at 242kV and transmits 200MW of power up a 1000-foot vertical shaft.
RevelStoke Hydro Project
Another example in an 1830m long inclined tunnel, is in British Columbia Hydro’s Revelstoke Project, in Canada. This hydroelectric plant included a multiple stage GIS arrangement, for which CGIT provided the best solution given the particular connection requirements and the installation constraints. Within the Power house, there are gas insulated breaker/grounding switch sets at each step up transformer output, and a dual set of 500kV GIS connecting the generator transformers output into two main feeder circuits. At the substation building end, another set of 500kV GIS modules connects the feeders to the outgoing overhead lines; a separate 230kV GIS receives incoming power for internal plant service.
The connections from the transformer breakers to the powerhouse GIS, and between the later and the substation GIS were all made with dual circuits of 500kV CGIT bus. The GIS-to-GIS portion runs almost entirely inside the 1830m long, inclined tunnel.
Another CGIT bus circuit connects the incoming 230kV power line bushings into the secondary GIS, from which low voltage cable lines bring the power into the rest of the plant.
The CGIT bus allowed a compact installation along the walls of the tunnel, and, because of its fully grounded enclosure, it permitted the use of the tunnel as the main access route to the powerhouse with minimum space interference and no electrical hazard to personnel.
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