This invention relates generally to gas-insulated power transmission lines, and more particularly to a power transmission line which utilizes two sheaths, one of carbon steel and the other of aluminum for the outer enclosure.
Compressed gas-insulated power transmission lines are a relatively recent commercial product which is gaining increased acceptance by the electric utility industry as an economical means for transmitting large blocks of electrical energy, and for use in special applications where conventional power transmission lines are inadequate. A typical compressed gas-insulated transmission line is generally comprised of an elongated outer sheath having disposed therein an inner conductor at high potential, with means disposed in the outer sheath to insulatably support the inner conductor within the outer sheath. An insulating gas, typical of which is sulfur hexafluoride at a pressure of 50 psi gauge, is utilized for electrically insulating the inner conductor from the outer sheath. The inner conductor is typically used to transmit energy at voltage ranges from 121-1200 KV. Both the inner conductor and the outer sheath are of good electrically conducting materials such as aluminum.
One of the drawbacks of gas-insulated power transmission lines which is hindering even more widespread applications of the lines is associated with the cost of the lines. One of the more costly components of the transmission lines is the outer enclosure utilized to contain the inner conductor and the insulating gas.
Present gas-insulated power transmission lines utilize a solid enclosure of aluminum to contain the inner conductor and the insulating gas. Aluminum is preferred because of the low loss it exhibits to the return current flowing through it induced by the conductor currents on the inner conductor. Typically, for grounded aluminum enclosures, the loss ratio (i.e., the ratio of enclosure loss to conductor loss) is of the order of one or less. However, the aluminum material utilized for the enclosure is itself expensive, and large quantities of the material must be utilized.
One method attempted in the prior art to minimize the cost of the outer enclosure has been to utilize carbon steel as the outer enclosure. Although carbon steel pipe is generally readily available at less expensive costs, the losses which result with respect to the return current are substantial. The loss ratio of carbon steel is generally of the order of three or higher. Therefore, because of the high-loss ratio, the use of carbon steel for the outer enclosure has generally occurred only when the current rating of a line is very low and the high electrical losses are acceptable.