IN A series of earlier papers a full report was given on the results of the Tidd High-Voltage Transmission Research Project and on the American Gas and Electric Company (AG&E) extra-high-voltage transmiission system, the design of which was based largely upon those results.1-6Results of network analyzer studies were also described showing the performance of future transmission system layouts at voltage levels varying from 287 kv to as high as 360 kv and with predicted system loads as high as 6,000,000 kw.At that time, basic tower designs, line insulation, conductor design, and tentative operating voltage levels of the proposed system had been determined. The decision to adopt 315 kv as the nominal rating of the new system was based upon an analysis of the combined results of the Tidd Research Project, the system planning studies carried out on the n-etwork analyzer and, in addition, a study of transmission costs at various voltages. WVhile the specific reasons for the selection of 315 kv (330 kv maximum) were described in these papers, a brief recapitulation of these reasons may be helpful:1. It was felt that the voltage level should be within the range for which a singleconductor design could be used without the necessity for going to twin conductors or to an excessively large-diameter single conductor, to take care of corona and radio influence requirements. 2. To carry over to the new high-voltage system the sleet-melting procedure already fully adopted and proved for the 132-kv system, it was necessary to rule out twinconductor lines entirely because of the excessive current such lines would require to melt sleet. With a conductor diameter and conductivity suitable for 315 kv, however, sleet melting was felt to be entirely feasible. 3. Substantial economies were offered by the use of double-circuit tower construction, which at that time was considered practicable for system levels as high as 315 kv, but at least questionable for voltages much above that figure. 4. The carrying over to the new highvoltage system of the hot-line maintenance C. A. IMBURGIA MEMBER AIEE procedure, which is now successfully used on the 192-kv system, was considered impracticable either with twin conductors or with any design using insulator strings longer than the 18 and 19 units adopted for 315 kv. 5. System planning studies using the a-c network analyzer indicated that 31.5-kv transmission would be fully adequate to take care of future system load requirements as then visualized.
Reasons for Changing Extra-High-Voltage System Rating from 315 Kv to 330 KvFollowing the initial adoption of 315 kv for the reasons just enumerated and the final determination of conductor size, conductivity, tower designs, etc, subsequent studies brought out the interesting possibility of operating these same lines at a somewhat higher voltage level without increasing line insulation or conductor diameter. This thought actually originated in connection with a suggestion that the proposed line insulation for 315kv operation might be reduced; the logical ...
