Publication: Electrical World
New York, NY, United States
Problems of Transmission at 220,000 Volts Are Being Solved
OPINION among operating engineers appears to be crystallizing in favor of a national standard of 220,000 volts for extra-long bulk-supply transmission lines. For this service the value advocated is an excellent one because, as is made clear by F. G. Baum elsewhere in this number of the ELECTRICAL WORLD, it enables the economical transmission of large amounts of power over distances as great as 1,000 miles and, besides, is a simple multiple of voltages now firmly established. However, engineers should not confuse long lines in which power is transmitted the entire length with long lines having intermediate feed-in and feed-out taps, since the latter serve chiefly for interchange of power and seldom if ever have to transmit energy the entire length.
For the 220,000-volt insulator string three alternatives have been discussed—(1) the use of larger and heavier insulator units, (2) grading, or the use of units of high electrostatic capacity at the line end of the string to ease the electric stress on these units, and (3) the application of metal shields in such a way as to lower the voltage gradient at the units next the line. The second and third methods, and especially the last, appear to offer the best solution according to the classic studies carried out in the Stanford University laboratories by Professor Ryan, Mr. Baum and others. A combination of grading and shielding is also being considered. These tests have shown that even after the units next the line have been relieved there is great danger that flashovers will strike in at the upper units, themselves under low unit stresses, unless this contingency is carefully guarded against. Mr. Baum summarizes the whole situation by saying that the line insulators must be made as reliable as the transformers and other apparatus before the insulation problem can be considered fully solved.
The voltage control of long lines cannot be accomplished satisfactorily at the generator alone. The heavy and ever-changing charging current along the line, not to mention changing loads at various points in the line, call for remedies which Mr. Baum happily compares to the loading of telephone lines. Synchronous condensers distributed along the line give the obvious and apparently complete solution of this problem.