Voltage regulation and insulation for large power, long distance transmission systems

Abstract

Heretofore the distance to which power could be transmitted has been limited. This limitation is now removed by a simple method of loading the line with synchronous condensers, so that the current and voltage may be kept practically in phase. High power factor and hence high efficiency result, and the voltage rises of the system are very much reduced, thus reducing insulation strains. A standard frequency of 60 cycles is advocated for the national system, and 220,000 volts is proposed as standard for extra large-power, long-distance transmission. The system of regulation proposed will result in practically constant voltage at all points of the line at all loads. And power may be taken from or supplied to the line at any point, and the power over sections of the line or over the entire line may be reversed and the constant voltage system maintained. A simple diagram is given, and this shows that for a 60-cycle, 220,000-volt line, the line-charging current supplies about two-thirds of the capacity current required for about 0.8 load or 820 amperes load current, and that for larger loads the synchronous condensers supply leading and for smaller loads lagging current. Thus it is seen that the transmission line has largely inherently the currents required for self-regulation, if we correct initially the power factor of the loads to near unity. Every induction motor added to the power system calls for a certain capacity current for correction of power factor to reduce the losses from motor through to the power station. Every synchronous motor added, instead of an induction motor, helps in the economy all along the line, improves the service and reduces the menace resulting from large lagging currents. Every synchronous motor added becomes an asset to the entire system. Power factor correction should be done largely at load centers, the final correction and regulation being accomplished by the transmission line capacity current and the synchronous condensers. The advantages of such a system are: Simpler and cheaper generators, transformers standardized for one voltage, insulation strains reduced and a safer system results, and with constant voltage the flow of power has the greatest possible flexibility. (So far as known the proposal and method of presenting is new). This will give a system power transmission comparable to railway transportation, with a flexibility not possible in the ordinary system which does not have the constant voltage feature. The problems of the line insulation are discussed, and especial attention is called to the necessity for low air and leakage resistance stresses. The leakage resistance stresses are most important. For best results these should be distributed as uniformly as possible over the insulator surfaces, under the worst conditions. Results of a large number of tests are given. A new diagram is given which results from analysis of experimental data, from which the characteristics of long strings of insulator strings may be calculated, knowing the constants of the units relatively. The wet and dry arc-over may be controlled if desired, as shown by the illustrations, but it is believed best to strive for the elimination of arcs, except for cases of accident. While the present insulators with some form of shielding or grading (and with a system of regulation as given in Part I) will no doubt give more satisfactory results for 220,000 volts than is now obtained on lower voltage lines, it is desirable that further work be done with a view to crystallizing the best method of handling the line insulation. There is here an opportunity for some pioneer work, which will give us all that is desired, resulting in a high factor of safety for the line insulation.