【正文】 the breaker is capable of carrying continuously without exceeding any of the temperature limitations. For the example above, a reasonable value of rated continuous current might be 600A. Transmission Line Tile overhead three phase power transmission line is the main energy corridor in apower system. One might assume that the circuit model would be trivial (ideal conductors), but three different phenomena produce effects that can not reasonably be ignored. In order of importance, they are the series voltages induced by the magic fields surrounding the conductors, the shunt displacement currents resulting from the electric fields between conductors, and the ohmic resistance of the conductor material. A forth, and minor, effect is the leakage conduction current that flows through contamination films on the insulators. The overhead neutrals of a typical overhead power transmission line are electrically in contact with the tower and therefore grounded. They primarily exist to provide lightning shielding for the phase conductors and also to carry zero sequence and harmonic currents that help to maintain balanced sinusoidal voltages. They are usually steel or aluminum and are small (diameter about lcm). The phase conductors are much larger (diameter about 5 cra), and are typically stranded aluminum surrounding a stranded steel cable (for increased tensile strength). Sometimes more than one (a bundle) prise a phase. All are bare(no insulating covering ) for heat dissipation reasons。 specifically, the worst icing and wind conditions within reason must be estimated because these factors also relate to the tower load. Architectural aesthetics should be considered particularly in populated areas. Tower width relates to available right of way, and the decision to use guyed or free standing towers must be made. Overhead minimum clearances for railroads, highways, structures, vegetation, and the earth must be established these dimensions fix tower height, along with tower spacing and permissible conductor sag. Wind excited conductor motions must be an ticipated and minimized. Line Voltage Regulation and Compensation The line impedance elements call have an effect on the voltage at all points along the line, the effect varying with line loading. Serious over voltages can occur at light loads and unacceptable low voltages can occur at heavy loads. The problem bees more serious as the line length increases. This effect can severely limit the acceptable operating range of practical long lines and justifies the cost of installing additional equipment to correct the situation. We observe that the problem in the loaded case is essentially caused by the series inductance. The insertion of series capacitance would have a cancellation effect and relieve the difficulty to some degree. Such a modification is referred to as series pensation. The over voltage condition is caused by the line shunt capacitance and this may at least partially be eliminated by the installation of shunt reactors (inductance). This is referred to as shunt pensation. The pensating elements should be located at points along the line spaced so as to keep the line voltage within required tolerances at all points. The economics of such in stallations must be considered as a part of the line cost. The pensating elements may be inserted or bypassed depending on the switch positions. Cables Insulated cables have many applications in the field of electric power. Small cables are used as extension cords around offices, homes, and factories. Larger cables are used for connections to machines that are movable over restricted distances. In some instances portable cables carry quite heavy electrical loads, as for example the case of electrically driven drag lines, which require several thousand horsepower for operation. Overhead cables find application in distribution circuits where tree conditions or proximity to buildings and other structures make the use of open wire lines impracticable. Underground cables are used in many situations, including major transmission circuits between large stations. Some cable power circuits operate at 765 kilovolts and carry loads of several hundred megawatts. Cables of even higher voltage will probably bee available soon. The characteristics of cables are quite different from those of overhead lines of the same voltage class. As tile conductors of cables are much closer together than are those of overhead lines, the inductance and inductive reactance of cables are much smaller than similar quantities for overhead lines. In some instances the resistance of a cable may be larger in magnitude than its inductive reactance. The close spacing of conductors and the presence of solid insulatio