Conductor is the main body of overhead line and is responsible for transmitting electric energy. Since the conductor is erected on the pole, it should often bear the effects of self weight, wind, rain, ice, snow, erosion of harmful gases and changes in air temperature. Therefore, the conductor is required not only to have good conductivity, but also to have sufficient mechanical strength and good corrosion resistance.
The conductors used in low-voltage overhead lines are divided into bare conductors and insulated conductors. According to the structure of conductor, it can be divided into single strand conductor, multi strand conductor and hollow conductor; According to the material of conductor, it is divided into copper conductor, aluminum conductor, steel core aluminum conductor and steel conductor.
How to select overhead conductors?
1) Low voltage overhead lines generally use bare stranded wires. Insulated conductors shall be selected only for household lines close to civil buildings, narrow streets, dense buildings and low overhead height. Single strand conductor or broken strand shall not be used for overhead lines.
2) Sufficient mechanical strength shall be ensured. The overhead conductor itself has a certain weight and is also affected by external forces such as wind, rain, ice and snow during operation, so it must have a certain mechanical strength. In order to avoid wire breakage, the cross-sectional area of overhead conductor shall not be less than l6mm2.
3) The allowable current carrying capacity of the conductor shall meet the load requirements. The actual load current of the conductor shall be less than the allowable current carrying capacity of the conductor. Correction factors for allowable current carrying capacity of aluminum strand and steel cored aluminum strand and allowable current carrying capacity of aluminum conductor
4) The voltage loss of the line should not be too large. Because the conductor has a certain resistance, voltage loss will occur when current passes through the conductor. The thinner and longer the conductor, the greater the load current, the greater the voltage loss, and the lower the voltage at the end of the line, which can not even meet the voltage requirements of electrical equipment. Therefore, generally, the voltage loss of the line shall not exceed 5%.
In the early days, copper was the preferred material for overhead conductors, but aluminum has replaced copper because it is much cheaper and lighter than copper conductors with the same resistance. Here are some materials that are considered good conductors.
Copper: copper has high conductivity and greater tensile strength. Therefore, hard drawn stranded copper is an excellent choice for overhead lines. Copper has a high current density, which means that the current carrying capacity per unit cross-sectional area is greater. Therefore, the cross-sectional area of copper conductor is relatively small. In addition, copper is durable and has high scrap value. However, copper is rarely used in overhead power lines due to its high cost and unavailability.
Aluminum: the conductivity of aluminum is about 60% of that of copper; This means that for the same resistance, the diameter of aluminum conductor is about 1.26 times that of copper conductor. However, the weight of an aluminum conductor is almost half that of an equivalent copper conductor. In addition, the tensile strength of aluminum is less than that of copper. Considering the comprehensive factors such as cost, conductivity, tensile strength and weight, aluminum has advantages over copper. Therefore, aluminum is widely used in overhead conductors.
Cadmium copper: cadmium copper alloys contain about 98% to 99% copper and up to 1.5% cadmium. Adding about 1% cadmium to copper can increase the tensile strength by up to 50%, while the conductivity decreases by only about 15%. Therefore, cadmium copper conductor can be used for ultra long span. However, due to the high cost of cadmium, this conductor may not be economical in many cases.
Other materials: there are many other metals and alloys that conduct electricity. Silver has better conductivity than copper, but it is not practical in most cases because of its high cost. Galvanized steel can also be used as a conductor. Although steel has very high tensile strength, due to its poor conductivity and high resistance, steel conductors are not suitable for effective power transmission. High strength alloys, such as phosphor bronze, can sometimes be used under extreme conditions.
As mentioned above, considering the comprehensive factors such as cost, conductivity, tensile strength and weight, aluminum conductor has advantages over copper conductor. Aluminum conductor has completely replaced copper conductor in overhead power lines because of its low cost and light weight. Although the diameter of aluminum conductor is larger than that of copper conductor with the same resistance, considering “corona”, this is actually an advantage. Corona decreases significantly with the increase of conductor diameter. The following four common types are used for overhead transmission and distribution of overhead conductors, from carrying generated power generation stations to end users.
In general, all types of conductors are stranded to increase flexibility. In addition to the very small cross-sectional area, solid lines are very difficult to handle, and they tend to crystallize at the support point due to their swing in the wind.
AAC: all aluminum conductor
AAAC: all aluminum alloy conductor
ACSR: aluminum conductor, steel reinforced
Acar: aluminum conductor, alloy reinforced