4 Conductors carry power from the generators to the load.In a grid,conductors may be classified as belonging to the transmission system,which carries large amounts of power at high voltages(typically more than 69 kV)from the generating centres to the load centres,or the distribution system,which feeds smaller amounts of power at lower voltages(typically less than 69 kV)from the load centres to nearby homes and industry. s Choice of conductors is based upon considerations such as cost,transmission losses and other desirable characteristics of the metal,like tensile strength. Copper,with lower resistivity than Aluminum,was the conductor of choice for most power systems.However,Aluminum has lower cost for the same current carrying capacity and is the primary metal used for transmission line conductors Overhead line conductors may be reinforced with steel or aluminum alloys. Electricity grid systems connect multiple generators and loads operating at the same frequency and number of phases,the commonest being three-phase at 50 or 60 Hz.However,there are other considerations.These considerations range from the obvious:How much power should the generator be able to supply; What is an acceptable length of time for starting the generator(some generators can take hours to start);Is the availability of the power source acceptable(some renewables are only available when the sun is shining or the wind is blowing): to the more technical:How should the generator start;What is the mechanical speed of operation for the turbine and consequently what is the number of poles required;What type of generator is suitable(synchronous or asynchronous) and what type of rotor(squirrel-cage rotor,wound rotor,salient pole rotor or cylindrical rotor)is appropriate? Electric power is the product of two quantities:current and voltage.These two quantities can vary with respect to time(AC power)or can be kept at constant levels(DC power).Solid state devices,which are products of the semiconductor revolution,make it possible to transform DC power to different voltages,build brushless DC machines and convert between AC and DC power. Nevertheless,devices utilizing solid state technology are often more expensive than their traditional counterparts,so AC power remains in widespread use. 46电气工程英语 46 4 Conductors carry power from the generators to the load. In a grid, conductors may be classified as belonging to the transmission system, which carries large amounts of power at high voltages (typically more than 69 kV) from the generating centres to the load centres, or the distribution system, which feeds smaller amounts of power at lower voltages (typically less than 69 kV) from the load centres to nearby homes and industry. s Choice of conductors is based upon considerations such as cost, transmission losses and other desirable characteristics of the metal, like tensile strength. Copper, with lower resistivity than Aluminum, was the conductor of choice for most power systems. However, Alun um has lower cost for the srune current ca ing capacity and is the primary metal used for transmission line conductors. Overhead line conductors may be reinforced with steel or aluminum alloys. 6 Electricity grid systems connect multiple generators and loads operating at the same frequency and number of phases, the commonest being three-phase at 50 or 60 Hz. However, there are other considerations. These considerations range from the obvious: How much power should the generator be able to supply; What is an acceptable length of time for starting the generator (some generators can take hours to start); Is the availability of the power source acceptable (some rencwables are only available when the sun is shining or the wind is blowing); to the more technical: How should the generator start; What is the mechanical speed of operation for the turbine and consequently what is the number of poles · required; What type of generator is suitable (synchronous or asynchronous) and what type of rotor (squirrel-cage rotor, wound rotor, salient pole rotor or cylin cal rotor) is appropriate? 1 Electric power is the product of two quantities: current and voltage. These 电气工程英语 two quantities can vary with respect to time (AC power) or can be kept at constant levels(DC powerJ. Solid state devices, which are products of the semiconductor revolution, make it possible to transform DC power to different voltages, build brushless DC machines and convert between AC and DC power. Nevertheless, devices utilizing solid state technology are often more expensive than their traditional counterparts, so AC power remains in widespread use