英语翻译

英语翻译
Fig.4.4 shows a minimum power system.The system consists of an energy source,a prime mover,a generator,and a load.The energy may be coal,gas,or oil burned in a furnace to heat water and generator steam in a boiler.The prime mover may be a steam-driven turbine,a hydraulic or water wheel,or an internal combustion engine.Each one of these prime movers has the ability to convert energy in the form of heat,falling water,or fuel into rotation of a shaft,which in turn will drive the generator.The electrical load on the generator may be lights,motors,heaters,or other devices,alone or in combination.The control system functions to keep speed of the machines substantially constant and the voltage within prescribed limits,even the road may change.The control system may include a man stationed in the power plant who watches a set of meters on the generator-output terminals and makes the necessary adjustments manually.In a modern station,the control system is a servomechanism that senses a generator-output conditions and automatically makes the necessary changes in energy input and field current to hold the electrical output within certain specifications.
Modern power systems are usually large-scale,geographically distributed,and with hundreds to thousands of generators operating in parallel and synchronously.They may vary in size and structure from one to another,but they all have basic characteristics:
(1)Are comprised of three-phase AC systems operating essentially at constant voltage.Generation and transmission facilities use three-phase equipment.Industrial loads are invariably three-phase; single-phase residential and commercial loads are distributed equally among the phases so as to effectively from a balanced three-phase system.
(2)Use synchronous machines for generation of electricity.Prime moves convert the primary energy ( fossil,nuclear,and hydraulic) to mechanical energy that is,in turn,converted to electrical energy by synchronous generators.
(3)Transmit power over significant distances to consumers spread over a wide area.This requires a transmission system comprising subsystems operating at different voltage levels.