Unit One Electric Power System Text A Components of Power SystemsModem power systems are usually largescale，geographically distributed，and with hundreds to thousands of generators operating in parallel and synchronouslyThey may vary in size and structure from one to another, but they all have the same basic characteristics：(1)Are comprised of threephase AC systems operating essentially at constant voltageGeneration and transmission facilities use threephase equipmentIndustrial loads are invariably three-phase；single-phase residential and commercial loads are distributed equally among the phases SO as to effectively form a balanced threephase system(2)Use synchronous machines for generation of electricityPrime movers 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 areaThisrequires a transmission system comprising subsystems operating at different voltage levelsThe basic elements of a modem power system in USAare shown in Fig61Electric power is produced at generating stations(GS)and transmitted to consumers through a complex network of individual components，including transmission lines，transformers，and switching devicesIt is common practice to classify the transmission network into the following subsystems：Transmission system；Subtransmission system；Distribution systemFig1 Basic elements of a power systemThe transmission system interconnects all major generating stations and main load centers inthe systemIt forms the backbone of the integrated power system and operates at the highestvoltage levels(typically,230 kV and above in USA )。The generator voltages are usually in therange of 11 to 35 kVThese are stepped up to the transmission voltage level，and power istransmitted to transmission substations where the voltages are stepped down to the subtransmission level(typically,69 to 1 38 kV)The generation and transmission subsystems are often referred to as the bulk power systemThe subtransmisson system transmits power in small quantifies from the transmission substations to the distribution substationsLarge industrial customers are commonly supplied directly from the subtransmission systemIn some systems，there iS no clear demarcation between subtransmission and transmission circuitsAs the system expands and higher voltage levels become necessary for transmission，the older transmission lines are often relegated to subtransmission functionThe distribution system represents the final stage in the transfer of power to the individual customersThe primary distribution voltage is typically between 40 kV and 345 kVSmallindustrial customers are supplied by primary feeders at this voltage levelThe secondary distribution feeders supply residential and commercial customers at 120240 VSmall generating plants located near the load are also connected to the subtransmission or distribution system directlyInterconnections to neighboring power systems are usually formed at the transmission system levelThe overall system thus consists of multiple generating sources and several layers of transmission networksThis provides a hiigh degree of structural redundancy that enables the system to withstand unusual contingencies without service disruption to the customersNew Words and Expressionsin parallel 并联地synchronously ad同步地facility n容易，方便灵活，熟练 (便利)条件(常用复数)设备，装置，工具，器材primary energy 一次能源fossil n化石，石块hydraulic a水力学的，水压的，液压的，液力的mechanical energy 机械能subsystem n子系统，分系统，辅助系统switching device 开关装置，开关设备subtransmission n二次输电subtransmission system 分支输电系统，二次输电系统distribution system 配电系统substation n变电所，变电站，分站，分所feeder n馈线，馈电电路redundancy n多余，冗余contingency n意外事故，偶然事故Notes1. Modem power systems are usually largescale，geographically distributed，and with hundreds to thousands of generators operating in parallel and synchronously现代电力系统通常是规模大，地域分布广，具有成百上千并列同步运行的发电机组。2.3.ExercisesITranslate the following words or expressions into English1. 23. 45. 6.7. 8. 9. 10.II Fill in the blanks according to the passage.IIn power plants convert the primary energy to mechanical energy, and convert mechanical energy to electric energy2 The generator voltages are usually stepped up to the transmission voltage level by 3 Residential and commercial customers are supplied by the secondary distribution feeders at 4 In USA，the voltages of the transmission system are typically 5. 6.7.8.9.10.III Answer the following questions according to the passage.1 What components is a modern power system comprised Of?2 What is the function of the distribution system?3 Wha