1、电力系统电力系统介绍随着电力工业的增长，与用于生成和处理当今大规模电能消费的电力生产、传输、分配系统相关的经济、工程问题也随之增多。这些系统构成了一个完整的电力系统。应该着重提到的是生成电能的工业，它与众不同之处在于其产品应按顾客要求即需即用。生成电的能源以煤、石油，或水库和湖泊中水的形式储存起来，以备将来所有需。但这并不会降低用户对发电机容量的需求。显然，对电力系统而言服务的连续性至关重要。没有哪种服务能完全避免可能出现的失误，而系统的成本明显依赖于其稳定性。因此，必须在稳定性与成本之间找到平衡点，而最终的选择应是负载大小、特点、可能出现中断的原因、用户要求等的综合体现。然而，网络可靠性的增
2、加是通过应用一定数量的生成单元和在发电站港湾各分区间以及在国内、国际电网传输线路中使用自动断路器得以实现的。事实上大型系统包括众多的发电站和由高容量传输线路连接的负载。这样，在不中断总体服务的前提下可以停止单个发电单元或一套输电线路的运作。当今生成和传输电力最普遍的系统是三相系统。相对于其他交流系统而言，它具有简便、节能的优点。尤其是在特定导体间电压、传输功率、传输距离和线耗的情况下，三相系统所需铜或铝仅为单相系统的75%。三相系统另一个重要优点是三相电机比单相电机效率更高。大规模电力生产的能源有：1. 从常规燃料（煤、石油或天然气）、城市废料燃烧或核燃料应用中得到的蒸汽；2. 水；3. 石油
3、中的柴油动力。其他可能的能源有太阳能、风能、潮汐能等，但没有一种超越了试点发电站阶段。在大型蒸汽发电站中，蒸汽中的热能通过涡轮轮转换为功。涡轮必须包括安装在轴承上并封闭于汽缸中的轴或转子。转子由汽缸四周喷嘴喷射出的蒸汽流带动而平衡地转动。蒸汽流撞击轴上的叶片。中央电站采用冷凝涡轮，即蒸汽在离开涡轮后会通过一冷凝器。冷凝器通过其导管中大量冷水的循环来达到冷凝的效果，从而提高蒸汽的膨胀率、后继效率及涡轮的输出功率。而涡轮则直接与大型发电机相连。涡轮中的蒸汽具有能动性。蒸汽进入涡轮时压力较高、体积较小，而离开时却压力较低、体积较大。蒸汽是由锅炉中的热水生成的。普通的锅炉有燃烧燃料的炉膛燃烧时产生的热
4、被传导至金属炉壁来生成与炉体内压力相等的蒸汽。在核电站中，蒸汽的生成是在反应堆的帮助下完成的。反应堆中受控制的铀或盥的裂变可提供使水激化所必需的热量，即反应堆代替了常规电站的蒸汽机。水电站是利用蕴藏在消遣的能来发电的。为了将这种能转换为功，我们使用了水轮机。现代水轮机可分为两类：脉冲式和压力式（又称反应式）。前者用于重要设备，佩尔顿轮是唯一的类型；对于后者而言，弗朗西斯涡轮或其改进型被广泛采用。在脉冲式涡轮中，整个水头在到达叶轮前都被转化为动能，因为水是通过喷嘴提供给叶轮的；而在压力式或反应式涡轮中，水通过其四周一系列引导叶版先直接导入叶片再提供给叶轮（或转子）。离开引导叶片的水有压力，并且以
5、一部分动能、一部分压力的形式来提供能量。对于低于10,000千伏安的发电站而言柴油机是出色的原动机。其优点是燃料成本低、预热时间短以及标准损耗低。此外，其所需冷却水量极少。柴油发电通常选择用于满足少量电力需求，如市政当局、宾馆及工厂等；医院通常备有独立的柴油发电机，以备紧急情况时使用。通过电线来传输电能是电力系统中的一个重大问题。而从下面研修目的目的架设输电线路又是必要的：1. 将电力从水电站输送到可能很远的负载中心；2. 从蒸汽站到相对较近负载中心电力的批量供应；3. 出于内部连接目的将电能在紧急情况下从一系统转换至另一系统。传输电压主要由经济因素决定。实际上，当距离、功率、功耗固定时，输电
6、线路中导体的重量与传输电压成反比。因此，出于经济方面的考虑，长距离传输时电压一定要高。当然，电压超高绝缘成本也就超高，要找到最佳电压必须通过减小导体横截面积来取得绝缘成本与经济节省之间的平衡。高压传输通常使用配以悬挂式绝缘设备的高架结构。称为路标铁塔用于负载绝缘设备，各导体悬挂于一组或一串绝缘体的底部。最常用的导体类型如下：多芯铜导线、空心铜导线和ACSR（用钢加强的铝线）导线。电力分配系统包括由高夺线供能的子电站与用户开关之间的所有部件。电力从子电站接收，并以电压标准按不同类型用户均能接受的持续程度分送到各用户。在大城市中，高架与地下布线方式均被采用。尽管地下布线方式比高架方式昂贵，但在高度
7、城市化地区却必不可少。而在小城镇和较疏散的大城市区域，整个分配系统通常都是高架式的。电力系统监控现代电力系统都是由许多部分组成的统一体，而每个部分又都影响着其他部分的特性。为了整个系统能正常运行，就必须在系统的许多不同地点监控其工作情况以确保系统处于最佳工作状态。用户方根关心的是供电频率和电压的变化范围要小。既然系统内的频率在任何地方都是相同的，因而只要在便于观测之处安装一个频率表就能进行监控与此相反，系统的电压在不同的测试点上会有很大的差别。因此，有必要在系统的某些关键测点上对电压进行不断的观测，以便提供满意的服务。给系统内每台发电机规定适当的负荷时间访问演出能使系统有效地运行。纵然一些新的
8、电厂各自都有较高的效率，但是由于设置地点不当，可能产生巨大的系统电能损失。发电机之间的负荷分配应使总的燃料费用最小。要在任何意相不到的情况下保证正常供电，所有发电机的总额定功率就必须略大于总负荷加上损耗值。这种富余电量通称为旋转备用容量，它可以弥补用户突然增加的负荷或某台发电机因需要检修而突然甩掉的负荷。为了向用户征收电费，就必须安装电表。在不同的电力系统之间有许多联结线。仪表必须安装在联接点上，以便记录从一个系统传输到另一个系统的电能。这种监测要连续进行，以确保相互输送的电力在合同允许的限度之内。为防止过截造成的损坏，有必要对各个主要设备的工作状态进行不断的监测。由于负载逐月增加，需要增加设
9、备容量的地点是可以查明的，附加设备的安装就有可能得到保障。因此，在日益扩大的电力系统的未来建设中，检测仪表起着指南的作用。有时，在紧急情况下，系统操作人员观测到时他的系统负荷超过了发电能力和输电能力。那么他就面临着甩掉部分负荷或者确切地说保持负荷不变这样的问题因而就有必要在中断供电造成损失最小的地区有选择地甩掉负荷。在这种情况下，系统操作人员依靠许许多多仪表进行工作。这些仪表显示着系统运行情况的信息。当设备的运行走出其设计极限时，仪表可以发出警报声，作为要求采取避免设备损坏的行动的预告。在出现像电力系统发生故障这种极端不利的情况时，有毛病的设备就会自动停止运行。监控电流、电压和其他指标的仪表必
10、须能识别有故障的设备，并使断路器开始工作，让有故障的设备停止运行，而让运行系统中的其余设备仍继续工作。电力系统上的那些众多而不同的电气设备和用户拥有的那些电气设备都是按照在特定范围内运行而设计的。凡超出这些设计限度的运行都是不合适的，因为这可能导致运行效率很低、能量损耗很多或者（在极端的情况下）使设备毁坏。仔细观察设备的运行情况就可以发现需要采取的修正措施。各种电气设备都不应该出现过量电流，因为它会产生过高的温度使运行效率降低而缩短设备的使用寿命。居民区电路上出现过量电流时，由于保险丝和电闸的作用会造成电路中断。马达内的过量电流有可能损坏绝缘而过早出现绝缘故障。电压不足会极大地降低白炽灯泡的亮
11、度，并可能使荧光灯不发光。供电电压过低会在马达内造成过量电流而有可能损坏马达的线圈。电压过高提高了灯泡的发光亮度，但是在许多情况下会严重缩短灯泡的使用寿命。给马达和变压器超夺供电能在绕组铁芯里产生过多的能量损失，有可能使保持铁芯或绕组线圈的绝缘遭到损坏。机器的超速旋转有可能对机器的转动部件造成结构上的损坏。用户生产设备的超速运行可能造成劣质产品。在两台发电机或一台发电机与一台同步马达之间存在的不协调状态会导致这两台机器有效功率传输的中断。不协调状态应立即查明，两台机器也应立即分开。然后，才可以使它们恢复同步继续运行。在电力系统的许多工作位置上和许多用户的家中都要安装许多各种型号的仪表。有了这些
12、仪表，就能连续监控供电的工作状态。Electric Power SystemsIntroduction of Electric Power SystemsAs the power industry grows, so do the economic and engineering problems connected with the generating, transmission and distribution systems used to produce and handle the vast quantities of electrical energy consumed toda
13、y. These systems together form an electrical power system.It is important to note that the industry that produces electrical energy is unique in that it manufactures its product at the very instant that it is required by the customer. Energy for the generation of electricity can be stored in the for
14、m of coal and oil, and of water in reservoirs and lakes, to meet future requirements, but this does not decrease the need for generator capacity to meet the customers demands.It is obvious that the problem of the continuity of service is very important for an electrical power system. No service can
15、be completely protected from the possibility of failure and clearly the cost of the system will depend on its requirements. However a net reliability gain is obtained by employing a certain number of generating units and by using automatic breakers for the separation into sections of the bus bars in
16、 generating stations and of the transmission lines in a national or international grid system. In fact a large system comprises numerous generating stations and loads interconnected by high-capacity transmission lines. An individual unit of generation or set of transmission lines can usually cease t
17、o function without interrupting the general service.The most usual system today for generation and for the general transmission of power is the three-phase system. In favor of this system are its simplicity and its simplicity and its saving with respect to other a.c. system. In particular, for a giv
18、en voltage between conductors, with a given power transmitted, with a given distance, and with a give line loss, the three-phase system requires only 75 per cent of the copper or aluminium needed in the single-phase system. Another important advantage of the three-phase system is that three-phase mo
19、tors are more efficient than single-phase ones. The sources of energy ofr large-scale electricity generation are:1. steam obtained by means of a conventional fuel (coal, oil or natural gas), the combustion of city refuse or the employment of nuclear fuel;2. water;3. diesel power from oil.There are o
20、ther possible sources of energy such as direct solar heat, wind power, tidal power, etc., but none of these has yet gone beyond the pilot-plant stage.In large steam power plants, the thermal energy stored in steam is converted into work by means of turbines. A turbine consists essentially of a shaft
21、 or rotor fixed in bearings and enclosed in a cylindrical casing. The rotor is made to turn smoothly by means of jets of steam from nozzles around the periphery of the turbine cylinder. These steam jets strike blades attached to the shaft. Central power stations employ condensing turbines in which t
22、he steam passes into a condenser after leaving the turbine. Condensation is effected by the circulation of large quantities of cold water through the tubes of the condenser,thus increasing the expansion ratio of the steam and the consequent efficiency and work output of the turbine. The turbines are
23、 connected directly to large electricity generators.In turbines the action of the steam is kinetic. There is progressive expansion of the steam from the high pressure and relatively small volume at which it leaves.Steam is made by heating water in a boiler. The usual boiler has a furnace in which fu
24、el is burned, and the heat given off during combustion is conducted through the metal walls of the boiler to generate steam at a pressure within the boiler vessel. In nuclear plants, steam is generated with the aid of a reactor in which the controlled fission of uranium or plutonium supplies the nec
25、essary heat for the vaporization of water. Thus the reactor rep; aces the steam generator of conventional plants.Use is made of the energy possessed by water in hydroelectric stations. In order to transform this energy into work, hydraulic turbines are used. Modern hydraulic turbines may be divided
26、into two classes: impulse turbines and pressure or reaction turbines. Of the former, the Pelton wheel is the only type used in important installations; of the latter, the Francis turbine or one of its modifications is universally employed.In an impulse turbine, the whole head of water is converted i
27、nto kinetic energy before the wheel is reached, as the water is supplied to the wheel through a nozzle. In the pressure or reaction turbine the wheel (or runner) is provided with vanes into which water is directed by means of a series of guide vanes around the whole periphery. The water leaving thes
28、e guide vanes is under pressure and supplies energy partly in the kinetic form and partly in the pressure form.The diesel engine is an excellent prime mover for electricity generation in plant below about 10,000 KVA. It has the advantage of low fuel cost, a brief warming-up period and low standing l
29、osses. Moreover it requires little cooling water. Diesel generation is generally chosen for small power requirements by municipalities, hotels and factories; hospitals often keep and independent diesel generator for emergency supply.The transmission of electrical energy by means of lines is a great
30、problem in electrical power systems. Transmission lines are essential for three purposes:1. To transmit power from a hydroelectric site to a load center perhaps a considerable distance away;2. For the bulk supply of power from steam stations to load centers a relatively short distance away;3. For in
31、terconnection purposes to transfer energy from one system to another in case of emergency.The transmission voltage is determined largely by economic factors. In fact, in a transmission line, if the distance, the power and the power loss are fixed, the total weight of the conductor varies inversely a
32、s the square of the transmission voltage. For the economic transmission of power over considerable distances the voltage must therefore be high. Naturally with higher voltages the insulation cost also rises and to find the optimum voltage we must strike a balance between this cost and the saving thr
33、ough the reduction of the cross-section of the conductors.For high voltages, overhead-line construction is generally used with suspension-type insulators. Steel tower, called pylons serve to carry the insulators, with each conductor suspended from the bottom of a group or string of insulator units.
34、The following types of conductor are those most commonly used: stranded copper conductors, hollow copper conductors and ACSR (aluminum cable, steel reinforced) conductors.Distribution includes all the parts of the electricity system between the power substations supplied from high-voltage transmissi
35、on lines and the consumers switch. Electric power is received from substations and distributed to the consumers at the voltage levels and with the degree of continuity that are acceptable to the various types of consumer. In large metropolitan systems both over head and underground distribution meth
36、ods are used. Although underground distribution is more expensive than an overhead system, it is virtually a necessary in heavily urbanized areas. In smaller towns and in the less congested districts of large cities, the entire distribution system is usually overhead.Electric System MonitoringA mode
37、rn electric power system is an assembly of many components each of which influences the behavior of every other part. Proper functioning of the system as a whole makes it necessary to monitor conditions existion at many different points on the system in order to assure optimum operation.The concern
38、of the customers is primarily that the frequency and voltage of the supply are held within certain rather narrow limits. Since frequency of the system is the same everywhere, it may be monitored by a single frequency meter located at any convenient point. In contrast the voltage of the system may be
39、 quite different at different points. Consequently, it is necessary to make continuous observation of the voltage at certain key points on the system in order to provide acceptable service.Efficient operation of the system is obtained by assigning proper load schedules to each of the generators on t
40、he system. Newer plants, although individually more efficient, may be located at points on the system where their loading occasions large system losses. It is desirable to operate with a division of the load between generators so that the total cost of fuel consumed is minimized. To provide reliabil
41、ity of the power supply in tile event of unexpected conditions, it is desirable to have the total kilowatt rating of all machines in operation somewhat greater than the total load plus losses. This excess of generation, known as spinning reserve, is then available for picking up suddenly applied cus
42、tomer loads or to pick up the load dropped by a generator that must be removed from service for emergency maintenance.Instrumentation is necessary to permit billing of Customers for energy used. Many interconnections exist between different power systems. Instruments must be provided at interchange
43、points to permit billing for energy transferred from one system to another. The continuous monitoring of energy transfer is necessary to assure that interchanged power is within the limits of contract agreements.The continuous measurement of conditions on major pieces of equipment is necessary to av
44、oid damage due to overload. As load increases from month to month, points at which additional capacity of equipment is required may be recognized and provision made for the installation of additional equipment. Thus instrumentation serves as a guide for future construction in a growing power system.
45、Occasionally, under emergency conditions, a system operator observes that his system load exceeds the ability of the available generating and transmission equipment. He is then faced with the problem of load shedding or ,more properly, load conservation. It is then necessary to drop selected loads w
46、here service interruption is least objectionable. In such an event, he relies on the many instruments which provide information relative to system-operation conditions.Instruments may sound alarms as advance warnings of conditions requiring action to avoid damage to equipment operating beyond its de
47、sign limitation. In the event of extreme conditions such as power-system faults, defective equipment is switched out of service automatically. Instruments that continuously monitor current voltage, and other quantities must be able to identify the faulted equipment and to bring about operation of th
48、e circuit breakers which remove it from service, while leaving in service all other equipment on the operating system.The many different electrical devices on a power system and those owned by the customers are designed for operation within certain specified ranges. Operation, excessive deterioratio
49、n, or (in extreme cases ) the destruction of the device. Careful attention to the conditions under which equipment is operating may indicate corrective action that must be taken.Overcurrent on all electrical devices is undesirable, as it produces excessive temperatures, inefficient operation, and reduced service life. Overcurrent in residential circuits may bring about disconnection of the circuit by fuse or breaker actio
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