Electric Power Systems.
The modern society depends on the electricity supply more heavily than ever before.It can not be imagined what the world should be if the electricity supply were interrupted all over the world.Electric power systems (or electric energy systems), providing electricity to the modern society, have become indispensable components of the industrial world.The first complete electric power system (comprising a generator, cable, fuse, meter, and loads) was built by Thomas Edison – the historic Pearl Street Station in New York City which began operation in September 1882.This was a DC system consisting of a steam-engine-driven DC generator supplying power to 59 customers within an area roughly 1.5 km in radius.The load, which consisted entirely of incandescent lamps, was supplied at 110 V through an underground cable system..Within a few years similar systems were in operation in most large cities throughout the world.With the development of motors by Frank Sprague in 1884, motor loads were added to such systems.This was the beginning of what would develop into one of the largest industries in the world.In spite of the initial widespread use of DC systems, they were almost completely superseded by AC systems.By 1886, the limitations of DC systems were becoming increasingly apparent.They could deliver power only a short distance from generators.
To keep transmiion power loes ( I 2 R ) and voltage drops to acceptable levels, voltage levels had to be high for long-distance power transmiion.Such high voltages were not acceptable for generation and consumption of power; therefore, a convenient means for voltage transformation became a neceity.The development of the transformer and AC transmiion by L.Gaulard and JD Gibbs of Paris, France, led to AC electric power systems.In 1889, the first AC transmiion line in North America was put into operation in Oregon between Willamette Falls and Portland.It was a single-phase line transmitting power at 4,000 V over a distance of 21 km.With the development of polyphase systems by Nikola Tesla, the AC system became even more attractive.By 1888, Tesla held several patents on AC motors, generators, transformers, and transmiion systems.Westinghouse bought the patents to these early inventions, and they formed the basis of the present-day AC systems.In the 1890s, there was considerable controversy over whether the electric utility industry should be standardized on DC or AC.By the turn of the century, the AC system had won out over the DC system for the following reasons:
（1） Voltage levels can be easily transformed in AC systems, thus
providing the flexibility for use of different voltages for generation, transmiion, and consumption.
（2） AC generators are much simpler than DC generators.
（3） AC motors are much simpler and cheaper than DC motors.
The first three-phase line in North America went into operation in 1893
——a 2,300 V, 12 km line in southern California.In the early period of AC power transmiion, frequency was not standardized.This poses a problem for interconnection.Eventually 60 Hz was adopted as standard in North America, although 50 Hz was used in many other countries.The increasing need for transmitting large amounts of power over longer distance created an incentive to use progreively high voltage levels.To avoid the proliferation of an unlimited number of voltages, the industry has standardized voltage levels.In USA, the standards are 115, 138, 161, and 230 kV for the high voltage (HV) cla, and 345, 500 and 765 kV for the extra-high voltage (EHV) cla.In China, the voltage levels in use are 10, 35, 110 for HV cla, and 220, 330 (only in Northwest China) and500 kVforEHVcla .Thefirst750kVtransmiion line will be built in the near future in Northwest China.With the development of the AC/DC converting equipment, high voltage DC (HVDC) transmiion systems have become more attractive and economical in special situations.The HVDC transmiion can be used for transmiion of large blocks of power over long distance, and providing an asynchronous link between systems where AC interconnection would be impractical because of system stability consideration or because nominal frequencies of the systems are different.The basic requirement to a power system is to provide an uninterrupted energy supply to customers with acceptable voltages and frequency.Because electricity can not be maively stored under a simple and economic way, the production and consumption of electricity must be done simultaneously.A fault or misoperation in any stages of a power system may poibly result in interruption of electricity supply to the customers.Therefore, a normal continuous operation of the power system to provide a reliable power supply to the customers is of paramount importance.Power system stability may be broadly defined as the property of a power system that enables it to remain in a state of operating equilibrium under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a disturbance..Instability in a power system may be manifested in many different ways depending on the system configuration and operating mode.Traditionally, the stability problem has been one of maintaining synchronous operation.Since power systems rely on synchronous machines for generation of electrical power, a neceary condition for satisfactory system operation is that all synchronous machines remain in synchronism or, colloquially \"in step\".This aspect of stability is influenced by the dynamics of generator rotor angles and power-angle relationships, and then referred to \" rotor angle stability \"
译文：
电力系统
现代社会比以往任何时候更多地依赖于电力供应。如果世界各地电力供应中断了,无法想象世界会变成什么样。电力系统（或电力能源系统），提供电力到现代社会，已成为产业界不可缺少的组成部分。历史上第一个完整的电力系统（包括发电机，电缆，熔断器，计量，加载）由托马斯爱迪生所建——纽约市珍珠街电站，始于1882年9月运作。这是一个直流系统组成蒸汽发动机驱动的直流发电机，供电范围面积约1.5公里，送给59个客户。他们的负载，其中包括白炽灯，通过地下电缆系统提供110V电压。一个个类似的系统在世界各地大多数大城市运行了数年。随着弗兰克斯普拉格在1884年对马达的发展，电机负载被添加到这些系统，从此开始发展成为世界上最大的产业之一。最初的直流系统被广泛使用，尽管如此，他们几乎完全被交流系统所取代。到1886年，直流系统的局限性也日益明显。他们从发电机提供功率只有很短的距离。
为了保持发射功率损失（I 2 R）和电压下降到可接受的水平，长途输电电压必须高。如此高的电压发电和电力消耗是可以接受的，因此，电压转换有一个方便的手段成为了必要。法国的L.巴黎戈拉尔和JD吉布斯发展了变压器和交流输电并引领了交流电力系统。1889年，在北美波特兰和威拉梅特大瀑布之间的俄勒冈州第一次实施交流传输线。这是一个单相线路传输为4,000伏，超过21公里距离的系统。随着交流的发展多相系统由尼古拉特斯拉，成为更具吸引力的。在1888年，尼古拉特斯拉取得多项交流专利，包括电动机，发电机，变压器和输电系统。西屋公司购买了这些早期的发明专利，并形成了现在交流系统的基础。 19世纪90年代，有很大的争议在于直流或交流电力行业是否应该统一。到了世纪之交时，下面的原因使交流系统赢过了直流系统：
（1）交流系统电压水平可以很容易地改变，从而提供了传输的灵活性，发电用不同的电压和消费。
（2）交流发电机比直流发电机简单得多。
（3）交流电机的马达比直流简单且便宜得多。
首次三相交流电线1893年投产于北美南加州- 1根 2300V， 12公里长的线路。在电力传输初期交流频率并不规范。有许多不同频率在使用：25，50，60，125，和133赫兹。这对互连的问题。最后北美的60赫兹标准获得通过，虽然
50赫兹在许多其他国家仍在使用。较长的距离越来越需要大量的电压传输这激励了他们逐步使用高压。为了避免电压增殖数值无限，业界标准了电压水平。在美国，标准是115，138， 161，和230千伏的高电压（高压）类，345，500和765千伏级的特高电压（超高压）。在中国，各级使用电压为10，35，110级高压， 220，330（仅在西北）和500千伏超高压类。第一个750 kVtransmiion线将在不久的将来建在中国西北地区。随着交流/直流转换设备的发展，高压直流（HVDC）传输系统已经成为更具吸引力和经济性的特殊情况。高压直流输电可用于输入大块输电和长距离输电，并提供不同系统间的异步连接，因为在交流联网系统间是不切实际的，因为稳定考虑，或因为系统间不同的频率。 基本要求到电源系统是提供一个客户可接受的电压和频率不间断的能源供应。由于电力无法用简单和经济的方法大量储存，电力的生产和消费必须同时进行。系统在任何阶段的故障或误操作可能导致给客户的电力供应中断。因此，一个正常的电力
系统能连续运行提供可靠的电力供应给客户是至关重要的。电力系统稳定，可广泛定义为干扰财产的权力系统，可继续经营的状态下正常运行的平衡条件和后向遭受恢复一个可以接受的平衡状态。 在电力系统的不稳定可能会表现在经营方式和多种不同的方式上，这取决于系统配置。传统上，稳定性问题一直是一个保持同步运行最主要的问题。由于电力系统的发电电力，一个令人满意的系统运行的必要条件是，依靠同步电机都留在同步或通俗的“步骤”。这一方面是受稳定的发电机转子的动态角度和功角的关系，然后提到“转子角稳定”。
