Electric Power System, components that transform other type of energy into electrical energy and transmit this energy to a consumer. The production and transmission of electricity is relatively efficient and inexpensive, although unlike other forms of energy, electricity is not easily stored and thus must generally be used as it is being produced.
Components of Electric Power System
A modern electric power system consists of six main components:(1) the power station, (2) a set of transformers to raise the generated power to the high voltages used on the transmission lines, (3) the transmission lines, (4) the substations at which the power is stepped down to the voltage on the distribution lines. (5) the distribution lines, and (6) the transformers that lower the distribution voltage to the level used by the consumer’s equipment.
Power Station The power station of a power system consists of a prime mover, such as a turbine driver by water, steam, or combustion gases that operate a system of electric motors and generators, Most of the world’s electric power is generated in steam plants driven by coal, oil, nuclear energy, or gas. A smaller percentage of the world’s electric power is generated by hydroelectric (waterpower), diesel, and internal-combustion plants.
Transformers Modern electric power systems use transformers to convert electricity into different voltages. With transformers, each stage of the system can be operated at an appropriate voltage. In a typical system, the generators at the power station deliver a voltage of from 1,000 to 26,000 volts (V). Transformers step this voltage up to values ranging from 138,000 to 765.000 V for the long-distance primary transmission line because higher voltages can be transmitted more efficiently over long distances. At the substation the voltage may be transformed down to levels of 69,000 to 138,000 V for further transfer on the distribution system. Another set of transformers step the voltage down again to a distribution level such as 2,400 or 4,160 V or 15,27, or 33 kilovolts (kV). Finally the voltage is transformed once again at the distribution transformer near the point of use to 240 or 120 V.
Transmission Lines The lines of high-voltage transmission systems are usually composed of wires of copper, aluminum, or aluminum-clad steel, which are suspended form tall latticework towers of steel by string of porcelain insulators. By the use of clad steel wires and high towers, the distance between towers can be increased, and the cost of the transmission line thus reduced. In modern installations with essentially straight paths, high-voltage lines may be built with as few as six towers to the kilometer. In som areas high-voltage lines are suspended from tall wooden poles spaced more closely together.
For lower voltage distribution lines, wooden poles are generally used safety hazard or are considered unattractive, insulated underground cables are used for distribution[1]. Some of these cables have a hollow core through which oil circulates under low pressure. The oil provides temporary protection from water damage to the enclosed wires should the cable develop a leak. Pipe-type cables in which three cables are enclosed in a pipe filled with oil under high pressure (14 kg per sq cm/200psi ) are frequently used. These cables are used for transmission of current at voltages as high as 345,000 V (or 345 kV).
Supplementary Equipment Any electric-distribution system involves a large amount of supplementary equipment to protect the generators, transformers, and the transmission lines themselves. The system often includes devices designed to regulate the voltage or other characteristics of power delivered to consumers
To protect all elements of a power system from short circuits and overloads, and for normal switching operations, circuit breakers are employed.
These breakers are large switches that are activated automatically in the event of a short circuit or other condition that produces a sudden rise of current. Because a current forms across the terminals of the circuit breaker at the moment when the current is interrupted, some large breakers (such as those used to protect a generator or a section of primary transmission line) are immersed in a liquid that in a poor conductor of electricity, such as oil, to quench the current . In large air-type circuit breakers, as well as in oil breakers, magnetic fields are used to break up the current. Small air-circuit breakers are used for protection in shops, factories, and in modern home installations. In residential electric wiring, fuses were once commonly employed for the same purpose. A fuse consists of a piece of alloy with a low melting point, inserted in the circuit, which melts, breaking the circuit if the current rises above a certain value, Most residences now use air-circuit breakers.
Power Failures
In most parts of the world, local or national electric utilities have joined in grid systems. The linking grids allow electricity generated in one area to be shared with others. Each utility that agrees to share an increased reserve capacity, use of larger, more efficient generators, and the ability to respond to local power failures by obtaining energy from a linking grid.[2]
These interconnected grids are large, complex systems that contain elements operated by different groups. These systems offer the opportunity for economic savings and improve overall reliability but can create a risk of widespread failure. For example, the worst blackout in the history of the United States and Canada occurred August 14.2003,when 61.800 megawatts of electrical power was lost in an area covering 50 million people. (One megawatt of electricity is roughly the amount needed to power 750 residential homes.) The blackout prompted calls to replace aging equipment and raised questions about the reliability of the national power grid.
Despite the potential for rare widespread problems, the interconnected grid system provides necessary backup and alternate paths for power flow, resulting in much higher overall reliability than is possible with isolated systems. National or regional grids can also cope with unexpected outages such as those caused by storms, earthquakes, landslides, and forest fires, or due to human error or deliberate acts sabotage.
Power Quality
In recent years electricity has been used to power more sophisticated and technically complex manufacturing processes, computers and computer networks, and a variety of other high-technology consumer goods. These products and processes are sensitive not only to the continuity of power supply but also to the constancy of electrical frequency and voltage. Consequently, utilities are taking new measures to provide the necessary reliability and quality of electrical power, such as by providing additional electrical equipment to assure that the constancy of electrical frequency and quality of electrical power, such as by providing additional electrical equipment to assure that the voltage and other characteristics of electrical power are constant.
Voltage regulation Long transmission lines have considerable inductance and capacitance .when a current flows through the line inductance and capacitance have the effect of varying the voltage on the line as the current varies. Thus the supply voltage varies with the load. Several kinds of devices are used to overcome this undesirable variation in an operation called regulation of the voltage. The devices include induction regulators and three-phase synchronous motors (called synchronous condensers), both of which vary the effective amount of inductance and capacitance in the transmission circuit.
Inductance and capacitance react with a tendency to nullify one another. When a load circuit has more inductive than capacitive reactance, as almost invariably occurs in large power systems, the amount of power delivered for a given voltage and current is less than when the two are equal. The ratio of these two amounts of power is called the power factor. Because transmission-line losses are proportional to current, capacitance is added to the circuit when possible, thus bringing the power factor as nearly as possible to 1.for this reason, large capacitors are frequently inserted as a part of power-transmission system.
World electric power production over the period from 1950 to 2003. The most recent year for which data are available, annual world electric power production and consumption rose from slightly less than 1 trillion kilowatt-hours ( kW.h) to 15.9 trillion Kw.h. a change also took place in the type of power generation. In 1950 about two-thirds of the world‘s electricity came from steam-generating sources and one-third from hydroelectric sources. In 2003 thermal sources produced 65 percent of the total. The growth in nuclear power slowed in some countries, notably the united states, in response to concerns about safety. Nuclear plants generated 20 percent of U.S. electricity in 2003; in France, the world leader, the figure was 78 percent.
Conservation
Much of the world’s electricity is produced from the use of nonrenewable resources, such as natural gas, coal, oil, and uranium. Coal, oil, and natural gas contain carbon, and burning these fossil fuels contributes to global emissions of carbon dioxide and other pollutants. Scientists believe that carton dioxide is the principal gas responsible for global warming, a steady rise in Earth’s surface temperature.
Consumers of electricity can save money and help protect the environment by eliminating unnecessary use of electricity, such as turning off lights when leaving a room. Other conservation methods include buying and using energy-efficient appliances and light bulbs, and using appliances, such as washing machines and dryers, at off-peak production hours when rates are lower. Consumers may also consider environmental measures such as purchasing “green power” when it is offered by a local utility. ”Green power” is usually more expensive but relies on renewable and environmentally friendly energy sources, such as wind turbines and geothermal power plants.
B Power System Automation
Power providers constantly deal with demands to increase productivity and reduce costs. This translates into the need for administrators, engineers, operators, planners, field crews, and others to collect and act on decision-making information. Power system vendors are following a trend to make devices smarter so they can create and communicate this information. The term “power system” describes the collection of devices that make up the physical systems that generate, transmit, and distribute power. The term “instrumentation and control (I&C) system” refers to the collection of devices that monitor, control, and protect the power system.
Power system automation refers to using I&C devices to perform automatic decision making and control of the power system.
Data Acquisition Data acquisition refers to acquiring, or collecting, data. This data is collected in the form of measured analog current or voltage values or the open or closed status of contact points. Acquired data can be used locally within the device collecting it, sent to another device in a substation, or sent from the substation to one or several databases for use by operators, engineers, planners, and administrators.
Power System Supervision Computer processes and personnel supervise, or monitor, the conditions and status of the power system using this acquired data. Operators and engineers monitor the information remotely on computer displays and graphical wall displays or locally, at the device, on front-panel displays and laptop computers.
Power System Control Control refers to sending command messages to a device to operate the I&C and power system devices. Traditional supervisory control and data acquisition (SCADA) systems rely on operators to supervise the system and initiate commands from an operator console on the master computer. Field personnel can also control devices using front-panel push buttons or a laptop comcuter.
Power System Automation System automation is the act of automatically controlling the power system via automated processes within computers and intelligent I&C devices. The processes rely on data acquisition, power system supervision, and power system control all working together in a coordinated automatic fashion. The commands are generated automatically and then transmitted in the same fashion as operator initiated commands.
I&C System IEDs I&C devices built using microprocessors are commonly referred to as intelligent electronic devices (IEDs). Microprocessors are single chip computers that allow the devices into which they are built to processes can be run in the IEDs, and communications are handled through a serial port like the communications ports on a computer. IEDs are found in the substation and on the pole-top.
Equipments for Power System Automation
Power system automation includes a variety of equipments. The principal items are listed and briefly described below.
Instrument Transformers Instrument transformers are used to sense power system current and voltage values. They are physically connected to power system apparatus and convert the actual power system signals, which include high voltage and current magnitudes, down to lower signal levels.
A 电力系统介绍
电力系统把其它形式的能源转化为电能并输送给用户。尽管不同于其它形式的能源，电能不容易储存，一旦生产出来，必须得到使用，但是电力的生产和传输相对高效和廉价。
电力系统的组成：当今的电力系统由六个主要部分组成：电站，升压变压器（将发出来的电升压至传输线所需高电压），传输线，变电站（电压降至配电线电压等级），配电线路和降压变压器（将配电电压降至用户设备使用的电压水平）。1、电站。电力系统的电站包括原动机，如由水，蒸汽驱动的涡轮，或者燃烧气体操控的电动机和发电机系统，世界上大多数的电能由煤炭、石油、核能或者燃气驱动的蒸汽发电厂产生。少量电能由水力，柴油和内燃机发电厂产生。2、变压器。现代电力系统使用变压器把电能转换为不同的电压。有了变压器，系统的每个阶段都能在合适的电压等级下运行。在典型的系统中，电站发电机发出的电压范围是1000伏到26000伏。变压器把电压升至138000到765000伏后，送至主传输线上。因为对于长距离传输，电压越高，效率越高。在变电站，电压被降至69000到138000伏，以便在配电系统中传输。另外一组变压器把电压进一步降至配电等级，如2400到4160伏，或者15，27，33KV。最终，在使用端，经配电变压器，电压再次被降至240V或120V。3、传输线。高压传输系统通常由铜线、铝线或者镀铜、镀铝的钢线组成，它们悬挂在高大钢格构塔架上成串的瓷质绝缘体上。由于含镀层钢线和铁塔的使用，增大了塔与塔之间的距离，降低了传输线的成本。在当前的直线安装中，每公里高压线只需建立6个铁塔。在一些地区，高压线悬挂于距离较近的木质电线杆上。对于低压配电线路，更多的使用木质电线杆，而不是铁塔。在城市和一些地区，明线存在安全危险或者被认为影响美观，所以使用绝缘地下电缆进行配电。一些电缆内核中空，供低压油循环。油可以为防止水对封闭线路的破坏提供临时保护。通常使用管式电缆，三根电缆放入线管中，并填满高压油。这些电缆用于传输高达345KV的电流。4、辅助设备。每个配电系统包含大量辅助设备来保护发电机、变压器和传输线。系统通常还包括用来调整电压或用户端其它电力特性的设备。为了保护电力系统设施，防止短路和过载，对于正常的开关操作，采用断路器。断路器是大型开关，在短路时或者电流突然上升的情况下自动切断电源。由于电流断开时，断路器触点两端会形成电流，一些大型断路器（如那些用来保护发电机和主输电线的断路器）通常浸入绝缘液体里面，如油，以熄灭电流。在大型空气开关和油断路器中，使用磁场来削弱电流。小型空气开关用于商场，工厂和现代家庭设备的保护。在住宅电气布线中，以前普遍采用保险丝。保险丝由熔点低的合金组成，安装在电路中，当电流超过一定值，它会熔断，切断电路。现在绝大多数住宅使用空气断路器。
供电故障：世界上大多数地方，局部或全国电力设施都连成电网。电网可以使发电实现区域共享。同意共享的每个电力企业可以获得不断增加的储备功率，使用更大、效率更高的发电机，从电网中获取电能以应对局部电力故障。互联的电网是大型复杂系统，包括被不同组织操控的部分。这些系统可以节约开支，提高整体可靠性，但是也带来了大范围停电的风险。例如，2003年8月14日，美国和加拿大发生了历史上最严重的停电事故。当时，这个区域61800兆瓦的电力供应中断，五千万人口受到影响。（一兆瓦大约可以满足750居民的用电需求）。停电事件迫切要求更新老化设备，提出关于全国电网可靠性的问题。尽管存在大范围停电危险，互联电网提供了必要的备份措施和供替换的线路，相对于孤立系统，其整体可靠性要高得多。国家或地区电网还可以应对由暴风雨、地震、泥石流、森林火灾、人员操作错误或者蓄意破坏造成的意外停电。供电质量 近年来，越来越多的精密复杂生产过程、计算机和网络及许多高科技消费品都使用电力为其提供能量。这些产品和生产过程对于供电的连续性和电压、频率的恒定性很敏感。于是，相关部门正采取新措施来保证供电的可靠性和质量。如提供附加的电气设备来保证电压和电能其它特性保持恒定。 1、电压调整。长距离传输线存在的电感和电容不容忽视。当电流流过线路时，随着电流的变化，电感和电容会对线路电压产生影响。这样，供电电压会随负荷变化。运行中，有几种设备用来克服这个波动，被称为电压调整。这些设备包括感应调节器、三相同步机（也称同步调相机），它们能够改变传输线路中的电感和电容的有效量。 电感和电容作用能相互抵消。当负载电流感性电抗大于容性电抗时，这种情况总是出现在大型电力系统中，对于给定的电压和电流，传送的功率小于两者相等的时候。这两个量功率之比称为功率因数。由于传输线损耗和电流成比例，如果可能，将在电路中使用电容，这样功率因数尽可能接近于1。正是这个原因，在电力传输系统中，经常使用大型电容器。2、世界电力生产。从1950年到2003年，最近一年的可用数据显示，每年世界电力生产和消费从小于1万亿千瓦时增长到15.9万亿千瓦时。同样，发电类型也发生了变化。在1950年，世界电力约2/3来自蒸汽源，约1/3 来自水电。2003年，热源生产65%的电能，水电却降至17%，核电占总量的16%。出于安全的考虑，在一些国家，特别是美国，核能的增长缓慢。2003年，美国电能的20%来自核电厂；在世界领先的法国，这个数字是78%。
保护世界上大多数电能的生产来自天然气、煤炭、石油和铀等不可再生资源。煤炭、石油、天然气含有碳元素，它们的燃烧加剧了二氧化碳和其它污染物的排放。科学家们认为，二氧化碳是导致全球变暖，地球表面温度上升的主要因素。电力用户通过节约用电，如离开房间时关闭电灯等措施消除不必要消耗，可以节省资金，有助于环境保护。其它保护措施包括购买和使用节能电器和灯泡，在费率较低的非用电高峰使用洗衣机和烘干机等电器。消费者也可以考虑环境措施，如购买当地公共部门提供的绿色能源等。绿色能源通常价格较贵，但依靠可再生和环境友好型资源，如风力轮机和地热发电厂。
B 电力系统自动化概述
提高生产力，降低成本，是电力供应商一直以来面对的问题。这就转化为需要管理者，工程师、操作员、计划者、现场人员和其它人员收集并执行决策信息。电力系统供应商遵从这一趋势，使设备变得智能化，这样，它们就可以创造并交流信息。术语“电力系统”描述的是用来产生、传输和分配电能的物理系统的组成设备集合。术语“设备和控制系统”指的是用来监视、控制和保护电力系统的设备集合。电力系统自动化指的是使用I&C 设备执行自动决策并对电力系统进行控制。
数据采集：数据采集指的是获取或者采集数据。采集的数据形式为测量的模拟电压或电流值，接触点的开关状态。采集到的数据可以被采集设备使用，发送到同一变电站的其它设备或者从变电站发送到一个或多个数据库供操作人员、工程师、计划人员和管理人员使用。
电力系统监视：有了获得的数据，计算机可以处理，人员可以监控电力系统的状况和状态。操作人员和工程师在远程可以通过计算机显示和图形墙显示，或者在现场，通过设备的前面板和笔记本电脑对信息进行监视。
电力系统控制：控制指的是对设备发送命令消息，实现对I&C和电力系统设备的操作。传统的监控和数据采集系统依赖于操作人员在主计算机的操控台监视系统并发送命令。现场人员也可以使用前面板按钮或笔记本电脑控制设备。
电力系统自动化：系统自动化是通过使用计算机和智能I&C设备的自动化过程而自动控制电力系统的行为。这个过程依赖于数据采集，电力系统监视和控制协调、自动地工作。像操作人员发送命令一样，命令自动生成，并以同样的方式传送。 I&C System IEDs：使用微处理器制造的I&C 设备通常指的是智能电子设备。微处理器是单片机，利用其制造的设备能够像计算机一样处理数据、接受命令和通信。在IED中，可以运行自动过程，像计算机通信端口一样，通过串口处理通信。在变电站和杆顶都能发现IED的身影。
电力系统自动化设备：电力系统自动化包括很多设备。下面列出主要设备并进行简单描述
。仪表（用）互感器：仪表（用）互感器用来检测电力系统电流和电压值。它们和电力系统设备连接在一起，把实际的电力系统信号，包括高电压，电流幅值，转换为小信号水平。
变换器：变换器把仪表（用）互感器输出的模拟信号从一种幅值变换到另一种，或者从一种类型变换到另一种，如把交流电流信号转换位直流电压信号。下载本文
