技术
protection relay
Protective relaying is that area of power system design concerned with minimizing service interruption and limiting damage to equipment when failures occur. The function of protective relaying is to cause the prompt removal of a defective element from a power system. The defective element may have a short circuit or it may be operating in an abnormal manner. Protective relaying systems are designed to detect such failures or abnormal conditions quickly (commensurate with system requirements) and to open a minimum of circuit breakers to isolate the defective element. The effect of quick isolation is threefold: (1) it minimizes or prevents damage to the defective element, thus reducing the time and expense of repairs and permitting quicker restoration of the element to service; (2) it minimizes the seriousness and duration of the defective elements affecting on the normal operation of the power system; and (3) it maximizes the power that can be transferred on power systems. The second and third points are of particular significance because they indicate the important role protective relaying plays in assuring maximum service reliability and in system design. The power that can be transmitted across system without the loss of synchronism is the function of fault clearing times. It is apparent that fast fault clearing times permit a higher power transfer than longer clearing times. High-speed clearing of faults can often provide a means for achieving higher power transfers and thereby defer investment in additional transmission facilities.
A protective relaying system is based on detecting fault conditions by continuously monitoring the power system variables such as current, voltage, power, frequency, and impedance. Measuring of currents and voltage is performed by instrument transformers of the potential type (PT) or current type (CT). Instrument transformers feed the measured variables to the relay system, which in turn, upon detecting a fault, commands circuit breaker (CB) to disconnect the faulted section of the system.
An electric power system is divided into several protective zones for generators, transformers, buses, transmission and distribution circuit, and motors. The division is such that zones are given adequate protection while keeping service interruption to a minimum. It is to be noted that each zone is overlapped to avoid unprotect (blind) areas. The connections of current transformers achieve the overlapping. The general philosophy of relay application is to divide the power system into zones that can be adequately protected by suitable protective equipment and can be disconnected from the power system in a minimum amount of time and with the least effect on the remainder of the power system. The protective relaying provided for each zone is divided into two categories: (1) primary relaying and (2) backup relaying. Primary relaying is the first line of defense when failures occur, and is connected to trip only the faulted element from the system. If a failure occurs in any primary zone the protective relays will operate to trip all of the breakers within that zone. If a breaker is omitted between two adjacent elements, both elements will be disconnected for a failure in either one. This latter arrangement is illustrated by the unit generator-transformer connection in the power plant. On bulk power generating and transmission systems, primary protection is designed to operate at high speed for all faults. Slower protection may be used in less important system areas but, in general, any system area will benefit by the fastest possible primary relaying.
If the fault is not cleared by the primary protection, backup relaying operates to clear the fault from the system. In general, backup relaying disconnects a greater portion of the system to isolate the fault. Backup protection is provided for possible failure in the primary relaying system and for possible circuit breaker failures. Any backup scheme must provide both relay backup as well as breaker backup. Ideally, the backup protection should be arranged so that anything that may cause the primary protection to fail will not also cause failure of the backup protection. Moreover, the backup protection must not operate until the primary protection has been given an opportunity to function. As a result, there is time delay associated with any backup operation. When a short circuit occurs, both the primary and the backup protection start to operate. If the primary protection clears the fault, the backup protection will reset without completing its function. If the fault is not cleared by the primary protection, the backup relaying will time out and trip the necessary breakers to clear the fault from the system.
There are two forms of backup protection in common use on power systems. They are remote backup and local backup.
The electric power industry has been one of the earliest users of the digital computer as a fundamental aid in the various design and analysis aspects of its activity. Computer-based systems have evolved to perform such complex tasks as generation control, economic dispatch (treated in chapter 11)and load-flow analysis for planning and operation , to name just a few application areas. research efforts directed at the prospect using digital computers to perform the tasks involved in power system protection date back to the mien-sixties and were motivated by the emergence of process-control computers a great deal of research is going on in this field, which is now referred to as computer relaying. Up to the early 1980s there had been no commercially availability protection systems offering digital computer-based relays. However, the availability of microprocessor technology has provided an impetus to computer relaying.*Microprocessors used as a replace*and solid state relays non provide a number of advantages while meeting the basic protection philosophy requirement of decentralization.
There are many perceived benefits of a digital relaying system:
The main elements of a digital computer-based relay are indicated in Figure 9-59. The input signals to the relay are analog (continuous) and digital power system variables. The digital inputs are of the order of five to ten and include status changes (on-off) of contacts and changes in voltage levels in a circuit. The analog signals are the 60-Hz currents and voltages. The number of analog signals needed depends on the relay function but is in the range of 3 to 30 in all cases. The analog signals are scaled down (attenuated) to acceptable computer input levels ( 10 volts maximum) and then converted to digital (discrete) form through analog/digital converters (ADC). These functions are performed in the block labeled “Analog Input Subsystem.”
The digital output of the relay is available through the computer’s parallel output port, five-to-ten digital outputs are sufficient for most applications.
The analog signals are sampled at a rate between 210 Hz to about 2000 Hz. The sampled signals are entered into the scratch pad (RAM) and are stored in a secondary data file for historical recording. A digital filter removes noise effects from the sampled signals. The relay logic program determines the functional operation of the relay and uses the filtered sampled signals to arrive at a trip or no trip decision which is then communicated to the system.
The heart of the relay logic program is a relaying algorithm that is designed to perform the intended relay function such as over currents detection, differential protection, or distance protection, etc. It is not our intention in this introductory text to purse this involved in a relaying algorithm, we discuss next one idea for peak current detection that is the function of a digital over current relay.
Microcomputer-based Relaying
A newer development in the field of power system protection is the use of computers (usually microcomputers) for relaying. Although computers provide the same protection as that supplied by conventional relays, there are some advantages to the use of computer-based relaying. The logic capability and application expansion possibilities for computer-based, relaying is much greater than for electromechanical devices. Computer-base relaying samples the values of the current, voltage, and by use of A/D converters, change these analog values to digital form and then send them to the computer. In the event of a fault, the computer can calculate the fault’s current values and characteristics, and settings can be changed merely by reprogramming. Computer-based relaying are also capable of locating faults, which has been one of the most popular features in their application. In addition, self-checking features can be built in and sequence of events information can be downloaded to remote computers for fast analysis of relaying operations.
Computer-based relaying system consists of subsystems with well defined functions. Although a specific subsystem may be different in some of its details, these subsystems are most likely to be incorporated in its design in some form. The block diagram in Figure 13-1 shows the principal subsystems of a computer-base relaying. The processor is the center of its organization. It is responsible for the execution of relaying programs, maintenance of various timing functions, and communicating with its peripheral equipment. Several types of memories are shown in Figure13-1-each of them serves a specific need. The Random Access Memory (RAM) holds the input sample data as they are brought in and processed. The Read Only Memory (ROM) or Programmable Read Only Memory (PROM) is used to store the programs permanently. In some cases the programs may execute directly from the ROM if its read time is short enough. If this is not the case, the programs must be copied from the ROM into the RAM during an initialization stage, and then the real-time execution would take place from the RAM. The Erasable PROM (EPROM) is needed for storing certain parameters (such as the relaying settings) which may be changed from time to time, but once it is set it must remain fixed even if the power supply to the computer is interrupted.
The relaying inputs are currents and voltages—or, to a lesser extent—digital signals indicating contact status. The analog signals must be converted to voltage signals suitable for conversion to digital from. The current and voltage signals obtained from current and voltage transformer secondary windings must be restricted to a full scale value of +10 volts. The current inputs must be converted to voltages by resistive shunts. As the normal current transformer secondary currents may be as high as hundreds of amperes, shunts of resistance of a few milliohms are needed to produce the desired voltage for the Analog to Digital Converter (ADC). An alternative arrangement would be to use an auxiliary current transformer to reduce the current to a lower level. An auxiliary current transformer serves another function: that of providing electrical isolation between the main CT secondary and the computer input system.
Since the digital computer can be programmed to perform several functions as long as it has the input and output signals needed for those functions. It is a simple matter to the relaying computer to do many other substation tasks, for example, measuring and monitoring flows and voltages in transformers and transmission lines, controlling the opening and closing of circuit breakers and switches, providing backup for other devices that have failed, are all functions that can be taken over by the relaying computer. With the program ability and communication capability, the computer-based relaying offers yet another possible advantage that is not easily realizable in a conventional system. This is the ability to change the relay characteristics (settings) as the system conditions warrant it. With reasonable prospects of having affordable computer-based relaying which can be dedicated to a single protection function, attention soon turned to the opportunities offered by computer-based relaying to integrate them into a substation, perhaps even a system-wide network. Integrated computer systems for substations which handle relaying, monitoring, and control tasks offer novel opportunities for improving overall system performance.
继 电 保 护
1. 继电器
当失效发生的时候继电器是以电力驱动的系统设计的那一个区域由于将服务中断减到最少而且限制对仪器的伤害有关。 给予保护的继电器的功能要引起提示来自一个以电力系统一种有缺陷的元素移动。有缺陷的元素可能有一个短路，或它可能以 非正常方式操作。 继电器系统很快地 ( 同量的由于系统需求) 被设计发现如此的失效或非正常情况和打开线路断路器的一个最小量隔离有缺陷的元素。快的隔离度的效果是三倍的:(1)它将或减到最少避免对有缺陷的元素伤害 ,如此减少修理的时间和费用而且允许元素的较快恢复维修;(2)它将严重和有缺陷的元素期间减到最少在以电力驱动的系统正常的操作上影响；和 (3) 它取可能被转移通功率系统的功率最大值。 因为他们指出重要的角色给予保护的继电器保证的最大值的游戏服务可靠性 , 所以秒和第三点是特别的重要性和在系统设计中。可能被传输过没有同步的损耗系统的功率是层错清扫时代的功能。很明显地 , 快速的层错清扫时代允许一个较高的功率移动胜于比较长的清扫时代。 层错的高速清扫能时常提供一个方法给达成较高的功率移动和藉此延期附加的传输设备的投资。
给予保护的继电器系统以藉着不断地监视功率系统变量 , 像是电流，电压，功率，频率和阻抗发现层错情况为基础。电流的测定和电压被潜在性类型 (PT) 或现在的类型互感器运行。 (CT)互感器在发现一个层错之上对继电器系统的过变量,命令线路断路器 (CB) 分离系统的被过失的线段。
一个电力系统被发电机，变压器，公共汽车，传输和分配线路 , 和电动机区分为一些保护的区域。以致于当保存对一个最小量的服务中断的时候 , 区域被给适当的保护。 它是到是着名的每个地域被重叠避免不保护 (盲) 区域。 电流互感器的连接达成重叠绕包。继电器一般要分开功率系统尽可能足够地适当的给予仪器保护，而且可能被在最小量的时间内从电力系统分离的区域和由于在以电力系统剩余元件方面的最少效果。继电器为每个区域提供被区分为二个种类: (1)主继电器(2) 后备继电器。 当失效时, 而且被连接只来自系统的错误值时候 ,主继电器是第一道防线。 如果失效继电器将会操作在那一个区域里面所有的断路器的任何区域中发生。如果一个断路器在二种毗连的元件之间被省略,两种元件当任意一个失效都将会被断开。 这一个后者的安排被某一个发电厂的发电机变压器连接举例说明。传输功率的系统,主保护被设计为快速操作。较慢的保护可能被用于后备保护。但是,大体上，任何的系统区域将会受益于快速继电器。
如果故障不是被主保护清除的,后备继电器操作切除来自系统故障。大体上, 后备继电器较好切除系统的故障。 后备保护继电器系统和对可能的线路断路器失效。其预备方案也一定要提供。 理论上，后备保护应该被安排以便可能引起要失败的主保护的任何事不会也引起后备保护的失效。 而且，后备保护不能操作直到主要的保护已经被给一个机会动作。结果,时间被其操作联合的延迟。 当一个短路发生的时候,主保护和后备保护开始操作。 如果主保护切除故障,后备保护将会不完成它的功能而重新设定。 如果故障不是被主保护切除的,后备继电器将会计时出并且断开断路器切除来自系统的故障。
通常使用功率的后备保护的二种系统。 他们是远后备和近后备保护。
(1) 远后备保护 。 在远后备继电器中，故障从系统失效已经发生那里被切除失效。
2. 微机继电器
电力工业已经是数传计算机的最早使用者之一如各种不同的设计和它的活度分析方面的一个基本的帮助。 计算机系统已经进展运行如此复杂的工作如各种计划交集运算的控制，经济的分配和负载- 流程分析，只是命名一些区域。在屏幕上指示使用要运行在以电力系统保护期限内是积极参与的工作数传计算机的研究藉着过程控制计算机的出现很多的研究回到风采- 六十和是给与动机正在这一个场中继续, 现在被称为计算机继电器。 达到 1980 年代早期商业地已经没有可用性保护系统提供数传计算机-建立继电器。 然而，微处理器技术的可用性已经提供一种动力给计算机继电器。微处理器用当做一代替固体继电器提供许多的利益当符合分散的基本保护需求的时候。
有传感的继电器系统:
1) 经济学: 藉由定态数传硬件的费用方面的减少,以传统的继电器的费用增加加倍。它合理的承担那一部计算机继电器是吸引人的替代选择。软件显影可能被期望藉由利用被呈现到基本继电器工作的产生微处理器的经济效益在外是
2) 可靠性: 一个数字系统不断活跃提供高度的自我发现意外的失效在数传里面继电器系统。
3) 柔性: 校订或修正藉由变更操作的情况制造必需可能被藉由利用数字系统特征适应。这会引导被修理和维修目的减少部份的详细目录
4) 系统交互作用: 在遥远的分局不断地检测系统表现的数传硬件的可用性能提高控制中心可用的信息程度。短暂的数据职位层错分析以被数字继电器检测的系统变量为基础可能被运行而且记录被外围设备。
对继电器的输入信号是类比 (连续的) 和数传以电力驱动的系统变量。 数传输入是五到十的次序而且包括接触件和状态变化 (开关) 方面的改变一个线路的电压电平。模拟信号是 60个赫兹的电流和电压。被需要的类比信号的数字在所有的情况在 3 到 30 的范围中仰赖继电器功能但是。模拟信号被依比例决定下 (变薄) 到可接受的计算机输入电平 (10 伏特最大值) 然后对数传 (不连续的) 形式经过类比/ 数传转换器转换。 (ADC) 这些功能在毛坯中被运行分类了 "类比输入次要系统".
数传继电器的输出是可得的经过计算机的平行输出端口,五-到-十数传输出对最大多数的要求是满足的。
模拟信号以在 210个赫兹之间的比率被抽取样品到大约 2000个赫兹。 采样信号被参与稿纸簿随机存取储存器) 而且被储存在一笔中级的数据文件中为历史的录制。来自采样信号的数字滤波器噪声效果。 继电器逻辑程序决定继电器的功能操作而且使用那过滤抽取样品了信号达成一个旅行或没有是然后沟通到系统的旅行判决。
继电器逻辑程序的核心是一个继电器被设计运行如此的继电器功能的运算法则当做超过电流检测，差别的保护或距离保护等。 它不是在这个介绍的本文中的我们意图这积极参与的在一个继电器运算法则中，我们为在电流继电器上的数传功能的峰值电流检测然后讨论。
3. 微机保护
电力系统保护领域中一个新发展是用计算机（通常为微型机）来进行继电保护。虽然计算机能提供于传统继电保护相同的保护，但它有一些优点。计算机继电保护比机电式继电保护装置在逻辑能力和应用的扩展方面要强的多。计算机继电保护可每秒进行多次电流、电压以及其它量的采样，通过A/D转换器将这些模拟量转变成数字量，然后送入计算机。在系统出现故障时，计算机可计算出故障电流的值和特性，整定值的改变仅改变编程就可以了。微机继电保护还能进行故障的定位，这已成为其应用时最受欢迎的特点之一。此外，可配置自检功能，事件顺序信息可下载到远方计算机以进行继电保护工作的快速分析。
计算机继电保护系统由一些功能明确的子系统组成。虽然各具体子系统可能在某些细节方面有所不同，但这些子系统很可能是以某种形式溶合在设计中。处理器是整个组合的中心。它负责继电保护程序的执行，各定时功能的保持，以及与外部设备的通讯。随机存储器（RAM）存放着送入进行处理下，若是读出时间足够短，则程序可直接从ROM执行。若不是这样，在初始化阶段，程序须从ROM复制到RAM中，然后从RAM实时执行。可擦写PROM（EPROM）用来存放某些参数（如继电保护整定值），这些参数可能经常变动，但一旦设置好，即使计算机电源断电，也保持不变。
继电保护的输入是电压和电流，或较少的是指示触头状态的数字信号。模拟信号须转成适合于变换到数字形式的电压信号。从电流互感器和电压互感器获得的电流信号和电压信号须限制到 10V的满额值。电流输入须由并联电阻变换成电压。由于电流互感器二次侧电流可高达数百安培，要产生提供给模—数转换器（ADC）的需要的电压就需要数微欧姆的并联电阻。另一种方法是采用中间电流互感器降低电流到较低的数值。中间电流互感器还起到另一个作用：对电流互感器二次侧与计算机输入系统进行电气隔离。
只要提供了各种功能所需要的输入和输出信号，数字计算机就可以编程来执行这些功能。要使继电保护的计算机执行变电站的其他任务是很容易的事。例如，监测变压器和输电线路中的潮流，控制断路器和开关的分闸和合闸，对故障设备提供备用，都是继电保护计算机可执行的功能。由于可编程能力和通信能力，计算机继电保护具有传统继电保护系统中不可能实现的优点。这就是按系统条件准许而改变继电器特性（整定值）的能力。随着相当看好计算机继电保护可提供半个保护功能，注意力很快就转到将计算机继电保护综合到变电站范围，甚至系统范围的机会。变电站综合计算机系统可进行继电保护、监测和控制、这对提高整个系统性能提供了新的机遇。
